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SQL Window Functions Explained Example Queries
Window functions are an advanced type of function in SQL. In this tutorial, you will learn about what window functions in SQL are, how they work and you'll also look at some of the clauses you can add in Windows
Window functions give you access to features like advanced analytics and data manipulation without the need to write complex queries.
In this lesson you will learn about what window functions are and how they work. Without further ado let's get started.
What is a Window Function?
Before learning exactly what a window function is, let's define the meaning of a term that will appear frequently in this article: result set.
In SQL, a result set is the data or result that is returned from a query. That is, it's the result (table) of running the code of a select statement.
For you to understand what a window function is, let's break the words down into pieces.
What exactly is a window in SQL?
A window is basically a set of rows or observations in a table or result set. In a table you may have more than one window depending on how you specify the query – you will learn about this shortly. A window is defined using the OVER() clause in SQL.
You will learn how to determine the number of windows in a result set later in this article.
What is a Function?
Functions are predefined in SQL and you use them to perform operations on data. They let you do things like aggregating data, formatting strings, extracting dates, and so on.
So windows functions are SQL functions that enable us to perform operations on a window – that is, a set of records.
The interesting thing about window functions is that with them you can specify the windows you want to apply the function on. For example, we can partition the full result set into various groups/windows.
Before we go into the syntax of Window functions, let's have a look at the categories of window functions.
Different Types of Window Functions
There are a lot of window functions that exist in SQL but they are primarily categorized into 3 different types:
- Aggregate window functions
- Value window functions
- Ranking window functions
Aggregate window functions are used to perform operations on sets of rows in a window(s). They include SUM(), MAX(), COUNT(), and others.
Rank window functions are used to rank rows in a window(s). They include RANK(), DENSE_RANK(), ROW_NUMBER(), and others.
Value window functions are like aggregate window functions that perform multiple operations in a window, but they're different from aggregate functions. They include things like LAG(), LEAD(), FIRST_VALUE(), and others. We will see their usefulness later in the section.
Sample Table
In this tutorial you will be working with a table called student_score which contains data such as student_id, student_name, dep_name and score.
You can create the table using the following code:
DROP TABLE IF EXISTS student_score;
CREATE TABLE student_score (
student_id SERIAL PRIMARY KEY,
student_name VARCHAR(30),
dep_name VARCHAR(40),
score INT
);
INSERT INTO student_score VALUES (11, 'Ibrahim', 'Computer Science', 80);
INSERT INTO student_score VALUES (7, 'Taiwo', 'Microbiology', 76);
INSERT INTO student_score VALUES (9, 'Nurain', 'Biochemistry', 80);
INSERT INTO student_score VALUES (8, 'Joel', 'Computer Science', 90);
INSERT INTO student_score VALUES (10, 'Mustapha', 'Industrial Chemistry', 78);
INSERT INTO student_score VALUES (5, 'Muritadoh', 'Biochemistry', 85);
INSERT INTO student_score VALUES (2, 'Yusuf', 'Biochemistry', 70);
INSERT INTO student_score VALUES (3, 'Habeebah', 'Microbiology', 80);
INSERT INTO student_score VALUES (1, 'Tomiwa', 'Microbiology', 65);
INSERT INTO student_score VALUES (4, 'Gbadebo', 'Computer Science', 80);
INSERT INTO student_score VALUES (12, 'Tolu', 'Computer Science', 67);Syntax for Window Functions
In a simple expression, a window function looks like this:
function(expression|column) OVER(
[ PARTITION BY expr_list optional]
[ ORDER BY order_list optional]
)Let's go over the syntax piece by piece:
function(expression|column) is the window function such as SUM() or RANK().
OVER() specifies that the function before it is a window function not an ordinary one. So when the SQL engine sees the over clause it will know that the function before the over clause is a window function.
The OVER() clause has some parameters which are optional depending on what you want to achieve. The first one being PARTITION BY.
The PARTITION BY divides the result set into different partitions/windows. For example if you specify the PARTITION BY clause by a column(s) then the result-set will be divided into different windows of the value of that column(s).
The expr_list in the PARTITION BY clause is:
expression | column_name [, expr_list ]Which means that the PARTITION BY can have an expression, a column, or more than one occurrence or an expression or column which must be separated by a comma. For example PARTITION BY column1, column2.
The next parameter ORDER BY is used to sort the observations in a window. The ORDER BY clause takes order_list which is:
expression | column_name [ ASC | DESC ]
[ NULLS FIRST | NULLS LAST ][, order_list ]where order_list can be a expression or column name and you can also specify the sort order (either ascending or descending), or you can sort any null values first or last. Also the order by can take many expressions or column names.
As stated earlier, the OVER() clause is used to specify the window in a result set. Now one thing to note is if any parameter is not specified in the OVER() clause the default number of windows in the result set will be one.
You use the PARTITION BY and ORDER BY parameters to determine or specify the numbers of windows. Let's go over an example.
How to Use a Window Function – Example
Let's go over an example of how to use a window function. Say for instance you want to compare the minimum score and maximum score from all the records in the table we created earlier. You can do that using a window function as shown below.
Remember that not specifying a partition clause in the OVER clause will cause all the windows to span through the entire dataset.
SELECT
*,
MAX(score) OVER() AS maximum_score,
MIN(score) OVER() AS minimum_score
FROM student_score;As you can see, we have the minimum and maximum salary across the entire dataset.
Table showing result of window function
Also, note that the above query can be also achieved using subqueries like this:
SELECT *,
(SELECT MAX(score) FROM student_score) AS maximum_score,
(SELECT MIN(score) FROM student_score) AS minimum_score
FROM student_score;As you can see, the window function is easier to comprehend compared to the subquery method which looks a bit more advanced.
How to Use a Window Function with PARTITION BY
Say, for instance, that you want to split the dataset into different partitions. Then you want to compare each record in each partition with an aggregate value or a calculated value of each partition. You can specify the PARTITION BY clause in the OVER function.
For example, say you want to compare the maximum score and average score in each department with the individual score. You can do this by specifying the PARTITION BY clause in the OVER statement and also use it with the aggregate function you want to use to achieve your desired result.
SELECT
*,
MAX(score)OVER(PARTITION BY dep_name) AS dep_maximum_score,
ROUND(AVG(score)OVER(PARTITION BY dep_name), 2) AS dep_average_score
FROM student_score;You can see that the PARTITION BY clause specified in the OVER() clause split the result set into 4 different partitions. This is because there are 4 different departments in the dep_name column (which are Biochemistry, Computer Science, Industrial Chemistry, and Microbiology).
Now after the PARTITION BY clause, you can then calculate the aggregate function for each record in the different departments.
You can see from the above image that the aggregate function MAX() and AVG() is calculated for each partition.
Other Examples of Window Functions
Let's go over some of the common window functions you will work with in SQL.
How to Use the ROW_NUMBER Function
You use ROW_NUMBER() to assign serial numbers to records in a window. Say we want to assign serial numbers to the records in a partition. For example, we want to add row numbers to the dataset based on their names in alphabetical order. You can do that using the following code:
SELECT
*,
ROW_NUMBER() OVER(ORDER BY student_name) AS name_serial_number
FROM student_score;
As you can see from the above image, the student_name with the smallest value (that is, the one that falls earliest in the alphabet) is Gbadebo since it starts with G. Then 1 is added as its row number which is followed by the name that begins with H, and so on.
How to Use the RANK Function
RANK(), as the name implies, lets you rank observations in a window but with gaps. Let's see what this means:
SELECT
*,
RANK()OVER(PARTITION BY dep_name ORDER BY score DESC)
FROM student_score;
As you can see in the above code, the result set was partitioned into different windows based on the department column. Then we used the ORDER BY clause to sort the student records based on their score in descending order in each partition. After that, we applied the RANK function.
Now concerning the gaps, as you can see in the highlighted part in the above image, two records in the Computer Science department have the same score (80). This caused both to be ranked with the value 2 (instead of one being ranked 2 and the other 3). So it doesn't know how to handle a tie, basically.
You can avoid this scenario using another window function called DENSE_RANK that ranks observations in a window without these gaps.
How to Use the DENSE_RANK Function
DENSE_RANK is similar to RANK except that it ranks observations in a window without gaps.
SELECT
*,
DENSE_RANK()OVER(PARTITION BY dep_name ORDER BY score DESC)
FROM student_score;
As you can see in the output above, when using DENSE_RANK, the next rank number (which is 3) was assigned to Tolu (unlike when using RANK which assigned Tolu a rank of 4, skipping 3 because of the tie).
How to Use the LAG Function
LAG is used to return the offset row before the current row within a window. By default it returns the previous row before the current row.
You typically use LAG when you want to compare the value of a previous row with the current row. It's commonly applied in time-series analysis. For example:
SELECT
*,
LAG(score) OVER(PARTITION BY dep_name ORDER BY score)
FROM student_score;
As shown in the first partition, the first record in the biochemistry partition (Yusuf's) does not have a previous value (that is, no record comes before it) so that's why null was returned. Then moving to the next record – Muritadoh's – it has a previous record, so it returns the previous value which is 70.
How to Use the Frame Clause in ORDER BY
Now you've learned some common window functions you might work with on a daily basis. So let's move on to learning another key concept related to the ORDER BY clause called the frame clause.
A frame clause, as the name implies, provides the frame (that is, the set of rows in a window) on which the function is to be applied. You use it to provide the offset of rows to be included or calculated with the current row (that is, the rows before or after the current row – the SQL engine process row one after the other).
Now before we look into how to specify a frame clause, let's look at some of the frame clause's assumptions:
- First, a frame clause does not apply to ranking functions. The ranking function only ranks the observation in the window based on the
ORDER BYclause. - When using an aggregate window function, you may not include the
ORDER BYclause. But when you use theORDER BYclause, it's a best practice to specify the frame clause for accurate results. What this means is say you want to use an aggregate window function and you want to also order the observations in that window by a column. It's best practice is to specify a frame clause so that you will get an accurate result. But if you are not ordering the observations in the window when using an aggregate function, you don't need to specify a frame clause.
You can specify a frame clause using two things – ROWS and RANGE. But in this part you will learn how to use the ROWS keyword since it is commonly used to specify a frame clause. The RANGE keyword is beyond the scope of this article.
The ROWS clause defines the frame in terms of the physical offset rows from the current rows. That is, it is used to specify the rows that will be used in conjunction with the current row for calculation.
For example the following frame clause ROWS BETWEEN 1 PRECEDING AND 1 FOLLOWING defines a frame that includes the current row, 1 row preceding it and 1 row following it.
Let's look at the keywords that you can use in conjunction with the ROWS clause:
N PRECEDINGis a keyword you use to specify the N rows that will be included in the calculation along with the current row. For example3 PRECEDINGmeans 3 rows preceding the current row.N FOLLOWINGworks likeN PRECEDINGexcepts that it works in an opposite manner.N FOLLOWINGspecifies the numbers of row after the current row.UNBOUNDED PRECEDINGmeans all rows before the current row.UNBOUNDED FOLLOWINGmeans all rows after the current row.CURRENT ROWis used to specify the current row.
For example, let's look at the below frame clause:
ROWS BETWEEN 2 PRECEDING AND CURRENT ROW will use less than or equal to 2 rows before the current row, along with the current row for the calculation.
Frame clause example
Let's look at an example. Say for instance you want to get the cumulative sum of all the student scores. You can do that by using a frame clause.
So first, to be able to do this, you need to first know the types of keywords you will specify in the frame clause.
Since you want to sum up all rows before the current row and the current row itself, you can use the UNBOUNDED PRECEDING keyword. Remember that this gets all rows before the current row and also uses the current row itself.
So the code to achieve that task is shown below:
SELECT
*,
SUM(score)OVER(ORDER BY student_id ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS cummulative_sum
FROM student_scoreLet's break down the window function code:
SUM(score)OVER(ORDER BY student_id ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW) AS cummulative_sum
Firstly in the OVER() clause, we sort the entire window – which is the whole dataset – using the student id.
Then we specify the frame clause which is ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW. This is all rows before the current row and the current row will be used for calculation.
The result is shown in the below image:
The first row in the dataset does not have any row before it. But since we also specify the CURRENT ROW keyword as the last frame, then the SQL engine finds its sum which equals 65.
Then moving to the second row. It has 1 row before it. So the SQL engine sums the score of the first row 65 with the current row which is 70. That is why the result is 135...and so on down the table.
When to Use a Window Function
You've learned what window functions are in this tutorial. Some practical cases where you can use them are:
- When you want to compare an aggregate value in a window with individual records in that window.
- When you want to do things like ranking, percentile, cumulative sum or running total, moving average, and so on.
Conclusion
In this tutorial, you've learned what window functions are, and you've also looked at some of the clauses you can add in Windows functions. One example is the PARTITION BY clause, which divides the result set into separate partitions or windows.
You also learned how to utilize the ORDER BY clause to order observations in a window and you saw various common examples of window functions.
Finally, you learned another advanced clause that you can use with window functions, the frame clause, which allows you to access more features of a window.
Thank you for reading all the way to the end.
Original article source at https://www.freecodecamp.org
#sql #database
What is GEEK
Buddha Community
1594369800
Introduction to Structured Query Language SQL pdf
SQL stands for Structured Query Language. SQL is a scripting language expected to store, control, and inquiry information put away in social databases. The main manifestation of SQL showed up in 1974, when a gathering in IBM built up the principal model of a social database. The primary business social database was discharged by Relational Software later turning out to be Oracle.
Models for SQL exist. In any case, the SQL that can be utilized on every last one of the major RDBMS today is in various flavors. This is because of two reasons:
1. The SQL order standard is genuinely intricate, and it isn’t handy to actualize the whole standard.
2. Every database seller needs an approach to separate its item from others.
Right now, contrasts are noted where fitting.
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1677668905
TS-mockito: Mocking Library for TypeScript
TS-mockito
Mocking library for TypeScript inspired by http://mockito.org/
1.x to 2.x migration guide
Main features
- Strongly typed
- IDE autocomplete
- Mock creation (
mock) (also abstract classes) #example - Spying on real objects (
spy) #example - Changing mock behavior (
when) via: - Checking if methods were called with given arguments (
verify) - Resetting mock (
reset,resetCalls) #example, #example - Capturing arguments passed to method (
capture) #example - Recording multiple behaviors #example
- Readable error messages (ex.
'Expected "convertNumberToString(strictEqual(3))" to be called 2 time(s). But has been called 1 time(s).')
Installation
npm install ts-mockito --save-dev
Usage
Basics
// Creating mock
let mockedFoo:Foo = mock(Foo);
// Getting instance from mock
let foo:Foo = instance(mockedFoo);
// Using instance in source code
foo.getBar(3);
foo.getBar(5);
// Explicit, readable verification
verify(mockedFoo.getBar(3)).called();
verify(mockedFoo.getBar(anything())).called();
Stubbing method calls
// Creating mock
let mockedFoo:Foo = mock(Foo);
// stub method before execution
when(mockedFoo.getBar(3)).thenReturn('three');
// Getting instance
let foo:Foo = instance(mockedFoo);
// prints three
console.log(foo.getBar(3));
// prints null, because "getBar(999)" was not stubbed
console.log(foo.getBar(999));
Stubbing getter value
// Creating mock
let mockedFoo:Foo = mock(Foo);
// stub getter before execution
when(mockedFoo.sampleGetter).thenReturn('three');
// Getting instance
let foo:Foo = instance(mockedFoo);
// prints three
console.log(foo.sampleGetter);
Stubbing property values that have no getters
Syntax is the same as with getter values.
Please note, that stubbing properties that don't have getters only works if Proxy object is available (ES6).
Call count verification
// Creating mock
let mockedFoo:Foo = mock(Foo);
// Getting instance
let foo:Foo = instance(mockedFoo);
// Some calls
foo.getBar(1);
foo.getBar(2);
foo.getBar(2);
foo.getBar(3);
// Call count verification
verify(mockedFoo.getBar(1)).once(); // was called with arg === 1 only once
verify(mockedFoo.getBar(2)).twice(); // was called with arg === 2 exactly two times
verify(mockedFoo.getBar(between(2, 3))).thrice(); // was called with arg between 2-3 exactly three times
verify(mockedFoo.getBar(anyNumber()).times(4); // was called with any number arg exactly four times
verify(mockedFoo.getBar(2)).atLeast(2); // was called with arg === 2 min two times
verify(mockedFoo.getBar(anything())).atMost(4); // was called with any argument max four times
verify(mockedFoo.getBar(4)).never(); // was never called with arg === 4
Call order verification
// Creating mock
let mockedFoo:Foo = mock(Foo);
let mockedBar:Bar = mock(Bar);
// Getting instance
let foo:Foo = instance(mockedFoo);
let bar:Bar = instance(mockedBar);
// Some calls
foo.getBar(1);
bar.getFoo(2);
// Call order verification
verify(mockedFoo.getBar(1)).calledBefore(mockedBar.getFoo(2)); // foo.getBar(1) has been called before bar.getFoo(2)
verify(mockedBar.getFoo(2)).calledAfter(mockedFoo.getBar(1)); // bar.getFoo(2) has been called before foo.getBar(1)
verify(mockedFoo.getBar(1)).calledBefore(mockedBar.getFoo(999999)); // throws error (mockedBar.getFoo(999999) has never been called)
Throwing errors
let mockedFoo:Foo = mock(Foo);
when(mockedFoo.getBar(10)).thenThrow(new Error('fatal error'));
let foo:Foo = instance(mockedFoo);
try {
foo.getBar(10);
} catch (error:Error) {
console.log(error.message); // 'fatal error'
}
Custom function
You can also stub method with your own implementation
let mockedFoo:Foo = mock(Foo);
let foo:Foo = instance(mockedFoo);
when(mockedFoo.sumTwoNumbers(anyNumber(), anyNumber())).thenCall((arg1:number, arg2:number) => {
return arg1 * arg2;
});
// prints '50' because we've changed sum method implementation to multiply!
console.log(foo.sumTwoNumbers(5, 10));
Resolving / rejecting promises
You can also stub method to resolve / reject promise
let mockedFoo:Foo = mock(Foo);
when(mockedFoo.fetchData("a")).thenResolve({id: "a", value: "Hello world"});
when(mockedFoo.fetchData("b")).thenReject(new Error("b does not exist"));
Resetting mock calls
You can reset just mock call counter
// Creating mock
let mockedFoo:Foo = mock(Foo);
// Getting instance
let foo:Foo = instance(mockedFoo);
// Some calls
foo.getBar(1);
foo.getBar(1);
verify(mockedFoo.getBar(1)).twice(); // getBar with arg "1" has been called twice
// Reset mock
resetCalls(mockedFoo);
// Call count verification
verify(mockedFoo.getBar(1)).never(); // has never been called after reset
You can also reset calls of multiple mocks at once resetCalls(firstMock, secondMock, thirdMock)
Resetting mock
Or reset mock call counter with all stubs
// Creating mock
let mockedFoo:Foo = mock(Foo);
when(mockedFoo.getBar(1)).thenReturn("one").
// Getting instance
let foo:Foo = instance(mockedFoo);
// Some calls
console.log(foo.getBar(1)); // "one" - as defined in stub
console.log(foo.getBar(1)); // "one" - as defined in stub
verify(mockedFoo.getBar(1)).twice(); // getBar with arg "1" has been called twice
// Reset mock
reset(mockedFoo);
// Call count verification
verify(mockedFoo.getBar(1)).never(); // has never been called after reset
console.log(foo.getBar(1)); // null - previously added stub has been removed
You can also reset multiple mocks at once reset(firstMock, secondMock, thirdMock)
Capturing method arguments
let mockedFoo:Foo = mock(Foo);
let foo:Foo = instance(mockedFoo);
// Call method
foo.sumTwoNumbers(1, 2);
// Check first arg captor values
const [firstArg, secondArg] = capture(mockedFoo.sumTwoNumbers).last();
console.log(firstArg); // prints 1
console.log(secondArg); // prints 2
You can also get other calls using first(), second(), byCallIndex(3) and more...
Recording multiple behaviors
You can set multiple returning values for same matching values
const mockedFoo:Foo = mock(Foo);
when(mockedFoo.getBar(anyNumber())).thenReturn('one').thenReturn('two').thenReturn('three');
const foo:Foo = instance(mockedFoo);
console.log(foo.getBar(1)); // one
console.log(foo.getBar(1)); // two
console.log(foo.getBar(1)); // three
console.log(foo.getBar(1)); // three - last defined behavior will be repeated infinitely
Another example with specific values
let mockedFoo:Foo = mock(Foo);
when(mockedFoo.getBar(1)).thenReturn('one').thenReturn('another one');
when(mockedFoo.getBar(2)).thenReturn('two');
let foo:Foo = instance(mockedFoo);
console.log(foo.getBar(1)); // one
console.log(foo.getBar(2)); // two
console.log(foo.getBar(1)); // another one
console.log(foo.getBar(1)); // another one - this is last defined behavior for arg '1' so it will be repeated
console.log(foo.getBar(2)); // two
console.log(foo.getBar(2)); // two - this is last defined behavior for arg '2' so it will be repeated
Short notation:
const mockedFoo:Foo = mock(Foo);
// You can specify return values as multiple thenReturn args
when(mockedFoo.getBar(anyNumber())).thenReturn('one', 'two', 'three');
const foo:Foo = instance(mockedFoo);
console.log(foo.getBar(1)); // one
console.log(foo.getBar(1)); // two
console.log(foo.getBar(1)); // three
console.log(foo.getBar(1)); // three - last defined behavior will be repeated infinity
Possible errors:
const mockedFoo:Foo = mock(Foo);
// When multiple matchers, matches same result:
when(mockedFoo.getBar(anyNumber())).thenReturn('one');
when(mockedFoo.getBar(3)).thenReturn('one');
const foo:Foo = instance(mockedFoo);
foo.getBar(3); // MultipleMatchersMatchSameStubError will be thrown, two matchers match same method call
Mocking interfaces
You can mock interfaces too, just instead of passing type to mock function, set mock function generic type Mocking interfaces requires Proxy implementation
let mockedFoo:Foo = mock<FooInterface>(); // instead of mock(FooInterface)
const foo: SampleGeneric<FooInterface> = instance(mockedFoo);
Mocking types
You can mock abstract classes
const mockedFoo: SampleAbstractClass = mock(SampleAbstractClass);
const foo: SampleAbstractClass = instance(mockedFoo);
You can also mock generic classes, but note that generic type is just needed by mock type definition
const mockedFoo: SampleGeneric<SampleInterface> = mock(SampleGeneric);
const foo: SampleGeneric<SampleInterface> = instance(mockedFoo);
Spying on real objects
You can partially mock an existing instance:
const foo: Foo = new Foo();
const spiedFoo = spy(foo);
when(spiedFoo.getBar(3)).thenReturn('one');
console.log(foo.getBar(3)); // 'one'
console.log(foo.getBaz()); // call to a real method
You can spy on plain objects too:
const foo = { bar: () => 42 };
const spiedFoo = spy(foo);
foo.bar();
console.log(capture(spiedFoo.bar).last()); // [42]
Thanks
- Szczepan Faber (https://www.linkedin.com/in/szczepiq)
- Sebastian Konkol (https://www.linkedin.com/in/sebastiankonkol)
- Clickmeeting (http://clickmeeting.com)
- Michał Stocki (https://github.com/michalstocki)
- Łukasz Bendykowski (https://github.com/viman)
- Andrey Ermakov (https://github.com/dreef3)
- Markus Ende (https://github.com/Markus-Ende)
- Thomas Hilzendegen (https://github.com/thomashilzendegen)
- Johan Blumenberg (https://github.com/johanblumenberg)
Download Details:
Author: NagRock
Source Code: https://github.com/NagRock/ts-mockito
License: MIT license
1661592007
A Collection Of Swift Tips & Tricks That I've Shared on Twitter
⚠️ This list is no longer being updated. For my latest Swift tips, checkout the "Tips" section on Swift by Sundell.
Swift tips & tricks ⚡️
One of the things I really love about Swift is how I keep finding interesting ways to use it in various situations, and when I do - I usually share them on Twitter. Here's a collection of all the tips & tricks that I've shared so far. Each entry has a link to the original tweet, if you want to respond with some feedback or question, which is always super welcome! 🚀
Also make sure to check out all of my other Swift content:
- 📖 My weekly articles, with a new post about Swift every Sunday!
- 🎧 My podcast which features special guests from around the community.
- 📬 My monthly newsletter - rounding up all of my content, including these tips, on the 1st of every month.
#102 Making async tests faster and more stable
🚀 Here are some quick tips to make async tests faster & more stable:
- 😴 Avoid sleep() - use expectations instead
- ⏱ Use generous timeouts to avoid flakiness on CI
- 🧐 Put all assertions at the end of each test, not inside closures
// BEFORE:
class MentionDetectorTests: XCTestCase {
func testDetectingMention() {
let detector = MentionDetector()
let string = "This test was written by @johnsundell."
detector.detectMentions(in: string) { mentions in
XCTAssertEqual(mentions, ["johnsundell"])
}
sleep(2)
}
}
// AFTER:
class MentionDetectorTests: XCTestCase {
func testDetectingMention() {
let detector = MentionDetector()
let string = "This test was written by @johnsundell."
var mentions: [String]?
let expectation = self.expectation(description: #function)
detector.detectMentions(in: string) {
mentions = $0
expectation.fulfill()
}
waitForExpectations(timeout: 10)
XCTAssertEqual(mentions, ["johnsundell"])
}
}
For more on async testing, check out "Unit testing asynchronous Swift code".
#101 Adding support for Apple Pencil double-taps
✍️ Adding support for the new Apple Pencil double-tap feature is super easy! All you have to do is to create a UIPencilInteraction, add it to a view, and implement one delegate method. Hopefully all pencil-compatible apps will soon adopt this.
let interaction = UIPencilInteraction()
interaction.delegate = self
view.addInteraction(interaction)
extension ViewController: UIPencilInteractionDelegate {
func pencilInteractionDidTap(_ interaction: UIPencilInteraction) {
// Handle pencil double-tap
}
}
For more on using this and other iPad Pro features, check out "Building iPad Pro features in Swift".
#100 Combining values with functions
😎 Here's a cool function that combines a value with a function to return a closure that captures that value, so that it can be called without any arguments. Super useful when working with closure-based APIs and we want to use some of our properties without having to capture self.
func combine<A, B>(_ value: A, with closure: @escaping (A) -> B) -> () -> B {
return { closure(value) }
}
// BEFORE:
class ProductViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
buyButton.handler = { [weak self] in
guard let self = self else {
return
}
self.productManager.startCheckout(for: self.product)
}
}
}
// AFTER:
class ProductViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
buyButton.handler = combine(product, with: productManager.startCheckout)
}
}
#99 Dependency injection using functions
💉 When I'm only using a single function from a dependency, I love to inject that function as a closure, instead of having to create a protocol and inject the whole object. Makes dependency injection & testing super simple.
final class ArticleLoader {
typealias Networking = (Endpoint) -> Future<Data>
private let networking: Networking
init(networking: @escaping Networking = URLSession.shared.load) {
self.networking = networking
}
func loadLatest() -> Future<[Article]> {
return networking(.latestArticles).decode()
}
}
For more on this technique, check out "Simple Swift dependency injection with functions".
#98 Using a custom exception handler
💥 It's cool that you can easily assign a closure as a custom NSException handler. This is super useful when building things in Playgrounds - since you can't use breakpoints - so instead of just signal SIGABRT, you'll get the full exception description if something goes wrong.
NSSetUncaughtExceptionHandler { exception in
print(exception)
}
#97 Using type aliases to give semantic meaning to primitives
❤️ I love that in Swift, we can use the type system to make our code so much more self-documenting - one way of doing so is to use type aliases to give the primitive types that we use a more semantic meaning.
extension List.Item {
// Using type aliases, we can give semantic meaning to the
// primitive types that we use, without having to introduce
// wrapper types.
typealias Index = Int
}
extension List {
enum Mutation {
// Our enum cases now become a lot more self-documenting,
// without having to add additional parameter labels to
// explain them.
case add(Item, Item.Index)
case update(Item, Item.Index)
case remove(Item.Index)
}
}
For more on self-documenting code, check out "Writing self-documenting Swift code".
#96 Specializing protocols using constraints
🤯 A little late night prototyping session reveals that protocol constraints can not only be applied to extensions - they can also be added to protocol definitions!
This is awesome, since it lets us easily define specialized protocols based on more generic ones.
protocol Component {
associatedtype Container
func add(to container: Container)
}
// Protocols that inherit from other protocols can include
// constraints to further specialize them.
protocol ViewComponent: Component where Container == UIView {
associatedtype View: UIView
var view: View { get }
}
extension ViewComponent {
func add(to container: UIView) {
container.addSubview(view)
}
}
For more on specializing protocols, check out "Specializing protocols in Swift".
#95 Unwrapping an optional or throwing an error
📦 Here's a super handy extension on Swift's Optional type, which gives us a really nice API for easily unwrapping an optional, or throwing an error in case the value turned out to be nil:
extension Optional {
func orThrow(_ errorExpression: @autoclosure () -> Error) throws -> Wrapped {
switch self {
case .some(let value):
return value
case .none:
throw errorExpression()
}
}
}
let file = try loadFile(at: path).orThrow(MissingFileError())
For more ways that optionals can be extended, check out "Extending optionals in Swift".
#94 Testing code that uses static APIs
👩🔬 Testing code that uses static APIs can be really tricky, but there's a way that it can often be done - using Swift's first class function capabilities!
Instead of accessing that static API directly, we can inject the function we want to use, which enables us to mock it!
// BEFORE
class FriendsLoader {
func loadFriends(then handler: @escaping (Result<[Friend]>) -> Void) {
Networking.loadData(from: .friends) { result in
...
}
}
}
// AFTER
class FriendsLoader {
typealias Handler<T> = (Result<T>) -> Void
typealias DataLoadingFunction = (Endpoint, @escaping Handler<Data>) -> Void
func loadFriends(using dataLoading: DataLoadingFunction = Networking.loadData,
then handler: @escaping Handler<[Friend]>) {
dataLoading(.friends) { result in
...
}
}
}
// MOCKING IN TESTS
let dataLoading: FriendsLoader.DataLoadingFunction = { _, handler in
handler(.success(mockData))
}
friendsLoader.loadFriends(using: dataLoading) { result in
...
}
#93 Matching multiple enum cases with associated values
🐾 Swift's pattern matching capabilities are so powerful! Two enum cases with associated values can even be matched and handled by the same switch case - which is super useful when handling state changes with similar data.
enum DownloadState {
case inProgress(progress: Double)
case paused(progress: Double)
case cancelled
case finished(Data)
}
func downloadStateDidChange(to state: DownloadState) {
switch state {
case .inProgress(let progress), .paused(let progress):
updateProgressView(with: progress)
case .cancelled:
showCancelledMessage()
case .finished(let data):
process(data)
}
}
#92 Multiline string literals
🅰 One really nice benefit of Swift multiline string literals - even for single lines of text - is that they don't require quotes to be escaped. Perfect when working with things like HTML, or creating a custom description for an object.
let html = highlighter.highlight("Array<String>")
XCTAssertEqual(html, """
<span class="type">Array</span><<span class="type">String</span>>
""")
#91 Reducing sequences
💎 While it's very common in functional programming, the reduce function might be a bit of a hidden gem in Swift. It provides a super useful way to transform a sequence into a single value.
extension Sequence where Element: Equatable {
func numberOfOccurrences(of target: Element) -> Int {
return reduce(0) { result, element in
guard element == target else {
return result
}
return result + 1
}
}
}
You can read more about transforming collections in "Transforming collections in Swift".
#90 Avoiding manual Codable implementations
📦 When I use Codable in Swift, I want to avoid manual implementations as much as possible, even when there's a mismatch between my code structure and the JSON I'm decoding.
One way that can often be achieved is to use private data containers combined with computed properties.
struct User: Codable {
let name: String
let age: Int
var homeTown: String { return originPlace.name }
private let originPlace: Place
}
private extension User {
struct Place: Codable {
let name: String
}
}
extension User {
struct Container: Codable {
let user: User
}
}
#89 Using feature flags instead of feature branches
🚢 Instead of using feature branches, I merge almost all of my code directly into master - and then I use feature flags to conditionally enable features when they're ready. That way I can avoid merge conflicts and keep shipping!
extension ListViewController {
func addSearchIfNeeded() {
// Rather than having to keep maintaining a separate
// feature branch for a new feature, we can use a flag
// to conditionally turn it on.
guard FeatureFlags.searchEnabled else {
return
}
let resultsVC = SearchResultsViewController()
let searchVC = UISearchController(
searchResultsController: resultsVC
)
searchVC.searchResultsUpdater = resultsVC
navigationItem.searchController = searchVC
}
}
You can read more about feature flags in "Feature flags in Swift".
#88 Lightweight data hierarchies using tuples
💾 Here I'm using tuples to create a lightweight hierarchy for my data, giving me a nice structure without having to introduce any additional types.
struct CodeSegment {
var tokens: (
previous: String?,
current: String
)
var delimiters: (
previous: Character?
next: Character?
)
}
handle(segment.tokens.current)
You can read more about tuples in "Using tuples as lightweight types in Swift"
#87 The rule of threes
3️⃣ Whenever I have 3 properties or local variables that share the same prefix, I usually try to extract them into their own method or type. That way I can avoid massive types & methods, and also increase readability, without falling into a "premature optimization" trap.
Before
public func generate() throws {
let contentFolder = try folder.subfolder(named: "content")
let articleFolder = try contentFolder.subfolder(named: "posts")
let articleProcessor = ContentProcessor(folder: articleFolder)
let articles = try articleProcessor.process()
...
}
After
public func generate() throws {
let contentFolder = try folder.subfolder(named: "content")
let articles = try processArticles(in: contentFolder)
...
}
private func processArticles(in folder: Folder) throws -> [ContentItem] {
let folder = try folder.subfolder(named: "posts")
let processor = ContentProcessor(folder: folder)
return try processor.process()
}
#86 Useful Codable extensions
👨🔧 Here's two extensions that I always add to the Encodable & Decodable protocols, which for me really make the Codable API nicer to use. By using type inference for decoding, a lot of boilerplate can be removed when the compiler is already able to infer the resulting type.
extension Encodable {
func encoded() throws -> Data {
return try JSONEncoder().encode(self)
}
}
extension Data {
func decoded<T: Decodable>() throws -> T {
return try JSONDecoder().decode(T.self, from: self)
}
}
let data = try user.encoded()
// By using a generic type in the decoded() method, the
// compiler can often infer the type we want to decode
// from the current context.
try userDidLogin(data.decoded())
// And if not, we can always supply the type, still making
// the call site read very nicely.
let otherUser = try data.decoded() as User
#85 Using shared UserDefaults suites
📦 UserDefaults is a lot more powerful than what it first might seem like. Not only can it store more complex values (like dates & dictionaries) and parse command line arguments - it also enables easy sharing of settings & lightweight data between apps in the same App Group.
let sharedDefaults = UserDefaults(suiteName: "my-app-group")!
let useDarkMode = sharedDefaults.bool(forKey: "dark-mode")
// This value is put into the shared suite.
sharedDefaults.set(true, forKey: "dark-mode")
// If you want to treat the shared settings as read-only (and add
// local overrides on top of them), you can simply add the shared
// suite to the standard UserDefaults.
let combinedDefaults = UserDefaults.standard
combinedDefaults.addSuite(named: "my-app-group")
// This value is a local override, not added to the shared suite.
combinedDefaults.set(true, forKey: "app-specific-override")
#84 Custom UIView backing layers
🎨 By overriding layerClass you can tell UIKit what CALayer class to use for a UIView's backing layer. That way you can reduce the amount of layers, and don't have to do any manual layout.
final class GradientView: UIView {
override class var layerClass: AnyClass { return CAGradientLayer.self }
var colors: (start: UIColor, end: UIColor)? {
didSet { updateLayer() }
}
private func updateLayer() {
let layer = self.layer as! CAGradientLayer
layer.colors = colors.map { [$0.start.cgColor, $0.end.cgColor] }
}
}
#83 Auto-Equatable enums with associated values
✅ That the compiler now automatically synthesizes Equatable conformances is such a huge upgrade for Swift! And the cool thing is that it works for all kinds of types - even for enums with associated values! Especially useful when using enums for verification in unit tests.
struct Article: Equatable {
let title: String
let text: String
}
struct User: Equatable {
let name: String
let age: Int
}
extension Navigator {
enum Destination: Equatable {
case profile(User)
case article(Article)
}
}
func testNavigatingToArticle() {
let article = Article(title: "Title", text: "Text")
controller.select(article)
XCTAssertEqual(navigator.destinations, [.article(article)])
}
#82 Defaults for associated types
🤝 Associated types can have defaults in Swift - which is super useful for types that are not easily inferred (for example when they're not used for a specific instance method or property).
protocol Identifiable {
associatedtype RawIdentifier: Codable = String
var id: Identifier<Self> { get }
}
struct User: Identifiable {
let id: Identifier<User>
let name: String
}
struct Group: Identifiable {
typealias RawIdentifier = Int
let id: Identifier<Group>
let name: String
}
#81 Creating a dedicated identifier type
🆔 If you want to avoid using plain strings as identifiers (which can increase both type safety & readability), it's really easy to create a custom Identifier type that feels just like a native Swift type, thanks to protocols!
More on this topic in "Type-safe identifiers in Swift".
struct Identifier: Hashable {
let string: String
}
extension Identifier: ExpressibleByStringLiteral {
init(stringLiteral value: String) {
string = value
}
}
extension Identifier: CustomStringConvertible {
var description: String {
return string
}
}
extension Identifier: Codable {
init(from decoder: Decoder) throws {
let container = try decoder.singleValueContainer()
string = try container.decode(String.self)
}
func encode(to encoder: Encoder) throws {
var container = encoder.singleValueContainer()
try container.encode(string)
}
}
struct Article: Codable {
let id: Identifier
let title: String
}
let article = Article(id: "my-article", title: "Hello world!")
#80 Assigning optional tuple members to variables
🙌 A really cool thing about using tuples to model the internal state of a Swift type, is that you can unwrap an optional tuple's members directly into local variables.
Very useful in order to group multiple optional values together for easy unwrapping & handling.
class ImageTransformer {
private var queue = [(image: UIImage, transform: Transform)]()
private func processNext() {
// When unwrapping an optional tuple, you can assign the members
// directly to local variables.
guard let (image, transform) = queue.first else {
return
}
let context = Context()
context.draw(image)
context.apply(transform)
...
}
}
#79 Struct convenience initializers
❤️ I love to structure my code using extensions in Swift. One big benefit of doing so when it comes to struct initializers, is that defining a convenience initializer doesn't remove the default one the compiler generates - best of both worlds!
struct Article {
let date: Date
var title: String
var text: String
var comments: [Comment]
}
extension Article {
init(title: String, text: String) {
self.init(date: Date(), title: title, text: text, comments: [])
}
}
let articleA = Article(title: "Best Cupcake Recipe", text: "...")
let articleB = Article(
date: Date(),
title: "Best Cupcake Recipe",
text: "...",
comments: [
Comment(user: currentUser, text: "Yep, can confirm!")
]
)
#78 Usages of throwing functions
🏈 A big benefit of using throwing functions for synchronous Swift APIs is that the caller can decide whether they want to treat the return value as optional (try?) or required (try).
func loadFile(named name: String) throws -> File {
guard let url = urlForFile(named: name) else {
throw File.Error.missing
}
do {
let data = try Data(contentsOf: url)
return File(url: url, data: data)
} catch {
throw File.Error.invalidData(error)
}
}
let requiredFile = try loadFile(named: "AppConfig.json")
let optionalFile = try? loadFile(named: "UserSettings.json")
#77 Nested generic types
🐝 Types that are nested in generics automatically inherit their parent's generic types - which is super useful when defining accessory types (for things like states or outcomes).
struct Task<Input, Output> {
typealias Closure = (Input) throws -> Output
let closure: Closure
}
extension Task {
enum Result {
case success(Output)
case failure(Error)
}
}
#76 Equatable & Hashable structures
🤖 Now that the Swift compiler automatically synthesizes Equatable & Hashable conformances for value types, it's easier than ever to setup model structures with nested types that are all Equatable/Hashable!
typealias Value = Hashable & Codable
struct User: Value {
var name: String
var age: Int
var lastLoginDate: Date?
var settings: Settings
}
extension User {
struct Settings: Value {
var itemsPerPage: Int
var theme: Theme
}
}
extension User.Settings {
enum Theme: String, Value {
case light
case dark
}
}
You can read more about using nested types in Swift here.
#75 Conditional conformances
🎉 Swift 4.1 is here! One of the key features it brings is conditional conformances, which lets you have a type only conform to a protocol under certain constraints.
protocol UnboxTransformable {
associatedtype RawValue
static func transform(_ value: RawValue) throws -> Self?
}
extension Array: UnboxTransformable where Element: UnboxTransformable {
typealias RawValue = [Element.RawValue]
static func transform(_ value: RawValue) throws -> [Element]? {
return try value.compactMap(Element.transform)
}
}
I also have an article with lots of more info on conditional conformances here. Paul Hudson also has a great overview of all Swift 4.1 features here.
#74 Generic type aliases
🕵️♀️ A cool thing about Swift type aliases is that they can be generic! Combine that with tuples and you can easily define simple generic types.
typealias Pair<T> = (T, T)
extension Game {
func calculateScore(for players: Pair<Player>) -> Int {
...
}
}
You can read more about using tuples as lightweight types here.
#73 Parsing command line arguments using UserDefaults
☑️ A really cool "hidden" feature of UserDefaults is that it contains any arguments that were passed to the app at launch!
Super useful both in Swift command line tools & scripts, but also to temporarily override a value when debugging iOS apps.
let defaults = UserDefaults.standard
let query = defaults.string(forKey: "query")
let resultCount = defaults.integer(forKey: "results")
#72 Using the & operator
👏 Swift's & operator is awesome! Not only can you use it to compose protocols, you can compose other types too! Very useful if you want to hide concrete types & implementation details.
protocol LoadableFromURL {
func load(from url: URL)
}
class ContentViewController: UIViewController, LoadableFromURL {
func load(from url: URL) {
...
}
}
class ViewControllerFactory {
func makeContentViewController() -> UIViewController & LoadableFromURL {
return ContentViewController()
}
}
#71 Capturing multiple values in mocks
🤗 When capturing values in mocks, using an array (instead of just a single value) makes it easy to verify that only a certain number of values were passed.
Perfect for protecting against "over-calling" something.
class UserManagerTests: XCTestCase {
func testObserversCalledWhenUserFirstLogsIn() {
let manager = UserManager()
let observer = ObserverMock()
manager.addObserver(observer)
// First login, observers should be notified
let user = User(id: 123, name: "John")
manager.userDidLogin(user)
XCTAssertEqual(observer.users, [user])
// If the same user logs in again, observers shouldn't be notified
manager.userDidLogin(user)
XCTAssertEqual(observer.users, [user])
}
}
private extension UserManagerTests {
class ObserverMock: UserManagerObserver {
private(set) var users = [User]()
func userDidChange(to user: User) {
users.append(user)
}
}
}
#70 Reducing the need for mocks
👋 When writing tests, you don't always need to create mocks - you can create stubs using real instances of things like errors, URLs & UserDefaults.
Here's how to do that for some common tasks/object types in Swift:
// Create errors using NSError (#function can be used to reference the name of the test)
let error = NSError(domain: #function, code: 1, userInfo: nil)
// Create non-optional URLs using file paths
let url = URL(fileURLWithPath: "Some/URL")
// Reference the test bundle using Bundle(for:)
let bundle = Bundle(for: type(of: self))
// Create an explicit UserDefaults object (instead of having to use a mock)
let userDefaults = UserDefaults(suiteName: #function)
// Create queues to control/await concurrent operations
let queue = DispatchQueue(label: #function)
For when you actually do need mocking, check out "Mocking in Swift".
#69 Using "then" as an external parameter label for closures
⏱ I've started using "then" as an external parameter label for completion handlers. Makes the call site read really nicely (Because I do ❤️ conversational API design) regardless of whether trailing closure syntax is used or not.
protocol DataLoader {
// Adding type aliases to protocols can be a great way to
// reduce verbosity for parameter types.
typealias Handler = (Result<Data>) -> Void
associatedtype Endpoint
func loadData(from endpoint: Endpoint, then handler: @escaping Handler)
}
loader.loadData(from: .messages) { result in
...
}
loader.loadData(from: .messages, then: { result in
...
})
#68 Combining lazily evaluated sequences with the builder pattern
😴 Combining lazily evaluated sequences with builder pattern-like properties can lead to some pretty sweet APIs for configurable sequences in Swift.
Also useful for queries & other things you "build up" and then execute.
// Extension adding builder pattern-like properties that return
// a new sequence value with the given configuration applied
extension FileSequence {
var recursive: FileSequence {
var sequence = self
sequence.isRecursive = true
return sequence
}
var includingHidden: FileSequence {
var sequence = self
sequence.includeHidden = true
return sequence
}
}
// BEFORE
let files = folder.makeFileSequence(recursive: true, includeHidden: true)
// AFTER
let files = folder.files.recursive.includingHidden
Want an intro to lazy sequences? Check out "Swift sequences: The art of being lazy".
#67 Faster & more stable UI tests
My top 3 tips for faster & more stable UI tests:
📱 Reset the app's state at the beginning of every test.
🆔 Use accessibility identifiers instead of UI strings.
⏱ Use expectations instead of waiting time.
func testOpeningArticle() {
// Launch the app with an argument that tells it to reset its state
let app = XCUIApplication()
app.launchArguments.append("--uitesting")
app.launch()
// Check that the app is displaying an activity indicator
let activityIndicator = app.activityIndicator.element
XCTAssertTrue(activityIndicator.exists)
// Wait for the loading indicator to disappear = content is ready
expectation(for: NSPredicate(format: "exists == 0"),
evaluatedWith: activityIndicator)
// Use a generous timeout in case the network is slow
waitForExpectations(timeout: 10)
// Tap the cell for the first article
app.tables.cells["Article.0"].tap()
// Assert that a label with the accessibility identifier "Article.Title" exists
let label = app.staticTexts["Article.Title"]
XCTAssertTrue(label.exists)
}
#66 Accessing the clipboard from a Swift script
📋 It's super easy to access the contents of the clipboard from a Swift script. A big benefit of Swift scripting is being able to use Cocoa's powerful APIs for Mac apps.
import Cocoa
let clipboard = NSPasteboard.general.string(forType: .string)
#65 Using tuples for view state
🎯 Using Swift tuples for view state can be a super nice way to group multiple properties together and render them reactively using the layout system.
By using a tuple we don't have to either introduce a new type or make our view model-aware.
class TextView: UIView {
var state: (title: String?, text: String?) {
// By telling UIKit that our view needs layout and binding our
// state in layoutSubviews, we can react to state changes without
// doing unnecessary layout work.
didSet { setNeedsLayout() }
}
private let titleLabel = UILabel()
private let textLabel = UILabel()
override func layoutSubviews() {
super.layoutSubviews()
titleLabel.text = state.title
textLabel.text = state.text
...
}
}
#64 Throwing tests and LocalizedError
⚾️ Swift tests can throw, which is super useful in order to avoid complicated logic or force unwrapping. By making errors conform to LocalizedError, you can also get a nice error message in Xcode if there's a failure.
class ImageCacheTests: XCTestCase {
func testCachingAndLoadingImage() throws {
let bundle = Bundle(for: type(of: self))
let cache = ImageCache(bundle: bundle)
// Bonus tip: You can easily load images from your test
// bundle using this UIImage initializer
let image = try require(UIImage(named: "sample", in: bundle, compatibleWith: nil))
try cache.cache(image, forKey: "key")
let cachedImage = try cache.image(forKey: "key")
XCTAssertEqual(image, cachedImage)
}
}
enum ImageCacheError {
case emptyKey
case dataConversionFailed
}
// When using throwing tests, making your errors conform to
// LocalizedError will render a much nicer error message in
// Xcode (per default only the error code is shown).
extension ImageCacheError: LocalizedError {
var errorDescription: String? {
switch self {
case .emptyKey:
return "An empty key was given"
case .dataConversionFailed:
return "Failed to convert the given image to Data"
}
}
}
For more information, and the implementation of the require method used above, check out "Avoiding force unwrapping in Swift unit tests".
#63 The difference between static and class properties
✍️ Unlike static properties, class properties can be overridden by subclasses (however, they can't be stored, only computed).
class TableViewCell: UITableViewCell {
class var preferredHeight: CGFloat { return 60 }
}
class TallTableViewCell: TableViewCell {
override class var preferredHeight: CGFloat { return 100 }
}
#62 Creating extensions with static factory methods
👨🎨 Creating extensions with static factory methods can be a great alternative to subclassing in Swift, especially for things like setting up UIViews, CALayers or other kinds of styling.
It also lets you remove a lot of styling & setup from your view controllers.
extension UILabel {
static func makeForTitle() -> UILabel {
let label = UILabel()
label.font = .boldSystemFont(ofSize: 24)
label.textColor = .darkGray
label.adjustsFontSizeToFitWidth = true
label.minimumScaleFactor = 0.75
return label
}
static func makeForText() -> UILabel {
let label = UILabel()
label.font = .systemFont(ofSize: 16)
label.textColor = .black
label.numberOfLines = 0
return label
}
}
class ArticleViewController: UIViewController {
lazy var titleLabel = UILabel.makeForTitle()
lazy var textLabel = UILabel.makeForText()
}
#61 Child view controller auto-resizing
🧒 An awesome thing about child view controllers is that they're automatically resized to match their parent, making them a super nice solution for things like loading & error views.
class ListViewController: UIViewController {
func loadItems() {
let loadingViewController = LoadingViewController()
add(loadingViewController)
dataLoader.loadItems { [weak self] result in
loadingViewController.remove()
self?.handle(result)
}
}
}
For more about child view controller (including the add and remove methods used above), check out "Using child view controllers as plugins in Swift".
#60 Using zip
🤐 Using the zip function in Swift you can easily combine two sequences. Super useful when using two sequences to do some work, since zip takes care of all the bounds-checking.
func render(titles: [String]) {
for (label, text) in zip(titleLabels, titles) {
print(text)
label.text = text
}
}
#59 Defining custom option sets
🎛 The awesome thing about option sets in Swift is that they can automatically either be passed as a single member or as a set. Even cooler is that you can easily define your own option sets as well, perfect for options and other non-exclusive values.
// Option sets are awesome, because you can easily pass them
// both using dot syntax and array literal syntax, like when
// using the UIView animation API:
UIView.animate(withDuration: 0.3,
delay: 0,
options: .allowUserInteraction,
animations: animations)
UIView.animate(withDuration: 0.3,
delay: 0,
options: [.allowUserInteraction, .layoutSubviews],
animations: animations)
// The cool thing is that you can easily define your own option
// sets as well, by defining a struct that has an Int rawValue,
// that will be used as a bit mask.
extension Cache {
struct Options: OptionSet {
static let saveToDisk = Options(rawValue: 1)
static let clearOnMemoryWarning = Options(rawValue: 1 << 1)
static let clearDaily = Options(rawValue: 1 << 2)
let rawValue: Int
}
}
// We can now use Cache.Options just like UIViewAnimationOptions:
Cache(options: .saveToDisk)
Cache(options: [.saveToDisk, .clearDaily])
#58 Using the where clause with associated types
🙌 Using the where clause when designing protocol-oriented APIs in Swift can let your implementations (or others' if it's open source) have a lot more freedom, especially when it comes to collections.
See "Using generic type constraints in Swift 4" for more info.
public protocol PathFinderMap {
associatedtype Node
// Using the 'where' clause for associated types, we can
// ensure that a type meets certain requirements (in this
// case that it's a sequence with Node elements).
associatedtype NodeSequence: Sequence where NodeSequence.Element == Node
// Instead of using a concrete type (like [Node]) here, we
// give implementors of this protocol more freedom while
// still meeting our requirements. For example, one
// implementation might use Set<Node>.
func neighbors(of node: Node) -> NodeSequence
}
#57 Using first class functions when iterating over a dictionary
👨🍳 Combine first class functions in Swift with the fact that Dictionary elements are (Key, Value) tuples and you can build yourself some pretty awesome functional chains when iterating over a Dictionary.
func makeActor(at coordinate: Coordinate, for building: Building) -> Actor {
let actor = Actor()
actor.position = coordinate.point
actor.animation = building.animation
return actor
}
func render(_ buildings: [Coordinate : Building]) {
buildings.map(makeActor).forEach(add)
}
#56 Calling instance methods as static functions
😎 In Swift, you can call any instance method as a static function and it will return a closure representing that method. This is how running tests using SPM on Linux works.
More about this topic in my blog post "First class functions in Swift".
// This produces a '() -> Void' closure which is a reference to the
// given view's 'removeFromSuperview' method.
let closure = UIView.removeFromSuperview(view)
// We can now call it just like we would any other closure, and it
// will run 'view.removeFromSuperview()'
closure()
// This is how running tests using the Swift Package Manager on Linux
// works, you return your test functions as closures:
extension UserManagerTests {
static var allTests = [
("testLoggingIn", testLoggingIn),
("testLoggingOut", testLoggingOut),
("testUserPermissions", testUserPermissions)
]
}
#55 Dropping suffixes from method names to support multiple arguments
👏 One really nice benefit of dropping suffixes from method names (and just using verbs, when possible) is that it becomes super easy to support both single and multiple arguments, and it works really well semantically.
extension UIView {
func add(_ subviews: UIView...) {
subviews.forEach(addSubview)
}
}
view.add(button)
view.add(label)
// By dropping the "Subview" suffix from the method name, both
// single and multiple arguments work really well semantically.
view.add(button, label)
#54 Constraining protocols to classes to ensure mutability
👽 Using the AnyObject (or class) constraint on protocols is not only useful when defining delegates (or other weak references), but also when you always want instances to be mutable without copying.
// By constraining a protocol with 'AnyObject' it can only be adopted
// by classes, which means all instances will always be mutable, and
// that it's the original instance (not a copy) that will be mutated.
protocol DataContainer: AnyObject {
var data: Data? { get set }
}
class UserSettingsManager {
private var settings: Settings
private let dataContainer: DataContainer
// Since DataContainer is a protocol, we an easily mock it in
// tests if we use dependency injection
init(settings: Settings, dataContainer: DataContainer) {
self.settings = settings
self.dataContainer = dataContainer
}
func saveSettings() throws {
let data = try settings.serialize()
// We can now assign properties on an instance of our protocol
// because the compiler knows it's always going to be a class
dataContainer.data = data
}
}
#53 String-based enums in string interpolation
🍣 Even if you define a custom raw value for a string-based enum in Swift, the full case name will be used in string interpolation.
Super useful when using separate raw values for JSON, while still wanting to use the full case name in other contexts.
extension Building {
// This enum has custom raw values that are used when decoding
// a value, for example from JSON.
enum Kind: String {
case castle = "C"
case town = "T"
case barracks = "B"
case goldMine = "G"
case camp = "CA"
case blacksmith = "BL"
}
var animation: Animation {
return Animation(
// When used in string interpolation, the full case name is still used.
// For 'castle' this will be 'buildings/castle'.
name: "buildings/\(kind)",
frameCount: frameCount,
frameDuration: frameDuration
)
}
}
#52 Expressively comparing a value with a list of candidates
👨🔬 Continuing to experiment with expressive ways of comparing a value with a list of candidates in Swift. Adding an extension on Equatable is probably my favorite approach so far.
extension Equatable {
func isAny(of candidates: Self...) -> Bool {
return candidates.contains(self)
}
}
let isHorizontal = direction.isAny(of: .left, .right)
See tip #35 for my previous experiment.
#51 UIView bounds and transforms
📐 A really interesting side-effect of a UIView's bounds being its rect within its own coordinate system is that transforms don't affect it at all. That's why it's usually a better fit than frame when doing layout calculations of subviews.
let view = UIView()
view.frame.size = CGSize(width: 100, height: 100)
view.transform = CGAffineTransform(scaleX: 2, y: 2)
print(view.frame) // (-50.0, -50.0, 200.0, 200.0)
print(view.bounds) // (0.0, 0.0, 100.0, 100.0)
#50 UIKit default arguments
👏 It's awesome that many UIKit APIs with completion handlers and other optional parameters import into Swift with default arguments (even though they are written in Objective-C). Getting rid of all those nil arguments is so nice!
// BEFORE: All parameters are specified, just like in Objective-C
viewController.present(modalViewController, animated: true, completion: nil)
modalViewController.dismiss(animated: true, completion: nil)
viewController.transition(from: loadingViewController,
to: contentViewController,
duration: 0.3,
options: [],
animations: animations,
completion: nil)
// AFTER: Since many UIKit APIs with completion handlers and other
// optional parameters import into Swift with default arguments,
// we can make our calls shorter
viewController.present(modalViewController, animated: true)
modalViewController.dismiss(animated: true)
viewController.transition(from: loadingViewController,
to: contentViewController,
duration: 0.3,
animations: animations)
#49 Avoiding Massive View Controllers
✂️ Avoiding Massive View Controllers is all about finding the right levels of abstraction and splitting things up.
My personal rule of thumb is that as soon as I have 3 methods or properties that have the same prefix, I break them out into their own type.
// BEFORE
class LoginViewController: UIViewController {
private lazy var signUpLabel = UILabel()
private lazy var signUpImageView = UIImageView()
private lazy var signUpButton = UIButton()
}
// AFTER
class LoginViewController: UIViewController {
private lazy var signUpView = SignUpView()
}
class SignUpView: UIView {
private lazy var label = UILabel()
private lazy var imageView = UIImageView()
private lazy var button = UIButton()
}
#48 Extending optionals
❤️ I love the fact that optionals are enums in Swift - it makes it so easy to extend them with convenience APIs for certain types. Especially useful when doing things like data validation on optional values.
func validateTextFields() -> Bool {
guard !usernameTextField.text.isNilOrEmpty else {
return false
}
...
return true
}
// Since all optionals are actual enum values in Swift, we can easily
// extend them for certain types, to add our own convenience APIs
extension Optional where Wrapped == String {
var isNilOrEmpty: Bool {
switch self {
case let string?:
return string.isEmpty
case nil:
return true
}
}
}
// Since strings are now Collections in Swift 4, you can even
// add this property to all optional collections:
extension Optional where Wrapped: Collection {
var isNilOrEmpty: Bool {
switch self {
case let collection?:
return collection.isEmpty
case nil:
return true
}
}
}
#47 Using where with for-loops
🗺 Using the where keyword can be a super nice way to quickly apply a filter in a for-loop in Swift. You can of course use map, filter and forEach, or guard, but for simple loops I think this is very expressive and nice.
func archiveMarkedPosts() {
for post in posts where post.isMarked {
archive(post)
}
}
func healAllies() {
for player in players where player.isAllied(to: currentPlayer) {
player.heal()
}
}
#46 Variable shadowing
👻 Variable shadowing can be super useful in Swift, especially when you want to create a local copy of a parameter value in order to use it as state within a closure.
init(repeatMode: RepeatMode, closure: @escaping () -> UpdateOutcome) {
// Shadow the argument with a local, mutable copy
var repeatMode = repeatMode
self.closure = {
// With shadowing, there's no risk of accidentially
// referring to the immutable version
switch repeatMode {
case .forever:
break
case .times(let count):
guard count > 0 else {
return .finished
}
// We can now capture the mutable version and use
// it for state in a closure
repeatMode = .times(count - 1)
}
return closure()
}
}
#45 Using dot syntax for static properties and initializers
✒️ Dot syntax is one of my favorite features of Swift. What's really cool is that it's not only for enums, any static method or property can be used with dot syntax - even initializers! Perfect for convenience APIs and default parameters.
public enum RepeatMode {
case times(Int)
case forever
}
public extension RepeatMode {
static var never: RepeatMode {
return .times(0)
}
static var once: RepeatMode {
return .times(1)
}
}
view.perform(animation, repeated: .once)
// To make default parameters more compact, you can even use init with dot syntax
class ImageLoader {
init(cache: Cache = .init(), decoder: ImageDecoder = .init()) {
...
}
}
#44 Calling functions as closures with a tuple as parameters
🚀 One really cool aspect of Swift having first class functions is that you can pass any function (or even initializer) as a closure, and even call it with a tuple containing its parameters!
// This function lets us treat any "normal" function or method as
// a closure and run it with a tuple that contains its parameters
func call<Input, Output>(_ function: (Input) -> Output, with input: Input) -> Output {
return function(input)
}
class ViewFactory {
func makeHeaderView() -> HeaderView {
// We can now pass an initializer as a closure, and a tuple
// containing its parameters
return call(HeaderView.init, with: loadTextStyles())
}
private func loadTextStyles() -> (font: UIFont, color: UIColor) {
return (theme.font, theme.textColor)
}
}
class HeaderView {
init(font: UIFont, textColor: UIColor) {
...
}
}
#43 Enabling static dependency injection
💉 If you've been struggling to test code that uses static APIs, here's a technique you can use to enable static dependency injection without having to modify any call sites:
// Before: Almost impossible to test due to the use of singletons
class Analytics {
static func log(_ event: Event) {
Database.shared.save(event)
let dictionary = event.serialize()
NetworkManager.shared.post(dictionary, to: eventURL)
}
}
// After: Much easier to test, since we can inject mocks as arguments
class Analytics {
static func log(_ event: Event,
database: Database = .shared,
networkManager: NetworkManager = .shared) {
database.save(event)
let dictionary = event.serialize()
networkManager.post(dictionary, to: eventURL)
}
}
#42 Type inference for lazy properties in Swift 4
🎉 In Swift 4, type inference works for lazy properties and you don't need to explicitly refer to self!
// Swift 3
class PurchaseView: UIView {
private lazy var buyButton: UIButton = self.makeBuyButton()
private func makeBuyButton() -> UIButton {
let button = UIButton()
button.setTitle("Buy", for: .normal)
button.setTitleColor(.blue, for: .normal)
return button
}
}
// Swift 4
class PurchaseView: UIView {
private lazy var buyButton = makeBuyButton()
private func makeBuyButton() -> UIButton {
let button = UIButton()
button.setTitle("Buy", for: .normal)
button.setTitleColor(.blue, for: .normal)
return button
}
}
#41 Converting Swift errors to NSError
😎 You can turn any Swift Error into an NSError, which is super useful when pattern matching with a code 👍. Also, switching on optionals is pretty cool!
let task = urlSession.dataTask(with: url) { data, _, error in
switch error {
case .some(let error as NSError) where error.code == NSURLErrorNotConnectedToInternet:
presenter.showOfflineView()
case .some(let error):
presenter.showGenericErrorView()
case .none:
presenter.renderContent(from: data)
}
}
task.resume()
Also make sure to check out Kostas Kremizas' tip about how you can pattern match directly against a member of URLError.
#40 Making UIImage macOS compatible
🖥 Here's an easy way to make iOS model code that uses UIImage macOS compatible - like me and Gui Rambo discussed on the Swift by Sundell Podcast.
// Either put this in a separate file that you only include in your macOS target or wrap the code in #if os(macOS) / #endif
import Cocoa
// Step 1: Typealias UIImage to NSImage
typealias UIImage = NSImage
// Step 2: You might want to add these APIs that UIImage has but NSImage doesn't.
extension NSImage {
var cgImage: CGImage? {
var proposedRect = CGRect(origin: .zero, size: size)
return cgImage(forProposedRect: &proposedRect,
context: nil,
hints: nil)
}
convenience init?(named name: String) {
self.init(named: Name(name))
}
}
// Step 3: Profit - you can now make your model code that uses UIImage cross-platform!
struct User {
let name: String
let profileImage: UIImage
}
#39 Internally mutable protocol-oriented APIs
🤖 You can easily define a protocol-oriented API that can only be mutated internally, by using an internal protocol that extends a public one.
// Declare a public protocol that acts as your immutable API
public protocol ModelHolder {
associatedtype Model
var model: Model { get }
}
// Declare an extended, internal protocol that provides a mutable API
internal protocol MutableModelHolder: ModelHolder {
var model: Model { get set }
}
// You can now implement the requirements using 'public internal(set)'
public class UserHolder: MutableModelHolder {
public internal(set) var model: User
internal init(model: User) {
self.model = model
}
}
#38 Switching on a set
🎛 You can switch on a set using array literals as cases in Swift! Can be really useful to avoid many if/else if statements.
class RoadTile: Tile {
var connectedDirections = Set<Direction>()
func render() {
switch connectedDirections {
case [.up, .down]:
image = UIImage(named: "road-vertical")
case [.left, .right]:
image = UIImage(named: "road-horizontal")
default:
image = UIImage(named: "road")
}
}
}
#37 Adding the current locale to cache keys
🌍 When caching localized content in an app, it's a good idea to add the current locale to all keys, to prevent bugs when switching languages.
func cache(_ content: Content, forKey key: String) throws {
let data = try wrap(content) as Data
let key = localize(key: key)
try storage.store(data, forKey: key)
}
func loadCachedContent(forKey key: String) -> Content? {
let key = localize(key: key)
let data = storage.loadData(forKey: key)
return data.flatMap { try? unbox(data: $0) }
}
private func localize(key: String) -> String {
return key + "-" + Bundle.main.preferredLocalizations[0]
}
#36 Setting up tests to avoid retain cycles with weak references
🚳 Here's an easy way to setup a test to avoid accidental retain cycles with object relationships (like weak delegates & observers) in Swift:
func testDelegateNotRetained() {
// Assign the delegate (weak) and also retain it using a local var
var delegate: Delegate? = DelegateMock()
controller.delegate = delegate
XCTAssertNotNil(controller.delegate)
// Release the local var, which should also release the weak reference
delegate = nil
XCTAssertNil(controller.delegate)
}
#35 Expressively matching a value against a list of candidates
👨🔬 Playing around with an expressive way to check if a value matches any of a list of candidates in Swift:
// Instead of multiple conditions like this:
if string == "One" || string == "Two" || string == "Three" {
}
// You can now do:
if string == any(of: "One", "Two", "Three") {
}
You can find a gist with the implementation here.
#34 Organizing code using extensions
👪 APIs in a Swift extension automatically inherit its access control level, making it a neat way to organize public, internal & private APIs.
public extension Animation {
init(textureNamed textureName: String) {
frames = [Texture(name: textureName)]
}
init(texturesNamed textureNames: [String], frameDuration: TimeInterval = 1) {
frames = textureNames.map(Texture.init)
self.frameDuration = frameDuration
}
init(image: Image) {
frames = [Texture(image: image)]
}
}
internal extension Animation {
func loadFrameImages() -> [Image] {
return frames.map { $0.loadImageIfNeeded() }
}
}
#33 Using map to transform an optional into a Result type
🗺 Using map you can transform an optional value into an optional Result type by simply passing in the enum case.
enum Result<Value> {
case value(Value)
case error(Error)
}
class Promise<Value> {
private var result: Result<Value>?
init(value: Value? = nil) {
result = value.map(Result.value)
}
}
#32 Assigning to self in struct initializers
👌 It's so nice that you can assign directly to self in struct initializers in Swift. Very useful when adding conformance to protocols.
extension Bool: AnswerConvertible {
public init(input: String) throws {
switch input.lowercased() {
case "y", "yes", "👍":
self = true
default:
self = false
}
}
}
#31 Recursively calling closures as inline functions
☎️ Defining Swift closures as inline functions enables you to recursively call them, which is super useful in things like custom sequences.
class Database {
func records(matching query: Query) -> AnySequence<Record> {
var recordIterator = loadRecords().makeIterator()
func iterate() -> Record? {
guard let nextRecord = recordIterator.next() else {
return nil
}
guard nextRecord.matches(query) else {
// Since the closure is an inline function, it can be recursively called,
// in this case in order to advance to the next item.
return iterate()
}
return nextRecord
}
// AnySequence/AnyIterator are part of the standard library and provide an easy way
// to define custom sequences using closures.
return AnySequence { AnyIterator(iterate) }
}
}
Rob Napier points out that using the above might cause crashes if used on a large databaset, since Swift has no guaranteed Tail Call Optimization (TCO).
Slava Pestov also points out that another benefit of inline functions vs closures is that they can have their own generic parameter list.
#30 Passing self to required Objective-C dependencies
🏖 Using lazy properties in Swift, you can pass self to required Objective-C dependencies without having to use force-unwrapped optionals.
class DataLoader: NSObject {
lazy var urlSession: URLSession = self.makeURLSession()
private func makeURLSession() -> URLSession {
return URLSession(configuration: .default, delegate: self, delegateQueue: .main)
}
}
class Renderer {
lazy var displayLink: CADisplayLink = self.makeDisplayLink()
private func makeDisplayLink() -> CADisplayLink {
return CADisplayLink(target: self, selector: #selector(screenDidRefresh))
}
}
#29 Making weak or lazy properties readonly
👓 If you have a property in Swift that needs to be weak or lazy, you can still make it readonly by using private(set).
class Node {
private(set) weak var parent: Node?
private(set) lazy var children = [Node]()
func add(child: Node) {
children.append(child)
child.parent = self
}
}
#28 Defining static URLs using string literals
🌏 Tired of using URL(string: "url")! for static URLs? Make URL conform to ExpressibleByStringLiteral and you can now simply use "url" instead.
extension URL: ExpressibleByStringLiteral {
// By using 'StaticString' we disable string interpolation, for safety
public init(stringLiteral value: StaticString) {
self = URL(string: "\(value)").require(hint: "Invalid URL string literal: \(value)")
}
}
// We can now define URLs using static string literals 🎉
let url: URL = "https://www.swiftbysundell.com"
let task = URLSession.shared.dataTask(with: "https://www.swiftbysundell.com")
// In Swift 3 or earlier, you also have to implement 2 additional initializers
extension URL {
public init(extendedGraphemeClusterLiteral value: StaticString) {
self.init(stringLiteral: value)
}
public init(unicodeScalarLiteral value: StaticString) {
self.init(stringLiteral: value)
}
}
To find the extension that adds the require() method on Optional that I use above, check out Require.
#27 Manipulating points, sizes and frames using math operators
✚ I'm always careful with operator overloading, but for manipulating things like sizes, points & frames I find them super useful.
extension CGSize {
static func *(lhs: CGSize, rhs: CGFloat) -> CGSize {
return CGSize(width: lhs.width * rhs, height: lhs.height * rhs)
}
}
button.frame.size = image.size * 2
If you like the above idea, check out CGOperators, which contains math operator overloads for all Core Graphics' vector types.
#26 Using closure types in generic constraints
🔗 You can use closure types in generic constraints in Swift. Enables nice APIs for handling sequences of closures.
extension Sequence where Element == () -> Void {
func callAll() {
forEach { $0() }
}
}
extension Sequence where Element == () -> String {
func joinedResults(separator: String) -> String {
return map { $0() }.joined(separator: separator)
}
}
callbacks.callAll()
let names = nameProviders.joinedResults(separator: ", ")
(If you're using Swift 3, you have to change Element to Iterator.Element)
#25 Using associated enum values to avoid state-specific optionals
🎉 Using associated enum values is a super nice way to encapsulate mutually exclusive state info (and avoiding state-specific optionals).
// BEFORE: Lots of state-specific, optional properties
class Player {
var isWaitingForMatchMaking: Bool
var invitingUser: User?
var numberOfLives: Int
var playerDefeatedBy: Player?
var roundDefeatedIn: Int?
}
// AFTER: All state-specific information is encapsulated in enum cases
class Player {
enum State {
case waitingForMatchMaking
case waitingForInviteResponse(from: User)
case active(numberOfLives: Int)
case defeated(by: Player, roundNumber: Int)
}
var state: State
}
#24 Using enums for async result types
👍 I really like using enums for all async result types, even boolean ones. Self-documenting, and makes the call site a lot nicer to read too!
protocol PushNotificationService {
// Before
func enablePushNotifications(completionHandler: @escaping (Bool) -> Void)
// After
func enablePushNotifications(completionHandler: @escaping (PushNotificationStatus) -> Void)
}
enum PushNotificationStatus {
case enabled
case disabled
}
service.enablePushNotifications { status in
if status == .enabled {
enableNotificationsButton.removeFromSuperview()
}
}
#23 Working on async code in a playground
🏃 Want to work on your async code in a Swift Playground? Just set needsIndefiniteExecution to true to keep it running:
import PlaygroundSupport
PlaygroundPage.current.needsIndefiniteExecution = true
DispatchQueue.main.asyncAfter(deadline: .now() + 3) {
let greeting = "Hello after 3 seconds"
print(greeting)
}
To stop the playground from executing, simply call PlaygroundPage.current.finishExecution().
#22 Overriding self with a weak reference
💦 Avoid memory leaks when accidentially refering to self in closures by overriding it locally with a weak reference:
Swift >= 4.2
dataLoader.loadData(from: url) { [weak self] result in
guard let self = self else {
return
}
self.cache(result)
...
Swift < 4.2
dataLoader.loadData(from: url) { [weak self] result in
guard let `self` = self else {
return
}
self.cache(result)
...
Note that the reason the above currently works is because of a compiler bug (which I hope gets turned into a properly supported feature soon).
#21 Using DispatchWorkItem
🕓 Using dispatch work items you can easily cancel a delayed asynchronous GCD task if you no longer need it:
let workItem = DispatchWorkItem {
// Your async code goes in here
}
// Execute the work item after 1 second
DispatchQueue.main.asyncAfter(deadline: .now() + 1, execute: workItem)
// You can cancel the work item if you no longer need it
workItem.cancel()
#20 Combining a sequence of functions
➕ While working on a new Swift developer tool (to be open sourced soon 😉), I came up with a pretty neat way of organizing its sequence of operations, by combining their functions into a closure:
internal func +<A, B, C>(lhs: @escaping (A) throws -> B,
rhs: @escaping (B) throws -> C) -> (A) throws -> C {
return { try rhs(lhs($0)) }
}
public func run() throws {
try (determineTarget + build + analyze + output)()
}
If you're familiar with the functional programming world, you might know the above technique as the pipe operator (thanks to Alexey Demedreckiy for pointing this out!)
#19 Chaining optionals with map() and flatMap()
🗺 Using map() and flatMap() on optionals you can chain multiple operations without having to use lengthy if lets or guards:
// BEFORE
guard let string = argument(at: 1) else {
return
}
guard let url = URL(string: string) else {
return
}
handle(url)
// AFTER
argument(at: 1).flatMap(URL.init).map(handle)
#18 Using self-executing closures for lazy properties
🚀 Using self-executing closures is a great way to encapsulate lazy property initialization:
class StoreViewController: UIViewController {
private lazy var collectionView: UICollectionView = {
let layout = UICollectionViewFlowLayout()
let view = UICollectionView(frame: self.view.bounds, collectionViewLayout: layout)
view.delegate = self
view.dataSource = self
return view
}()
override func viewDidLoad() {
super.viewDidLoad()
view.addSubview(collectionView)
}
}
#17 Speeding up Swift package tests
⚡️ You can speed up your Swift package tests using the --parallel flag. For Marathon, the tests execute 3 times faster that way!
swift test --parallel
#16 Avoiding mocking UserDefaults
🛠 Struggling with mocking UserDefaults in a test? The good news is: you don't need mocking - just create a real instance:
class LoginTests: XCTestCase {
private var userDefaults: UserDefaults!
private var manager: LoginManager!
override func setUp() {
super.setup()
userDefaults = UserDefaults(suiteName: #file)
userDefaults.removePersistentDomain(forName: #file)
manager = LoginManager(userDefaults: userDefaults)
}
}
#15 Using variadic parameters
👍 Using variadic parameters in Swift, you can create some really nice APIs that take a list of objects without having to use an array:
extension Canvas {
func add(_ shapes: Shape...) {
shapes.forEach(add)
}
}
let circle = Circle(center: CGPoint(x: 5, y: 5), radius: 5)
let lineA = Line(start: .zero, end: CGPoint(x: 10, y: 10))
let lineB = Line(start: CGPoint(x: 0, y: 10), end: CGPoint(x: 10, y: 0))
let canvas = Canvas()
canvas.add(circle, lineA, lineB)
canvas.render()
#14 Referring to enum cases with associated values as closures
😮 Just like you can refer to a Swift function as a closure, you can do the same thing with enum cases with associated values:
enum UnboxPath {
case key(String)
case keyPath(String)
}
struct UserSchema {
static let name = key("name")
static let age = key("age")
static let posts = key("posts")
private static let key = UnboxPath.key
}
#13 Using the === operator to compare objects by instance
📈 The === operator lets you check if two objects are the same instance. Very useful when verifying that an array contains an instance in a test:
protocol InstanceEquatable: class, Equatable {}
extension InstanceEquatable {
static func ==(lhs: Self, rhs: Self) -> Bool {
return lhs === rhs
}
}
extension Enemy: InstanceEquatable {}
func testDestroyingEnemy() {
player.attack(enemy)
XCTAssertTrue(player.destroyedEnemies.contains(enemy))
}
#12 Calling initializers with dot syntax and passing them as closures
😎 Cool thing about Swift initializers: you can call them using dot syntax and pass them as closures! Perfect for mocking dates in tests.
class Logger {
private let storage: LogStorage
private let dateProvider: () -> Date
init(storage: LogStorage = .init(), dateProvider: @escaping () -> Date = Date.init) {
self.storage = storage
self.dateProvider = dateProvider
}
func log(event: Event) {
storage.store(event: event, date: dateProvider())
}
}
#11 Structuring UI tests as extensions on XCUIApplication
📱 Most of my UI testing logic is now categories on XCUIApplication. Makes the test cases really easy to read:
func testLoggingInAndOut() {
XCTAssertFalse(app.userIsLoggedIn)
app.launch()
app.login()
XCTAssertTrue(app.userIsLoggedIn)
app.logout()
XCTAssertFalse(app.userIsLoggedIn)
}
func testDisplayingCategories() {
XCTAssertFalse(app.isDisplayingCategories)
app.launch()
app.login()
app.goToCategories()
XCTAssertTrue(app.isDisplayingCategories)
}
#10 Avoiding default cases in switch statements
🙂 It’s a good idea to avoid “default” cases when switching on Swift enums - it’ll “force you” to update your logic when a new case is added:
enum State {
case loggedIn
case loggedOut
case onboarding
}
func handle(_ state: State) {
switch state {
case .loggedIn:
showMainUI()
case .loggedOut:
showLoginUI()
// Compiler error: Switch must be exhaustive
}
}
#9 Using the guard statement in many different scopes
💂 It's really cool that you can use Swift's 'guard' statement to exit out of pretty much any scope, not only return from functions:
// You can use the 'guard' statement to...
for string in strings {
// ...continue an iteration
guard shouldProcess(string) else {
continue
}
// ...or break it
guard !shouldBreak(for: string) else {
break
}
// ...or return
guard !shouldReturn(for: string) else {
return
}
// ..or throw an error
guard string.isValid else {
throw StringError.invalid(string)
}
// ...or exit the program
guard !shouldExit(for: string) else {
exit(1)
}
}
#8 Passing functions & operators as closures
❤️ Love how you can pass functions & operators as closures in Swift. For example, it makes the syntax for sorting arrays really nice!
let array = [3, 9, 1, 4, 6, 2]
let sorted = array.sorted(by: <)
#7 Using #function for UserDefaults key consistency
🗝 Here's a neat little trick I use to get UserDefault key consistency in Swift (#function expands to the property name in getters/setters). Just remember to write a good suite of tests that'll guard you against bugs when changing property names.
extension UserDefaults {
var onboardingCompleted: Bool {
get { return bool(forKey: #function) }
set { set(newValue, forKey: #function) }
}
}
#6 Using a name already taken by the standard library
📛 Want to use a name already taken by the standard library for a nested type? No problem - just use Swift. to disambiguate:
extension Command {
enum Error: Swift.Error {
case missing
case invalid(String)
}
}
#5 Using Wrap to implement Equatable
📦 Playing around with using Wrap to implement Equatable for any type, primarily for testing:
protocol AutoEquatable: Equatable {}
extension AutoEquatable {
static func ==(lhs: Self, rhs: Self) -> Bool {
let lhsData = try! wrap(lhs) as Data
let rhsData = try! wrap(rhs) as Data
return lhsData == rhsData
}
}
#4 Using typealiases to reduce the length of method signatures
📏 One thing that I find really useful in Swift is to use typealiases to reduce the length of method signatures in generic types:
public class PathFinder<Object: PathFinderObject> {
public typealias Map = Object.Map
public typealias Node = Map.Node
public typealias Path = PathFinderPath<Object>
public static func possiblePaths(for object: Object, at rootNode: Node, on map: Map) -> Path.Sequence {
return .init(object: object, rootNode: rootNode, map: map)
}
}
#3 Referencing either external or internal parameter name when writing docs
📖 You can reference either the external or internal parameter label when writing Swift docs - and they get parsed the same:
// EITHER:
class Foo {
/**
* - parameter string: A string
*/
func bar(with string: String) {}
}
// OR:
class Foo {
/**
* - parameter with: A string
*/
func bar(with string: String) {}
}
#2 Using auto closures
👍 Finding more and more uses for auto closures in Swift. Can enable some pretty nice APIs:
extension Dictionary {
mutating func value(for key: Key, orAdd valueClosure: @autoclosure () -> Value) -> Value {
if let value = self[key] {
return value
}
let value = valueClosure()
self[key] = value
return value
}
}
#1 Namespacing with nested types
🚀 I’ve started to become a really big fan of nested types in Swift. Love the additional namespacing it gives you!
public struct Map {
public struct Model {
public let size: Size
public let theme: Theme
public var terrain: [Position : Terrain.Model]
public var units: [Position : Unit.Model]
public var buildings: [Position : Building.Model]
}
public enum Direction {
case up
case right
case down
case left
}
public struct Position {
public var x: Int
public var y: Int
}
public enum Size: String {
case small = "S"
case medium = "M"
case large = "L"
case extraLarge = "XL"
}
}Download Details:
Author: JohnSundell
Source code: https://github.com/JohnSundell/SwiftTips
License: MIT license
#swift
1666245660
A Collection Of Swift Tips & Tricks That I've Shared on Twitter
Swift tips & tricks ⚡️
One of the things I really love about Swift is how I keep finding interesting ways to use it in various situations, and when I do - I usually share them on Twitter. Here's a collection of all the tips & tricks that I've shared so far. Each entry has a link to the original tweet, if you want to respond with some feedback or question, which is always super welcome! 🚀
⚠️ This list is no longer being updated. For my latest Swift tips, checkout the "Tips" section on Swift by Sundell.
Also make sure to check out all of my other Swift content:
- 📖 My weekly articles, with a new post about Swift every Sunday!
- 🎧 My podcast which features special guests from around the community.
- 📬 My monthly newsletter - rounding up all of my content, including these tips, on the 1st of every month.
102 Making async tests faster and more stable
🚀 Here are some quick tips to make async tests faster & more stable:
- 😴 Avoid sleep() - use expectations instead
- ⏱ Use generous timeouts to avoid flakiness on CI
- 🧐 Put all assertions at the end of each test, not inside closures
// BEFORE:
class MentionDetectorTests: XCTestCase {
func testDetectingMention() {
let detector = MentionDetector()
let string = "This test was written by @johnsundell."
detector.detectMentions(in: string) { mentions in
XCTAssertEqual(mentions, ["johnsundell"])
}
sleep(2)
}
}
// AFTER:
class MentionDetectorTests: XCTestCase {
func testDetectingMention() {
let detector = MentionDetector()
let string = "This test was written by @johnsundell."
var mentions: [String]?
let expectation = self.expectation(description: #function)
detector.detectMentions(in: string) {
mentions = $0
expectation.fulfill()
}
waitForExpectations(timeout: 10)
XCTAssertEqual(mentions, ["johnsundell"])
}
}
For more on async testing, check out "Unit testing asynchronous Swift code".
101 Adding support for Apple Pencil double-taps
✍️ Adding support for the new Apple Pencil double-tap feature is super easy! All you have to do is to create a UIPencilInteraction, add it to a view, and implement one delegate method. Hopefully all pencil-compatible apps will soon adopt this.
let interaction = UIPencilInteraction()
interaction.delegate = self
view.addInteraction(interaction)
extension ViewController: UIPencilInteractionDelegate {
func pencilInteractionDidTap(_ interaction: UIPencilInteraction) {
// Handle pencil double-tap
}
}
For more on using this and other iPad Pro features, check out "Building iPad Pro features in Swift".
100 Combining values with functions
😎 Here's a cool function that combines a value with a function to return a closure that captures that value, so that it can be called without any arguments. Super useful when working with closure-based APIs and we want to use some of our properties without having to capture self.
func combine<A, B>(_ value: A, with closure: @escaping (A) -> B) -> () -> B {
return { closure(value) }
}
// BEFORE:
class ProductViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
buyButton.handler = { [weak self] in
guard let self = self else {
return
}
self.productManager.startCheckout(for: self.product)
}
}
}
// AFTER:
class ProductViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
buyButton.handler = combine(product, with: productManager.startCheckout)
}
}
99 Dependency injection using functions
💉 When I'm only using a single function from a dependency, I love to inject that function as a closure, instead of having to create a protocol and inject the whole object. Makes dependency injection & testing super simple.
final class ArticleLoader {
typealias Networking = (Endpoint) -> Future<Data>
private let networking: Networking
init(networking: @escaping Networking = URLSession.shared.load) {
self.networking = networking
}
func loadLatest() -> Future<[Article]> {
return networking(.latestArticles).decode()
}
}
For more on this technique, check out "Simple Swift dependency injection with functions".
98 Using a custom exception handler
💥 It's cool that you can easily assign a closure as a custom NSException handler. This is super useful when building things in Playgrounds - since you can't use breakpoints - so instead of just signal SIGABRT, you'll get the full exception description if something goes wrong.
NSSetUncaughtExceptionHandler { exception in
print(exception)
}
97 Using type aliases to give semantic meaning to primitives
❤️ I love that in Swift, we can use the type system to make our code so much more self-documenting - one way of doing so is to use type aliases to give the primitive types that we use a more semantic meaning.
extension List.Item {
// Using type aliases, we can give semantic meaning to the
// primitive types that we use, without having to introduce
// wrapper types.
typealias Index = Int
}
extension List {
enum Mutation {
// Our enum cases now become a lot more self-documenting,
// without having to add additional parameter labels to
// explain them.
case add(Item, Item.Index)
case update(Item, Item.Index)
case remove(Item.Index)
}
}
For more on self-documenting code, check out "Writing self-documenting Swift code".
96 Specializing protocols using constraints
🤯 A little late night prototyping session reveals that protocol constraints can not only be applied to extensions - they can also be added to protocol definitions!
This is awesome, since it lets us easily define specialized protocols based on more generic ones.
protocol Component {
associatedtype Container
func add(to container: Container)
}
// Protocols that inherit from other protocols can include
// constraints to further specialize them.
protocol ViewComponent: Component where Container == UIView {
associatedtype View: UIView
var view: View { get }
}
extension ViewComponent {
func add(to container: UIView) {
container.addSubview(view)
}
}
For more on specializing protocols, check out "Specializing protocols in Swift".
95 Unwrapping an optional or throwing an error
📦 Here's a super handy extension on Swift's Optional type, which gives us a really nice API for easily unwrapping an optional, or throwing an error in case the value turned out to be nil:
extension Optional {
func orThrow(_ errorExpression: @autoclosure () -> Error) throws -> Wrapped {
switch self {
case .some(let value):
return value
case .none:
throw errorExpression()
}
}
}
let file = try loadFile(at: path).orThrow(MissingFileError())
For more ways that optionals can be extended, check out "Extending optionals in Swift".
94 Testing code that uses static APIs
👩🔬 Testing code that uses static APIs can be really tricky, but there's a way that it can often be done - using Swift's first class function capabilities!
Instead of accessing that static API directly, we can inject the function we want to use, which enables us to mock it!
// BEFORE
class FriendsLoader {
func loadFriends(then handler: @escaping (Result<[Friend]>) -> Void) {
Networking.loadData(from: .friends) { result in
...
}
}
}
// AFTER
class FriendsLoader {
typealias Handler<T> = (Result<T>) -> Void
typealias DataLoadingFunction = (Endpoint, @escaping Handler<Data>) -> Void
func loadFriends(using dataLoading: DataLoadingFunction = Networking.loadData,
then handler: @escaping Handler<[Friend]>) {
dataLoading(.friends) { result in
...
}
}
}
// MOCKING IN TESTS
let dataLoading: FriendsLoader.DataLoadingFunction = { _, handler in
handler(.success(mockData))
}
friendsLoader.loadFriends(using: dataLoading) { result in
...
}
93 Matching multiple enum cases with associated values
🐾 Swift's pattern matching capabilities are so powerful! Two enum cases with associated values can even be matched and handled by the same switch case - which is super useful when handling state changes with similar data.
enum DownloadState {
case inProgress(progress: Double)
case paused(progress: Double)
case cancelled
case finished(Data)
}
func downloadStateDidChange(to state: DownloadState) {
switch state {
case .inProgress(let progress), .paused(let progress):
updateProgressView(with: progress)
case .cancelled:
showCancelledMessage()
case .finished(let data):
process(data)
}
}
92 Multiline string literals
🅰 One really nice benefit of Swift multiline string literals - even for single lines of text - is that they don't require quotes to be escaped. Perfect when working with things like HTML, or creating a custom description for an object.
let html = highlighter.highlight("Array<String>")
XCTAssertEqual(html, """
<span class="type">Array</span><<span class="type">String</span>>
""")
91 Reducing sequences
💎 While it's very common in functional programming, the reduce function might be a bit of a hidden gem in Swift. It provides a super useful way to transform a sequence into a single value.
extension Sequence where Element: Equatable {
func numberOfOccurrences(of target: Element) -> Int {
return reduce(0) { result, element in
guard element == target else {
return result
}
return result + 1
}
}
}
You can read more about transforming collections in "Transforming collections in Swift".
90 Avoiding manual Codable implementations
📦 When I use Codable in Swift, I want to avoid manual implementations as much as possible, even when there's a mismatch between my code structure and the JSON I'm decoding.
One way that can often be achieved is to use private data containers combined with computed properties.
struct User: Codable {
let name: String
let age: Int
var homeTown: String { return originPlace.name }
private let originPlace: Place
}
private extension User {
struct Place: Codable {
let name: String
}
}
extension User {
struct Container: Codable {
let user: User
}
}
89 Using feature flags instead of feature branches
🚢 Instead of using feature branches, I merge almost all of my code directly into master - and then I use feature flags to conditionally enable features when they're ready. That way I can avoid merge conflicts and keep shipping!
extension ListViewController {
func addSearchIfNeeded() {
// Rather than having to keep maintaining a separate
// feature branch for a new feature, we can use a flag
// to conditionally turn it on.
guard FeatureFlags.searchEnabled else {
return
}
let resultsVC = SearchResultsViewController()
let searchVC = UISearchController(
searchResultsController: resultsVC
)
searchVC.searchResultsUpdater = resultsVC
navigationItem.searchController = searchVC
}
}
You can read more about feature flags in "Feature flags in Swift".
88 Lightweight data hierarchies using tuples
💾 Here I'm using tuples to create a lightweight hierarchy for my data, giving me a nice structure without having to introduce any additional types.
struct CodeSegment {
var tokens: (
previous: String?,
current: String
)
var delimiters: (
previous: Character?
next: Character?
)
}
handle(segment.tokens.current)
You can read more about tuples in "Using tuples as lightweight types in Swift"
87 The rule of threes
3️⃣ Whenever I have 3 properties or local variables that share the same prefix, I usually try to extract them into their own method or type. That way I can avoid massive types & methods, and also increase readability, without falling into a "premature optimization" trap.
Before
public func generate() throws {
let contentFolder = try folder.subfolder(named: "content")
let articleFolder = try contentFolder.subfolder(named: "posts")
let articleProcessor = ContentProcessor(folder: articleFolder)
let articles = try articleProcessor.process()
...
}
After
public func generate() throws {
let contentFolder = try folder.subfolder(named: "content")
let articles = try processArticles(in: contentFolder)
...
}
private func processArticles(in folder: Folder) throws -> [ContentItem] {
let folder = try folder.subfolder(named: "posts")
let processor = ContentProcessor(folder: folder)
return try processor.process()
}
86 Useful Codable extensions
👨🔧 Here's two extensions that I always add to the Encodable & Decodable protocols, which for me really make the Codable API nicer to use. By using type inference for decoding, a lot of boilerplate can be removed when the compiler is already able to infer the resulting type.
extension Encodable {
func encoded() throws -> Data {
return try JSONEncoder().encode(self)
}
}
extension Data {
func decoded<T: Decodable>() throws -> T {
return try JSONDecoder().decode(T.self, from: self)
}
}
let data = try user.encoded()
// By using a generic type in the decoded() method, the
// compiler can often infer the type we want to decode
// from the current context.
try userDidLogin(data.decoded())
// And if not, we can always supply the type, still making
// the call site read very nicely.
let otherUser = try data.decoded() as User
85 Using shared UserDefaults suites
📦 UserDefaults is a lot more powerful than what it first might seem like. Not only can it store more complex values (like dates & dictionaries) and parse command line arguments - it also enables easy sharing of settings & lightweight data between apps in the same App Group.
let sharedDefaults = UserDefaults(suiteName: "my-app-group")!
let useDarkMode = sharedDefaults.bool(forKey: "dark-mode")
// This value is put into the shared suite.
sharedDefaults.set(true, forKey: "dark-mode")
// If you want to treat the shared settings as read-only (and add
// local overrides on top of them), you can simply add the shared
// suite to the standard UserDefaults.
let combinedDefaults = UserDefaults.standard
combinedDefaults.addSuite(named: "my-app-group")
// This value is a local override, not added to the shared suite.
combinedDefaults.set(true, forKey: "app-specific-override")
84 Custom UIView backing layers
🎨 By overriding layerClass you can tell UIKit what CALayer class to use for a UIView's backing layer. That way you can reduce the amount of layers, and don't have to do any manual layout.
final class GradientView: UIView {
override class var layerClass: AnyClass { return CAGradientLayer.self }
var colors: (start: UIColor, end: UIColor)? {
didSet { updateLayer() }
}
private func updateLayer() {
let layer = self.layer as! CAGradientLayer
layer.colors = colors.map { [$0.start.cgColor, $0.end.cgColor] }
}
}
83 Auto-Equatable enums with associated values
✅ That the compiler now automatically synthesizes Equatable conformances is such a huge upgrade for Swift! And the cool thing is that it works for all kinds of types - even for enums with associated values! Especially useful when using enums for verification in unit tests.
struct Article: Equatable {
let title: String
let text: String
}
struct User: Equatable {
let name: String
let age: Int
}
extension Navigator {
enum Destination: Equatable {
case profile(User)
case article(Article)
}
}
func testNavigatingToArticle() {
let article = Article(title: "Title", text: "Text")
controller.select(article)
XCTAssertEqual(navigator.destinations, [.article(article)])
}
82 Defaults for associated types
🤝 Associated types can have defaults in Swift - which is super useful for types that are not easily inferred (for example when they're not used for a specific instance method or property).
protocol Identifiable {
associatedtype RawIdentifier: Codable = String
var id: Identifier<Self> { get }
}
struct User: Identifiable {
let id: Identifier<User>
let name: String
}
struct Group: Identifiable {
typealias RawIdentifier = Int
let id: Identifier<Group>
let name: String
}
81 Creating a dedicated identifier type
🆔 If you want to avoid using plain strings as identifiers (which can increase both type safety & readability), it's really easy to create a custom Identifier type that feels just like a native Swift type, thanks to protocols!
More on this topic in "Type-safe identifiers in Swift".
struct Identifier: Hashable {
let string: String
}
extension Identifier: ExpressibleByStringLiteral {
init(stringLiteral value: String) {
string = value
}
}
extension Identifier: CustomStringConvertible {
var description: String {
return string
}
}
extension Identifier: Codable {
init(from decoder: Decoder) throws {
let container = try decoder.singleValueContainer()
string = try container.decode(String.self)
}
func encode(to encoder: Encoder) throws {
var container = encoder.singleValueContainer()
try container.encode(string)
}
}
struct Article: Codable {
let id: Identifier
let title: String
}
let article = Article(id: "my-article", title: "Hello world!")
80 Assigning optional tuple members to variables
🙌 A really cool thing about using tuples to model the internal state of a Swift type, is that you can unwrap an optional tuple's members directly into local variables.
Very useful in order to group multiple optional values together for easy unwrapping & handling.
class ImageTransformer {
private var queue = [(image: UIImage, transform: Transform)]()
private func processNext() {
// When unwrapping an optional tuple, you can assign the members
// directly to local variables.
guard let (image, transform) = queue.first else {
return
}
let context = Context()
context.draw(image)
context.apply(transform)
...
}
}
79 Struct convenience initializers
❤️ I love to structure my code using extensions in Swift. One big benefit of doing so when it comes to struct initializers, is that defining a convenience initializer doesn't remove the default one the compiler generates - best of both worlds!
struct Article {
let date: Date
var title: String
var text: String
var comments: [Comment]
}
extension Article {
init(title: String, text: String) {
self.init(date: Date(), title: title, text: text, comments: [])
}
}
let articleA = Article(title: "Best Cupcake Recipe", text: "...")
let articleB = Article(
date: Date(),
title: "Best Cupcake Recipe",
text: "...",
comments: [
Comment(user: currentUser, text: "Yep, can confirm!")
]
)
78 Usages of throwing functions
🏈 A big benefit of using throwing functions for synchronous Swift APIs is that the caller can decide whether they want to treat the return value as optional (try?) or required (try).
func loadFile(named name: String) throws -> File {
guard let url = urlForFile(named: name) else {
throw File.Error.missing
}
do {
let data = try Data(contentsOf: url)
return File(url: url, data: data)
} catch {
throw File.Error.invalidData(error)
}
}
let requiredFile = try loadFile(named: "AppConfig.json")
let optionalFile = try? loadFile(named: "UserSettings.json")
77 Nested generic types
🐝 Types that are nested in generics automatically inherit their parent's generic types - which is super useful when defining accessory types (for things like states or outcomes).
struct Task<Input, Output> {
typealias Closure = (Input) throws -> Output
let closure: Closure
}
extension Task {
enum Result {
case success(Output)
case failure(Error)
}
}
76 Equatable & Hashable structures
🤖 Now that the Swift compiler automatically synthesizes Equatable & Hashable conformances for value types, it's easier than ever to setup model structures with nested types that are all Equatable/Hashable!
typealias Value = Hashable & Codable
struct User: Value {
var name: String
var age: Int
var lastLoginDate: Date?
var settings: Settings
}
extension User {
struct Settings: Value {
var itemsPerPage: Int
var theme: Theme
}
}
extension User.Settings {
enum Theme: String, Value {
case light
case dark
}
}
You can read more about using nested types in Swift here.
75 Conditional conformances
🎉 Swift 4.1 is here! One of the key features it brings is conditional conformances, which lets you have a type only conform to a protocol under certain constraints.
protocol UnboxTransformable {
associatedtype RawValue
static func transform(_ value: RawValue) throws -> Self?
}
extension Array: UnboxTransformable where Element: UnboxTransformable {
typealias RawValue = [Element.RawValue]
static func transform(_ value: RawValue) throws -> [Element]? {
return try value.compactMap(Element.transform)
}
}
I also have an article with lots of more info on conditional conformances here. Paul Hudson also has a great overview of all Swift 4.1 features here.
74 Generic type aliases
🕵️♀️ A cool thing about Swift type aliases is that they can be generic! Combine that with tuples and you can easily define simple generic types.
typealias Pair<T> = (T, T)
extension Game {
func calculateScore(for players: Pair<Player>) -> Int {
...
}
}
You can read more about using tuples as lightweight types here.
73 Parsing command line arguments using UserDefaults
☑️ A really cool "hidden" feature of UserDefaults is that it contains any arguments that were passed to the app at launch!
Super useful both in Swift command line tools & scripts, but also to temporarily override a value when debugging iOS apps.
let defaults = UserDefaults.standard
let query = defaults.string(forKey: "query")
let resultCount = defaults.integer(forKey: "results")
72 Using the & operator
👏 Swift's & operator is awesome! Not only can you use it to compose protocols, you can compose other types too! Very useful if you want to hide concrete types & implementation details.
protocol LoadableFromURL {
func load(from url: URL)
}
class ContentViewController: UIViewController, LoadableFromURL {
func load(from url: URL) {
...
}
}
class ViewControllerFactory {
func makeContentViewController() -> UIViewController & LoadableFromURL {
return ContentViewController()
}
}
71 Capturing multiple values in mocks
🤗 When capturing values in mocks, using an array (instead of just a single value) makes it easy to verify that only a certain number of values were passed.
Perfect for protecting against "over-calling" something.
class UserManagerTests: XCTestCase {
func testObserversCalledWhenUserFirstLogsIn() {
let manager = UserManager()
let observer = ObserverMock()
manager.addObserver(observer)
// First login, observers should be notified
let user = User(id: 123, name: "John")
manager.userDidLogin(user)
XCTAssertEqual(observer.users, [user])
// If the same user logs in again, observers shouldn't be notified
manager.userDidLogin(user)
XCTAssertEqual(observer.users, [user])
}
}
private extension UserManagerTests {
class ObserverMock: UserManagerObserver {
private(set) var users = [User]()
func userDidChange(to user: User) {
users.append(user)
}
}
}
70 Reducing the need for mocks
👋 When writing tests, you don't always need to create mocks - you can create stubs using real instances of things like errors, URLs & UserDefaults.
Here's how to do that for some common tasks/object types in Swift:
// Create errors using NSError (#function can be used to reference the name of the test)
let error = NSError(domain: #function, code: 1, userInfo: nil)
// Create non-optional URLs using file paths
let url = URL(fileURLWithPath: "Some/URL")
// Reference the test bundle using Bundle(for:)
let bundle = Bundle(for: type(of: self))
// Create an explicit UserDefaults object (instead of having to use a mock)
let userDefaults = UserDefaults(suiteName: #function)
// Create queues to control/await concurrent operations
let queue = DispatchQueue(label: #function)
For when you actually do need mocking, check out "Mocking in Swift".
69 Using "then" as an external parameter label for closures
⏱ I've started using "then" as an external parameter label for completion handlers. Makes the call site read really nicely (Because I do ❤️ conversational API design) regardless of whether trailing closure syntax is used or not.
protocol DataLoader {
// Adding type aliases to protocols can be a great way to
// reduce verbosity for parameter types.
typealias Handler = (Result<Data>) -> Void
associatedtype Endpoint
func loadData(from endpoint: Endpoint, then handler: @escaping Handler)
}
loader.loadData(from: .messages) { result in
...
}
loader.loadData(from: .messages, then: { result in
...
})
68 Combining lazily evaluated sequences with the builder pattern
😴 Combining lazily evaluated sequences with builder pattern-like properties can lead to some pretty sweet APIs for configurable sequences in Swift.
Also useful for queries & other things you "build up" and then execute.
// Extension adding builder pattern-like properties that return
// a new sequence value with the given configuration applied
extension FileSequence {
var recursive: FileSequence {
var sequence = self
sequence.isRecursive = true
return sequence
}
var includingHidden: FileSequence {
var sequence = self
sequence.includeHidden = true
return sequence
}
}
// BEFORE
let files = folder.makeFileSequence(recursive: true, includeHidden: true)
// AFTER
let files = folder.files.recursive.includingHidden
Want an intro to lazy sequences? Check out "Swift sequences: The art of being lazy".
67 Faster & more stable UI tests
My top 3 tips for faster & more stable UI tests:
📱 Reset the app's state at the beginning of every test.
🆔 Use accessibility identifiers instead of UI strings.
⏱ Use expectations instead of waiting time.
func testOpeningArticle() {
// Launch the app with an argument that tells it to reset its state
let app = XCUIApplication()
app.launchArguments.append("--uitesting")
app.launch()
// Check that the app is displaying an activity indicator
let activityIndicator = app.activityIndicator.element
XCTAssertTrue(activityIndicator.exists)
// Wait for the loading indicator to disappear = content is ready
expectation(for: NSPredicate(format: "exists == 0"),
evaluatedWith: activityIndicator)
// Use a generous timeout in case the network is slow
waitForExpectations(timeout: 10)
// Tap the cell for the first article
app.tables.cells["Article.0"].tap()
// Assert that a label with the accessibility identifier "Article.Title" exists
let label = app.staticTexts["Article.Title"]
XCTAssertTrue(label.exists)
}
66 Accessing the clipboard from a Swift script
📋 It's super easy to access the contents of the clipboard from a Swift script. A big benefit of Swift scripting is being able to use Cocoa's powerful APIs for Mac apps.
import Cocoa
let clipboard = NSPasteboard.general.string(forType: .string)
65 Using tuples for view state
🎯 Using Swift tuples for view state can be a super nice way to group multiple properties together and render them reactively using the layout system.
By using a tuple we don't have to either introduce a new type or make our view model-aware.
class TextView: UIView {
var state: (title: String?, text: String?) {
// By telling UIKit that our view needs layout and binding our
// state in layoutSubviews, we can react to state changes without
// doing unnecessary layout work.
didSet { setNeedsLayout() }
}
private let titleLabel = UILabel()
private let textLabel = UILabel()
override func layoutSubviews() {
super.layoutSubviews()
titleLabel.text = state.title
textLabel.text = state.text
...
}
}
64 Throwing tests and LocalizedError
⚾️ Swift tests can throw, which is super useful in order to avoid complicated logic or force unwrapping. By making errors conform to LocalizedError, you can also get a nice error message in Xcode if there's a failure.
class ImageCacheTests: XCTestCase {
func testCachingAndLoadingImage() throws {
let bundle = Bundle(for: type(of: self))
let cache = ImageCache(bundle: bundle)
// Bonus tip: You can easily load images from your test
// bundle using this UIImage initializer
let image = try require(UIImage(named: "sample", in: bundle, compatibleWith: nil))
try cache.cache(image, forKey: "key")
let cachedImage = try cache.image(forKey: "key")
XCTAssertEqual(image, cachedImage)
}
}
enum ImageCacheError {
case emptyKey
case dataConversionFailed
}
// When using throwing tests, making your errors conform to
// LocalizedError will render a much nicer error message in
// Xcode (per default only the error code is shown).
extension ImageCacheError: LocalizedError {
var errorDescription: String? {
switch self {
case .emptyKey:
return "An empty key was given"
case .dataConversionFailed:
return "Failed to convert the given image to Data"
}
}
}
For more information, and the implementation of the require method used above, check out "Avoiding force unwrapping in Swift unit tests".
63 The difference between static and class properties
✍️ Unlike static properties, class properties can be overridden by subclasses (however, they can't be stored, only computed).
class TableViewCell: UITableViewCell {
class var preferredHeight: CGFloat { return 60 }
}
class TallTableViewCell: TableViewCell {
override class var preferredHeight: CGFloat { return 100 }
}
62 Creating extensions with static factory methods
👨🎨 Creating extensions with static factory methods can be a great alternative to subclassing in Swift, especially for things like setting up UIViews, CALayers or other kinds of styling.
It also lets you remove a lot of styling & setup from your view controllers.
extension UILabel {
static func makeForTitle() -> UILabel {
let label = UILabel()
label.font = .boldSystemFont(ofSize: 24)
label.textColor = .darkGray
label.adjustsFontSizeToFitWidth = true
label.minimumScaleFactor = 0.75
return label
}
static func makeForText() -> UILabel {
let label = UILabel()
label.font = .systemFont(ofSize: 16)
label.textColor = .black
label.numberOfLines = 0
return label
}
}
class ArticleViewController: UIViewController {
lazy var titleLabel = UILabel.makeForTitle()
lazy var textLabel = UILabel.makeForText()
}
61 Child view controller auto-resizing
🧒 An awesome thing about child view controllers is that they're automatically resized to match their parent, making them a super nice solution for things like loading & error views.
class ListViewController: UIViewController {
func loadItems() {
let loadingViewController = LoadingViewController()
add(loadingViewController)
dataLoader.loadItems { [weak self] result in
loadingViewController.remove()
self?.handle(result)
}
}
}
For more about child view controller (including the add and remove methods used above), check out "Using child view controllers as plugins in Swift".
60 Using zip
🤐 Using the zip function in Swift you can easily combine two sequences. Super useful when using two sequences to do some work, since zip takes care of all the bounds-checking.
func render(titles: [String]) {
for (label, text) in zip(titleLabels, titles) {
print(text)
label.text = text
}
}
59 Defining custom option sets
🎛 The awesome thing about option sets in Swift is that they can automatically either be passed as a single member or as a set. Even cooler is that you can easily define your own option sets as well, perfect for options and other non-exclusive values.
// Option sets are awesome, because you can easily pass them
// both using dot syntax and array literal syntax, like when
// using the UIView animation API:
UIView.animate(withDuration: 0.3,
delay: 0,
options: .allowUserInteraction,
animations: animations)
UIView.animate(withDuration: 0.3,
delay: 0,
options: [.allowUserInteraction, .layoutSubviews],
animations: animations)
// The cool thing is that you can easily define your own option
// sets as well, by defining a struct that has an Int rawValue,
// that will be used as a bit mask.
extension Cache {
struct Options: OptionSet {
static let saveToDisk = Options(rawValue: 1)
static let clearOnMemoryWarning = Options(rawValue: 1 << 1)
static let clearDaily = Options(rawValue: 1 << 2)
let rawValue: Int
}
}
// We can now use Cache.Options just like UIViewAnimationOptions:
Cache(options: .saveToDisk)
Cache(options: [.saveToDisk, .clearDaily])
58 Using the where clause with associated types
🙌 Using the where clause when designing protocol-oriented APIs in Swift can let your implementations (or others' if it's open source) have a lot more freedom, especially when it comes to collections.
See "Using generic type constraints in Swift 4" for more info.
public protocol PathFinderMap {
associatedtype Node
// Using the 'where' clause for associated types, we can
// ensure that a type meets certain requirements (in this
// case that it's a sequence with Node elements).
associatedtype NodeSequence: Sequence where NodeSequence.Element == Node
// Instead of using a concrete type (like [Node]) here, we
// give implementors of this protocol more freedom while
// still meeting our requirements. For example, one
// implementation might use Set<Node>.
func neighbors(of node: Node) -> NodeSequence
}
57 Using first class functions when iterating over a dictionary
👨🍳 Combine first class functions in Swift with the fact that Dictionary elements are (Key, Value) tuples and you can build yourself some pretty awesome functional chains when iterating over a Dictionary.
func makeActor(at coordinate: Coordinate, for building: Building) -> Actor {
let actor = Actor()
actor.position = coordinate.point
actor.animation = building.animation
return actor
}
func render(_ buildings: [Coordinate : Building]) {
buildings.map(makeActor).forEach(add)
}
56 Calling instance methods as static functions
😎 In Swift, you can call any instance method as a static function and it will return a closure representing that method. This is how running tests using SPM on Linux works.
More about this topic in my blog post "First class functions in Swift".
// This produces a '() -> Void' closure which is a reference to the
// given view's 'removeFromSuperview' method.
let closure = UIView.removeFromSuperview(view)
// We can now call it just like we would any other closure, and it
// will run 'view.removeFromSuperview()'
closure()
// This is how running tests using the Swift Package Manager on Linux
// works, you return your test functions as closures:
extension UserManagerTests {
static var allTests = [
("testLoggingIn", testLoggingIn),
("testLoggingOut", testLoggingOut),
("testUserPermissions", testUserPermissions)
]
}
55 Dropping suffixes from method names to support multiple arguments
👏 One really nice benefit of dropping suffixes from method names (and just using verbs, when possible) is that it becomes super easy to support both single and multiple arguments, and it works really well semantically.
extension UIView {
func add(_ subviews: UIView...) {
subviews.forEach(addSubview)
}
}
view.add(button)
view.add(label)
// By dropping the "Subview" suffix from the method name, both
// single and multiple arguments work really well semantically.
view.add(button, label)
54 Constraining protocols to classes to ensure mutability
👽 Using the AnyObject (or class) constraint on protocols is not only useful when defining delegates (or other weak references), but also when you always want instances to be mutable without copying.
// By constraining a protocol with 'AnyObject' it can only be adopted
// by classes, which means all instances will always be mutable, and
// that it's the original instance (not a copy) that will be mutated.
protocol DataContainer: AnyObject {
var data: Data? { get set }
}
class UserSettingsManager {
private var settings: Settings
private let dataContainer: DataContainer
// Since DataContainer is a protocol, we an easily mock it in
// tests if we use dependency injection
init(settings: Settings, dataContainer: DataContainer) {
self.settings = settings
self.dataContainer = dataContainer
}
func saveSettings() throws {
let data = try settings.serialize()
// We can now assign properties on an instance of our protocol
// because the compiler knows it's always going to be a class
dataContainer.data = data
}
}
53 String-based enums in string interpolation
🍣 Even if you define a custom raw value for a string-based enum in Swift, the full case name will be used in string interpolation.
Super useful when using separate raw values for JSON, while still wanting to use the full case name in other contexts.
extension Building {
// This enum has custom raw values that are used when decoding
// a value, for example from JSON.
enum Kind: String {
case castle = "C"
case town = "T"
case barracks = "B"
case goldMine = "G"
case camp = "CA"
case blacksmith = "BL"
}
var animation: Animation {
return Animation(
// When used in string interpolation, the full case name is still used.
// For 'castle' this will be 'buildings/castle'.
name: "buildings/\(kind)",
frameCount: frameCount,
frameDuration: frameDuration
)
}
}
52 Expressively comparing a value with a list of candidates
👨🔬 Continuing to experiment with expressive ways of comparing a value with a list of candidates in Swift. Adding an extension on Equatable is probably my favorite approach so far.
extension Equatable {
func isAny(of candidates: Self...) -> Bool {
return candidates.contains(self)
}
}
let isHorizontal = direction.isAny(of: .left, .right)
See tip 35 for my previous experiment.
51 UIView bounds and transforms
📐 A really interesting side-effect of a UIView's bounds being its rect within its own coordinate system is that transforms don't affect it at all. That's why it's usually a better fit than frame when doing layout calculations of subviews.
let view = UIView()
view.frame.size = CGSize(width: 100, height: 100)
view.transform = CGAffineTransform(scaleX: 2, y: 2)
print(view.frame) // (-50.0, -50.0, 200.0, 200.0)
print(view.bounds) // (0.0, 0.0, 100.0, 100.0)
50 UIKit default arguments
👏 It's awesome that many UIKit APIs with completion handlers and other optional parameters import into Swift with default arguments (even though they are written in Objective-C). Getting rid of all those nil arguments is so nice!
// BEFORE: All parameters are specified, just like in Objective-C
viewController.present(modalViewController, animated: true, completion: nil)
modalViewController.dismiss(animated: true, completion: nil)
viewController.transition(from: loadingViewController,
to: contentViewController,
duration: 0.3,
options: [],
animations: animations,
completion: nil)
// AFTER: Since many UIKit APIs with completion handlers and other
// optional parameters import into Swift with default arguments,
// we can make our calls shorter
viewController.present(modalViewController, animated: true)
modalViewController.dismiss(animated: true)
viewController.transition(from: loadingViewController,
to: contentViewController,
duration: 0.3,
animations: animations)
49 Avoiding Massive View Controllers
✂️ Avoiding Massive View Controllers is all about finding the right levels of abstraction and splitting things up.
My personal rule of thumb is that as soon as I have 3 methods or properties that have the same prefix, I break them out into their own type.
// BEFORE
class LoginViewController: UIViewController {
private lazy var signUpLabel = UILabel()
private lazy var signUpImageView = UIImageView()
private lazy var signUpButton = UIButton()
}
// AFTER
class LoginViewController: UIViewController {
private lazy var signUpView = SignUpView()
}
class SignUpView: UIView {
private lazy var label = UILabel()
private lazy var imageView = UIImageView()
private lazy var button = UIButton()
}
48 Extending optionals
❤️ I love the fact that optionals are enums in Swift - it makes it so easy to extend them with convenience APIs for certain types. Especially useful when doing things like data validation on optional values.
func validateTextFields() -> Bool {
guard !usernameTextField.text.isNilOrEmpty else {
return false
}
...
return true
}
// Since all optionals are actual enum values in Swift, we can easily
// extend them for certain types, to add our own convenience APIs
extension Optional where Wrapped == String {
var isNilOrEmpty: Bool {
switch self {
case let string?:
return string.isEmpty
case nil:
return true
}
}
}
// Since strings are now Collections in Swift 4, you can even
// add this property to all optional collections:
extension Optional where Wrapped: Collection {
var isNilOrEmpty: Bool {
switch self {
case let collection?:
return collection.isEmpty
case nil:
return true
}
}
}
47 Using where with for-loops
🗺 Using the where keyword can be a super nice way to quickly apply a filter in a for-loop in Swift. You can of course use map, filter and forEach, or guard, but for simple loops I think this is very expressive and nice.
func archiveMarkedPosts() {
for post in posts where post.isMarked {
archive(post)
}
}
func healAllies() {
for player in players where player.isAllied(to: currentPlayer) {
player.heal()
}
}
46 Variable shadowing
👻 Variable shadowing can be super useful in Swift, especially when you want to create a local copy of a parameter value in order to use it as state within a closure.
init(repeatMode: RepeatMode, closure: @escaping () -> UpdateOutcome) {
// Shadow the argument with a local, mutable copy
var repeatMode = repeatMode
self.closure = {
// With shadowing, there's no risk of accidentially
// referring to the immutable version
switch repeatMode {
case .forever:
break
case .times(let count):
guard count > 0 else {
return .finished
}
// We can now capture the mutable version and use
// it for state in a closure
repeatMode = .times(count - 1)
}
return closure()
}
}
45 Using dot syntax for static properties and initializers
✒️ Dot syntax is one of my favorite features of Swift. What's really cool is that it's not only for enums, any static method or property can be used with dot syntax - even initializers! Perfect for convenience APIs and default parameters.
public enum RepeatMode {
case times(Int)
case forever
}
public extension RepeatMode {
static var never: RepeatMode {
return .times(0)
}
static var once: RepeatMode {
return .times(1)
}
}
view.perform(animation, repeated: .once)
// To make default parameters more compact, you can even use init with dot syntax
class ImageLoader {
init(cache: Cache = .init(), decoder: ImageDecoder = .init()) {
...
}
}
44 Calling functions as closures with a tuple as parameters
🚀 One really cool aspect of Swift having first class functions is that you can pass any function (or even initializer) as a closure, and even call it with a tuple containing its parameters!
// This function lets us treat any "normal" function or method as
// a closure and run it with a tuple that contains its parameters
func call<Input, Output>(_ function: (Input) -> Output, with input: Input) -> Output {
return function(input)
}
class ViewFactory {
func makeHeaderView() -> HeaderView {
// We can now pass an initializer as a closure, and a tuple
// containing its parameters
return call(HeaderView.init, with: loadTextStyles())
}
private func loadTextStyles() -> (font: UIFont, color: UIColor) {
return (theme.font, theme.textColor)
}
}
class HeaderView {
init(font: UIFont, textColor: UIColor) {
...
}
}
43 Enabling static dependency injection
💉 If you've been struggling to test code that uses static APIs, here's a technique you can use to enable static dependency injection without having to modify any call sites:
// Before: Almost impossible to test due to the use of singletons
class Analytics {
static func log(_ event: Event) {
Database.shared.save(event)
let dictionary = event.serialize()
NetworkManager.shared.post(dictionary, to: eventURL)
}
}
// After: Much easier to test, since we can inject mocks as arguments
class Analytics {
static func log(_ event: Event,
database: Database = .shared,
networkManager: NetworkManager = .shared) {
database.save(event)
let dictionary = event.serialize()
networkManager.post(dictionary, to: eventURL)
}
}
42 Type inference for lazy properties in Swift 4
🎉 In Swift 4, type inference works for lazy properties and you don't need to explicitly refer to self!
// Swift 3
class PurchaseView: UIView {
private lazy var buyButton: UIButton = self.makeBuyButton()
private func makeBuyButton() -> UIButton {
let button = UIButton()
button.setTitle("Buy", for: .normal)
button.setTitleColor(.blue, for: .normal)
return button
}
}
// Swift 4
class PurchaseView: UIView {
private lazy var buyButton = makeBuyButton()
private func makeBuyButton() -> UIButton {
let button = UIButton()
button.setTitle("Buy", for: .normal)
button.setTitleColor(.blue, for: .normal)
return button
}
}
41 Converting Swift errors to NSError
😎 You can turn any Swift Error into an NSError, which is super useful when pattern matching with a code 👍. Also, switching on optionals is pretty cool!
let task = urlSession.dataTask(with: url) { data, _, error in
switch error {
case .some(let error as NSError) where error.code == NSURLErrorNotConnectedToInternet:
presenter.showOfflineView()
case .some(let error):
presenter.showGenericErrorView()
case .none:
presenter.renderContent(from: data)
}
}
task.resume()
Also make sure to check out Kostas Kremizas' tip about how you can pattern match directly against a member of URLError.
40 Making UIImage macOS compatible
🖥 Here's an easy way to make iOS model code that uses UIImage macOS compatible - like me and Gui Rambo discussed on the Swift by Sundell Podcast.
// Either put this in a separate file that you only include in your macOS target or wrap the code in #if os(macOS) / #endif
import Cocoa
// Step 1: Typealias UIImage to NSImage
typealias UIImage = NSImage
// Step 2: You might want to add these APIs that UIImage has but NSImage doesn't.
extension NSImage {
var cgImage: CGImage? {
var proposedRect = CGRect(origin: .zero, size: size)
return cgImage(forProposedRect: &proposedRect,
context: nil,
hints: nil)
}
convenience init?(named name: String) {
self.init(named: Name(name))
}
}
// Step 3: Profit - you can now make your model code that uses UIImage cross-platform!
struct User {
let name: String
let profileImage: UIImage
}
39 Internally mutable protocol-oriented APIs
🤖 You can easily define a protocol-oriented API that can only be mutated internally, by using an internal protocol that extends a public one.
// Declare a public protocol that acts as your immutable API
public protocol ModelHolder {
associatedtype Model
var model: Model { get }
}
// Declare an extended, internal protocol that provides a mutable API
internal protocol MutableModelHolder: ModelHolder {
var model: Model { get set }
}
// You can now implement the requirements using 'public internal(set)'
public class UserHolder: MutableModelHolder {
public internal(set) var model: User
internal init(model: User) {
self.model = model
}
}
38 Switching on a set
🎛 You can switch on a set using array literals as cases in Swift! Can be really useful to avoid many if/else if statements.
class RoadTile: Tile {
var connectedDirections = Set<Direction>()
func render() {
switch connectedDirections {
case [.up, .down]:
image = UIImage(named: "road-vertical")
case [.left, .right]:
image = UIImage(named: "road-horizontal")
default:
image = UIImage(named: "road")
}
}
}
37 Adding the current locale to cache keys
🌍 When caching localized content in an app, it's a good idea to add the current locale to all keys, to prevent bugs when switching languages.
func cache(_ content: Content, forKey key: String) throws {
let data = try wrap(content) as Data
let key = localize(key: key)
try storage.store(data, forKey: key)
}
func loadCachedContent(forKey key: String) -> Content? {
let key = localize(key: key)
let data = storage.loadData(forKey: key)
return data.flatMap { try? unbox(data: $0) }
}
private func localize(key: String) -> String {
return key + "-" + Bundle.main.preferredLocalizations[0]
}
36 Setting up tests to avoid retain cycles with weak references
🚳 Here's an easy way to setup a test to avoid accidental retain cycles with object relationships (like weak delegates & observers) in Swift:
func testDelegateNotRetained() {
// Assign the delegate (weak) and also retain it using a local var
var delegate: Delegate? = DelegateMock()
controller.delegate = delegate
XCTAssertNotNil(controller.delegate)
// Release the local var, which should also release the weak reference
delegate = nil
XCTAssertNil(controller.delegate)
}
35 Expressively matching a value against a list of candidates
👨🔬 Playing around with an expressive way to check if a value matches any of a list of candidates in Swift:
// Instead of multiple conditions like this:
if string == "One" || string == "Two" || string == "Three" {
}
// You can now do:
if string == any(of: "One", "Two", "Three") {
}
You can find a gist with the implementation here.
34 Organizing code using extensions
👪 APIs in a Swift extension automatically inherit its access control level, making it a neat way to organize public, internal & private APIs.
public extension Animation {
init(textureNamed textureName: String) {
frames = [Texture(name: textureName)]
}
init(texturesNamed textureNames: [String], frameDuration: TimeInterval = 1) {
frames = textureNames.map(Texture.init)
self.frameDuration = frameDuration
}
init(image: Image) {
frames = [Texture(image: image)]
}
}
internal extension Animation {
func loadFrameImages() -> [Image] {
return frames.map { $0.loadImageIfNeeded() }
}
}
33 Using map to transform an optional into a Result type
🗺 Using map you can transform an optional value into an optional Result type by simply passing in the enum case.
enum Result<Value> {
case value(Value)
case error(Error)
}
class Promise<Value> {
private var result: Result<Value>?
init(value: Value? = nil) {
result = value.map(Result.value)
}
}
32 Assigning to self in struct initializers
👌 It's so nice that you can assign directly to self in struct initializers in Swift. Very useful when adding conformance to protocols.
extension Bool: AnswerConvertible {
public init(input: String) throws {
switch input.lowercased() {
case "y", "yes", "👍":
self = true
default:
self = false
}
}
}
31 Recursively calling closures as inline functions
☎️ Defining Swift closures as inline functions enables you to recursively call them, which is super useful in things like custom sequences.
class Database {
func records(matching query: Query) -> AnySequence<Record> {
var recordIterator = loadRecords().makeIterator()
func iterate() -> Record? {
guard let nextRecord = recordIterator.next() else {
return nil
}
guard nextRecord.matches(query) else {
// Since the closure is an inline function, it can be recursively called,
// in this case in order to advance to the next item.
return iterate()
}
return nextRecord
}
// AnySequence/AnyIterator are part of the standard library and provide an easy way
// to define custom sequences using closures.
return AnySequence { AnyIterator(iterate) }
}
}
Rob Napier points out that using the above might cause crashes if used on a large databaset, since Swift has no guaranteed Tail Call Optimization (TCO).
Slava Pestov also points out that another benefit of inline functions vs closures is that they can have their own generic parameter list.
30 Passing self to required Objective-C dependencies
🏖 Using lazy properties in Swift, you can pass self to required Objective-C dependencies without having to use force-unwrapped optionals.
class DataLoader: NSObject {
lazy var urlSession: URLSession = self.makeURLSession()
private func makeURLSession() -> URLSession {
return URLSession(configuration: .default, delegate: self, delegateQueue: .main)
}
}
class Renderer {
lazy var displayLink: CADisplayLink = self.makeDisplayLink()
private func makeDisplayLink() -> CADisplayLink {
return CADisplayLink(target: self, selector: #selector(screenDidRefresh))
}
}
29 Making weak or lazy properties readonly
👓 If you have a property in Swift that needs to be weak or lazy, you can still make it readonly by using private(set).
class Node {
private(set) weak var parent: Node?
private(set) lazy var children = [Node]()
func add(child: Node) {
children.append(child)
child.parent = self
}
}
28 Defining static URLs using string literals
🌏 Tired of using URL(string: "url")! for static URLs? Make URL conform to ExpressibleByStringLiteral and you can now simply use "url" instead.
extension URL: ExpressibleByStringLiteral {
// By using 'StaticString' we disable string interpolation, for safety
public init(stringLiteral value: StaticString) {
self = URL(string: "\(value)").require(hint: "Invalid URL string literal: \(value)")
}
}
// We can now define URLs using static string literals 🎉
let url: URL = "https://www.swiftbysundell.com"
let task = URLSession.shared.dataTask(with: "https://www.swiftbysundell.com")
// In Swift 3 or earlier, you also have to implement 2 additional initializers
extension URL {
public init(extendedGraphemeClusterLiteral value: StaticString) {
self.init(stringLiteral: value)
}
public init(unicodeScalarLiteral value: StaticString) {
self.init(stringLiteral: value)
}
}
To find the extension that adds the require() method on Optional that I use above, check out Require.
27 Manipulating points, sizes and frames using math operators
✚ I'm always careful with operator overloading, but for manipulating things like sizes, points & frames I find them super useful.
extension CGSize {
static func *(lhs: CGSize, rhs: CGFloat) -> CGSize {
return CGSize(width: lhs.width * rhs, height: lhs.height * rhs)
}
}
button.frame.size = image.size * 2
If you like the above idea, check out CGOperators, which contains math operator overloads for all Core Graphics' vector types.
26 Using closure types in generic constraints
🔗 You can use closure types in generic constraints in Swift. Enables nice APIs for handling sequences of closures.
extension Sequence where Element == () -> Void {
func callAll() {
forEach { $0() }
}
}
extension Sequence where Element == () -> String {
func joinedResults(separator: String) -> String {
return map { $0() }.joined(separator: separator)
}
}
callbacks.callAll()
let names = nameProviders.joinedResults(separator: ", ")
(If you're using Swift 3, you have to change Element to Iterator.Element)
25 Using associated enum values to avoid state-specific optionals
🎉 Using associated enum values is a super nice way to encapsulate mutually exclusive state info (and avoiding state-specific optionals).
// BEFORE: Lots of state-specific, optional properties
class Player {
var isWaitingForMatchMaking: Bool
var invitingUser: User?
var numberOfLives: Int
var playerDefeatedBy: Player?
var roundDefeatedIn: Int?
}
// AFTER: All state-specific information is encapsulated in enum cases
class Player {
enum State {
case waitingForMatchMaking
case waitingForInviteResponse(from: User)
case active(numberOfLives: Int)
case defeated(by: Player, roundNumber: Int)
}
var state: State
}
24 Using enums for async result types
👍 I really like using enums for all async result types, even boolean ones. Self-documenting, and makes the call site a lot nicer to read too!
protocol PushNotificationService {
// Before
func enablePushNotifications(completionHandler: @escaping (Bool) -> Void)
// After
func enablePushNotifications(completionHandler: @escaping (PushNotificationStatus) -> Void)
}
enum PushNotificationStatus {
case enabled
case disabled
}
service.enablePushNotifications { status in
if status == .enabled {
enableNotificationsButton.removeFromSuperview()
}
}
23 Working on async code in a playground
🏃 Want to work on your async code in a Swift Playground? Just set needsIndefiniteExecution to true to keep it running:
import PlaygroundSupport
PlaygroundPage.current.needsIndefiniteExecution = true
DispatchQueue.main.asyncAfter(deadline: .now() + 3) {
let greeting = "Hello after 3 seconds"
print(greeting)
}
To stop the playground from executing, simply call PlaygroundPage.current.finishExecution().
22 Overriding self with a weak reference
💦 Avoid memory leaks when accidentially refering to self in closures by overriding it locally with a weak reference:
Swift >= 4.2
dataLoader.loadData(from: url) { [weak self] result in
guard let self = self else {
return
}
self.cache(result)
...
Swift < 4.2
dataLoader.loadData(from: url) { [weak self] result in
guard let `self` = self else {
return
}
self.cache(result)
...
Note that the reason the above currently works is because of a compiler bug (which I hope gets turned into a properly supported feature soon).
21 Using DispatchWorkItem
🕓 Using dispatch work items you can easily cancel a delayed asynchronous GCD task if you no longer need it:
let workItem = DispatchWorkItem {
// Your async code goes in here
}
// Execute the work item after 1 second
DispatchQueue.main.asyncAfter(deadline: .now() + 1, execute: workItem)
// You can cancel the work item if you no longer need it
workItem.cancel()
20 Combining a sequence of functions
➕ While working on a new Swift developer tool (to be open sourced soon 😉), I came up with a pretty neat way of organizing its sequence of operations, by combining their functions into a closure:
internal func +<A, B, C>(lhs: @escaping (A) throws -> B,
rhs: @escaping (B) throws -> C) -> (A) throws -> C {
return { try rhs(lhs($0)) }
}
public func run() throws {
try (determineTarget + build + analyze + output)()
}
If you're familiar with the functional programming world, you might know the above technique as the pipe operator (thanks to Alexey Demedreckiy for pointing this out!)
19 Chaining optionals with map() and flatMap()
🗺 Using map() and flatMap() on optionals you can chain multiple operations without having to use lengthy if lets or guards:
// BEFORE
guard let string = argument(at: 1) else {
return
}
guard let url = URL(string: string) else {
return
}
handle(url)
// AFTER
argument(at: 1).flatMap(URL.init).map(handle)
18 Using self-executing closures for lazy properties
🚀 Using self-executing closures is a great way to encapsulate lazy property initialization:
class StoreViewController: UIViewController {
private lazy var collectionView: UICollectionView = {
let layout = UICollectionViewFlowLayout()
let view = UICollectionView(frame: self.view.bounds, collectionViewLayout: layout)
view.delegate = self
view.dataSource = self
return view
}()
override func viewDidLoad() {
super.viewDidLoad()
view.addSubview(collectionView)
}
}
17 Speeding up Swift package tests
⚡️ You can speed up your Swift package tests using the --parallel flag. For Marathon, the tests execute 3 times faster that way!
swift test --parallel
16 Avoiding mocking UserDefaults
🛠 Struggling with mocking UserDefaults in a test? The good news is: you don't need mocking - just create a real instance:
class LoginTests: XCTestCase {
private var userDefaults: UserDefaults!
private var manager: LoginManager!
override func setUp() {
super.setup()
userDefaults = UserDefaults(suiteName: #file)
userDefaults.removePersistentDomain(forName: #file)
manager = LoginManager(userDefaults: userDefaults)
}
}
15 Using variadic parameters
👍 Using variadic parameters in Swift, you can create some really nice APIs that take a list of objects without having to use an array:
extension Canvas {
func add(_ shapes: Shape...) {
shapes.forEach(add)
}
}
let circle = Circle(center: CGPoint(x: 5, y: 5), radius: 5)
let lineA = Line(start: .zero, end: CGPoint(x: 10, y: 10))
let lineB = Line(start: CGPoint(x: 0, y: 10), end: CGPoint(x: 10, y: 0))
let canvas = Canvas()
canvas.add(circle, lineA, lineB)
canvas.render()
14 Referring to enum cases with associated values as closures
😮 Just like you can refer to a Swift function as a closure, you can do the same thing with enum cases with associated values:
enum UnboxPath {
case key(String)
case keyPath(String)
}
struct UserSchema {
static let name = key("name")
static let age = key("age")
static let posts = key("posts")
private static let key = UnboxPath.key
}
13 Using the === operator to compare objects by instance
📈 The === operator lets you check if two objects are the same instance. Very useful when verifying that an array contains an instance in a test:
protocol InstanceEquatable: class, Equatable {}
extension InstanceEquatable {
static func ==(lhs: Self, rhs: Self) -> Bool {
return lhs === rhs
}
}
extension Enemy: InstanceEquatable {}
func testDestroyingEnemy() {
player.attack(enemy)
XCTAssertTrue(player.destroyedEnemies.contains(enemy))
}
12 Calling initializers with dot syntax and passing them as closures
😎 Cool thing about Swift initializers: you can call them using dot syntax and pass them as closures! Perfect for mocking dates in tests.
class Logger {
private let storage: LogStorage
private let dateProvider: () -> Date
init(storage: LogStorage = .init(), dateProvider: @escaping () -> Date = Date.init) {
self.storage = storage
self.dateProvider = dateProvider
}
func log(event: Event) {
storage.store(event: event, date: dateProvider())
}
}
11 Structuring UI tests as extensions on XCUIApplication
📱 Most of my UI testing logic is now categories on XCUIApplication. Makes the test cases really easy to read:
func testLoggingInAndOut() {
XCTAssertFalse(app.userIsLoggedIn)
app.launch()
app.login()
XCTAssertTrue(app.userIsLoggedIn)
app.logout()
XCTAssertFalse(app.userIsLoggedIn)
}
func testDisplayingCategories() {
XCTAssertFalse(app.isDisplayingCategories)
app.launch()
app.login()
app.goToCategories()
XCTAssertTrue(app.isDisplayingCategories)
}
10 Avoiding default cases in switch statements
🙂 It’s a good idea to avoid “default” cases when switching on Swift enums - it’ll “force you” to update your logic when a new case is added:
enum State {
case loggedIn
case loggedOut
case onboarding
}
func handle(_ state: State) {
switch state {
case .loggedIn:
showMainUI()
case .loggedOut:
showLoginUI()
// Compiler error: Switch must be exhaustive
}
}
9 Using the guard statement in many different scopes
💂 It's really cool that you can use Swift's 'guard' statement to exit out of pretty much any scope, not only return from functions:
// You can use the 'guard' statement to...
for string in strings {
// ...continue an iteration
guard shouldProcess(string) else {
continue
}
// ...or break it
guard !shouldBreak(for: string) else {
break
}
// ...or return
guard !shouldReturn(for: string) else {
return
}
// ..or throw an error
guard string.isValid else {
throw StringError.invalid(string)
}
// ...or exit the program
guard !shouldExit(for: string) else {
exit(1)
}
}
8 Passing functions & operators as closures
❤️ Love how you can pass functions & operators as closures in Swift. For example, it makes the syntax for sorting arrays really nice!
let array = [3, 9, 1, 4, 6, 2]
let sorted = array.sorted(by: <)
7 Using #function for UserDefaults key consistency
🗝 Here's a neat little trick I use to get UserDefault key consistency in Swift (#function expands to the property name in getters/setters). Just remember to write a good suite of tests that'll guard you against bugs when changing property names.
extension UserDefaults {
var onboardingCompleted: Bool {
get { return bool(forKey: #function) }
set { set(newValue, forKey: #function) }
}
}
6 Using a name already taken by the standard library
📛 Want to use a name already taken by the standard library for a nested type? No problem - just use Swift. to disambiguate:
extension Command {
enum Error: Swift.Error {
case missing
case invalid(String)
}
}
5 Using Wrap to implement Equatable
📦 Playing around with using Wrap to implement Equatable for any type, primarily for testing:
protocol AutoEquatable: Equatable {}
extension AutoEquatable {
static func ==(lhs: Self, rhs: Self) -> Bool {
let lhsData = try! wrap(lhs) as Data
let rhsData = try! wrap(rhs) as Data
return lhsData == rhsData
}
}
4 Using typealiases to reduce the length of method signatures
📏 One thing that I find really useful in Swift is to use typealiases to reduce the length of method signatures in generic types:
public class PathFinder<Object: PathFinderObject> {
public typealias Map = Object.Map
public typealias Node = Map.Node
public typealias Path = PathFinderPath<Object>
public static func possiblePaths(for object: Object, at rootNode: Node, on map: Map) -> Path.Sequence {
return .init(object: object, rootNode: rootNode, map: map)
}
}
3 Referencing either external or internal parameter name when writing docs
📖 You can reference either the external or internal parameter label when writing Swift docs - and they get parsed the same:
// EITHER:
class Foo {
/**
* - parameter string: A string
*/
func bar(with string: String) {}
}
// OR:
class Foo {
/**
* - parameter with: A string
*/
func bar(with string: String) {}
}
2 Using auto closures
👍 Finding more and more uses for auto closures in Swift. Can enable some pretty nice APIs:
extension Dictionary {
mutating func value(for key: Key, orAdd valueClosure: @autoclosure () -> Value) -> Value {
if let value = self[key] {
return value
}
let value = valueClosure()
self[key] = value
return value
}
}
1 Namespacing with nested types
🚀 I’ve started to become a really big fan of nested types in Swift. Love the additional namespacing it gives you!
public struct Map {
public struct Model {
public let size: Size
public let theme: Theme
public var terrain: [Position : Terrain.Model]
public var units: [Position : Unit.Model]
public var buildings: [Position : Building.Model]
}
public enum Direction {
case up
case right
case down
case left
}
public struct Position {
public var x: Int
public var y: Int
}
public enum Size: String {
case small = "S"
case medium = "M"
case large = "L"
case extraLarge = "XL"
}
}Download Details:
Author: JohnSundell
Source Code: https://github.com/JohnSundell/SwiftTips
License: MIT license
1620591780
Split Query Window in SQL Server Management Studio(SSMS)
Introduction
Sometime we may write a long query in our query window and we may need to refer the objects or other part of the T-SQL frequently. For that we may be scrolling up and down in the query window. But we have an awesome option for that in the SQL Query Window, which many of us may not be aware of.
#sql server #sql query #query window #sql #ssms