Karim Aya

Karim Aya

1561514078

A Metaphorical Introduction to Functional JavaScript

A Metaphorical Introduction to Functional JavaScript - Functional JavaScript isn’t a tool, framework, 3rd-party addon, npm module, or anything else you’d add on. Functional Programming is an approach for writing code, and Functional JavaScript (FJS) is how to use that approach for JavaScript.

Like all approaches, it has benefits and drawbacks, tradeoffs one makes, people who like and dislike it, and different lobbying groups in international governments.

I’m firmly in the “like” camp for FJS. I’ve been writing JavaScript (poorly and otherwise) for several years and wish I’d learned about it from the start. I’ve found the benefits of FJS well worth the learning curve, and it’s resulted in code that’s more readable, flexible, and maintainable. I struggled to nail down what it means to write FJS, but once I did I couldn’t go back.

From the name, you’d expect Functional Programming to just be about writing lots of functions. In a way that’s true, but there are several rules one must follow to achieve that. Rules that are often hard to understand both at first glance and after some Google searches.

  1. Use Pure Functions
  2. Don’t Mutate State
  3. Declarative, not Imperative
  4. Higher Order Functions

In this article, I’m going to try and break down these elements of FJS in ways that are easier to understand. This isn’t an exhaustive guide by any means but is meant to be a jumping off point so people can better understand more detailed, thorough resources to learn FJS.

Let’s begin!

Keep Functions Pure

Using Pure Functions and avoiding State Mutation are perhaps the most important parts of writing FJS. Instead of starting with the usual definitions, I’m going to indulge myself and explain them with an imaginary dinner party.

An Angel and a Mutant Enter a Dinner Party…

First, imagine an angel. The angel radiates a soft, pure white with glittering wings and a serene face. They bob gently over the ground and moves with smooth yet purposeful grace. No living person can see this angel, and it passes through anything it touches.

Let’s say this angel was in the corner of a crowded dinner party. You tell the angel they need to move across the room and stand next to the punch bowl. The angel nods and starts floating toward this spot. No one can see or touch it. No one’s conversations are disturbed and no one needs to move out of their way. Thanks to all this, the angel takes the shortest possible route to the punch bowl. If the dinner party filled with entirely new guests, the angel could do this again along the exact path.

Now imagine almost the exact opposite of this angel: a radioactive mutant. The mutant was once human but has transformed into something grotesque. They could have any grotesque feature you want: waving tentacles, eyes all over their back, feet that are webbed and clawed, a t-shirt with a pop culture reference decades out of date, or they own a real estate business. Whatever you choose, this mutant is scary and you can’t look at it too long.

Let’s say this mutant had the same task: move from the corner of a dinner party to the punch bowl. You could imagine how horrible that would go. People would be screaming and pushing away from the mutant constantly. Plus its radioactivity would start giving random people different mutations, and guests would run from them too. The mutant would need to push and shove along an unpredictable path to reach that spot. If you restarted this scenario at a party with different guests, the radiation would make different people mutate, and the humans would panic in new ways. The mutant would need to take a different, but just as rough, route to the punch bowl.

To be a Pure Function

As you may have guessed, the angel has all the qualities of a pure function.

  1. No external state is changed. The angel goes across the room without anyone or anything changing. A pure function does its job without anything outside the function changing either.
  2. The same input has the same results. The angel takes the same exact path to the same path every time. A pure function, when given the same input, returns the same result each time.

And if the name wasn’t a big enough giveaway, the mutant has all the qualities of a function that mutates state.

  1. Variables outside the function are affected. The mutant affects other people by scaring party guests and making other people mutate. Impure functions change variables that exist outside of them, on purpose or by accident.
  2. The same input can have different results. The mutant makes random people mutate, which will change the type of panic and therefore the path the mutant takes each time. Impure functions return different values due to how they affect outside variables each time.

Here’s some actual JavaScript to drive all this home. Is the below addNumberfunction an angel or a mutant?

let number = 0;

let addNumber = x => {
  number += x;
  return number;
}

addNumber is a mutant since it changes number, a variable outside the function. These changes mean we could run this function twice with the same parameter and get different results.

addNumber(5) // 5
addNumber(5) // 10 (which is not 5)

If we wanted a pure angel function, we’d rewrite one like this.

let number = 0;

let addNumbers = (x, y) => x + y;

Instead of relying on an outside variable, we make both numbers variables that we pass in. This keeps all the function’s variables in its own scope, and the same inputs give the same results.

addNumbers(number, 5); // 5
addNumbers(number, 5); // 5 (which is 5)!

FJS uses pure functions since they’re like angels. Angels are good and mutants are bad. Don’t let the mutants win. Use pure functions.

Be Declarative, not Imperative

I’ve had a hard time understanding the difference between declarative and imperative programming for the longest time. Before anything else, know that declarative and imperative programming are both valid approaches with their own ups and downs. Functional programming just favors being declarative.

As for the specifics, let’s imagine two different beings again. This time it’ll be a southern belle and a stable boy. We ask both of them to fetch us a bucket of milk and give them an empty bucket for the job.

The southern belle is haughty and hates getting her hands dirty. She handles this task by summoning her servant and saying “I do declare, if there is a cow outside, bring me a bucket of milk with a bucket like this!” The servant bows, examines the bucket, leaves, and returns with a bucket of milk. It’s in a different bucket that looks identical to the one we gave her. The southern belle takes the milk and hands it to us.

The stable boy enjoys getting his hands dirty. He handles this task by taking the bucket, going to the barn, finding a cow, and going through all the motions to milk it. He chooses the right cow, milks the cow, fills our bucket with the milk, and carries it back to us himself.

Both people got us the bucket of milk, albeit in very different ways. The southern belle wasn’t involved in the actual steps to get the milk, she focused on what she needed and used her servant to get it. Meanwhile, the stable boy focused on how to get the milk and went through all the steps.

At its core, that’s the difference between declarative and imperative programming. Declarative programming solves a problem based on what it needs, and avoids direct DOM or variable manipulation. This is a good fit for pure functions since they’re about giving you new data and objects to avoid mutating state. Meanwhile, imperative programming changes the DOM and manipulates state, but in a more focused way that gives better control when done right.

When you’re not writing JavaScript to manipulate DOMs, I’ve approached declarative programming by declaring new variables instead of mutating existing ones.

For example, let’s say you had to write a function that doubled all numbers in an array. An imperative approach would manipulate the given array directly and redefine each item.

const doubleArray = array => {
  for (i = 0; i < array.length; i++) {
    array[i] += array[i];
  }

  return array;
}

This is the code equivalent of the stable boy taking the array, doubling each item in it, and giving you a mutated version of the array. The declarative version looks quite different.

const doubleArray = array => array.map(item => item * 2);

This declarative version gives the work to another function, in this case, map, which already has built-in logic to go through each item (we’ll cover this in a bit). This returns an array separate from the original and the first array isn’t mutated, making this a pure function! As a result, this function is simpler, cleaner, safer to use, and much more in line with FJS.

The southern belle is simply declaring she wants an array with double the values, and her servant (map) is returning a different array to meet her request.

Use the Right FJS Tools

Okay, enough metaphors. Let’s get into the nitty-gritty-codey ways to write FJS. First let’s cover some of the tools you’ll be using the most to write your pure, imperative functions.

Arrow functions

Arrow functions were added with ES6 and their main benefit is a shorter, sweeter function syntax. FJS means writing lots of functions, so we might as well make it easier.

Before arrow functions, a basic “add five to a number” function would look like this.

const addFive = function(number) {
  return number + 5;
}

Simple functions like this can be written without the function keyword or the explicit return.

const addFive = number => number + 5;

The variable first identifies the arguments, in this case number. You could also use parenthesis for no arguments, like with (), or for multiple arguments, like with (number1, number2).

After that is the arrow, shown as =>. Whatever expression follows is automatically returned, in this case, that’s number with five added.

More complex functions can use braces for extra lines, but you’ll lose the implicit return and need to write it out. Not as good, but still better than the first syntax.

const addFive = number => {
  // more code here
  return number + 5;
};

Array Prototype Methods

Each array has several powerful tools built into them that’ll cover most, if not all, of your FJS needs. Calling them returns new, modified arrays you can easily assign to new variables. They’re similar to the southern belle’s servant from the declarative metaphor - they’re already there, do the work for you, and give you new objects based on what you started with.

Let’s start with one of the most basic methods, map. It takes each item in an array, runs it through a function to get a new value, and replaces the old value with this new one. Once it does that for each item, it returns a newly updated array.

Here’s a tweaked example of the declarative code example from before, but using map to double array values.

[2, 4, 6].map(item => item * 2);
// [4, 8, 12]

You’re basically using map to pull out each array object as item and say “Replace this item with item * 2.”

You could also write the doubling function separately to make the code even more functional. Plus you can assign what map returns to an entirely different variable.

const double       = (item) => item * 2,
      array        = [2, 4, 6],
      doubledArray = array.map(double);

console.log(array);        // [2, 4, 6]
console.log(doubledArray); // [4, 8, 12]
// The original array hasn't been mutated!

There are many great methods to learn, and covering them all is another post altogether. Check out my study repo for a quick look at different array prototype methods, or just google around for them!

Bonus: Chain Array Prototype Methods

One more fun fact you should know: array methods can be chained together! This lets you combine different array changes quickly and without breaking FJS rules.

Let’s say we wanted to double each array value, then filter out the ones lower than five (filter is another useful method to learn later). We just need to write one extra function and add another method to our array.

const double         = (item) => item * 2,
      higherThanFive = (item) => item > 5,
      array          = [2, 4, 6],
      doubledArray   = array.map(double).filter(higherThanFive);

console.log(array);        // [2, 4, 6]
console.log(doubledArray); // [8, 12]

Lastly, many people (like myself) often use different spacing when chaining to keep it readable. The below variable is the same as the above but easier to understand at a glance.

doubledArray   = array
                  .map(double)
                  .filter(higherThanFive);

Remember Reduce

reduce is a prototype method I want to highlight since it’s arguably the most powerful. On its own, it can recreate almost any other prototype method and can make more complex and powerful ones. Reduce is another blog post too, so once again I refer you to my study notebook (or Google) to quickly learn more about <a href="https://maxx1128.gitbook.io/study-notebook/javascript/reducefunction" target="_blank">reduce</a>. Just remember these important facts:

  1. Reduce is great for complex merging or manipulation of data, as long as you make sure another prototype method doesn’t already do what you need.
  2. Everything about other prototype methods, such as chaining and passing in functions, applies to reduce.

reduce is one of your most powerful FJS tools, so learn it well.

Higher-order functions

Now that we have these new ways to write functions, next is new ways to manage them. One of the best methods for FJS is making use of higher-order functions, of HOFs. Several code examples so far have made use of HOFs, but having a clearer definition of it helps with making full use of it.

HOFs are functions that take other functions as arguments. Remember that functions are first-class citizens in the JavaScript kingdom, so they can be:

  • Saved to variables
  • Passed to other functions
  • Returned from other functions

I wish I’d learned to make use of HOFs earlier in my career. It helped me write more functions that abstracted logic away to make programs more readable and maintainable. So getting into the mindset of “functions passing around functions” is great for FJS, but also better JavaScript in general.

Let’s say I needed to test if many numbers were divisible by a group of other numbers. One’s first instinct may be to write each function out like this.

const divisibleby3 = (n) => n % 3 === 0,
      divisibleby5 = (n) => n % 5 === 0,
      divisibleby7 = (n) => n % 7 === 0;

divisibleBy3(6);  // true
divisibleBy5(14); // false
divisibleBy7(28); // false

This works, but you have to repeat the same expressions over and over. A solution using HOFs would look like this and get the same results.

const divideBy = (x) => (y) => y % x === 0;

const divisibleBy3 = divideBy(3),
      divisibleBy5 = divideBy(5),
      divisibleBy7 = divideBy(7);

divisibleBy3(6);  // true
divisibleBy5(14); // false
divisibleBy7(28); // true

This is complicated, so let’s break it down.

  1. The divideBy function takes one argument, x, and saves it while returning another function. So when we call divideBy(3), we’re saving x as part of the function being returned every time.
  2. We can save this function to a variable, such as divisibleBy3. This makes sense since we’ve already made 3 part of the function returned each time.
  3. Since divideBy returns a function, we can now call divisibleBy3 like a normal function. It uses both the y variable it gets at the call, and the x variable it got before.

All this is an example of “currying” functions or functions that return other functions until they eventually give final functions like divisibleBy3. As you can see, currying in JavaScript has many examples of HOFs in use. Currying is great if you have functions with some, but not all, of their logic in common. You can create a template with the logic they have in common (a comparison operator) and pass in logic specific to each one (the number used in the operation).

If you don’t want to save the first part of a curried function to a variable, you can use the shortcut of calling multiple arguments at the same time.

const divideBy = (x) => (y) => y % x === 0;

divideBy(3)(6);  // true
divideBy(5)(14); // false
divideBy(7)(28); // true

As you can see, curried functions help your code whether you use them once or dozens of times in your program!

Curried functions are also tough to understand, so I have another note that better breaks currying down if you need it.

Here’s another example of HOFs that takes a function as the fn argument. The function accepting it refers to it like any other variable.

const performMultipleTimes = (times, x, fn) => {
  for (let i = 0; i < times; i++) {
    fn(x);
  }
}

This function takes three arguments:

  1. The number of times to repeat the loop
  2. The argument passed into the fn function
  3. The fn function

The function is being called inside the loop, so performMultipleTimes lets us call a function many times while only writing it once. All we need to do is pass one function another function, which we can do by storing the function in a variable. HOFs at work once again!

const logString = s => console.log(s);

performMultipleTimes(3, 'Greetings!', logString);
// Greetings!
// Greetings!
// Greetings!

If you don’t use this function anywhere else and don’t want to save it for later, you can also pass it directly. You can do this with or without the bracket syntax.

performMultipleTimes(3, 'Greetings!', (s) => console.log(s));

performMultipleTimes(3, 'Greetings!', (s) => {
  const newString = `I am here to say '${s}'`;
  console.log(newString);
});

Understanding HOFs is great but is especially for FJS. This style is about focusing on the power of functions, and passing functions around effectively can multiply their power and modularity.

However, this was hard to grasp for me and it might be the same for you. So if you (understandably) still have trouble, this chapter from Eloquent JavaScript does a great job breaking HOFs down further.

FJS is a Style, not an Absolute

A final note on FJS: it’s a style of writing JavaScript that exists on a spectrum. It isn’t simply “this is or is not FJS.” You can write code with elements of FJS (like pure functions or being declarative) without needing to follow the rules. Each is a preference that, as they’re put together, make your JavaScript closer to the functional paradigm.

FJS can be tough to grasp, as you can tell by the number of links I reference for extra reading. But understanding these four topics will help you build a solid foundation for learning more. This is what holds true for me since they made everything click into place for every other article I read on FJS. Hopefully, this post can do the same for anyone else looking to learn and use it.

If you really want to go deeper into FJS, I recommend [Kyle Simpson’s book “Functional-Light JS.”](https://github.com/getify/Functional-Light-JSKyle Simpson’s book “Functional-Light JS.””) It goes into much deeper detail on functional programming and JavaScript, and you can read it for free on Github!

#javascript #web-development

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A Metaphorical Introduction to Functional JavaScript

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:

#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>&lt;<span class="type">String</span>&gt;
""")

#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 

Vincent Lab

Vincent Lab

1605017502

The Difference Between Regular Functions and Arrow Functions in JavaScript

Other then the syntactical differences. The main difference is the way the this keyword behaves? In an arrow function, the this keyword remains the same throughout the life-cycle of the function and is always bound to the value of this in the closest non-arrow parent function. Arrow functions can never be constructor functions so they can never be invoked with the new keyword. And they can never have duplicate named parameters like a regular function not using strict mode.

Here are a few code examples to show you some of the differences
this.name = "Bob";

const person = {
name: “Jon”,

<span style="color: #008000">// Regular function</span>
func1: <span style="color: #0000ff">function</span> () {
    console.log(<span style="color: #0000ff">this</span>);
},

<span style="color: #008000">// Arrow function</span>
func2: () =&gt; {
    console.log(<span style="color: #0000ff">this</span>);
}

}

person.func1(); // Call the Regular function
// Output: {name:“Jon”, func1:[Function: func1], func2:[Function: func2]}

person.func2(); // Call the Arrow function
// Output: {name:“Bob”}

The new keyword with an arrow function
const person = (name) => console.log("Your name is " + name);
const bob = new person("Bob");
// Uncaught TypeError: person is not a constructor

If you want to see a visual presentation on the differences, then you can see the video below:

#arrow functions #javascript #regular functions #arrow functions vs normal functions #difference between functions and arrow functions

Madyson  Reilly

Madyson Reilly

1601069940

Function Expression vs Function Declaration in JavaScript

Function Expression vs Function Declaration in JavaScript.

It was until during one of the JavaScript mock interviews did I came across the term function expression.

The question was: What is the difference between these two syntax?

function x(){

}

let x = function(){

}

I was clueless for a moment. After thinking a little, I could only come up with: the second syntax invokes an _anonymous _function and is assigned to a variable.

I was alien to the term hoisting.

In this article, we will acquaint ourselves with three simple terms: function declaration,_ function expression, _and hoisting.

What is function declaration?

Function declaration is also known as _function statement. _It contains the name of the function, parameters, and a return statement. **Naming the function **is what sets function declaration apart. Parameters and return statement is optional.

Image for post

Function Declaration

What is function expression?

Function expression also has a name, parameters, and return statement. All of which are optional. The important thing to bear in mind is: the function here is _assigned _to a JavaScript variable.

Image for post

Function Expression

#function-expression #function-declaration #functions-in-javascript #coding #javascript #express

Lowa Alice

Lowa Alice

1624410000

JavaScript Factory Functions

JavaScript factory functions made simple.

📺 The video in this post was made by Programming with Mosh
The origin of the article: https://www.youtube.com/watch?v=jpegXpQpb3o&list=PLTjRvDozrdlxEIuOBZkMAK5uiqp8rHUax&index=9
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Thanks for visiting and watching! Please don’t forget to leave a like, comment and share!

#javascript #factory functions #functions #javascript factory functions

Introduction With Basic JavaScript

The world’s most misunderstood programming language is JavaScript but JavaScript is now used by an incredible number of high-profile applications. So, it’s an important skill for any web or mobile developer to enrich the deeper knowledge in it.

Unlike most programming languages, the JavaScript language has no concept of input or output. It is designed to run as a scripting language in a host environment, and it is up to the host environment to provide mechanisms for communicating with the outside world.

Its syntax is based on the Java and C languages — many structures from those languages apply to JavaScript as well. JavaScript supports object-oriented programming with object prototypes, instead of classes. JavaScript also supports functional programming — because they are objects, functions may be stored in variables and passed around like any other object.

Let’s start off by looking at the building blocks of any language: the types. JavaScript programs manipulate values, and those values all belong to a type. JavaScript’s types are:

· Number

· String

· Boolean

· Function

· Object

· Symbol

and undefined and null, which are … slightly odd. And Array, which is a special kind of object. Date and RegExp, which are objects that you get for free. And to be technically accurate, functions are just a special type of object. So the type of diagram looks like this:

#beginner-javascript #javascript #javascript-introduction #javascript-fundamental #basic-javascritp