1598426580
How to Use If, Else and Elif in Lambda Functions in Python
Python lambda function is the function that is defined without a name. The anonymous functions are defined using a lambda keyword. In this article, we will discuss how to use if, else if and else in lambda functions in Python.
Syntax of Lambda function
lambda arguments: expression
The lambda functions can have any number of parameters but only one expression. That expression is evaluated and returned. Lambda functions can be used wherever a function objects are required.
See the following example of a Lambda function in Python.
## app.py
cube = lambda x: x * x * x
print(cube(11))
#python #lambda
What is GEEK
Buddha Community
1667425440
Pdf2gerb: Perl Script Converts PDF Files to Gerber format
pdf2gerb
Perl script converts PDF files to Gerber format
Pdf2Gerb generates Gerber 274X photoplotting and Excellon drill files from PDFs of a PCB. Up to three PDFs are used: the top copper layer, the bottom copper layer (for 2-sided PCBs), and an optional silk screen layer. The PDFs can be created directly from any PDF drawing software, or a PDF print driver can be used to capture the Print output if the drawing software does not directly support output to PDF.
The general workflow is as follows:
- Design the PCB using your favorite CAD or drawing software.
- Print the top and bottom copper and top silk screen layers to a PDF file.
- Run Pdf2Gerb on the PDFs to create Gerber and Excellon files.
- Use a Gerber viewer to double-check the output against the original PCB design.
- Make adjustments as needed.
- Submit the files to a PCB manufacturer.
Please note that Pdf2Gerb does NOT perform DRC (Design Rule Checks), as these will vary according to individual PCB manufacturer conventions and capabilities. Also note that Pdf2Gerb is not perfect, so the output files must always be checked before submitting them. As of version 1.6, Pdf2Gerb supports most PCB elements, such as round and square pads, round holes, traces, SMD pads, ground planes, no-fill areas, and panelization. However, because it interprets the graphical output of a Print function, there are limitations in what it can recognize (or there may be bugs).
See docs/Pdf2Gerb.pdf for install/setup, config, usage, and other info.
pdf2gerb_cfg.pm
#Pdf2Gerb config settings:
#Put this file in same folder/directory as pdf2gerb.pl itself (global settings),
#or copy to another folder/directory with PDFs if you want PCB-specific settings.
#There is only one user of this file, so we don't need a custom package or namespace.
#NOTE: all constants defined in here will be added to main namespace.
#package pdf2gerb_cfg;
use strict; #trap undef vars (easier debug)
use warnings; #other useful info (easier debug)
##############################################################################################
#configurable settings:
#change values here instead of in main pfg2gerb.pl file
use constant WANT_COLORS => ($^O !~ m/Win/); #ANSI colors no worky on Windows? this must be set < first DebugPrint() call
#just a little warning; set realistic expectations:
#DebugPrint("${\(CYAN)}Pdf2Gerb.pl ${\(VERSION)}, $^O O/S\n${\(YELLOW)}${\(BOLD)}${\(ITALIC)}This is EXPERIMENTAL software. \nGerber files MAY CONTAIN ERRORS. Please CHECK them before fabrication!${\(RESET)}", 0); #if WANT_DEBUG
use constant METRIC => FALSE; #set to TRUE for metric units (only affect final numbers in output files, not internal arithmetic)
use constant APERTURE_LIMIT => 0; #34; #max #apertures to use; generate warnings if too many apertures are used (0 to not check)
use constant DRILL_FMT => '2.4'; #'2.3'; #'2.4' is the default for PCB fab; change to '2.3' for CNC
use constant WANT_DEBUG => 0; #10; #level of debug wanted; higher == more, lower == less, 0 == none
use constant GERBER_DEBUG => 0; #level of debug to include in Gerber file; DON'T USE FOR FABRICATION
use constant WANT_STREAMS => FALSE; #TRUE; #save decompressed streams to files (for debug)
use constant WANT_ALLINPUT => FALSE; #TRUE; #save entire input stream (for debug ONLY)
#DebugPrint(sprintf("${\(CYAN)}DEBUG: stdout %d, gerber %d, want streams? %d, all input? %d, O/S: $^O, Perl: $]${\(RESET)}\n", WANT_DEBUG, GERBER_DEBUG, WANT_STREAMS, WANT_ALLINPUT), 1);
#DebugPrint(sprintf("max int = %d, min int = %d\n", MAXINT, MININT), 1);
#define standard trace and pad sizes to reduce scaling or PDF rendering errors:
#This avoids weird aperture settings and replaces them with more standardized values.
#(I'm not sure how photoplotters handle strange sizes).
#Fewer choices here gives more accurate mapping in the final Gerber files.
#units are in inches
use constant TOOL_SIZES => #add more as desired
(
#round or square pads (> 0) and drills (< 0):
.010, -.001, #tiny pads for SMD; dummy drill size (too small for practical use, but needed so StandardTool will use this entry)
.031, -.014, #used for vias
.041, -.020, #smallest non-filled plated hole
.051, -.025,
.056, -.029, #useful for IC pins
.070, -.033,
.075, -.040, #heavier leads
# .090, -.043, #NOTE: 600 dpi is not high enough resolution to reliably distinguish between .043" and .046", so choose 1 of the 2 here
.100, -.046,
.115, -.052,
.130, -.061,
.140, -.067,
.150, -.079,
.175, -.088,
.190, -.093,
.200, -.100,
.220, -.110,
.160, -.125, #useful for mounting holes
#some additional pad sizes without holes (repeat a previous hole size if you just want the pad size):
.090, -.040, #want a .090 pad option, but use dummy hole size
.065, -.040, #.065 x .065 rect pad
.035, -.040, #.035 x .065 rect pad
#traces:
.001, #too thin for real traces; use only for board outlines
.006, #minimum real trace width; mainly used for text
.008, #mainly used for mid-sized text, not traces
.010, #minimum recommended trace width for low-current signals
.012,
.015, #moderate low-voltage current
.020, #heavier trace for power, ground (even if a lighter one is adequate)
.025,
.030, #heavy-current traces; be careful with these ones!
.040,
.050,
.060,
.080,
.100,
.120,
);
#Areas larger than the values below will be filled with parallel lines:
#This cuts down on the number of aperture sizes used.
#Set to 0 to always use an aperture or drill, regardless of size.
use constant { MAX_APERTURE => max((TOOL_SIZES)) + .004, MAX_DRILL => -min((TOOL_SIZES)) + .004 }; #max aperture and drill sizes (plus a little tolerance)
#DebugPrint(sprintf("using %d standard tool sizes: %s, max aper %.3f, max drill %.3f\n", scalar((TOOL_SIZES)), join(", ", (TOOL_SIZES)), MAX_APERTURE, MAX_DRILL), 1);
#NOTE: Compare the PDF to the original CAD file to check the accuracy of the PDF rendering and parsing!
#for example, the CAD software I used generated the following circles for holes:
#CAD hole size: parsed PDF diameter: error:
# .014 .016 +.002
# .020 .02267 +.00267
# .025 .026 +.001
# .029 .03167 +.00267
# .033 .036 +.003
# .040 .04267 +.00267
#This was usually ~ .002" - .003" too big compared to the hole as displayed in the CAD software.
#To compensate for PDF rendering errors (either during CAD Print function or PDF parsing logic), adjust the values below as needed.
#units are pixels; for example, a value of 2.4 at 600 dpi = .0004 inch, 2 at 600 dpi = .0033"
use constant
{
HOLE_ADJUST => -0.004 * 600, #-2.6, #holes seemed to be slightly oversized (by .002" - .004"), so shrink them a little
RNDPAD_ADJUST => -0.003 * 600, #-2, #-2.4, #round pads seemed to be slightly oversized, so shrink them a little
SQRPAD_ADJUST => +0.001 * 600, #+.5, #square pads are sometimes too small by .00067, so bump them up a little
RECTPAD_ADJUST => 0, #(pixels) rectangular pads seem to be okay? (not tested much)
TRACE_ADJUST => 0, #(pixels) traces seemed to be okay?
REDUCE_TOLERANCE => .001, #(inches) allow this much variation when reducing circles and rects
};
#Also, my CAD's Print function or the PDF print driver I used was a little off for circles, so define some additional adjustment values here:
#Values are added to X/Y coordinates; units are pixels; for example, a value of 1 at 600 dpi would be ~= .002 inch
use constant
{
CIRCLE_ADJUST_MINX => 0,
CIRCLE_ADJUST_MINY => -0.001 * 600, #-1, #circles were a little too high, so nudge them a little lower
CIRCLE_ADJUST_MAXX => +0.001 * 600, #+1, #circles were a little too far to the left, so nudge them a little to the right
CIRCLE_ADJUST_MAXY => 0,
SUBST_CIRCLE_CLIPRECT => FALSE, #generate circle and substitute for clip rects (to compensate for the way some CAD software draws circles)
WANT_CLIPRECT => TRUE, #FALSE, #AI doesn't need clip rect at all? should be on normally?
RECT_COMPLETION => FALSE, #TRUE, #fill in 4th side of rect when 3 sides found
};
#allow .012 clearance around pads for solder mask:
#This value effectively adjusts pad sizes in the TOOL_SIZES list above (only for solder mask layers).
use constant SOLDER_MARGIN => +.012; #units are inches
#line join/cap styles:
use constant
{
CAP_NONE => 0, #butt (none); line is exact length
CAP_ROUND => 1, #round cap/join; line overhangs by a semi-circle at either end
CAP_SQUARE => 2, #square cap/join; line overhangs by a half square on either end
CAP_OVERRIDE => FALSE, #cap style overrides drawing logic
};
#number of elements in each shape type:
use constant
{
RECT_SHAPELEN => 6, #x0, y0, x1, y1, count, "rect" (start, end corners)
LINE_SHAPELEN => 6, #x0, y0, x1, y1, count, "line" (line seg)
CURVE_SHAPELEN => 10, #xstart, ystart, x0, y0, x1, y1, xend, yend, count, "curve" (bezier 2 points)
CIRCLE_SHAPELEN => 5, #x, y, 5, count, "circle" (center + radius)
};
#const my %SHAPELEN =
#Readonly my %SHAPELEN =>
our %SHAPELEN =
(
rect => RECT_SHAPELEN,
line => LINE_SHAPELEN,
curve => CURVE_SHAPELEN,
circle => CIRCLE_SHAPELEN,
);
#panelization:
#This will repeat the entire body the number of times indicated along the X or Y axes (files grow accordingly).
#Display elements that overhang PCB boundary can be squashed or left as-is (typically text or other silk screen markings).
#Set "overhangs" TRUE to allow overhangs, FALSE to truncate them.
#xpad and ypad allow margins to be added around outer edge of panelized PCB.
use constant PANELIZE => {'x' => 1, 'y' => 1, 'xpad' => 0, 'ypad' => 0, 'overhangs' => TRUE}; #number of times to repeat in X and Y directions
# Set this to 1 if you need TurboCAD support.
#$turboCAD = FALSE; #is this still needed as an option?
#CIRCAD pad generation uses an appropriate aperture, then moves it (stroke) "a little" - we use this to find pads and distinguish them from PCB holes.
use constant PAD_STROKE => 0.3; #0.0005 * 600; #units are pixels
#convert very short traces to pads or holes:
use constant TRACE_MINLEN => .001; #units are inches
#use constant ALWAYS_XY => TRUE; #FALSE; #force XY even if X or Y doesn't change; NOTE: needs to be TRUE for all pads to show in FlatCAM and ViewPlot
use constant REMOVE_POLARITY => FALSE; #TRUE; #set to remove subtractive (negative) polarity; NOTE: must be FALSE for ground planes
#PDF uses "points", each point = 1/72 inch
#combined with a PDF scale factor of .12, this gives 600 dpi resolution (1/72 * .12 = 600 dpi)
use constant INCHES_PER_POINT => 1/72; #0.0138888889; #multiply point-size by this to get inches
# The precision used when computing a bezier curve. Higher numbers are more precise but slower (and generate larger files).
#$bezierPrecision = 100;
use constant BEZIER_PRECISION => 36; #100; #use const; reduced for faster rendering (mainly used for silk screen and thermal pads)
# Ground planes and silk screen or larger copper rectangles or circles are filled line-by-line using this resolution.
use constant FILL_WIDTH => .01; #fill at most 0.01 inch at a time
# The max number of characters to read into memory
use constant MAX_BYTES => 10 * M; #bumped up to 10 MB, use const
use constant DUP_DRILL1 => TRUE; #FALSE; #kludge: ViewPlot doesn't load drill files that are too small so duplicate first tool
my $runtime = time(); #Time::HiRes::gettimeofday(); #measure my execution time
print STDERR "Loaded config settings from '${\(__FILE__)}'.\n";
1; #last value must be truthful to indicate successful load
#############################################################################################
#junk/experiment:
#use Package::Constants;
#use Exporter qw(import); #https://perldoc.perl.org/Exporter.html
#my $caller = "pdf2gerb::";
#sub cfg
#{
# my $proto = shift;
# my $class = ref($proto) || $proto;
# my $settings =
# {
# $WANT_DEBUG => 990, #10; #level of debug wanted; higher == more, lower == less, 0 == none
# };
# bless($settings, $class);
# return $settings;
#}
#use constant HELLO => "hi there2"; #"main::HELLO" => "hi there";
#use constant GOODBYE => 14; #"main::GOODBYE" => 12;
#print STDERR "read cfg file\n";
#our @EXPORT_OK = Package::Constants->list(__PACKAGE__); #https://www.perlmonks.org/?node_id=1072691; NOTE: "_OK" skips short/common names
#print STDERR scalar(@EXPORT_OK) . " consts exported:\n";
#foreach(@EXPORT_OK) { print STDERR "$_\n"; }
#my $val = main::thing("xyz");
#print STDERR "caller gave me $val\n";
#foreach my $arg (@ARGV) { print STDERR "arg $arg\n"; }Download Details:
Author: swannman
Source Code: https://github.com/swannman/pdf2gerb
License: GPL-3.0 license
1626775355
Why use Python for Software Development
No programming language is pretty much as diverse as Python. It enables building cutting edge applications effortlessly. Developers are as yet investigating the full capability of end-to-end Python development services in various areas.
By areas, we mean FinTech, HealthTech, InsureTech, Cybersecurity, and that's just the beginning. These are New Economy areas, and Python has the ability to serve every one of them. The vast majority of them require massive computational abilities. Python's code is dynamic and powerful - equipped for taking care of the heavy traffic and substantial algorithmic capacities.
Programming advancement is multidimensional today. Endeavor programming requires an intelligent application with AI and ML capacities. Shopper based applications require information examination to convey a superior client experience. Netflix, Trello, and Amazon are genuine instances of such applications. Python assists with building them effortlessly.
5 Reasons to Utilize Python for Programming Web Apps
Python can do such numerous things that developers can't discover enough reasons to admire it. Python application development isn't restricted to web and enterprise applications. It is exceptionally adaptable and superb for a wide range of uses.
Robust frameworks
Python is known for its tools and frameworks. There's a structure for everything. Django is helpful for building web applications, venture applications, logical applications, and mathematical processing. Flask is another web improvement framework with no conditions.
Web2Py, CherryPy, and Falcon offer incredible capabilities to customize Python development services. A large portion of them are open-source frameworks that allow quick turn of events.
Simple to read and compose
Python has an improved sentence structure - one that is like the English language. New engineers for Python can undoubtedly understand where they stand in the development process. The simplicity of composing allows quick application building.
The motivation behind building Python, as said by its maker Guido Van Rossum, was to empower even beginner engineers to comprehend the programming language. The simple coding likewise permits developers to roll out speedy improvements without getting confused by pointless subtleties.
Utilized by the best
Alright - Python isn't simply one more programming language. It should have something, which is the reason the business giants use it. Furthermore, that too for different purposes. Developers at Google use Python to assemble framework organization systems, parallel information pusher, code audit, testing and QA, and substantially more. Netflix utilizes Python web development services for its recommendation algorithm and media player.
Massive community support
Python has a steadily developing community that offers enormous help. From amateurs to specialists, there's everybody. There are a lot of instructional exercises, documentation, and guides accessible for Python web development solutions.
Today, numerous universities start with Python, adding to the quantity of individuals in the community. Frequently, Python designers team up on various tasks and help each other with algorithmic, utilitarian, and application critical thinking.
Progressive applications
Python is the greatest supporter of data science, Machine Learning, and Artificial Intelligence at any enterprise software development company. Its utilization cases in cutting edge applications are the most compelling motivation for its prosperity. Python is the second most well known tool after R for data analytics.
The simplicity of getting sorted out, overseeing, and visualizing information through unique libraries makes it ideal for data based applications. TensorFlow for neural networks and OpenCV for computer vision are two of Python's most well known use cases for Machine learning applications.
Summary
Thinking about the advances in programming and innovation, Python is a YES for an assorted scope of utilizations. Game development, web application development services, GUI advancement, ML and AI improvement, Enterprise and customer applications - every one of them uses Python to its full potential.
The disadvantages of Python web improvement arrangements are regularly disregarded by developers and organizations because of the advantages it gives. They focus on quality over speed and performance over blunders. That is the reason it's a good idea to utilize Python for building the applications of the future.
#python development services #python development company #python app development #python development #python in web development #python software development
1666245660
A Collection Of Swift Tips & Tricks That I've Shared on Twitter
Swift tips & tricks ⚡️
One of the things I really love about Swift is how I keep finding interesting ways to use it in various situations, and when I do - I usually share them on Twitter. Here's a collection of all the tips & tricks that I've shared so far. Each entry has a link to the original tweet, if you want to respond with some feedback or question, which is always super welcome! 🚀
⚠️ This list is no longer being updated. For my latest Swift tips, checkout the "Tips" section on Swift by Sundell.
Also make sure to check out all of my other Swift content:
- 📖 My weekly articles, with a new post about Swift every Sunday!
- 🎧 My podcast which features special guests from around the community.
- 📬 My monthly newsletter - rounding up all of my content, including these tips, on the 1st of every month.
102 Making async tests faster and more stable
🚀 Here are some quick tips to make async tests faster & more stable:
- 😴 Avoid sleep() - use expectations instead
- ⏱ Use generous timeouts to avoid flakiness on CI
- 🧐 Put all assertions at the end of each test, not inside closures
// BEFORE:
class MentionDetectorTests: XCTestCase {
func testDetectingMention() {
let detector = MentionDetector()
let string = "This test was written by @johnsundell."
detector.detectMentions(in: string) { mentions in
XCTAssertEqual(mentions, ["johnsundell"])
}
sleep(2)
}
}
// AFTER:
class MentionDetectorTests: XCTestCase {
func testDetectingMention() {
let detector = MentionDetector()
let string = "This test was written by @johnsundell."
var mentions: [String]?
let expectation = self.expectation(description: #function)
detector.detectMentions(in: string) {
mentions = $0
expectation.fulfill()
}
waitForExpectations(timeout: 10)
XCTAssertEqual(mentions, ["johnsundell"])
}
}
For more on async testing, check out "Unit testing asynchronous Swift code".
101 Adding support for Apple Pencil double-taps
✍️ Adding support for the new Apple Pencil double-tap feature is super easy! All you have to do is to create a UIPencilInteraction, add it to a view, and implement one delegate method. Hopefully all pencil-compatible apps will soon adopt this.
let interaction = UIPencilInteraction()
interaction.delegate = self
view.addInteraction(interaction)
extension ViewController: UIPencilInteractionDelegate {
func pencilInteractionDidTap(_ interaction: UIPencilInteraction) {
// Handle pencil double-tap
}
}
For more on using this and other iPad Pro features, check out "Building iPad Pro features in Swift".
100 Combining values with functions
😎 Here's a cool function that combines a value with a function to return a closure that captures that value, so that it can be called without any arguments. Super useful when working with closure-based APIs and we want to use some of our properties without having to capture self.
func combine<A, B>(_ value: A, with closure: @escaping (A) -> B) -> () -> B {
return { closure(value) }
}
// BEFORE:
class ProductViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
buyButton.handler = { [weak self] in
guard let self = self else {
return
}
self.productManager.startCheckout(for: self.product)
}
}
}
// AFTER:
class ProductViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
buyButton.handler = combine(product, with: productManager.startCheckout)
}
}
99 Dependency injection using functions
💉 When I'm only using a single function from a dependency, I love to inject that function as a closure, instead of having to create a protocol and inject the whole object. Makes dependency injection & testing super simple.
final class ArticleLoader {
typealias Networking = (Endpoint) -> Future<Data>
private let networking: Networking
init(networking: @escaping Networking = URLSession.shared.load) {
self.networking = networking
}
func loadLatest() -> Future<[Article]> {
return networking(.latestArticles).decode()
}
}
For more on this technique, check out "Simple Swift dependency injection with functions".
98 Using a custom exception handler
💥 It's cool that you can easily assign a closure as a custom NSException handler. This is super useful when building things in Playgrounds - since you can't use breakpoints - so instead of just signal SIGABRT, you'll get the full exception description if something goes wrong.
NSSetUncaughtExceptionHandler { exception in
print(exception)
}
97 Using type aliases to give semantic meaning to primitives
❤️ I love that in Swift, we can use the type system to make our code so much more self-documenting - one way of doing so is to use type aliases to give the primitive types that we use a more semantic meaning.
extension List.Item {
// Using type aliases, we can give semantic meaning to the
// primitive types that we use, without having to introduce
// wrapper types.
typealias Index = Int
}
extension List {
enum Mutation {
// Our enum cases now become a lot more self-documenting,
// without having to add additional parameter labels to
// explain them.
case add(Item, Item.Index)
case update(Item, Item.Index)
case remove(Item.Index)
}
}
For more on self-documenting code, check out "Writing self-documenting Swift code".
96 Specializing protocols using constraints
🤯 A little late night prototyping session reveals that protocol constraints can not only be applied to extensions - they can also be added to protocol definitions!
This is awesome, since it lets us easily define specialized protocols based on more generic ones.
protocol Component {
associatedtype Container
func add(to container: Container)
}
// Protocols that inherit from other protocols can include
// constraints to further specialize them.
protocol ViewComponent: Component where Container == UIView {
associatedtype View: UIView
var view: View { get }
}
extension ViewComponent {
func add(to container: UIView) {
container.addSubview(view)
}
}
For more on specializing protocols, check out "Specializing protocols in Swift".
95 Unwrapping an optional or throwing an error
📦 Here's a super handy extension on Swift's Optional type, which gives us a really nice API for easily unwrapping an optional, or throwing an error in case the value turned out to be nil:
extension Optional {
func orThrow(_ errorExpression: @autoclosure () -> Error) throws -> Wrapped {
switch self {
case .some(let value):
return value
case .none:
throw errorExpression()
}
}
}
let file = try loadFile(at: path).orThrow(MissingFileError())
For more ways that optionals can be extended, check out "Extending optionals in Swift".
94 Testing code that uses static APIs
👩🔬 Testing code that uses static APIs can be really tricky, but there's a way that it can often be done - using Swift's first class function capabilities!
Instead of accessing that static API directly, we can inject the function we want to use, which enables us to mock it!
// BEFORE
class FriendsLoader {
func loadFriends(then handler: @escaping (Result<[Friend]>) -> Void) {
Networking.loadData(from: .friends) { result in
...
}
}
}
// AFTER
class FriendsLoader {
typealias Handler<T> = (Result<T>) -> Void
typealias DataLoadingFunction = (Endpoint, @escaping Handler<Data>) -> Void
func loadFriends(using dataLoading: DataLoadingFunction = Networking.loadData,
then handler: @escaping Handler<[Friend]>) {
dataLoading(.friends) { result in
...
}
}
}
// MOCKING IN TESTS
let dataLoading: FriendsLoader.DataLoadingFunction = { _, handler in
handler(.success(mockData))
}
friendsLoader.loadFriends(using: dataLoading) { result in
...
}
93 Matching multiple enum cases with associated values
🐾 Swift's pattern matching capabilities are so powerful! Two enum cases with associated values can even be matched and handled by the same switch case - which is super useful when handling state changes with similar data.
enum DownloadState {
case inProgress(progress: Double)
case paused(progress: Double)
case cancelled
case finished(Data)
}
func downloadStateDidChange(to state: DownloadState) {
switch state {
case .inProgress(let progress), .paused(let progress):
updateProgressView(with: progress)
case .cancelled:
showCancelledMessage()
case .finished(let data):
process(data)
}
}
92 Multiline string literals
🅰 One really nice benefit of Swift multiline string literals - even for single lines of text - is that they don't require quotes to be escaped. Perfect when working with things like HTML, or creating a custom description for an object.
let html = highlighter.highlight("Array<String>")
XCTAssertEqual(html, """
<span class="type">Array</span><<span class="type">String</span>>
""")
91 Reducing sequences
💎 While it's very common in functional programming, the reduce function might be a bit of a hidden gem in Swift. It provides a super useful way to transform a sequence into a single value.
extension Sequence where Element: Equatable {
func numberOfOccurrences(of target: Element) -> Int {
return reduce(0) { result, element in
guard element == target else {
return result
}
return result + 1
}
}
}
You can read more about transforming collections in "Transforming collections in Swift".
90 Avoiding manual Codable implementations
📦 When I use Codable in Swift, I want to avoid manual implementations as much as possible, even when there's a mismatch between my code structure and the JSON I'm decoding.
One way that can often be achieved is to use private data containers combined with computed properties.
struct User: Codable {
let name: String
let age: Int
var homeTown: String { return originPlace.name }
private let originPlace: Place
}
private extension User {
struct Place: Codable {
let name: String
}
}
extension User {
struct Container: Codable {
let user: User
}
}
89 Using feature flags instead of feature branches
🚢 Instead of using feature branches, I merge almost all of my code directly into master - and then I use feature flags to conditionally enable features when they're ready. That way I can avoid merge conflicts and keep shipping!
extension ListViewController {
func addSearchIfNeeded() {
// Rather than having to keep maintaining a separate
// feature branch for a new feature, we can use a flag
// to conditionally turn it on.
guard FeatureFlags.searchEnabled else {
return
}
let resultsVC = SearchResultsViewController()
let searchVC = UISearchController(
searchResultsController: resultsVC
)
searchVC.searchResultsUpdater = resultsVC
navigationItem.searchController = searchVC
}
}
You can read more about feature flags in "Feature flags in Swift".
88 Lightweight data hierarchies using tuples
💾 Here I'm using tuples to create a lightweight hierarchy for my data, giving me a nice structure without having to introduce any additional types.
struct CodeSegment {
var tokens: (
previous: String?,
current: String
)
var delimiters: (
previous: Character?
next: Character?
)
}
handle(segment.tokens.current)
You can read more about tuples in "Using tuples as lightweight types in Swift"
87 The rule of threes
3️⃣ Whenever I have 3 properties or local variables that share the same prefix, I usually try to extract them into their own method or type. That way I can avoid massive types & methods, and also increase readability, without falling into a "premature optimization" trap.
Before
public func generate() throws {
let contentFolder = try folder.subfolder(named: "content")
let articleFolder = try contentFolder.subfolder(named: "posts")
let articleProcessor = ContentProcessor(folder: articleFolder)
let articles = try articleProcessor.process()
...
}
After
public func generate() throws {
let contentFolder = try folder.subfolder(named: "content")
let articles = try processArticles(in: contentFolder)
...
}
private func processArticles(in folder: Folder) throws -> [ContentItem] {
let folder = try folder.subfolder(named: "posts")
let processor = ContentProcessor(folder: folder)
return try processor.process()
}
86 Useful Codable extensions
👨🔧 Here's two extensions that I always add to the Encodable & Decodable protocols, which for me really make the Codable API nicer to use. By using type inference for decoding, a lot of boilerplate can be removed when the compiler is already able to infer the resulting type.
extension Encodable {
func encoded() throws -> Data {
return try JSONEncoder().encode(self)
}
}
extension Data {
func decoded<T: Decodable>() throws -> T {
return try JSONDecoder().decode(T.self, from: self)
}
}
let data = try user.encoded()
// By using a generic type in the decoded() method, the
// compiler can often infer the type we want to decode
// from the current context.
try userDidLogin(data.decoded())
// And if not, we can always supply the type, still making
// the call site read very nicely.
let otherUser = try data.decoded() as User
85 Using shared UserDefaults suites
📦 UserDefaults is a lot more powerful than what it first might seem like. Not only can it store more complex values (like dates & dictionaries) and parse command line arguments - it also enables easy sharing of settings & lightweight data between apps in the same App Group.
let sharedDefaults = UserDefaults(suiteName: "my-app-group")!
let useDarkMode = sharedDefaults.bool(forKey: "dark-mode")
// This value is put into the shared suite.
sharedDefaults.set(true, forKey: "dark-mode")
// If you want to treat the shared settings as read-only (and add
// local overrides on top of them), you can simply add the shared
// suite to the standard UserDefaults.
let combinedDefaults = UserDefaults.standard
combinedDefaults.addSuite(named: "my-app-group")
// This value is a local override, not added to the shared suite.
combinedDefaults.set(true, forKey: "app-specific-override")
84 Custom UIView backing layers
🎨 By overriding layerClass you can tell UIKit what CALayer class to use for a UIView's backing layer. That way you can reduce the amount of layers, and don't have to do any manual layout.
final class GradientView: UIView {
override class var layerClass: AnyClass { return CAGradientLayer.self }
var colors: (start: UIColor, end: UIColor)? {
didSet { updateLayer() }
}
private func updateLayer() {
let layer = self.layer as! CAGradientLayer
layer.colors = colors.map { [$0.start.cgColor, $0.end.cgColor] }
}
}
83 Auto-Equatable enums with associated values
✅ That the compiler now automatically synthesizes Equatable conformances is such a huge upgrade for Swift! And the cool thing is that it works for all kinds of types - even for enums with associated values! Especially useful when using enums for verification in unit tests.
struct Article: Equatable {
let title: String
let text: String
}
struct User: Equatable {
let name: String
let age: Int
}
extension Navigator {
enum Destination: Equatable {
case profile(User)
case article(Article)
}
}
func testNavigatingToArticle() {
let article = Article(title: "Title", text: "Text")
controller.select(article)
XCTAssertEqual(navigator.destinations, [.article(article)])
}
82 Defaults for associated types
🤝 Associated types can have defaults in Swift - which is super useful for types that are not easily inferred (for example when they're not used for a specific instance method or property).
protocol Identifiable {
associatedtype RawIdentifier: Codable = String
var id: Identifier<Self> { get }
}
struct User: Identifiable {
let id: Identifier<User>
let name: String
}
struct Group: Identifiable {
typealias RawIdentifier = Int
let id: Identifier<Group>
let name: String
}
81 Creating a dedicated identifier type
🆔 If you want to avoid using plain strings as identifiers (which can increase both type safety & readability), it's really easy to create a custom Identifier type that feels just like a native Swift type, thanks to protocols!
More on this topic in "Type-safe identifiers in Swift".
struct Identifier: Hashable {
let string: String
}
extension Identifier: ExpressibleByStringLiteral {
init(stringLiteral value: String) {
string = value
}
}
extension Identifier: CustomStringConvertible {
var description: String {
return string
}
}
extension Identifier: Codable {
init(from decoder: Decoder) throws {
let container = try decoder.singleValueContainer()
string = try container.decode(String.self)
}
func encode(to encoder: Encoder) throws {
var container = encoder.singleValueContainer()
try container.encode(string)
}
}
struct Article: Codable {
let id: Identifier
let title: String
}
let article = Article(id: "my-article", title: "Hello world!")
80 Assigning optional tuple members to variables
🙌 A really cool thing about using tuples to model the internal state of a Swift type, is that you can unwrap an optional tuple's members directly into local variables.
Very useful in order to group multiple optional values together for easy unwrapping & handling.
class ImageTransformer {
private var queue = [(image: UIImage, transform: Transform)]()
private func processNext() {
// When unwrapping an optional tuple, you can assign the members
// directly to local variables.
guard let (image, transform) = queue.first else {
return
}
let context = Context()
context.draw(image)
context.apply(transform)
...
}
}
79 Struct convenience initializers
❤️ I love to structure my code using extensions in Swift. One big benefit of doing so when it comes to struct initializers, is that defining a convenience initializer doesn't remove the default one the compiler generates - best of both worlds!
struct Article {
let date: Date
var title: String
var text: String
var comments: [Comment]
}
extension Article {
init(title: String, text: String) {
self.init(date: Date(), title: title, text: text, comments: [])
}
}
let articleA = Article(title: "Best Cupcake Recipe", text: "...")
let articleB = Article(
date: Date(),
title: "Best Cupcake Recipe",
text: "...",
comments: [
Comment(user: currentUser, text: "Yep, can confirm!")
]
)
78 Usages of throwing functions
🏈 A big benefit of using throwing functions for synchronous Swift APIs is that the caller can decide whether they want to treat the return value as optional (try?) or required (try).
func loadFile(named name: String) throws -> File {
guard let url = urlForFile(named: name) else {
throw File.Error.missing
}
do {
let data = try Data(contentsOf: url)
return File(url: url, data: data)
} catch {
throw File.Error.invalidData(error)
}
}
let requiredFile = try loadFile(named: "AppConfig.json")
let optionalFile = try? loadFile(named: "UserSettings.json")
77 Nested generic types
🐝 Types that are nested in generics automatically inherit their parent's generic types - which is super useful when defining accessory types (for things like states or outcomes).
struct Task<Input, Output> {
typealias Closure = (Input) throws -> Output
let closure: Closure
}
extension Task {
enum Result {
case success(Output)
case failure(Error)
}
}
76 Equatable & Hashable structures
🤖 Now that the Swift compiler automatically synthesizes Equatable & Hashable conformances for value types, it's easier than ever to setup model structures with nested types that are all Equatable/Hashable!
typealias Value = Hashable & Codable
struct User: Value {
var name: String
var age: Int
var lastLoginDate: Date?
var settings: Settings
}
extension User {
struct Settings: Value {
var itemsPerPage: Int
var theme: Theme
}
}
extension User.Settings {
enum Theme: String, Value {
case light
case dark
}
}
You can read more about using nested types in Swift here.
75 Conditional conformances
🎉 Swift 4.1 is here! One of the key features it brings is conditional conformances, which lets you have a type only conform to a protocol under certain constraints.
protocol UnboxTransformable {
associatedtype RawValue
static func transform(_ value: RawValue) throws -> Self?
}
extension Array: UnboxTransformable where Element: UnboxTransformable {
typealias RawValue = [Element.RawValue]
static func transform(_ value: RawValue) throws -> [Element]? {
return try value.compactMap(Element.transform)
}
}
I also have an article with lots of more info on conditional conformances here. Paul Hudson also has a great overview of all Swift 4.1 features here.
74 Generic type aliases
🕵️♀️ A cool thing about Swift type aliases is that they can be generic! Combine that with tuples and you can easily define simple generic types.
typealias Pair<T> = (T, T)
extension Game {
func calculateScore(for players: Pair<Player>) -> Int {
...
}
}
You can read more about using tuples as lightweight types here.
73 Parsing command line arguments using UserDefaults
☑️ A really cool "hidden" feature of UserDefaults is that it contains any arguments that were passed to the app at launch!
Super useful both in Swift command line tools & scripts, but also to temporarily override a value when debugging iOS apps.
let defaults = UserDefaults.standard
let query = defaults.string(forKey: "query")
let resultCount = defaults.integer(forKey: "results")
72 Using the & operator
👏 Swift's & operator is awesome! Not only can you use it to compose protocols, you can compose other types too! Very useful if you want to hide concrete types & implementation details.
protocol LoadableFromURL {
func load(from url: URL)
}
class ContentViewController: UIViewController, LoadableFromURL {
func load(from url: URL) {
...
}
}
class ViewControllerFactory {
func makeContentViewController() -> UIViewController & LoadableFromURL {
return ContentViewController()
}
}
71 Capturing multiple values in mocks
🤗 When capturing values in mocks, using an array (instead of just a single value) makes it easy to verify that only a certain number of values were passed.
Perfect for protecting against "over-calling" something.
class UserManagerTests: XCTestCase {
func testObserversCalledWhenUserFirstLogsIn() {
let manager = UserManager()
let observer = ObserverMock()
manager.addObserver(observer)
// First login, observers should be notified
let user = User(id: 123, name: "John")
manager.userDidLogin(user)
XCTAssertEqual(observer.users, [user])
// If the same user logs in again, observers shouldn't be notified
manager.userDidLogin(user)
XCTAssertEqual(observer.users, [user])
}
}
private extension UserManagerTests {
class ObserverMock: UserManagerObserver {
private(set) var users = [User]()
func userDidChange(to user: User) {
users.append(user)
}
}
}
70 Reducing the need for mocks
👋 When writing tests, you don't always need to create mocks - you can create stubs using real instances of things like errors, URLs & UserDefaults.
Here's how to do that for some common tasks/object types in Swift:
// Create errors using NSError (#function can be used to reference the name of the test)
let error = NSError(domain: #function, code: 1, userInfo: nil)
// Create non-optional URLs using file paths
let url = URL(fileURLWithPath: "Some/URL")
// Reference the test bundle using Bundle(for:)
let bundle = Bundle(for: type(of: self))
// Create an explicit UserDefaults object (instead of having to use a mock)
let userDefaults = UserDefaults(suiteName: #function)
// Create queues to control/await concurrent operations
let queue = DispatchQueue(label: #function)
For when you actually do need mocking, check out "Mocking in Swift".
69 Using "then" as an external parameter label for closures
⏱ I've started using "then" as an external parameter label for completion handlers. Makes the call site read really nicely (Because I do ❤️ conversational API design) regardless of whether trailing closure syntax is used or not.
protocol DataLoader {
// Adding type aliases to protocols can be a great way to
// reduce verbosity for parameter types.
typealias Handler = (Result<Data>) -> Void
associatedtype Endpoint
func loadData(from endpoint: Endpoint, then handler: @escaping Handler)
}
loader.loadData(from: .messages) { result in
...
}
loader.loadData(from: .messages, then: { result in
...
})
68 Combining lazily evaluated sequences with the builder pattern
😴 Combining lazily evaluated sequences with builder pattern-like properties can lead to some pretty sweet APIs for configurable sequences in Swift.
Also useful for queries & other things you "build up" and then execute.
// Extension adding builder pattern-like properties that return
// a new sequence value with the given configuration applied
extension FileSequence {
var recursive: FileSequence {
var sequence = self
sequence.isRecursive = true
return sequence
}
var includingHidden: FileSequence {
var sequence = self
sequence.includeHidden = true
return sequence
}
}
// BEFORE
let files = folder.makeFileSequence(recursive: true, includeHidden: true)
// AFTER
let files = folder.files.recursive.includingHidden
Want an intro to lazy sequences? Check out "Swift sequences: The art of being lazy".
67 Faster & more stable UI tests
My top 3 tips for faster & more stable UI tests:
📱 Reset the app's state at the beginning of every test.
🆔 Use accessibility identifiers instead of UI strings.
⏱ Use expectations instead of waiting time.
func testOpeningArticle() {
// Launch the app with an argument that tells it to reset its state
let app = XCUIApplication()
app.launchArguments.append("--uitesting")
app.launch()
// Check that the app is displaying an activity indicator
let activityIndicator = app.activityIndicator.element
XCTAssertTrue(activityIndicator.exists)
// Wait for the loading indicator to disappear = content is ready
expectation(for: NSPredicate(format: "exists == 0"),
evaluatedWith: activityIndicator)
// Use a generous timeout in case the network is slow
waitForExpectations(timeout: 10)
// Tap the cell for the first article
app.tables.cells["Article.0"].tap()
// Assert that a label with the accessibility identifier "Article.Title" exists
let label = app.staticTexts["Article.Title"]
XCTAssertTrue(label.exists)
}
66 Accessing the clipboard from a Swift script
📋 It's super easy to access the contents of the clipboard from a Swift script. A big benefit of Swift scripting is being able to use Cocoa's powerful APIs for Mac apps.
import Cocoa
let clipboard = NSPasteboard.general.string(forType: .string)
65 Using tuples for view state
🎯 Using Swift tuples for view state can be a super nice way to group multiple properties together and render them reactively using the layout system.
By using a tuple we don't have to either introduce a new type or make our view model-aware.
class TextView: UIView {
var state: (title: String?, text: String?) {
// By telling UIKit that our view needs layout and binding our
// state in layoutSubviews, we can react to state changes without
// doing unnecessary layout work.
didSet { setNeedsLayout() }
}
private let titleLabel = UILabel()
private let textLabel = UILabel()
override func layoutSubviews() {
super.layoutSubviews()
titleLabel.text = state.title
textLabel.text = state.text
...
}
}
64 Throwing tests and LocalizedError
⚾️ Swift tests can throw, which is super useful in order to avoid complicated logic or force unwrapping. By making errors conform to LocalizedError, you can also get a nice error message in Xcode if there's a failure.
class ImageCacheTests: XCTestCase {
func testCachingAndLoadingImage() throws {
let bundle = Bundle(for: type(of: self))
let cache = ImageCache(bundle: bundle)
// Bonus tip: You can easily load images from your test
// bundle using this UIImage initializer
let image = try require(UIImage(named: "sample", in: bundle, compatibleWith: nil))
try cache.cache(image, forKey: "key")
let cachedImage = try cache.image(forKey: "key")
XCTAssertEqual(image, cachedImage)
}
}
enum ImageCacheError {
case emptyKey
case dataConversionFailed
}
// When using throwing tests, making your errors conform to
// LocalizedError will render a much nicer error message in
// Xcode (per default only the error code is shown).
extension ImageCacheError: LocalizedError {
var errorDescription: String? {
switch self {
case .emptyKey:
return "An empty key was given"
case .dataConversionFailed:
return "Failed to convert the given image to Data"
}
}
}
For more information, and the implementation of the require method used above, check out "Avoiding force unwrapping in Swift unit tests".
63 The difference between static and class properties
✍️ Unlike static properties, class properties can be overridden by subclasses (however, they can't be stored, only computed).
class TableViewCell: UITableViewCell {
class var preferredHeight: CGFloat { return 60 }
}
class TallTableViewCell: TableViewCell {
override class var preferredHeight: CGFloat { return 100 }
}
62 Creating extensions with static factory methods
👨🎨 Creating extensions with static factory methods can be a great alternative to subclassing in Swift, especially for things like setting up UIViews, CALayers or other kinds of styling.
It also lets you remove a lot of styling & setup from your view controllers.
extension UILabel {
static func makeForTitle() -> UILabel {
let label = UILabel()
label.font = .boldSystemFont(ofSize: 24)
label.textColor = .darkGray
label.adjustsFontSizeToFitWidth = true
label.minimumScaleFactor = 0.75
return label
}
static func makeForText() -> UILabel {
let label = UILabel()
label.font = .systemFont(ofSize: 16)
label.textColor = .black
label.numberOfLines = 0
return label
}
}
class ArticleViewController: UIViewController {
lazy var titleLabel = UILabel.makeForTitle()
lazy var textLabel = UILabel.makeForText()
}
61 Child view controller auto-resizing
🧒 An awesome thing about child view controllers is that they're automatically resized to match their parent, making them a super nice solution for things like loading & error views.
class ListViewController: UIViewController {
func loadItems() {
let loadingViewController = LoadingViewController()
add(loadingViewController)
dataLoader.loadItems { [weak self] result in
loadingViewController.remove()
self?.handle(result)
}
}
}
For more about child view controller (including the add and remove methods used above), check out "Using child view controllers as plugins in Swift".
60 Using zip
🤐 Using the zip function in Swift you can easily combine two sequences. Super useful when using two sequences to do some work, since zip takes care of all the bounds-checking.
func render(titles: [String]) {
for (label, text) in zip(titleLabels, titles) {
print(text)
label.text = text
}
}
59 Defining custom option sets
🎛 The awesome thing about option sets in Swift is that they can automatically either be passed as a single member or as a set. Even cooler is that you can easily define your own option sets as well, perfect for options and other non-exclusive values.
// Option sets are awesome, because you can easily pass them
// both using dot syntax and array literal syntax, like when
// using the UIView animation API:
UIView.animate(withDuration: 0.3,
delay: 0,
options: .allowUserInteraction,
animations: animations)
UIView.animate(withDuration: 0.3,
delay: 0,
options: [.allowUserInteraction, .layoutSubviews],
animations: animations)
// The cool thing is that you can easily define your own option
// sets as well, by defining a struct that has an Int rawValue,
// that will be used as a bit mask.
extension Cache {
struct Options: OptionSet {
static let saveToDisk = Options(rawValue: 1)
static let clearOnMemoryWarning = Options(rawValue: 1 << 1)
static let clearDaily = Options(rawValue: 1 << 2)
let rawValue: Int
}
}
// We can now use Cache.Options just like UIViewAnimationOptions:
Cache(options: .saveToDisk)
Cache(options: [.saveToDisk, .clearDaily])
58 Using the where clause with associated types
🙌 Using the where clause when designing protocol-oriented APIs in Swift can let your implementations (or others' if it's open source) have a lot more freedom, especially when it comes to collections.
See "Using generic type constraints in Swift 4" for more info.
public protocol PathFinderMap {
associatedtype Node
// Using the 'where' clause for associated types, we can
// ensure that a type meets certain requirements (in this
// case that it's a sequence with Node elements).
associatedtype NodeSequence: Sequence where NodeSequence.Element == Node
// Instead of using a concrete type (like [Node]) here, we
// give implementors of this protocol more freedom while
// still meeting our requirements. For example, one
// implementation might use Set<Node>.
func neighbors(of node: Node) -> NodeSequence
}
57 Using first class functions when iterating over a dictionary
👨🍳 Combine first class functions in Swift with the fact that Dictionary elements are (Key, Value) tuples and you can build yourself some pretty awesome functional chains when iterating over a Dictionary.
func makeActor(at coordinate: Coordinate, for building: Building) -> Actor {
let actor = Actor()
actor.position = coordinate.point
actor.animation = building.animation
return actor
}
func render(_ buildings: [Coordinate : Building]) {
buildings.map(makeActor).forEach(add)
}
56 Calling instance methods as static functions
😎 In Swift, you can call any instance method as a static function and it will return a closure representing that method. This is how running tests using SPM on Linux works.
More about this topic in my blog post "First class functions in Swift".
// This produces a '() -> Void' closure which is a reference to the
// given view's 'removeFromSuperview' method.
let closure = UIView.removeFromSuperview(view)
// We can now call it just like we would any other closure, and it
// will run 'view.removeFromSuperview()'
closure()
// This is how running tests using the Swift Package Manager on Linux
// works, you return your test functions as closures:
extension UserManagerTests {
static var allTests = [
("testLoggingIn", testLoggingIn),
("testLoggingOut", testLoggingOut),
("testUserPermissions", testUserPermissions)
]
}
55 Dropping suffixes from method names to support multiple arguments
👏 One really nice benefit of dropping suffixes from method names (and just using verbs, when possible) is that it becomes super easy to support both single and multiple arguments, and it works really well semantically.
extension UIView {
func add(_ subviews: UIView...) {
subviews.forEach(addSubview)
}
}
view.add(button)
view.add(label)
// By dropping the "Subview" suffix from the method name, both
// single and multiple arguments work really well semantically.
view.add(button, label)
54 Constraining protocols to classes to ensure mutability
👽 Using the AnyObject (or class) constraint on protocols is not only useful when defining delegates (or other weak references), but also when you always want instances to be mutable without copying.
// By constraining a protocol with 'AnyObject' it can only be adopted
// by classes, which means all instances will always be mutable, and
// that it's the original instance (not a copy) that will be mutated.
protocol DataContainer: AnyObject {
var data: Data? { get set }
}
class UserSettingsManager {
private var settings: Settings
private let dataContainer: DataContainer
// Since DataContainer is a protocol, we an easily mock it in
// tests if we use dependency injection
init(settings: Settings, dataContainer: DataContainer) {
self.settings = settings
self.dataContainer = dataContainer
}
func saveSettings() throws {
let data = try settings.serialize()
// We can now assign properties on an instance of our protocol
// because the compiler knows it's always going to be a class
dataContainer.data = data
}
}
53 String-based enums in string interpolation
🍣 Even if you define a custom raw value for a string-based enum in Swift, the full case name will be used in string interpolation.
Super useful when using separate raw values for JSON, while still wanting to use the full case name in other contexts.
extension Building {
// This enum has custom raw values that are used when decoding
// a value, for example from JSON.
enum Kind: String {
case castle = "C"
case town = "T"
case barracks = "B"
case goldMine = "G"
case camp = "CA"
case blacksmith = "BL"
}
var animation: Animation {
return Animation(
// When used in string interpolation, the full case name is still used.
// For 'castle' this will be 'buildings/castle'.
name: "buildings/\(kind)",
frameCount: frameCount,
frameDuration: frameDuration
)
}
}
52 Expressively comparing a value with a list of candidates
👨🔬 Continuing to experiment with expressive ways of comparing a value with a list of candidates in Swift. Adding an extension on Equatable is probably my favorite approach so far.
extension Equatable {
func isAny(of candidates: Self...) -> Bool {
return candidates.contains(self)
}
}
let isHorizontal = direction.isAny(of: .left, .right)
See tip 35 for my previous experiment.
51 UIView bounds and transforms
📐 A really interesting side-effect of a UIView's bounds being its rect within its own coordinate system is that transforms don't affect it at all. That's why it's usually a better fit than frame when doing layout calculations of subviews.
let view = UIView()
view.frame.size = CGSize(width: 100, height: 100)
view.transform = CGAffineTransform(scaleX: 2, y: 2)
print(view.frame) // (-50.0, -50.0, 200.0, 200.0)
print(view.bounds) // (0.0, 0.0, 100.0, 100.0)
50 UIKit default arguments
👏 It's awesome that many UIKit APIs with completion handlers and other optional parameters import into Swift with default arguments (even though they are written in Objective-C). Getting rid of all those nil arguments is so nice!
// BEFORE: All parameters are specified, just like in Objective-C
viewController.present(modalViewController, animated: true, completion: nil)
modalViewController.dismiss(animated: true, completion: nil)
viewController.transition(from: loadingViewController,
to: contentViewController,
duration: 0.3,
options: [],
animations: animations,
completion: nil)
// AFTER: Since many UIKit APIs with completion handlers and other
// optional parameters import into Swift with default arguments,
// we can make our calls shorter
viewController.present(modalViewController, animated: true)
modalViewController.dismiss(animated: true)
viewController.transition(from: loadingViewController,
to: contentViewController,
duration: 0.3,
animations: animations)
49 Avoiding Massive View Controllers
✂️ Avoiding Massive View Controllers is all about finding the right levels of abstraction and splitting things up.
My personal rule of thumb is that as soon as I have 3 methods or properties that have the same prefix, I break them out into their own type.
// BEFORE
class LoginViewController: UIViewController {
private lazy var signUpLabel = UILabel()
private lazy var signUpImageView = UIImageView()
private lazy var signUpButton = UIButton()
}
// AFTER
class LoginViewController: UIViewController {
private lazy var signUpView = SignUpView()
}
class SignUpView: UIView {
private lazy var label = UILabel()
private lazy var imageView = UIImageView()
private lazy var button = UIButton()
}
48 Extending optionals
❤️ I love the fact that optionals are enums in Swift - it makes it so easy to extend them with convenience APIs for certain types. Especially useful when doing things like data validation on optional values.
func validateTextFields() -> Bool {
guard !usernameTextField.text.isNilOrEmpty else {
return false
}
...
return true
}
// Since all optionals are actual enum values in Swift, we can easily
// extend them for certain types, to add our own convenience APIs
extension Optional where Wrapped == String {
var isNilOrEmpty: Bool {
switch self {
case let string?:
return string.isEmpty
case nil:
return true
}
}
}
// Since strings are now Collections in Swift 4, you can even
// add this property to all optional collections:
extension Optional where Wrapped: Collection {
var isNilOrEmpty: Bool {
switch self {
case let collection?:
return collection.isEmpty
case nil:
return true
}
}
}
47 Using where with for-loops
🗺 Using the where keyword can be a super nice way to quickly apply a filter in a for-loop in Swift. You can of course use map, filter and forEach, or guard, but for simple loops I think this is very expressive and nice.
func archiveMarkedPosts() {
for post in posts where post.isMarked {
archive(post)
}
}
func healAllies() {
for player in players where player.isAllied(to: currentPlayer) {
player.heal()
}
}
46 Variable shadowing
👻 Variable shadowing can be super useful in Swift, especially when you want to create a local copy of a parameter value in order to use it as state within a closure.
init(repeatMode: RepeatMode, closure: @escaping () -> UpdateOutcome) {
// Shadow the argument with a local, mutable copy
var repeatMode = repeatMode
self.closure = {
// With shadowing, there's no risk of accidentially
// referring to the immutable version
switch repeatMode {
case .forever:
break
case .times(let count):
guard count > 0 else {
return .finished
}
// We can now capture the mutable version and use
// it for state in a closure
repeatMode = .times(count - 1)
}
return closure()
}
}
45 Using dot syntax for static properties and initializers
✒️ Dot syntax is one of my favorite features of Swift. What's really cool is that it's not only for enums, any static method or property can be used with dot syntax - even initializers! Perfect for convenience APIs and default parameters.
public enum RepeatMode {
case times(Int)
case forever
}
public extension RepeatMode {
static var never: RepeatMode {
return .times(0)
}
static var once: RepeatMode {
return .times(1)
}
}
view.perform(animation, repeated: .once)
// To make default parameters more compact, you can even use init with dot syntax
class ImageLoader {
init(cache: Cache = .init(), decoder: ImageDecoder = .init()) {
...
}
}
44 Calling functions as closures with a tuple as parameters
🚀 One really cool aspect of Swift having first class functions is that you can pass any function (or even initializer) as a closure, and even call it with a tuple containing its parameters!
// This function lets us treat any "normal" function or method as
// a closure and run it with a tuple that contains its parameters
func call<Input, Output>(_ function: (Input) -> Output, with input: Input) -> Output {
return function(input)
}
class ViewFactory {
func makeHeaderView() -> HeaderView {
// We can now pass an initializer as a closure, and a tuple
// containing its parameters
return call(HeaderView.init, with: loadTextStyles())
}
private func loadTextStyles() -> (font: UIFont, color: UIColor) {
return (theme.font, theme.textColor)
}
}
class HeaderView {
init(font: UIFont, textColor: UIColor) {
...
}
}
43 Enabling static dependency injection
💉 If you've been struggling to test code that uses static APIs, here's a technique you can use to enable static dependency injection without having to modify any call sites:
// Before: Almost impossible to test due to the use of singletons
class Analytics {
static func log(_ event: Event) {
Database.shared.save(event)
let dictionary = event.serialize()
NetworkManager.shared.post(dictionary, to: eventURL)
}
}
// After: Much easier to test, since we can inject mocks as arguments
class Analytics {
static func log(_ event: Event,
database: Database = .shared,
networkManager: NetworkManager = .shared) {
database.save(event)
let dictionary = event.serialize()
networkManager.post(dictionary, to: eventURL)
}
}
42 Type inference for lazy properties in Swift 4
🎉 In Swift 4, type inference works for lazy properties and you don't need to explicitly refer to self!
// Swift 3
class PurchaseView: UIView {
private lazy var buyButton: UIButton = self.makeBuyButton()
private func makeBuyButton() -> UIButton {
let button = UIButton()
button.setTitle("Buy", for: .normal)
button.setTitleColor(.blue, for: .normal)
return button
}
}
// Swift 4
class PurchaseView: UIView {
private lazy var buyButton = makeBuyButton()
private func makeBuyButton() -> UIButton {
let button = UIButton()
button.setTitle("Buy", for: .normal)
button.setTitleColor(.blue, for: .normal)
return button
}
}
41 Converting Swift errors to NSError
😎 You can turn any Swift Error into an NSError, which is super useful when pattern matching with a code 👍. Also, switching on optionals is pretty cool!
let task = urlSession.dataTask(with: url) { data, _, error in
switch error {
case .some(let error as NSError) where error.code == NSURLErrorNotConnectedToInternet:
presenter.showOfflineView()
case .some(let error):
presenter.showGenericErrorView()
case .none:
presenter.renderContent(from: data)
}
}
task.resume()
Also make sure to check out Kostas Kremizas' tip about how you can pattern match directly against a member of URLError.
40 Making UIImage macOS compatible
🖥 Here's an easy way to make iOS model code that uses UIImage macOS compatible - like me and Gui Rambo discussed on the Swift by Sundell Podcast.
// Either put this in a separate file that you only include in your macOS target or wrap the code in #if os(macOS) / #endif
import Cocoa
// Step 1: Typealias UIImage to NSImage
typealias UIImage = NSImage
// Step 2: You might want to add these APIs that UIImage has but NSImage doesn't.
extension NSImage {
var cgImage: CGImage? {
var proposedRect = CGRect(origin: .zero, size: size)
return cgImage(forProposedRect: &proposedRect,
context: nil,
hints: nil)
}
convenience init?(named name: String) {
self.init(named: Name(name))
}
}
// Step 3: Profit - you can now make your model code that uses UIImage cross-platform!
struct User {
let name: String
let profileImage: UIImage
}
39 Internally mutable protocol-oriented APIs
🤖 You can easily define a protocol-oriented API that can only be mutated internally, by using an internal protocol that extends a public one.
// Declare a public protocol that acts as your immutable API
public protocol ModelHolder {
associatedtype Model
var model: Model { get }
}
// Declare an extended, internal protocol that provides a mutable API
internal protocol MutableModelHolder: ModelHolder {
var model: Model { get set }
}
// You can now implement the requirements using 'public internal(set)'
public class UserHolder: MutableModelHolder {
public internal(set) var model: User
internal init(model: User) {
self.model = model
}
}
38 Switching on a set
🎛 You can switch on a set using array literals as cases in Swift! Can be really useful to avoid many if/else if statements.
class RoadTile: Tile {
var connectedDirections = Set<Direction>()
func render() {
switch connectedDirections {
case [.up, .down]:
image = UIImage(named: "road-vertical")
case [.left, .right]:
image = UIImage(named: "road-horizontal")
default:
image = UIImage(named: "road")
}
}
}
37 Adding the current locale to cache keys
🌍 When caching localized content in an app, it's a good idea to add the current locale to all keys, to prevent bugs when switching languages.
func cache(_ content: Content, forKey key: String) throws {
let data = try wrap(content) as Data
let key = localize(key: key)
try storage.store(data, forKey: key)
}
func loadCachedContent(forKey key: String) -> Content? {
let key = localize(key: key)
let data = storage.loadData(forKey: key)
return data.flatMap { try? unbox(data: $0) }
}
private func localize(key: String) -> String {
return key + "-" + Bundle.main.preferredLocalizations[0]
}
36 Setting up tests to avoid retain cycles with weak references
🚳 Here's an easy way to setup a test to avoid accidental retain cycles with object relationships (like weak delegates & observers) in Swift:
func testDelegateNotRetained() {
// Assign the delegate (weak) and also retain it using a local var
var delegate: Delegate? = DelegateMock()
controller.delegate = delegate
XCTAssertNotNil(controller.delegate)
// Release the local var, which should also release the weak reference
delegate = nil
XCTAssertNil(controller.delegate)
}
35 Expressively matching a value against a list of candidates
👨🔬 Playing around with an expressive way to check if a value matches any of a list of candidates in Swift:
// Instead of multiple conditions like this:
if string == "One" || string == "Two" || string == "Three" {
}
// You can now do:
if string == any(of: "One", "Two", "Three") {
}
You can find a gist with the implementation here.
34 Organizing code using extensions
👪 APIs in a Swift extension automatically inherit its access control level, making it a neat way to organize public, internal & private APIs.
public extension Animation {
init(textureNamed textureName: String) {
frames = [Texture(name: textureName)]
}
init(texturesNamed textureNames: [String], frameDuration: TimeInterval = 1) {
frames = textureNames.map(Texture.init)
self.frameDuration = frameDuration
}
init(image: Image) {
frames = [Texture(image: image)]
}
}
internal extension Animation {
func loadFrameImages() -> [Image] {
return frames.map { $0.loadImageIfNeeded() }
}
}
33 Using map to transform an optional into a Result type
🗺 Using map you can transform an optional value into an optional Result type by simply passing in the enum case.
enum Result<Value> {
case value(Value)
case error(Error)
}
class Promise<Value> {
private var result: Result<Value>?
init(value: Value? = nil) {
result = value.map(Result.value)
}
}
32 Assigning to self in struct initializers
👌 It's so nice that you can assign directly to self in struct initializers in Swift. Very useful when adding conformance to protocols.
extension Bool: AnswerConvertible {
public init(input: String) throws {
switch input.lowercased() {
case "y", "yes", "👍":
self = true
default:
self = false
}
}
}
31 Recursively calling closures as inline functions
☎️ Defining Swift closures as inline functions enables you to recursively call them, which is super useful in things like custom sequences.
class Database {
func records(matching query: Query) -> AnySequence<Record> {
var recordIterator = loadRecords().makeIterator()
func iterate() -> Record? {
guard let nextRecord = recordIterator.next() else {
return nil
}
guard nextRecord.matches(query) else {
// Since the closure is an inline function, it can be recursively called,
// in this case in order to advance to the next item.
return iterate()
}
return nextRecord
}
// AnySequence/AnyIterator are part of the standard library and provide an easy way
// to define custom sequences using closures.
return AnySequence { AnyIterator(iterate) }
}
}
Rob Napier points out that using the above might cause crashes if used on a large databaset, since Swift has no guaranteed Tail Call Optimization (TCO).
Slava Pestov also points out that another benefit of inline functions vs closures is that they can have their own generic parameter list.
30 Passing self to required Objective-C dependencies
🏖 Using lazy properties in Swift, you can pass self to required Objective-C dependencies without having to use force-unwrapped optionals.
class DataLoader: NSObject {
lazy var urlSession: URLSession = self.makeURLSession()
private func makeURLSession() -> URLSession {
return URLSession(configuration: .default, delegate: self, delegateQueue: .main)
}
}
class Renderer {
lazy var displayLink: CADisplayLink = self.makeDisplayLink()
private func makeDisplayLink() -> CADisplayLink {
return CADisplayLink(target: self, selector: #selector(screenDidRefresh))
}
}
29 Making weak or lazy properties readonly
👓 If you have a property in Swift that needs to be weak or lazy, you can still make it readonly by using private(set).
class Node {
private(set) weak var parent: Node?
private(set) lazy var children = [Node]()
func add(child: Node) {
children.append(child)
child.parent = self
}
}
28 Defining static URLs using string literals
🌏 Tired of using URL(string: "url")! for static URLs? Make URL conform to ExpressibleByStringLiteral and you can now simply use "url" instead.
extension URL: ExpressibleByStringLiteral {
// By using 'StaticString' we disable string interpolation, for safety
public init(stringLiteral value: StaticString) {
self = URL(string: "\(value)").require(hint: "Invalid URL string literal: \(value)")
}
}
// We can now define URLs using static string literals 🎉
let url: URL = "https://www.swiftbysundell.com"
let task = URLSession.shared.dataTask(with: "https://www.swiftbysundell.com")
// In Swift 3 or earlier, you also have to implement 2 additional initializers
extension URL {
public init(extendedGraphemeClusterLiteral value: StaticString) {
self.init(stringLiteral: value)
}
public init(unicodeScalarLiteral value: StaticString) {
self.init(stringLiteral: value)
}
}
To find the extension that adds the require() method on Optional that I use above, check out Require.
27 Manipulating points, sizes and frames using math operators
✚ I'm always careful with operator overloading, but for manipulating things like sizes, points & frames I find them super useful.
extension CGSize {
static func *(lhs: CGSize, rhs: CGFloat) -> CGSize {
return CGSize(width: lhs.width * rhs, height: lhs.height * rhs)
}
}
button.frame.size = image.size * 2
If you like the above idea, check out CGOperators, which contains math operator overloads for all Core Graphics' vector types.
26 Using closure types in generic constraints
🔗 You can use closure types in generic constraints in Swift. Enables nice APIs for handling sequences of closures.
extension Sequence where Element == () -> Void {
func callAll() {
forEach { $0() }
}
}
extension Sequence where Element == () -> String {
func joinedResults(separator: String) -> String {
return map { $0() }.joined(separator: separator)
}
}
callbacks.callAll()
let names = nameProviders.joinedResults(separator: ", ")
(If you're using Swift 3, you have to change Element to Iterator.Element)
25 Using associated enum values to avoid state-specific optionals
🎉 Using associated enum values is a super nice way to encapsulate mutually exclusive state info (and avoiding state-specific optionals).
// BEFORE: Lots of state-specific, optional properties
class Player {
var isWaitingForMatchMaking: Bool
var invitingUser: User?
var numberOfLives: Int
var playerDefeatedBy: Player?
var roundDefeatedIn: Int?
}
// AFTER: All state-specific information is encapsulated in enum cases
class Player {
enum State {
case waitingForMatchMaking
case waitingForInviteResponse(from: User)
case active(numberOfLives: Int)
case defeated(by: Player, roundNumber: Int)
}
var state: State
}
24 Using enums for async result types
👍 I really like using enums for all async result types, even boolean ones. Self-documenting, and makes the call site a lot nicer to read too!
protocol PushNotificationService {
// Before
func enablePushNotifications(completionHandler: @escaping (Bool) -> Void)
// After
func enablePushNotifications(completionHandler: @escaping (PushNotificationStatus) -> Void)
}
enum PushNotificationStatus {
case enabled
case disabled
}
service.enablePushNotifications { status in
if status == .enabled {
enableNotificationsButton.removeFromSuperview()
}
}
23 Working on async code in a playground
🏃 Want to work on your async code in a Swift Playground? Just set needsIndefiniteExecution to true to keep it running:
import PlaygroundSupport
PlaygroundPage.current.needsIndefiniteExecution = true
DispatchQueue.main.asyncAfter(deadline: .now() + 3) {
let greeting = "Hello after 3 seconds"
print(greeting)
}
To stop the playground from executing, simply call PlaygroundPage.current.finishExecution().
22 Overriding self with a weak reference
💦 Avoid memory leaks when accidentially refering to self in closures by overriding it locally with a weak reference:
Swift >= 4.2
dataLoader.loadData(from: url) { [weak self] result in
guard let self = self else {
return
}
self.cache(result)
...
Swift < 4.2
dataLoader.loadData(from: url) { [weak self] result in
guard let `self` = self else {
return
}
self.cache(result)
...
Note that the reason the above currently works is because of a compiler bug (which I hope gets turned into a properly supported feature soon).
21 Using DispatchWorkItem
🕓 Using dispatch work items you can easily cancel a delayed asynchronous GCD task if you no longer need it:
let workItem = DispatchWorkItem {
// Your async code goes in here
}
// Execute the work item after 1 second
DispatchQueue.main.asyncAfter(deadline: .now() + 1, execute: workItem)
// You can cancel the work item if you no longer need it
workItem.cancel()
20 Combining a sequence of functions
➕ While working on a new Swift developer tool (to be open sourced soon 😉), I came up with a pretty neat way of organizing its sequence of operations, by combining their functions into a closure:
internal func +<A, B, C>(lhs: @escaping (A) throws -> B,
rhs: @escaping (B) throws -> C) -> (A) throws -> C {
return { try rhs(lhs($0)) }
}
public func run() throws {
try (determineTarget + build + analyze + output)()
}
If you're familiar with the functional programming world, you might know the above technique as the pipe operator (thanks to Alexey Demedreckiy for pointing this out!)
19 Chaining optionals with map() and flatMap()
🗺 Using map() and flatMap() on optionals you can chain multiple operations without having to use lengthy if lets or guards:
// BEFORE
guard let string = argument(at: 1) else {
return
}
guard let url = URL(string: string) else {
return
}
handle(url)
// AFTER
argument(at: 1).flatMap(URL.init).map(handle)
18 Using self-executing closures for lazy properties
🚀 Using self-executing closures is a great way to encapsulate lazy property initialization:
class StoreViewController: UIViewController {
private lazy var collectionView: UICollectionView = {
let layout = UICollectionViewFlowLayout()
let view = UICollectionView(frame: self.view.bounds, collectionViewLayout: layout)
view.delegate = self
view.dataSource = self
return view
}()
override func viewDidLoad() {
super.viewDidLoad()
view.addSubview(collectionView)
}
}
17 Speeding up Swift package tests
⚡️ You can speed up your Swift package tests using the --parallel flag. For Marathon, the tests execute 3 times faster that way!
swift test --parallel
16 Avoiding mocking UserDefaults
🛠 Struggling with mocking UserDefaults in a test? The good news is: you don't need mocking - just create a real instance:
class LoginTests: XCTestCase {
private var userDefaults: UserDefaults!
private var manager: LoginManager!
override func setUp() {
super.setup()
userDefaults = UserDefaults(suiteName: #file)
userDefaults.removePersistentDomain(forName: #file)
manager = LoginManager(userDefaults: userDefaults)
}
}
15 Using variadic parameters
👍 Using variadic parameters in Swift, you can create some really nice APIs that take a list of objects without having to use an array:
extension Canvas {
func add(_ shapes: Shape...) {
shapes.forEach(add)
}
}
let circle = Circle(center: CGPoint(x: 5, y: 5), radius: 5)
let lineA = Line(start: .zero, end: CGPoint(x: 10, y: 10))
let lineB = Line(start: CGPoint(x: 0, y: 10), end: CGPoint(x: 10, y: 0))
let canvas = Canvas()
canvas.add(circle, lineA, lineB)
canvas.render()
14 Referring to enum cases with associated values as closures
😮 Just like you can refer to a Swift function as a closure, you can do the same thing with enum cases with associated values:
enum UnboxPath {
case key(String)
case keyPath(String)
}
struct UserSchema {
static let name = key("name")
static let age = key("age")
static let posts = key("posts")
private static let key = UnboxPath.key
}
13 Using the === operator to compare objects by instance
📈 The === operator lets you check if two objects are the same instance. Very useful when verifying that an array contains an instance in a test:
protocol InstanceEquatable: class, Equatable {}
extension InstanceEquatable {
static func ==(lhs: Self, rhs: Self) -> Bool {
return lhs === rhs
}
}
extension Enemy: InstanceEquatable {}
func testDestroyingEnemy() {
player.attack(enemy)
XCTAssertTrue(player.destroyedEnemies.contains(enemy))
}
12 Calling initializers with dot syntax and passing them as closures
😎 Cool thing about Swift initializers: you can call them using dot syntax and pass them as closures! Perfect for mocking dates in tests.
class Logger {
private let storage: LogStorage
private let dateProvider: () -> Date
init(storage: LogStorage = .init(), dateProvider: @escaping () -> Date = Date.init) {
self.storage = storage
self.dateProvider = dateProvider
}
func log(event: Event) {
storage.store(event: event, date: dateProvider())
}
}
11 Structuring UI tests as extensions on XCUIApplication
📱 Most of my UI testing logic is now categories on XCUIApplication. Makes the test cases really easy to read:
func testLoggingInAndOut() {
XCTAssertFalse(app.userIsLoggedIn)
app.launch()
app.login()
XCTAssertTrue(app.userIsLoggedIn)
app.logout()
XCTAssertFalse(app.userIsLoggedIn)
}
func testDisplayingCategories() {
XCTAssertFalse(app.isDisplayingCategories)
app.launch()
app.login()
app.goToCategories()
XCTAssertTrue(app.isDisplayingCategories)
}
10 Avoiding default cases in switch statements
🙂 It’s a good idea to avoid “default” cases when switching on Swift enums - it’ll “force you” to update your logic when a new case is added:
enum State {
case loggedIn
case loggedOut
case onboarding
}
func handle(_ state: State) {
switch state {
case .loggedIn:
showMainUI()
case .loggedOut:
showLoginUI()
// Compiler error: Switch must be exhaustive
}
}
9 Using the guard statement in many different scopes
💂 It's really cool that you can use Swift's 'guard' statement to exit out of pretty much any scope, not only return from functions:
// You can use the 'guard' statement to...
for string in strings {
// ...continue an iteration
guard shouldProcess(string) else {
continue
}
// ...or break it
guard !shouldBreak(for: string) else {
break
}
// ...or return
guard !shouldReturn(for: string) else {
return
}
// ..or throw an error
guard string.isValid else {
throw StringError.invalid(string)
}
// ...or exit the program
guard !shouldExit(for: string) else {
exit(1)
}
}
8 Passing functions & operators as closures
❤️ Love how you can pass functions & operators as closures in Swift. For example, it makes the syntax for sorting arrays really nice!
let array = [3, 9, 1, 4, 6, 2]
let sorted = array.sorted(by: <)
7 Using #function for UserDefaults key consistency
🗝 Here's a neat little trick I use to get UserDefault key consistency in Swift (#function expands to the property name in getters/setters). Just remember to write a good suite of tests that'll guard you against bugs when changing property names.
extension UserDefaults {
var onboardingCompleted: Bool {
get { return bool(forKey: #function) }
set { set(newValue, forKey: #function) }
}
}
6 Using a name already taken by the standard library
📛 Want to use a name already taken by the standard library for a nested type? No problem - just use Swift. to disambiguate:
extension Command {
enum Error: Swift.Error {
case missing
case invalid(String)
}
}
5 Using Wrap to implement Equatable
📦 Playing around with using Wrap to implement Equatable for any type, primarily for testing:
protocol AutoEquatable: Equatable {}
extension AutoEquatable {
static func ==(lhs: Self, rhs: Self) -> Bool {
let lhsData = try! wrap(lhs) as Data
let rhsData = try! wrap(rhs) as Data
return lhsData == rhsData
}
}
4 Using typealiases to reduce the length of method signatures
📏 One thing that I find really useful in Swift is to use typealiases to reduce the length of method signatures in generic types:
public class PathFinder<Object: PathFinderObject> {
public typealias Map = Object.Map
public typealias Node = Map.Node
public typealias Path = PathFinderPath<Object>
public static func possiblePaths(for object: Object, at rootNode: Node, on map: Map) -> Path.Sequence {
return .init(object: object, rootNode: rootNode, map: map)
}
}
3 Referencing either external or internal parameter name when writing docs
📖 You can reference either the external or internal parameter label when writing Swift docs - and they get parsed the same:
// EITHER:
class Foo {
/**
* - parameter string: A string
*/
func bar(with string: String) {}
}
// OR:
class Foo {
/**
* - parameter with: A string
*/
func bar(with string: String) {}
}
2 Using auto closures
👍 Finding more and more uses for auto closures in Swift. Can enable some pretty nice APIs:
extension Dictionary {
mutating func value(for key: Key, orAdd valueClosure: @autoclosure () -> Value) -> Value {
if let value = self[key] {
return value
}
let value = valueClosure()
self[key] = value
return value
}
}
1 Namespacing with nested types
🚀 I’ve started to become a really big fan of nested types in Swift. Love the additional namespacing it gives you!
public struct Map {
public struct Model {
public let size: Size
public let theme: Theme
public var terrain: [Position : Terrain.Model]
public var units: [Position : Unit.Model]
public var buildings: [Position : Building.Model]
}
public enum Direction {
case up
case right
case down
case left
}
public struct Position {
public var x: Int
public var y: Int
}
public enum Size: String {
case small = "S"
case medium = "M"
case large = "L"
case extraLarge = "XL"
}
}Download Details:
Author: JohnSundell
Source Code: https://github.com/JohnSundell/SwiftTips
License: MIT license
1661592007
A Collection Of Swift Tips & Tricks That I've Shared on Twitter
⚠️ This list is no longer being updated. For my latest Swift tips, checkout the "Tips" section on Swift by Sundell.
Swift tips & tricks ⚡️
One of the things I really love about Swift is how I keep finding interesting ways to use it in various situations, and when I do - I usually share them on Twitter. Here's a collection of all the tips & tricks that I've shared so far. Each entry has a link to the original tweet, if you want to respond with some feedback or question, which is always super welcome! 🚀
Also make sure to check out all of my other Swift content:
- 📖 My weekly articles, with a new post about Swift every Sunday!
- 🎧 My podcast which features special guests from around the community.
- 📬 My monthly newsletter - rounding up all of my content, including these tips, on the 1st of every month.
#102 Making async tests faster and more stable
🚀 Here are some quick tips to make async tests faster & more stable:
- 😴 Avoid sleep() - use expectations instead
- ⏱ Use generous timeouts to avoid flakiness on CI
- 🧐 Put all assertions at the end of each test, not inside closures
// BEFORE:
class MentionDetectorTests: XCTestCase {
func testDetectingMention() {
let detector = MentionDetector()
let string = "This test was written by @johnsundell."
detector.detectMentions(in: string) { mentions in
XCTAssertEqual(mentions, ["johnsundell"])
}
sleep(2)
}
}
// AFTER:
class MentionDetectorTests: XCTestCase {
func testDetectingMention() {
let detector = MentionDetector()
let string = "This test was written by @johnsundell."
var mentions: [String]?
let expectation = self.expectation(description: #function)
detector.detectMentions(in: string) {
mentions = $0
expectation.fulfill()
}
waitForExpectations(timeout: 10)
XCTAssertEqual(mentions, ["johnsundell"])
}
}
For more on async testing, check out "Unit testing asynchronous Swift code".
#101 Adding support for Apple Pencil double-taps
✍️ Adding support for the new Apple Pencil double-tap feature is super easy! All you have to do is to create a UIPencilInteraction, add it to a view, and implement one delegate method. Hopefully all pencil-compatible apps will soon adopt this.
let interaction = UIPencilInteraction()
interaction.delegate = self
view.addInteraction(interaction)
extension ViewController: UIPencilInteractionDelegate {
func pencilInteractionDidTap(_ interaction: UIPencilInteraction) {
// Handle pencil double-tap
}
}
For more on using this and other iPad Pro features, check out "Building iPad Pro features in Swift".
#100 Combining values with functions
😎 Here's a cool function that combines a value with a function to return a closure that captures that value, so that it can be called without any arguments. Super useful when working with closure-based APIs and we want to use some of our properties without having to capture self.
func combine<A, B>(_ value: A, with closure: @escaping (A) -> B) -> () -> B {
return { closure(value) }
}
// BEFORE:
class ProductViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
buyButton.handler = { [weak self] in
guard let self = self else {
return
}
self.productManager.startCheckout(for: self.product)
}
}
}
// AFTER:
class ProductViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
buyButton.handler = combine(product, with: productManager.startCheckout)
}
}
#99 Dependency injection using functions
💉 When I'm only using a single function from a dependency, I love to inject that function as a closure, instead of having to create a protocol and inject the whole object. Makes dependency injection & testing super simple.
final class ArticleLoader {
typealias Networking = (Endpoint) -> Future<Data>
private let networking: Networking
init(networking: @escaping Networking = URLSession.shared.load) {
self.networking = networking
}
func loadLatest() -> Future<[Article]> {
return networking(.latestArticles).decode()
}
}
For more on this technique, check out "Simple Swift dependency injection with functions".
#98 Using a custom exception handler
💥 It's cool that you can easily assign a closure as a custom NSException handler. This is super useful when building things in Playgrounds - since you can't use breakpoints - so instead of just signal SIGABRT, you'll get the full exception description if something goes wrong.
NSSetUncaughtExceptionHandler { exception in
print(exception)
}
#97 Using type aliases to give semantic meaning to primitives
❤️ I love that in Swift, we can use the type system to make our code so much more self-documenting - one way of doing so is to use type aliases to give the primitive types that we use a more semantic meaning.
extension List.Item {
// Using type aliases, we can give semantic meaning to the
// primitive types that we use, without having to introduce
// wrapper types.
typealias Index = Int
}
extension List {
enum Mutation {
// Our enum cases now become a lot more self-documenting,
// without having to add additional parameter labels to
// explain them.
case add(Item, Item.Index)
case update(Item, Item.Index)
case remove(Item.Index)
}
}
For more on self-documenting code, check out "Writing self-documenting Swift code".
#96 Specializing protocols using constraints
🤯 A little late night prototyping session reveals that protocol constraints can not only be applied to extensions - they can also be added to protocol definitions!
This is awesome, since it lets us easily define specialized protocols based on more generic ones.
protocol Component {
associatedtype Container
func add(to container: Container)
}
// Protocols that inherit from other protocols can include
// constraints to further specialize them.
protocol ViewComponent: Component where Container == UIView {
associatedtype View: UIView
var view: View { get }
}
extension ViewComponent {
func add(to container: UIView) {
container.addSubview(view)
}
}
For more on specializing protocols, check out "Specializing protocols in Swift".
#95 Unwrapping an optional or throwing an error
📦 Here's a super handy extension on Swift's Optional type, which gives us a really nice API for easily unwrapping an optional, or throwing an error in case the value turned out to be nil:
extension Optional {
func orThrow(_ errorExpression: @autoclosure () -> Error) throws -> Wrapped {
switch self {
case .some(let value):
return value
case .none:
throw errorExpression()
}
}
}
let file = try loadFile(at: path).orThrow(MissingFileError())
For more ways that optionals can be extended, check out "Extending optionals in Swift".
#94 Testing code that uses static APIs
👩🔬 Testing code that uses static APIs can be really tricky, but there's a way that it can often be done - using Swift's first class function capabilities!
Instead of accessing that static API directly, we can inject the function we want to use, which enables us to mock it!
// BEFORE
class FriendsLoader {
func loadFriends(then handler: @escaping (Result<[Friend]>) -> Void) {
Networking.loadData(from: .friends) { result in
...
}
}
}
// AFTER
class FriendsLoader {
typealias Handler<T> = (Result<T>) -> Void
typealias DataLoadingFunction = (Endpoint, @escaping Handler<Data>) -> Void
func loadFriends(using dataLoading: DataLoadingFunction = Networking.loadData,
then handler: @escaping Handler<[Friend]>) {
dataLoading(.friends) { result in
...
}
}
}
// MOCKING IN TESTS
let dataLoading: FriendsLoader.DataLoadingFunction = { _, handler in
handler(.success(mockData))
}
friendsLoader.loadFriends(using: dataLoading) { result in
...
}
#93 Matching multiple enum cases with associated values
🐾 Swift's pattern matching capabilities are so powerful! Two enum cases with associated values can even be matched and handled by the same switch case - which is super useful when handling state changes with similar data.
enum DownloadState {
case inProgress(progress: Double)
case paused(progress: Double)
case cancelled
case finished(Data)
}
func downloadStateDidChange(to state: DownloadState) {
switch state {
case .inProgress(let progress), .paused(let progress):
updateProgressView(with: progress)
case .cancelled:
showCancelledMessage()
case .finished(let data):
process(data)
}
}
#92 Multiline string literals
🅰 One really nice benefit of Swift multiline string literals - even for single lines of text - is that they don't require quotes to be escaped. Perfect when working with things like HTML, or creating a custom description for an object.
let html = highlighter.highlight("Array<String>")
XCTAssertEqual(html, """
<span class="type">Array</span><<span class="type">String</span>>
""")
#91 Reducing sequences
💎 While it's very common in functional programming, the reduce function might be a bit of a hidden gem in Swift. It provides a super useful way to transform a sequence into a single value.
extension Sequence where Element: Equatable {
func numberOfOccurrences(of target: Element) -> Int {
return reduce(0) { result, element in
guard element == target else {
return result
}
return result + 1
}
}
}
You can read more about transforming collections in "Transforming collections in Swift".
#90 Avoiding manual Codable implementations
📦 When I use Codable in Swift, I want to avoid manual implementations as much as possible, even when there's a mismatch between my code structure and the JSON I'm decoding.
One way that can often be achieved is to use private data containers combined with computed properties.
struct User: Codable {
let name: String
let age: Int
var homeTown: String { return originPlace.name }
private let originPlace: Place
}
private extension User {
struct Place: Codable {
let name: String
}
}
extension User {
struct Container: Codable {
let user: User
}
}
#89 Using feature flags instead of feature branches
🚢 Instead of using feature branches, I merge almost all of my code directly into master - and then I use feature flags to conditionally enable features when they're ready. That way I can avoid merge conflicts and keep shipping!
extension ListViewController {
func addSearchIfNeeded() {
// Rather than having to keep maintaining a separate
// feature branch for a new feature, we can use a flag
// to conditionally turn it on.
guard FeatureFlags.searchEnabled else {
return
}
let resultsVC = SearchResultsViewController()
let searchVC = UISearchController(
searchResultsController: resultsVC
)
searchVC.searchResultsUpdater = resultsVC
navigationItem.searchController = searchVC
}
}
You can read more about feature flags in "Feature flags in Swift".
#88 Lightweight data hierarchies using tuples
💾 Here I'm using tuples to create a lightweight hierarchy for my data, giving me a nice structure without having to introduce any additional types.
struct CodeSegment {
var tokens: (
previous: String?,
current: String
)
var delimiters: (
previous: Character?
next: Character?
)
}
handle(segment.tokens.current)
You can read more about tuples in "Using tuples as lightweight types in Swift"
#87 The rule of threes
3️⃣ Whenever I have 3 properties or local variables that share the same prefix, I usually try to extract them into their own method or type. That way I can avoid massive types & methods, and also increase readability, without falling into a "premature optimization" trap.
Before
public func generate() throws {
let contentFolder = try folder.subfolder(named: "content")
let articleFolder = try contentFolder.subfolder(named: "posts")
let articleProcessor = ContentProcessor(folder: articleFolder)
let articles = try articleProcessor.process()
...
}
After
public func generate() throws {
let contentFolder = try folder.subfolder(named: "content")
let articles = try processArticles(in: contentFolder)
...
}
private func processArticles(in folder: Folder) throws -> [ContentItem] {
let folder = try folder.subfolder(named: "posts")
let processor = ContentProcessor(folder: folder)
return try processor.process()
}
#86 Useful Codable extensions
👨🔧 Here's two extensions that I always add to the Encodable & Decodable protocols, which for me really make the Codable API nicer to use. By using type inference for decoding, a lot of boilerplate can be removed when the compiler is already able to infer the resulting type.
extension Encodable {
func encoded() throws -> Data {
return try JSONEncoder().encode(self)
}
}
extension Data {
func decoded<T: Decodable>() throws -> T {
return try JSONDecoder().decode(T.self, from: self)
}
}
let data = try user.encoded()
// By using a generic type in the decoded() method, the
// compiler can often infer the type we want to decode
// from the current context.
try userDidLogin(data.decoded())
// And if not, we can always supply the type, still making
// the call site read very nicely.
let otherUser = try data.decoded() as User
#85 Using shared UserDefaults suites
📦 UserDefaults is a lot more powerful than what it first might seem like. Not only can it store more complex values (like dates & dictionaries) and parse command line arguments - it also enables easy sharing of settings & lightweight data between apps in the same App Group.
let sharedDefaults = UserDefaults(suiteName: "my-app-group")!
let useDarkMode = sharedDefaults.bool(forKey: "dark-mode")
// This value is put into the shared suite.
sharedDefaults.set(true, forKey: "dark-mode")
// If you want to treat the shared settings as read-only (and add
// local overrides on top of them), you can simply add the shared
// suite to the standard UserDefaults.
let combinedDefaults = UserDefaults.standard
combinedDefaults.addSuite(named: "my-app-group")
// This value is a local override, not added to the shared suite.
combinedDefaults.set(true, forKey: "app-specific-override")
#84 Custom UIView backing layers
🎨 By overriding layerClass you can tell UIKit what CALayer class to use for a UIView's backing layer. That way you can reduce the amount of layers, and don't have to do any manual layout.
final class GradientView: UIView {
override class var layerClass: AnyClass { return CAGradientLayer.self }
var colors: (start: UIColor, end: UIColor)? {
didSet { updateLayer() }
}
private func updateLayer() {
let layer = self.layer as! CAGradientLayer
layer.colors = colors.map { [$0.start.cgColor, $0.end.cgColor] }
}
}
#83 Auto-Equatable enums with associated values
✅ That the compiler now automatically synthesizes Equatable conformances is such a huge upgrade for Swift! And the cool thing is that it works for all kinds of types - even for enums with associated values! Especially useful when using enums for verification in unit tests.
struct Article: Equatable {
let title: String
let text: String
}
struct User: Equatable {
let name: String
let age: Int
}
extension Navigator {
enum Destination: Equatable {
case profile(User)
case article(Article)
}
}
func testNavigatingToArticle() {
let article = Article(title: "Title", text: "Text")
controller.select(article)
XCTAssertEqual(navigator.destinations, [.article(article)])
}
#82 Defaults for associated types
🤝 Associated types can have defaults in Swift - which is super useful for types that are not easily inferred (for example when they're not used for a specific instance method or property).
protocol Identifiable {
associatedtype RawIdentifier: Codable = String
var id: Identifier<Self> { get }
}
struct User: Identifiable {
let id: Identifier<User>
let name: String
}
struct Group: Identifiable {
typealias RawIdentifier = Int
let id: Identifier<Group>
let name: String
}
#81 Creating a dedicated identifier type
🆔 If you want to avoid using plain strings as identifiers (which can increase both type safety & readability), it's really easy to create a custom Identifier type that feels just like a native Swift type, thanks to protocols!
More on this topic in "Type-safe identifiers in Swift".
struct Identifier: Hashable {
let string: String
}
extension Identifier: ExpressibleByStringLiteral {
init(stringLiteral value: String) {
string = value
}
}
extension Identifier: CustomStringConvertible {
var description: String {
return string
}
}
extension Identifier: Codable {
init(from decoder: Decoder) throws {
let container = try decoder.singleValueContainer()
string = try container.decode(String.self)
}
func encode(to encoder: Encoder) throws {
var container = encoder.singleValueContainer()
try container.encode(string)
}
}
struct Article: Codable {
let id: Identifier
let title: String
}
let article = Article(id: "my-article", title: "Hello world!")
#80 Assigning optional tuple members to variables
🙌 A really cool thing about using tuples to model the internal state of a Swift type, is that you can unwrap an optional tuple's members directly into local variables.
Very useful in order to group multiple optional values together for easy unwrapping & handling.
class ImageTransformer {
private var queue = [(image: UIImage, transform: Transform)]()
private func processNext() {
// When unwrapping an optional tuple, you can assign the members
// directly to local variables.
guard let (image, transform) = queue.first else {
return
}
let context = Context()
context.draw(image)
context.apply(transform)
...
}
}
#79 Struct convenience initializers
❤️ I love to structure my code using extensions in Swift. One big benefit of doing so when it comes to struct initializers, is that defining a convenience initializer doesn't remove the default one the compiler generates - best of both worlds!
struct Article {
let date: Date
var title: String
var text: String
var comments: [Comment]
}
extension Article {
init(title: String, text: String) {
self.init(date: Date(), title: title, text: text, comments: [])
}
}
let articleA = Article(title: "Best Cupcake Recipe", text: "...")
let articleB = Article(
date: Date(),
title: "Best Cupcake Recipe",
text: "...",
comments: [
Comment(user: currentUser, text: "Yep, can confirm!")
]
)
#78 Usages of throwing functions
🏈 A big benefit of using throwing functions for synchronous Swift APIs is that the caller can decide whether they want to treat the return value as optional (try?) or required (try).
func loadFile(named name: String) throws -> File {
guard let url = urlForFile(named: name) else {
throw File.Error.missing
}
do {
let data = try Data(contentsOf: url)
return File(url: url, data: data)
} catch {
throw File.Error.invalidData(error)
}
}
let requiredFile = try loadFile(named: "AppConfig.json")
let optionalFile = try? loadFile(named: "UserSettings.json")
#77 Nested generic types
🐝 Types that are nested in generics automatically inherit their parent's generic types - which is super useful when defining accessory types (for things like states or outcomes).
struct Task<Input, Output> {
typealias Closure = (Input) throws -> Output
let closure: Closure
}
extension Task {
enum Result {
case success(Output)
case failure(Error)
}
}
#76 Equatable & Hashable structures
🤖 Now that the Swift compiler automatically synthesizes Equatable & Hashable conformances for value types, it's easier than ever to setup model structures with nested types that are all Equatable/Hashable!
typealias Value = Hashable & Codable
struct User: Value {
var name: String
var age: Int
var lastLoginDate: Date?
var settings: Settings
}
extension User {
struct Settings: Value {
var itemsPerPage: Int
var theme: Theme
}
}
extension User.Settings {
enum Theme: String, Value {
case light
case dark
}
}
You can read more about using nested types in Swift here.
#75 Conditional conformances
🎉 Swift 4.1 is here! One of the key features it brings is conditional conformances, which lets you have a type only conform to a protocol under certain constraints.
protocol UnboxTransformable {
associatedtype RawValue
static func transform(_ value: RawValue) throws -> Self?
}
extension Array: UnboxTransformable where Element: UnboxTransformable {
typealias RawValue = [Element.RawValue]
static func transform(_ value: RawValue) throws -> [Element]? {
return try value.compactMap(Element.transform)
}
}
I also have an article with lots of more info on conditional conformances here. Paul Hudson also has a great overview of all Swift 4.1 features here.
#74 Generic type aliases
🕵️♀️ A cool thing about Swift type aliases is that they can be generic! Combine that with tuples and you can easily define simple generic types.
typealias Pair<T> = (T, T)
extension Game {
func calculateScore(for players: Pair<Player>) -> Int {
...
}
}
You can read more about using tuples as lightweight types here.
#73 Parsing command line arguments using UserDefaults
☑️ A really cool "hidden" feature of UserDefaults is that it contains any arguments that were passed to the app at launch!
Super useful both in Swift command line tools & scripts, but also to temporarily override a value when debugging iOS apps.
let defaults = UserDefaults.standard
let query = defaults.string(forKey: "query")
let resultCount = defaults.integer(forKey: "results")
#72 Using the & operator
👏 Swift's & operator is awesome! Not only can you use it to compose protocols, you can compose other types too! Very useful if you want to hide concrete types & implementation details.
protocol LoadableFromURL {
func load(from url: URL)
}
class ContentViewController: UIViewController, LoadableFromURL {
func load(from url: URL) {
...
}
}
class ViewControllerFactory {
func makeContentViewController() -> UIViewController & LoadableFromURL {
return ContentViewController()
}
}
#71 Capturing multiple values in mocks
🤗 When capturing values in mocks, using an array (instead of just a single value) makes it easy to verify that only a certain number of values were passed.
Perfect for protecting against "over-calling" something.
class UserManagerTests: XCTestCase {
func testObserversCalledWhenUserFirstLogsIn() {
let manager = UserManager()
let observer = ObserverMock()
manager.addObserver(observer)
// First login, observers should be notified
let user = User(id: 123, name: "John")
manager.userDidLogin(user)
XCTAssertEqual(observer.users, [user])
// If the same user logs in again, observers shouldn't be notified
manager.userDidLogin(user)
XCTAssertEqual(observer.users, [user])
}
}
private extension UserManagerTests {
class ObserverMock: UserManagerObserver {
private(set) var users = [User]()
func userDidChange(to user: User) {
users.append(user)
}
}
}
#70 Reducing the need for mocks
👋 When writing tests, you don't always need to create mocks - you can create stubs using real instances of things like errors, URLs & UserDefaults.
Here's how to do that for some common tasks/object types in Swift:
// Create errors using NSError (#function can be used to reference the name of the test)
let error = NSError(domain: #function, code: 1, userInfo: nil)
// Create non-optional URLs using file paths
let url = URL(fileURLWithPath: "Some/URL")
// Reference the test bundle using Bundle(for:)
let bundle = Bundle(for: type(of: self))
// Create an explicit UserDefaults object (instead of having to use a mock)
let userDefaults = UserDefaults(suiteName: #function)
// Create queues to control/await concurrent operations
let queue = DispatchQueue(label: #function)
For when you actually do need mocking, check out "Mocking in Swift".
#69 Using "then" as an external parameter label for closures
⏱ I've started using "then" as an external parameter label for completion handlers. Makes the call site read really nicely (Because I do ❤️ conversational API design) regardless of whether trailing closure syntax is used or not.
protocol DataLoader {
// Adding type aliases to protocols can be a great way to
// reduce verbosity for parameter types.
typealias Handler = (Result<Data>) -> Void
associatedtype Endpoint
func loadData(from endpoint: Endpoint, then handler: @escaping Handler)
}
loader.loadData(from: .messages) { result in
...
}
loader.loadData(from: .messages, then: { result in
...
})
#68 Combining lazily evaluated sequences with the builder pattern
😴 Combining lazily evaluated sequences with builder pattern-like properties can lead to some pretty sweet APIs for configurable sequences in Swift.
Also useful for queries & other things you "build up" and then execute.
// Extension adding builder pattern-like properties that return
// a new sequence value with the given configuration applied
extension FileSequence {
var recursive: FileSequence {
var sequence = self
sequence.isRecursive = true
return sequence
}
var includingHidden: FileSequence {
var sequence = self
sequence.includeHidden = true
return sequence
}
}
// BEFORE
let files = folder.makeFileSequence(recursive: true, includeHidden: true)
// AFTER
let files = folder.files.recursive.includingHidden
Want an intro to lazy sequences? Check out "Swift sequences: The art of being lazy".
#67 Faster & more stable UI tests
My top 3 tips for faster & more stable UI tests:
📱 Reset the app's state at the beginning of every test.
🆔 Use accessibility identifiers instead of UI strings.
⏱ Use expectations instead of waiting time.
func testOpeningArticle() {
// Launch the app with an argument that tells it to reset its state
let app = XCUIApplication()
app.launchArguments.append("--uitesting")
app.launch()
// Check that the app is displaying an activity indicator
let activityIndicator = app.activityIndicator.element
XCTAssertTrue(activityIndicator.exists)
// Wait for the loading indicator to disappear = content is ready
expectation(for: NSPredicate(format: "exists == 0"),
evaluatedWith: activityIndicator)
// Use a generous timeout in case the network is slow
waitForExpectations(timeout: 10)
// Tap the cell for the first article
app.tables.cells["Article.0"].tap()
// Assert that a label with the accessibility identifier "Article.Title" exists
let label = app.staticTexts["Article.Title"]
XCTAssertTrue(label.exists)
}
#66 Accessing the clipboard from a Swift script
📋 It's super easy to access the contents of the clipboard from a Swift script. A big benefit of Swift scripting is being able to use Cocoa's powerful APIs for Mac apps.
import Cocoa
let clipboard = NSPasteboard.general.string(forType: .string)
#65 Using tuples for view state
🎯 Using Swift tuples for view state can be a super nice way to group multiple properties together and render them reactively using the layout system.
By using a tuple we don't have to either introduce a new type or make our view model-aware.
class TextView: UIView {
var state: (title: String?, text: String?) {
// By telling UIKit that our view needs layout and binding our
// state in layoutSubviews, we can react to state changes without
// doing unnecessary layout work.
didSet { setNeedsLayout() }
}
private let titleLabel = UILabel()
private let textLabel = UILabel()
override func layoutSubviews() {
super.layoutSubviews()
titleLabel.text = state.title
textLabel.text = state.text
...
}
}
#64 Throwing tests and LocalizedError
⚾️ Swift tests can throw, which is super useful in order to avoid complicated logic or force unwrapping. By making errors conform to LocalizedError, you can also get a nice error message in Xcode if there's a failure.
class ImageCacheTests: XCTestCase {
func testCachingAndLoadingImage() throws {
let bundle = Bundle(for: type(of: self))
let cache = ImageCache(bundle: bundle)
// Bonus tip: You can easily load images from your test
// bundle using this UIImage initializer
let image = try require(UIImage(named: "sample", in: bundle, compatibleWith: nil))
try cache.cache(image, forKey: "key")
let cachedImage = try cache.image(forKey: "key")
XCTAssertEqual(image, cachedImage)
}
}
enum ImageCacheError {
case emptyKey
case dataConversionFailed
}
// When using throwing tests, making your errors conform to
// LocalizedError will render a much nicer error message in
// Xcode (per default only the error code is shown).
extension ImageCacheError: LocalizedError {
var errorDescription: String? {
switch self {
case .emptyKey:
return "An empty key was given"
case .dataConversionFailed:
return "Failed to convert the given image to Data"
}
}
}
For more information, and the implementation of the require method used above, check out "Avoiding force unwrapping in Swift unit tests".
#63 The difference between static and class properties
✍️ Unlike static properties, class properties can be overridden by subclasses (however, they can't be stored, only computed).
class TableViewCell: UITableViewCell {
class var preferredHeight: CGFloat { return 60 }
}
class TallTableViewCell: TableViewCell {
override class var preferredHeight: CGFloat { return 100 }
}
#62 Creating extensions with static factory methods
👨🎨 Creating extensions with static factory methods can be a great alternative to subclassing in Swift, especially for things like setting up UIViews, CALayers or other kinds of styling.
It also lets you remove a lot of styling & setup from your view controllers.
extension UILabel {
static func makeForTitle() -> UILabel {
let label = UILabel()
label.font = .boldSystemFont(ofSize: 24)
label.textColor = .darkGray
label.adjustsFontSizeToFitWidth = true
label.minimumScaleFactor = 0.75
return label
}
static func makeForText() -> UILabel {
let label = UILabel()
label.font = .systemFont(ofSize: 16)
label.textColor = .black
label.numberOfLines = 0
return label
}
}
class ArticleViewController: UIViewController {
lazy var titleLabel = UILabel.makeForTitle()
lazy var textLabel = UILabel.makeForText()
}
#61 Child view controller auto-resizing
🧒 An awesome thing about child view controllers is that they're automatically resized to match their parent, making them a super nice solution for things like loading & error views.
class ListViewController: UIViewController {
func loadItems() {
let loadingViewController = LoadingViewController()
add(loadingViewController)
dataLoader.loadItems { [weak self] result in
loadingViewController.remove()
self?.handle(result)
}
}
}
For more about child view controller (including the add and remove methods used above), check out "Using child view controllers as plugins in Swift".
#60 Using zip
🤐 Using the zip function in Swift you can easily combine two sequences. Super useful when using two sequences to do some work, since zip takes care of all the bounds-checking.
func render(titles: [String]) {
for (label, text) in zip(titleLabels, titles) {
print(text)
label.text = text
}
}
#59 Defining custom option sets
🎛 The awesome thing about option sets in Swift is that they can automatically either be passed as a single member or as a set. Even cooler is that you can easily define your own option sets as well, perfect for options and other non-exclusive values.
// Option sets are awesome, because you can easily pass them
// both using dot syntax and array literal syntax, like when
// using the UIView animation API:
UIView.animate(withDuration: 0.3,
delay: 0,
options: .allowUserInteraction,
animations: animations)
UIView.animate(withDuration: 0.3,
delay: 0,
options: [.allowUserInteraction, .layoutSubviews],
animations: animations)
// The cool thing is that you can easily define your own option
// sets as well, by defining a struct that has an Int rawValue,
// that will be used as a bit mask.
extension Cache {
struct Options: OptionSet {
static let saveToDisk = Options(rawValue: 1)
static let clearOnMemoryWarning = Options(rawValue: 1 << 1)
static let clearDaily = Options(rawValue: 1 << 2)
let rawValue: Int
}
}
// We can now use Cache.Options just like UIViewAnimationOptions:
Cache(options: .saveToDisk)
Cache(options: [.saveToDisk, .clearDaily])
#58 Using the where clause with associated types
🙌 Using the where clause when designing protocol-oriented APIs in Swift can let your implementations (or others' if it's open source) have a lot more freedom, especially when it comes to collections.
See "Using generic type constraints in Swift 4" for more info.
public protocol PathFinderMap {
associatedtype Node
// Using the 'where' clause for associated types, we can
// ensure that a type meets certain requirements (in this
// case that it's a sequence with Node elements).
associatedtype NodeSequence: Sequence where NodeSequence.Element == Node
// Instead of using a concrete type (like [Node]) here, we
// give implementors of this protocol more freedom while
// still meeting our requirements. For example, one
// implementation might use Set<Node>.
func neighbors(of node: Node) -> NodeSequence
}
#57 Using first class functions when iterating over a dictionary
👨🍳 Combine first class functions in Swift with the fact that Dictionary elements are (Key, Value) tuples and you can build yourself some pretty awesome functional chains when iterating over a Dictionary.
func makeActor(at coordinate: Coordinate, for building: Building) -> Actor {
let actor = Actor()
actor.position = coordinate.point
actor.animation = building.animation
return actor
}
func render(_ buildings: [Coordinate : Building]) {
buildings.map(makeActor).forEach(add)
}
#56 Calling instance methods as static functions
😎 In Swift, you can call any instance method as a static function and it will return a closure representing that method. This is how running tests using SPM on Linux works.
More about this topic in my blog post "First class functions in Swift".
// This produces a '() -> Void' closure which is a reference to the
// given view's 'removeFromSuperview' method.
let closure = UIView.removeFromSuperview(view)
// We can now call it just like we would any other closure, and it
// will run 'view.removeFromSuperview()'
closure()
// This is how running tests using the Swift Package Manager on Linux
// works, you return your test functions as closures:
extension UserManagerTests {
static var allTests = [
("testLoggingIn", testLoggingIn),
("testLoggingOut", testLoggingOut),
("testUserPermissions", testUserPermissions)
]
}
#55 Dropping suffixes from method names to support multiple arguments
👏 One really nice benefit of dropping suffixes from method names (and just using verbs, when possible) is that it becomes super easy to support both single and multiple arguments, and it works really well semantically.
extension UIView {
func add(_ subviews: UIView...) {
subviews.forEach(addSubview)
}
}
view.add(button)
view.add(label)
// By dropping the "Subview" suffix from the method name, both
// single and multiple arguments work really well semantically.
view.add(button, label)
#54 Constraining protocols to classes to ensure mutability
👽 Using the AnyObject (or class) constraint on protocols is not only useful when defining delegates (or other weak references), but also when you always want instances to be mutable without copying.
// By constraining a protocol with 'AnyObject' it can only be adopted
// by classes, which means all instances will always be mutable, and
// that it's the original instance (not a copy) that will be mutated.
protocol DataContainer: AnyObject {
var data: Data? { get set }
}
class UserSettingsManager {
private var settings: Settings
private let dataContainer: DataContainer
// Since DataContainer is a protocol, we an easily mock it in
// tests if we use dependency injection
init(settings: Settings, dataContainer: DataContainer) {
self.settings = settings
self.dataContainer = dataContainer
}
func saveSettings() throws {
let data = try settings.serialize()
// We can now assign properties on an instance of our protocol
// because the compiler knows it's always going to be a class
dataContainer.data = data
}
}
#53 String-based enums in string interpolation
🍣 Even if you define a custom raw value for a string-based enum in Swift, the full case name will be used in string interpolation.
Super useful when using separate raw values for JSON, while still wanting to use the full case name in other contexts.
extension Building {
// This enum has custom raw values that are used when decoding
// a value, for example from JSON.
enum Kind: String {
case castle = "C"
case town = "T"
case barracks = "B"
case goldMine = "G"
case camp = "CA"
case blacksmith = "BL"
}
var animation: Animation {
return Animation(
// When used in string interpolation, the full case name is still used.
// For 'castle' this will be 'buildings/castle'.
name: "buildings/\(kind)",
frameCount: frameCount,
frameDuration: frameDuration
)
}
}
#52 Expressively comparing a value with a list of candidates
👨🔬 Continuing to experiment with expressive ways of comparing a value with a list of candidates in Swift. Adding an extension on Equatable is probably my favorite approach so far.
extension Equatable {
func isAny(of candidates: Self...) -> Bool {
return candidates.contains(self)
}
}
let isHorizontal = direction.isAny(of: .left, .right)
See tip #35 for my previous experiment.
#51 UIView bounds and transforms
📐 A really interesting side-effect of a UIView's bounds being its rect within its own coordinate system is that transforms don't affect it at all. That's why it's usually a better fit than frame when doing layout calculations of subviews.
let view = UIView()
view.frame.size = CGSize(width: 100, height: 100)
view.transform = CGAffineTransform(scaleX: 2, y: 2)
print(view.frame) // (-50.0, -50.0, 200.0, 200.0)
print(view.bounds) // (0.0, 0.0, 100.0, 100.0)
#50 UIKit default arguments
👏 It's awesome that many UIKit APIs with completion handlers and other optional parameters import into Swift with default arguments (even though they are written in Objective-C). Getting rid of all those nil arguments is so nice!
// BEFORE: All parameters are specified, just like in Objective-C
viewController.present(modalViewController, animated: true, completion: nil)
modalViewController.dismiss(animated: true, completion: nil)
viewController.transition(from: loadingViewController,
to: contentViewController,
duration: 0.3,
options: [],
animations: animations,
completion: nil)
// AFTER: Since many UIKit APIs with completion handlers and other
// optional parameters import into Swift with default arguments,
// we can make our calls shorter
viewController.present(modalViewController, animated: true)
modalViewController.dismiss(animated: true)
viewController.transition(from: loadingViewController,
to: contentViewController,
duration: 0.3,
animations: animations)
#49 Avoiding Massive View Controllers
✂️ Avoiding Massive View Controllers is all about finding the right levels of abstraction and splitting things up.
My personal rule of thumb is that as soon as I have 3 methods or properties that have the same prefix, I break them out into their own type.
// BEFORE
class LoginViewController: UIViewController {
private lazy var signUpLabel = UILabel()
private lazy var signUpImageView = UIImageView()
private lazy var signUpButton = UIButton()
}
// AFTER
class LoginViewController: UIViewController {
private lazy var signUpView = SignUpView()
}
class SignUpView: UIView {
private lazy var label = UILabel()
private lazy var imageView = UIImageView()
private lazy var button = UIButton()
}
#48 Extending optionals
❤️ I love the fact that optionals are enums in Swift - it makes it so easy to extend them with convenience APIs for certain types. Especially useful when doing things like data validation on optional values.
func validateTextFields() -> Bool {
guard !usernameTextField.text.isNilOrEmpty else {
return false
}
...
return true
}
// Since all optionals are actual enum values in Swift, we can easily
// extend them for certain types, to add our own convenience APIs
extension Optional where Wrapped == String {
var isNilOrEmpty: Bool {
switch self {
case let string?:
return string.isEmpty
case nil:
return true
}
}
}
// Since strings are now Collections in Swift 4, you can even
// add this property to all optional collections:
extension Optional where Wrapped: Collection {
var isNilOrEmpty: Bool {
switch self {
case let collection?:
return collection.isEmpty
case nil:
return true
}
}
}
#47 Using where with for-loops
🗺 Using the where keyword can be a super nice way to quickly apply a filter in a for-loop in Swift. You can of course use map, filter and forEach, or guard, but for simple loops I think this is very expressive and nice.
func archiveMarkedPosts() {
for post in posts where post.isMarked {
archive(post)
}
}
func healAllies() {
for player in players where player.isAllied(to: currentPlayer) {
player.heal()
}
}
#46 Variable shadowing
👻 Variable shadowing can be super useful in Swift, especially when you want to create a local copy of a parameter value in order to use it as state within a closure.
init(repeatMode: RepeatMode, closure: @escaping () -> UpdateOutcome) {
// Shadow the argument with a local, mutable copy
var repeatMode = repeatMode
self.closure = {
// With shadowing, there's no risk of accidentially
// referring to the immutable version
switch repeatMode {
case .forever:
break
case .times(let count):
guard count > 0 else {
return .finished
}
// We can now capture the mutable version and use
// it for state in a closure
repeatMode = .times(count - 1)
}
return closure()
}
}
#45 Using dot syntax for static properties and initializers
✒️ Dot syntax is one of my favorite features of Swift. What's really cool is that it's not only for enums, any static method or property can be used with dot syntax - even initializers! Perfect for convenience APIs and default parameters.
public enum RepeatMode {
case times(Int)
case forever
}
public extension RepeatMode {
static var never: RepeatMode {
return .times(0)
}
static var once: RepeatMode {
return .times(1)
}
}
view.perform(animation, repeated: .once)
// To make default parameters more compact, you can even use init with dot syntax
class ImageLoader {
init(cache: Cache = .init(), decoder: ImageDecoder = .init()) {
...
}
}
#44 Calling functions as closures with a tuple as parameters
🚀 One really cool aspect of Swift having first class functions is that you can pass any function (or even initializer) as a closure, and even call it with a tuple containing its parameters!
// This function lets us treat any "normal" function or method as
// a closure and run it with a tuple that contains its parameters
func call<Input, Output>(_ function: (Input) -> Output, with input: Input) -> Output {
return function(input)
}
class ViewFactory {
func makeHeaderView() -> HeaderView {
// We can now pass an initializer as a closure, and a tuple
// containing its parameters
return call(HeaderView.init, with: loadTextStyles())
}
private func loadTextStyles() -> (font: UIFont, color: UIColor) {
return (theme.font, theme.textColor)
}
}
class HeaderView {
init(font: UIFont, textColor: UIColor) {
...
}
}
#43 Enabling static dependency injection
💉 If you've been struggling to test code that uses static APIs, here's a technique you can use to enable static dependency injection without having to modify any call sites:
// Before: Almost impossible to test due to the use of singletons
class Analytics {
static func log(_ event: Event) {
Database.shared.save(event)
let dictionary = event.serialize()
NetworkManager.shared.post(dictionary, to: eventURL)
}
}
// After: Much easier to test, since we can inject mocks as arguments
class Analytics {
static func log(_ event: Event,
database: Database = .shared,
networkManager: NetworkManager = .shared) {
database.save(event)
let dictionary = event.serialize()
networkManager.post(dictionary, to: eventURL)
}
}
#42 Type inference for lazy properties in Swift 4
🎉 In Swift 4, type inference works for lazy properties and you don't need to explicitly refer to self!
// Swift 3
class PurchaseView: UIView {
private lazy var buyButton: UIButton = self.makeBuyButton()
private func makeBuyButton() -> UIButton {
let button = UIButton()
button.setTitle("Buy", for: .normal)
button.setTitleColor(.blue, for: .normal)
return button
}
}
// Swift 4
class PurchaseView: UIView {
private lazy var buyButton = makeBuyButton()
private func makeBuyButton() -> UIButton {
let button = UIButton()
button.setTitle("Buy", for: .normal)
button.setTitleColor(.blue, for: .normal)
return button
}
}
#41 Converting Swift errors to NSError
😎 You can turn any Swift Error into an NSError, which is super useful when pattern matching with a code 👍. Also, switching on optionals is pretty cool!
let task = urlSession.dataTask(with: url) { data, _, error in
switch error {
case .some(let error as NSError) where error.code == NSURLErrorNotConnectedToInternet:
presenter.showOfflineView()
case .some(let error):
presenter.showGenericErrorView()
case .none:
presenter.renderContent(from: data)
}
}
task.resume()
Also make sure to check out Kostas Kremizas' tip about how you can pattern match directly against a member of URLError.
#40 Making UIImage macOS compatible
🖥 Here's an easy way to make iOS model code that uses UIImage macOS compatible - like me and Gui Rambo discussed on the Swift by Sundell Podcast.
// Either put this in a separate file that you only include in your macOS target or wrap the code in #if os(macOS) / #endif
import Cocoa
// Step 1: Typealias UIImage to NSImage
typealias UIImage = NSImage
// Step 2: You might want to add these APIs that UIImage has but NSImage doesn't.
extension NSImage {
var cgImage: CGImage? {
var proposedRect = CGRect(origin: .zero, size: size)
return cgImage(forProposedRect: &proposedRect,
context: nil,
hints: nil)
}
convenience init?(named name: String) {
self.init(named: Name(name))
}
}
// Step 3: Profit - you can now make your model code that uses UIImage cross-platform!
struct User {
let name: String
let profileImage: UIImage
}
#39 Internally mutable protocol-oriented APIs
🤖 You can easily define a protocol-oriented API that can only be mutated internally, by using an internal protocol that extends a public one.
// Declare a public protocol that acts as your immutable API
public protocol ModelHolder {
associatedtype Model
var model: Model { get }
}
// Declare an extended, internal protocol that provides a mutable API
internal protocol MutableModelHolder: ModelHolder {
var model: Model { get set }
}
// You can now implement the requirements using 'public internal(set)'
public class UserHolder: MutableModelHolder {
public internal(set) var model: User
internal init(model: User) {
self.model = model
}
}
#38 Switching on a set
🎛 You can switch on a set using array literals as cases in Swift! Can be really useful to avoid many if/else if statements.
class RoadTile: Tile {
var connectedDirections = Set<Direction>()
func render() {
switch connectedDirections {
case [.up, .down]:
image = UIImage(named: "road-vertical")
case [.left, .right]:
image = UIImage(named: "road-horizontal")
default:
image = UIImage(named: "road")
}
}
}
#37 Adding the current locale to cache keys
🌍 When caching localized content in an app, it's a good idea to add the current locale to all keys, to prevent bugs when switching languages.
func cache(_ content: Content, forKey key: String) throws {
let data = try wrap(content) as Data
let key = localize(key: key)
try storage.store(data, forKey: key)
}
func loadCachedContent(forKey key: String) -> Content? {
let key = localize(key: key)
let data = storage.loadData(forKey: key)
return data.flatMap { try? unbox(data: $0) }
}
private func localize(key: String) -> String {
return key + "-" + Bundle.main.preferredLocalizations[0]
}
#36 Setting up tests to avoid retain cycles with weak references
🚳 Here's an easy way to setup a test to avoid accidental retain cycles with object relationships (like weak delegates & observers) in Swift:
func testDelegateNotRetained() {
// Assign the delegate (weak) and also retain it using a local var
var delegate: Delegate? = DelegateMock()
controller.delegate = delegate
XCTAssertNotNil(controller.delegate)
// Release the local var, which should also release the weak reference
delegate = nil
XCTAssertNil(controller.delegate)
}
#35 Expressively matching a value against a list of candidates
👨🔬 Playing around with an expressive way to check if a value matches any of a list of candidates in Swift:
// Instead of multiple conditions like this:
if string == "One" || string == "Two" || string == "Three" {
}
// You can now do:
if string == any(of: "One", "Two", "Three") {
}
You can find a gist with the implementation here.
#34 Organizing code using extensions
👪 APIs in a Swift extension automatically inherit its access control level, making it a neat way to organize public, internal & private APIs.
public extension Animation {
init(textureNamed textureName: String) {
frames = [Texture(name: textureName)]
}
init(texturesNamed textureNames: [String], frameDuration: TimeInterval = 1) {
frames = textureNames.map(Texture.init)
self.frameDuration = frameDuration
}
init(image: Image) {
frames = [Texture(image: image)]
}
}
internal extension Animation {
func loadFrameImages() -> [Image] {
return frames.map { $0.loadImageIfNeeded() }
}
}
#33 Using map to transform an optional into a Result type
🗺 Using map you can transform an optional value into an optional Result type by simply passing in the enum case.
enum Result<Value> {
case value(Value)
case error(Error)
}
class Promise<Value> {
private var result: Result<Value>?
init(value: Value? = nil) {
result = value.map(Result.value)
}
}
#32 Assigning to self in struct initializers
👌 It's so nice that you can assign directly to self in struct initializers in Swift. Very useful when adding conformance to protocols.
extension Bool: AnswerConvertible {
public init(input: String) throws {
switch input.lowercased() {
case "y", "yes", "👍":
self = true
default:
self = false
}
}
}
#31 Recursively calling closures as inline functions
☎️ Defining Swift closures as inline functions enables you to recursively call them, which is super useful in things like custom sequences.
class Database {
func records(matching query: Query) -> AnySequence<Record> {
var recordIterator = loadRecords().makeIterator()
func iterate() -> Record? {
guard let nextRecord = recordIterator.next() else {
return nil
}
guard nextRecord.matches(query) else {
// Since the closure is an inline function, it can be recursively called,
// in this case in order to advance to the next item.
return iterate()
}
return nextRecord
}
// AnySequence/AnyIterator are part of the standard library and provide an easy way
// to define custom sequences using closures.
return AnySequence { AnyIterator(iterate) }
}
}
Rob Napier points out that using the above might cause crashes if used on a large databaset, since Swift has no guaranteed Tail Call Optimization (TCO).
Slava Pestov also points out that another benefit of inline functions vs closures is that they can have their own generic parameter list.
#30 Passing self to required Objective-C dependencies
🏖 Using lazy properties in Swift, you can pass self to required Objective-C dependencies without having to use force-unwrapped optionals.
class DataLoader: NSObject {
lazy var urlSession: URLSession = self.makeURLSession()
private func makeURLSession() -> URLSession {
return URLSession(configuration: .default, delegate: self, delegateQueue: .main)
}
}
class Renderer {
lazy var displayLink: CADisplayLink = self.makeDisplayLink()
private func makeDisplayLink() -> CADisplayLink {
return CADisplayLink(target: self, selector: #selector(screenDidRefresh))
}
}
#29 Making weak or lazy properties readonly
👓 If you have a property in Swift that needs to be weak or lazy, you can still make it readonly by using private(set).
class Node {
private(set) weak var parent: Node?
private(set) lazy var children = [Node]()
func add(child: Node) {
children.append(child)
child.parent = self
}
}
#28 Defining static URLs using string literals
🌏 Tired of using URL(string: "url")! for static URLs? Make URL conform to ExpressibleByStringLiteral and you can now simply use "url" instead.
extension URL: ExpressibleByStringLiteral {
// By using 'StaticString' we disable string interpolation, for safety
public init(stringLiteral value: StaticString) {
self = URL(string: "\(value)").require(hint: "Invalid URL string literal: \(value)")
}
}
// We can now define URLs using static string literals 🎉
let url: URL = "https://www.swiftbysundell.com"
let task = URLSession.shared.dataTask(with: "https://www.swiftbysundell.com")
// In Swift 3 or earlier, you also have to implement 2 additional initializers
extension URL {
public init(extendedGraphemeClusterLiteral value: StaticString) {
self.init(stringLiteral: value)
}
public init(unicodeScalarLiteral value: StaticString) {
self.init(stringLiteral: value)
}
}
To find the extension that adds the require() method on Optional that I use above, check out Require.
#27 Manipulating points, sizes and frames using math operators
✚ I'm always careful with operator overloading, but for manipulating things like sizes, points & frames I find them super useful.
extension CGSize {
static func *(lhs: CGSize, rhs: CGFloat) -> CGSize {
return CGSize(width: lhs.width * rhs, height: lhs.height * rhs)
}
}
button.frame.size = image.size * 2
If you like the above idea, check out CGOperators, which contains math operator overloads for all Core Graphics' vector types.
#26 Using closure types in generic constraints
🔗 You can use closure types in generic constraints in Swift. Enables nice APIs for handling sequences of closures.
extension Sequence where Element == () -> Void {
func callAll() {
forEach { $0() }
}
}
extension Sequence where Element == () -> String {
func joinedResults(separator: String) -> String {
return map { $0() }.joined(separator: separator)
}
}
callbacks.callAll()
let names = nameProviders.joinedResults(separator: ", ")
(If you're using Swift 3, you have to change Element to Iterator.Element)
#25 Using associated enum values to avoid state-specific optionals
🎉 Using associated enum values is a super nice way to encapsulate mutually exclusive state info (and avoiding state-specific optionals).
// BEFORE: Lots of state-specific, optional properties
class Player {
var isWaitingForMatchMaking: Bool
var invitingUser: User?
var numberOfLives: Int
var playerDefeatedBy: Player?
var roundDefeatedIn: Int?
}
// AFTER: All state-specific information is encapsulated in enum cases
class Player {
enum State {
case waitingForMatchMaking
case waitingForInviteResponse(from: User)
case active(numberOfLives: Int)
case defeated(by: Player, roundNumber: Int)
}
var state: State
}
#24 Using enums for async result types
👍 I really like using enums for all async result types, even boolean ones. Self-documenting, and makes the call site a lot nicer to read too!
protocol PushNotificationService {
// Before
func enablePushNotifications(completionHandler: @escaping (Bool) -> Void)
// After
func enablePushNotifications(completionHandler: @escaping (PushNotificationStatus) -> Void)
}
enum PushNotificationStatus {
case enabled
case disabled
}
service.enablePushNotifications { status in
if status == .enabled {
enableNotificationsButton.removeFromSuperview()
}
}
#23 Working on async code in a playground
🏃 Want to work on your async code in a Swift Playground? Just set needsIndefiniteExecution to true to keep it running:
import PlaygroundSupport
PlaygroundPage.current.needsIndefiniteExecution = true
DispatchQueue.main.asyncAfter(deadline: .now() + 3) {
let greeting = "Hello after 3 seconds"
print(greeting)
}
To stop the playground from executing, simply call PlaygroundPage.current.finishExecution().
#22 Overriding self with a weak reference
💦 Avoid memory leaks when accidentially refering to self in closures by overriding it locally with a weak reference:
Swift >= 4.2
dataLoader.loadData(from: url) { [weak self] result in
guard let self = self else {
return
}
self.cache(result)
...
Swift < 4.2
dataLoader.loadData(from: url) { [weak self] result in
guard let `self` = self else {
return
}
self.cache(result)
...
Note that the reason the above currently works is because of a compiler bug (which I hope gets turned into a properly supported feature soon).
#21 Using DispatchWorkItem
🕓 Using dispatch work items you can easily cancel a delayed asynchronous GCD task if you no longer need it:
let workItem = DispatchWorkItem {
// Your async code goes in here
}
// Execute the work item after 1 second
DispatchQueue.main.asyncAfter(deadline: .now() + 1, execute: workItem)
// You can cancel the work item if you no longer need it
workItem.cancel()
#20 Combining a sequence of functions
➕ While working on a new Swift developer tool (to be open sourced soon 😉), I came up with a pretty neat way of organizing its sequence of operations, by combining their functions into a closure:
internal func +<A, B, C>(lhs: @escaping (A) throws -> B,
rhs: @escaping (B) throws -> C) -> (A) throws -> C {
return { try rhs(lhs($0)) }
}
public func run() throws {
try (determineTarget + build + analyze + output)()
}
If you're familiar with the functional programming world, you might know the above technique as the pipe operator (thanks to Alexey Demedreckiy for pointing this out!)
#19 Chaining optionals with map() and flatMap()
🗺 Using map() and flatMap() on optionals you can chain multiple operations without having to use lengthy if lets or guards:
// BEFORE
guard let string = argument(at: 1) else {
return
}
guard let url = URL(string: string) else {
return
}
handle(url)
// AFTER
argument(at: 1).flatMap(URL.init).map(handle)
#18 Using self-executing closures for lazy properties
🚀 Using self-executing closures is a great way to encapsulate lazy property initialization:
class StoreViewController: UIViewController {
private lazy var collectionView: UICollectionView = {
let layout = UICollectionViewFlowLayout()
let view = UICollectionView(frame: self.view.bounds, collectionViewLayout: layout)
view.delegate = self
view.dataSource = self
return view
}()
override func viewDidLoad() {
super.viewDidLoad()
view.addSubview(collectionView)
}
}
#17 Speeding up Swift package tests
⚡️ You can speed up your Swift package tests using the --parallel flag. For Marathon, the tests execute 3 times faster that way!
swift test --parallel
#16 Avoiding mocking UserDefaults
🛠 Struggling with mocking UserDefaults in a test? The good news is: you don't need mocking - just create a real instance:
class LoginTests: XCTestCase {
private var userDefaults: UserDefaults!
private var manager: LoginManager!
override func setUp() {
super.setup()
userDefaults = UserDefaults(suiteName: #file)
userDefaults.removePersistentDomain(forName: #file)
manager = LoginManager(userDefaults: userDefaults)
}
}
#15 Using variadic parameters
👍 Using variadic parameters in Swift, you can create some really nice APIs that take a list of objects without having to use an array:
extension Canvas {
func add(_ shapes: Shape...) {
shapes.forEach(add)
}
}
let circle = Circle(center: CGPoint(x: 5, y: 5), radius: 5)
let lineA = Line(start: .zero, end: CGPoint(x: 10, y: 10))
let lineB = Line(start: CGPoint(x: 0, y: 10), end: CGPoint(x: 10, y: 0))
let canvas = Canvas()
canvas.add(circle, lineA, lineB)
canvas.render()
#14 Referring to enum cases with associated values as closures
😮 Just like you can refer to a Swift function as a closure, you can do the same thing with enum cases with associated values:
enum UnboxPath {
case key(String)
case keyPath(String)
}
struct UserSchema {
static let name = key("name")
static let age = key("age")
static let posts = key("posts")
private static let key = UnboxPath.key
}
#13 Using the === operator to compare objects by instance
📈 The === operator lets you check if two objects are the same instance. Very useful when verifying that an array contains an instance in a test:
protocol InstanceEquatable: class, Equatable {}
extension InstanceEquatable {
static func ==(lhs: Self, rhs: Self) -> Bool {
return lhs === rhs
}
}
extension Enemy: InstanceEquatable {}
func testDestroyingEnemy() {
player.attack(enemy)
XCTAssertTrue(player.destroyedEnemies.contains(enemy))
}
#12 Calling initializers with dot syntax and passing them as closures
😎 Cool thing about Swift initializers: you can call them using dot syntax and pass them as closures! Perfect for mocking dates in tests.
class Logger {
private let storage: LogStorage
private let dateProvider: () -> Date
init(storage: LogStorage = .init(), dateProvider: @escaping () -> Date = Date.init) {
self.storage = storage
self.dateProvider = dateProvider
}
func log(event: Event) {
storage.store(event: event, date: dateProvider())
}
}
#11 Structuring UI tests as extensions on XCUIApplication
📱 Most of my UI testing logic is now categories on XCUIApplication. Makes the test cases really easy to read:
func testLoggingInAndOut() {
XCTAssertFalse(app.userIsLoggedIn)
app.launch()
app.login()
XCTAssertTrue(app.userIsLoggedIn)
app.logout()
XCTAssertFalse(app.userIsLoggedIn)
}
func testDisplayingCategories() {
XCTAssertFalse(app.isDisplayingCategories)
app.launch()
app.login()
app.goToCategories()
XCTAssertTrue(app.isDisplayingCategories)
}
#10 Avoiding default cases in switch statements
🙂 It’s a good idea to avoid “default” cases when switching on Swift enums - it’ll “force you” to update your logic when a new case is added:
enum State {
case loggedIn
case loggedOut
case onboarding
}
func handle(_ state: State) {
switch state {
case .loggedIn:
showMainUI()
case .loggedOut:
showLoginUI()
// Compiler error: Switch must be exhaustive
}
}
#9 Using the guard statement in many different scopes
💂 It's really cool that you can use Swift's 'guard' statement to exit out of pretty much any scope, not only return from functions:
// You can use the 'guard' statement to...
for string in strings {
// ...continue an iteration
guard shouldProcess(string) else {
continue
}
// ...or break it
guard !shouldBreak(for: string) else {
break
}
// ...or return
guard !shouldReturn(for: string) else {
return
}
// ..or throw an error
guard string.isValid else {
throw StringError.invalid(string)
}
// ...or exit the program
guard !shouldExit(for: string) else {
exit(1)
}
}
#8 Passing functions & operators as closures
❤️ Love how you can pass functions & operators as closures in Swift. For example, it makes the syntax for sorting arrays really nice!
let array = [3, 9, 1, 4, 6, 2]
let sorted = array.sorted(by: <)
#7 Using #function for UserDefaults key consistency
🗝 Here's a neat little trick I use to get UserDefault key consistency in Swift (#function expands to the property name in getters/setters). Just remember to write a good suite of tests that'll guard you against bugs when changing property names.
extension UserDefaults {
var onboardingCompleted: Bool {
get { return bool(forKey: #function) }
set { set(newValue, forKey: #function) }
}
}
#6 Using a name already taken by the standard library
📛 Want to use a name already taken by the standard library for a nested type? No problem - just use Swift. to disambiguate:
extension Command {
enum Error: Swift.Error {
case missing
case invalid(String)
}
}
#5 Using Wrap to implement Equatable
📦 Playing around with using Wrap to implement Equatable for any type, primarily for testing:
protocol AutoEquatable: Equatable {}
extension AutoEquatable {
static func ==(lhs: Self, rhs: Self) -> Bool {
let lhsData = try! wrap(lhs) as Data
let rhsData = try! wrap(rhs) as Data
return lhsData == rhsData
}
}
#4 Using typealiases to reduce the length of method signatures
📏 One thing that I find really useful in Swift is to use typealiases to reduce the length of method signatures in generic types:
public class PathFinder<Object: PathFinderObject> {
public typealias Map = Object.Map
public typealias Node = Map.Node
public typealias Path = PathFinderPath<Object>
public static func possiblePaths(for object: Object, at rootNode: Node, on map: Map) -> Path.Sequence {
return .init(object: object, rootNode: rootNode, map: map)
}
}
#3 Referencing either external or internal parameter name when writing docs
📖 You can reference either the external or internal parameter label when writing Swift docs - and they get parsed the same:
// EITHER:
class Foo {
/**
* - parameter string: A string
*/
func bar(with string: String) {}
}
// OR:
class Foo {
/**
* - parameter with: A string
*/
func bar(with string: String) {}
}
#2 Using auto closures
👍 Finding more and more uses for auto closures in Swift. Can enable some pretty nice APIs:
extension Dictionary {
mutating func value(for key: Key, orAdd valueClosure: @autoclosure () -> Value) -> Value {
if let value = self[key] {
return value
}
let value = valueClosure()
self[key] = value
return value
}
}
#1 Namespacing with nested types
🚀 I’ve started to become a really big fan of nested types in Swift. Love the additional namespacing it gives you!
public struct Map {
public struct Model {
public let size: Size
public let theme: Theme
public var terrain: [Position : Terrain.Model]
public var units: [Position : Unit.Model]
public var buildings: [Position : Building.Model]
}
public enum Direction {
case up
case right
case down
case left
}
public struct Position {
public var x: Int
public var y: Int
}
public enum Size: String {
case small = "S"
case medium = "M"
case large = "L"
case extraLarge = "XL"
}
}Download Details:
Author: JohnSundell
Source code: https://github.com/JohnSundell/SwiftTips
License: MIT license
#swift
1655024460
What Are Python Lambda Functions ?
Learning Python Takes Time
Traveling the road from rookie to Pythonista can take a while and varies from person to person. Most people start learning Python by reviewing the basic data structures for numbers, Booleans and strings, and then move to complex data structures like lists and dictionaries. From there you learn loops and if/else control logic, and eventually learn to write reusable code via functions.
Once you start exploring functions, you’ll come across lambda functions which can seem pretty intimidating at first. Similarly to list comprehensions, lambda functions allow you to write succinct code. Often something that would take several lines as a defined function can be done in one line using a lambda function!
Reviewing Python Functions
Typical Python functions are simply a self-contained set of instructions designed to perform a specific task. They are important to master and understand because they allow us to keep code organized by breaking it into smaller, reusable chunks. If we’re writing a large program, utilizing functions can make the code easier to read and debug too.
In Python, we define a function using the def keyword, then give the function a name along with any necessary arguments that impact the function body’s execution. Often times, the return keyword is used to terminate the function and return the desired output to whatever called the function. Here is an example function:
#python example function
def my_function(string):
return print(string) 
testing my_function
The example my_function takes a string as an argument and prints the string as the returned output.
What Is A Lambda Function?
To take your Python skills to the next level, you need to master both normal functions and lambda functions. Lambda functions are great for making code shorter and more concise, which is the way of the Pythonista! We’ll take a look at a few simple examples to review the syntax, and then look at some actual use cases for lambda functions that will make you look like a pro.
As we see in the previous example, normal functions are defined using def and given a name. Lambda functions are defined using the lambda keyword and are nameless. They are anonymous functions that are not defined in any namespace, and they are intended for single use.
When I first learned about lambda functions, it seemed a bit tricky to wrap my head around. The syntax can feel confusing since a lambda function can take any number of arguments, but can only have one expression. Here are a couple simple examples:
#lambda syntax
#lambda <arguments> : <return expression>#simple example
lambda_example = lambda x: x + x#multiple arguments example
multiple_arguments = lambda x, y, z: (x + y) * z
Testing lambda function examples
Notice the multiple arguments are separated by commas. When calling the function, the caller provides the arguments. The return expression is defined after the colon (:). There is a single return expression, but the return expression can be simple or complex. It could even be another function, which makes lambda functions a powerful tool in your Python programming toolbox.
Using Lambda Functions
Let’s take a look at how we can use lambda to write functions the Pythonic way. Below we have a function that checks a list of strings for the word “two.” It returns the list of strings along with True or False depending on whether the word was in the strings:
sentences = ['Sentence one.', 'Sentence two.', 'Sentence three.']def contains_two(text):
answers = []
for s in txt:
if 'two' in s:
answers.append(True)
else:
answers.append(False)
return zip(answers, text)
Testing contains_two function
Instead of defining a function and using a for loop to go through the list, we can rewrite this succinctly using Python’s map() function along with a lambda function.
contains_two_lambda = map(lambda x: (True, x) if 'two' in x else (False, x), sentences)
Testing contains_two_lambda function
The map() function takes a function and a sequence as arguments, running the function on the given sequence. The function we pass to map() is a lambda function, allowing us to perform the same thing our defined function does in only one line of code!
Lambda functions work well with functions like map(), reduce(), and filter(), three built-in Python functions that take a function as arguments. Lambda functions are also popular in data munging as they can work well with pandas dataframes. They can make applying logical operations to columns and rows concise one-liners.
Using Lambda Functions With Pandas DataFrames
As an analyst, I find myself using Pandas DataFrames all the time. Since DataFames are a popular data structure, let’s take a look at how easy it is to use lambda functions for transforming data in dataframes. Let’s create a simple pandas dataframe with some mock data:
#import dependencies
import pandas as pd#create dataframe
df = pd.DataFrame({
'Occupation': ['Data Tech','Sales','Analyst','Engineer'],
'Name': ['John Smith', 'Jodie Whales', 'Eric Kleppen', 'Richard Heart'],
'Salary': [50000, 75000, 80000, 100000],
'YearsAtCompany': [2, 3, 1, 3],})
Example dataframe
Next, let's create a new column called newSalary using a lambda function. If I’m giving out raises based on the column YearsAtCompany and want to see what a person’s new salary will be, I can use the apply() function similar to how we used map() in the previous example:
df['newSalary'] =
df['Salary'].apply(lambda x: x * (1 + df['YearsAtCompany']/10))
Verifying newSalary column
Notice that the apply() function takes the lambda function as an argument and applies the function to each row in the Salary column. The expression in the lambda function uses YearsAtCompany to determine the size of the raise.
Wrapping Up
Writing Python like a pro takes practice, so don’t worry if you feel intimidated by new concepts like lambda functions. The syntax looks strange, but lambda functions allow you to write concise, one-time-use functions that are great for applying logical operations.
Although they can only perform one expression, the expression can be complex and even include another function. Lambda functions can work well with series of data, as even data structures like Pandas DataFrames.
This story was originally published at https://betterprogramming.pub/become-a-python-programmer-what-are-lambda-functions-1de065686edf