Our overarching goals are clarity, consistency and brevity, in that order.
- Correctness
- Naming
- Code Organization
- Spacing
- Comments
- Classes and Structures
- Function Declarations
- Function Calls
- Closure Expressions
- Types
- Working with Storyboards
- Functions vs Methods
- Memory Management
- Access Control
- Control Flow
- Third Party Imports
- Golden Path
- Semicolons
- Parentheses
- Multi-line String Literals
- No Emoji
- No #imageLiteral or #colorLiteral
- Organization and Bundle Identifier
- Copyright Statement
- References
Strive to make your code compile without warnings. This rule informs many style decisions such as using #selector types instead of string literals.
Descriptive and consistent naming makes software easier to read and understand. Use the Swift naming conventions described in the API Design Guidelines. Some key takeaways include:
- striving for clarity at the call site
- prioritizing clarity over brevity
- using
camelCase(notsnake_case) - using
UpperCamelCasefor types and protocols,lowerCamelCasefor everything else - including all needed words while omitting needless words
- using names based on roles, not types
- sometimes compensating for weak type information
- striving for fluent usage
- beginning factory methods with
make - naming methods for their side effects
- verb methods follow the -ed, -ing rule for the non-mutating version
- noun methods follow the formX rule for the mutating version
- boolean types should read like assertions
- protocols that describe what something is should read as nouns
- protocols that describe a capability should end in -able or -ible
- using terms that don't surprise experts or confuse beginners
- generally avoiding abbreviations
- using precedent for names
- preferring methods and properties to free functions
- casing acronyms and initialisms uniformly up or down
- giving the same base name to methods that share the same meaning
- avoiding overloads on return type
- choosing good parameter names that serve as documentation
- preferring to name the first parameter instead of including its name in the method name, except as mentioned under Delegates
- labeling closure and tuple parameters
- taking advantage of default parameters
Preferred:
import UIKit
private let maximumWidgetCount = 100
class WidgetContainer {
var widgetButton: UIButton!
let widgetHeightPercentage = 0.85
}Not Preferred:
let MAX_WIDGET_COUNT = 100
class app_widgetContainer {
var wBut: UIButton!
let wHeightPct = 0.85
}Abbreviations and acronyms should generally be avoided. Following the API Design Guidelines, abbreviations and initialisms that appear in all uppercase should be uniformly uppercase or lowercase. Examples:
Preferred
let urlString: URLString
let userID: UserIDNot Preferred
let uRLString: UrlString
let userId: UserIdFor functions and init methods, prefer named parameters for all arguments unless the context is very clear. Include external parameter names if it makes function calls more readable.
import SpriteKit
func dateFromString(_ dateString: String) -> Date { NSDate() }
func convertPointAt(column: Int, row: Int) -> CGPoint { CGPoint(x: 0, y: 0) }
func timedAction(afterDelay delay: TimeInterval, perform action: SKAction) -> SKAction! { SKAction() }would be called like this:
dateFromString("2014-03-14")
convertPointAt(column: 42, row: 13)
timedAction(afterDelay: 1.0, perform: SKAction())For methods, follow the standard Apple convention of referring to the first parameter in the method name:
class Counter {
func combineWith(otherCounter: Counter, options: Dictionary<Int, String>?) { }
func incrementBy(amount: Int) { }
}Following Apple's API Design Guidelines, protocols names that describe what something is should be a noun. Examples: Collection, WidgetFactory. Protocols names that describe an ability should end in -ing, -able, or -ible. Examples: Equatable, Resizing.
Following Apple's API Design Guidelines, use lowerCamelCase for enumeration values.
enum ShapeType {
case rectangle
case square
case rightTriangle
case equilateralTriangle
}When referring to methods in prose, being unambiguous is critical. To refer to a method name, use the simplest form possible.
- Write the method name with no parameters. Example: Next, you need to call
addTarget. - Write the method name with argument labels. Example: Next, you need to call
addTarget(_:action:). - Write the full method name with argument labels and types. Example: Next, you need to call
addTarget(_: Any?, action: Selector?).
For the above example using UIGestureRecognizer, 1 is unambiguous and preferred.
Pro Tip: You can use Xcode's jump bar to lookup methods with argument labels. If you’re particularly good at mashing lots of keys simultaneously, put the cursor in the method name and press Shift-Control-Option-Command-C (all 4 modifier keys) and Xcode will kindly put the signature on your clipboard.
Swift types are automatically namespaced by the module that contains them and you should not add a class prefix such as RW. If two names from different modules collide you can disambiguate by prefixing the type name with the module name. However, only specify the module name when there is possibility for confusion, which should be rare.
import SomeModule
let myClass = MyModule.UsefulClass()When creating custom delegate methods, an unnamed first parameter should be the delegate source. (UIKit contains numerous examples of this.)
Preferred:
func namePickerView(_ namePickerView: NamePickerView, didSelectName name: String)
func namePickerViewShouldReload(_ namePickerView: NamePickerView) -> BoolNot Preferred:
func didSelectName(namePicker: NamePickerViewController, name: String)
func namePickerShouldReload() -> BoolUse compiler inferred context to write shorter, clear code. (Also see Type Inference.)
Preferred:
let selector = #selector(viewDidLoad)
view.backgroundColor = .red
let toView = context.view(forKey: .to)
let view = UIView(frame: .zero)Not Preferred:
let selector = #selector(ViewController.viewDidLoad)
view.backgroundColor = UIColor.red
let toView = context.view(forKey: UITransitionContextViewKey.to)
let view = UIView(frame: CGRect.zero)Generic type parameters should be descriptive, upper camel case names. When a type name doesn't have a meaningful relationship or role, use a traditional single uppercase letter such as T, U, or V.
Preferred:
struct Stack<Element> { ... }
func write<Target: OutputStream>(to target: inout Target)
func swap<T>(_ a: inout T, _ b: inout T)Not Preferred:
struct Stack<T> { ... }
func write<target: OutputStream>(to target: inout target)
func swap<Thing>(_ a: inout Thing, _ b: inout Thing)Use US English spelling to match Apple's API.
Preferred:
let color = "red"Not Preferred:
let colour = "red"Use extensions to organize your code into logical blocks of functionality. Each extension should be set off with a // MARK: - comment to keep things well-organized.
In particular, when adding protocol conformance to a model, prefer adding a separate extension for the protocol methods. This keeps the related methods grouped together with the protocol and can simplify instructions to add a protocol to a class with its associated methods.
Preferred:
class MyViewController: UIViewController {
// class stuff here
}
// MARK: - UITableViewDataSource
extension MyViewController: UITableViewDataSource {
// table view data source methods
}
// MARK: - UIScrollViewDelegate
extension MyViewController: UIScrollViewDelegate {
// scroll view delegate methods
}Not Preferred:
class MyViewController: UIViewController, UITableViewDataSource, UIScrollViewDelegate {
// all methods
}Since the compiler does not allow you to re-declare protocol conformance in a derived class, it is not always required to replicate the extension groups of the base class. This is especially true if the derived class is a terminal class and a small number of methods are being overridden. When to preserve the extension groups is left to the discretion of the author.
For UIKit view controllers, consider grouping lifecycle, custom accessors, and IBAction in separate class extensions.
Variable binding introducers should be placed next to the variable being bound to clarify what associated values are being bound.
Preferred
enum Foo {
case bar
case baz(Bool)
case fred(Bool)
}
let firstSwitch = (Foo.bar, Foo.baz(false))
switch firstSwitch {
case (.bar, .baz(let quux)):
// do stuff with quux
break
default:
break
}
let secondSwitch = (Foo.bar, Foo.baz(false), Foo.fred(true))
switch secondSwitch {
case (.bar, .baz(let quux), .fred(var corge)):
break
default:
break
}Not Preferred:
switch firstSwitch {
case let (.bar, .baz(quux)):
// do stuff with quux
break
default:
break
}
switch secondSwitch {
case let (.bar, .baz(quux), .fred(corge)):
break
default:
break
}When binding variables to associated values of custom types, a label can be omitted. However, for Foundation types or in cases that are generic enough to need more clarity, a label should be used.
Preferred
//case authenticated(User):
//[...]
//case authenticated(username: String):Not Preferred
//case authenticated(u):Unused (dead) code, including Xcode template code and placeholder comments should be removed. An exception is when your tutorial or book instructs the user to use the commented code.
Methods whose implementation simply calls the super class should also be removed. This includes any empty/unused UIApplicationDelegate methods.
Preferred:
override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return Database.contacts.count
}Not Preferred:
override func didReceiveMemoryWarning() {
super.didReceiveMemoryWarning()
// Dispose of any resources that can be recreated.
}
override func numberOfSections(in tableView: UITableView) -> Int {
// #warning Incomplete implementation, return the number of sections
return 1
}
override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
// #warning Incomplete implementation, return the number of rows
return Database.contacts.count
}Import only the modules a source file requires. For example, don't import UIKit when importing Foundation will suffice. Likewise, don't import Foundation if you must import UIKit.
Preferred:
import UIKit
var view: UIView
var deviceModels: [String]Preferred:
import Foundation
var deviceModels: [String]Not Preferred:
import UIKit
import Foundation
var view: UIView
var deviceModels: [String]Not Preferred:
import UIKit
var deviceModels: [String]- Use the Xcode defaults for tab spacing (4 spaces and spaces instead of tabs).
- Method braces and other braces (
if/else/switch/whileetc.) always open on the same line as the statement but close on a new line. - Tip: You can re-indent by selecting some code (or Command-A to select all) and then Control-I (or Editor ▸ Structure ▸ Re-Indent in the menu). Some of the Xcode template code will have 4-space tabs hard coded, so this is a good way to fix that.
Preferred:
if let user = User.loggedInUser {
if user.isHappy {
// Be happy
} else {
// Turn that frown upside down!
}
}Not Preferred:
if user.isHappy
{
// Do something
}
else {
// Do something else
}-
There should be one blank line between methods and up to one blank line between type declarations to aid in visual clarity and organization. Whitespace within methods should separate functionality, but having too many sections in a method often means you should refactor into several methods.
-
There should be no blank lines after an opening brace or before a closing brace.
-
Closing parentheses should not appear on a line by themselves.
Preferred:
let user = try await getUser(
for: userID,
on: connection)Not Preferred:
let user = try await getUser(
for: userID,
on: connection
)- Colons always have no space on the left and one space on the right. Exceptions are the ternary operator
? :, empty dictionary[:]and#selectorsyntaxaddTarget(_:action:).
Preferred:
class TestDatabase: Database {
var data: [String: CGFloat] = ["A": 1.2, "B": 3.2]
}Not Preferred:
class TestDatabase : Database {
var data :[String:CGFloat] = ["A" : 1.2, "B":3.2]
}-
Long lines should be wrapped at around 70 characters. A hard limit is intentionally not specified.
-
Avoid trailing whitespaces at the ends of lines.
-
Add a single newline character at the end of each file.
When they are needed, use comments to explain why a particular piece of code does something. Comments must be kept up-to-date or deleted.
Avoid block comments inline with code, as the code should be as self-documenting as possible. Exception: This does not apply to those comments used to generate documentation.
Avoid the use of C-style comments (/* ... */). Prefer the use of double- or triple-slash.
If you are developing a framework or reusable component, it is preferable to add the Xcode Markdown style comments to public functions and classes. This enables the Quick Help functionality for other developers, which can be helpful during integration of frameworks. Either the triple slash /// style comments or the /** comment styles can be used for the block comments, but the triple slash is generally preferred as this is the style Xcode uses in the "Add Documentation" command that can be found under Editor -> Structure -> Add Documentation
/// Decodes a JSON payload from the API into the corresponding Person model object. The JSON
/// payload must contain the following keys: name, birthDate, age.
///
/// - parameter json: JSON payload from the /persons endpoint
///
/// - throws: MissingKeyException on a missing required key
///
/// - returns: the JSON populated Person object
public static func decode(json: AnyObject) throws -> Person {}Care should be taken to make the comments meaningful and add additional information or clarity than what can be inferred by the function declaration. Additional documentation on the Markdown style can be found here.
Remember, structs have value semantics. Use structs for things that do not have an identity. An array that contains [a, b, c] is really the same as another array that contains [a, b, c] and they are completely interchangeable. It doesn't matter whether you use the first array or the second, because they represent the exact same thing. That's why arrays are structs.
Classes have reference semantics. Use classes for things that do have an identity or a specific life cycle. You would model a person as a class because two person objects are two different things. Just because two people have the same name and birthdate, doesn't mean they are the same person. But the person's birthdate would be a struct because a date of 3 March 1950 is the same as any other date object for 3 March 1950. The date itself doesn't have an identity.
Sometimes, things should be structs but need to conform to AnyObject or are historically modeled as classes already (NSDate, NSSet). Try to follow these guidelines as closely as possible.
Here's an example of a well-styled class definition:
class Circle: Shape {
var x: Int, y: Int
var radius: Double
var diameter: Double {
get {
return radius * 2
}
set {
radius = newValue / 2
}
}
init(x: Int, y: Int, radius: Double) {
self.x = x
self.y = y
self.radius = radius
}
convenience init(x: Int, y: Int, diameter: Double) {
self.init(x: x, y: y, radius: diameter / 2)
}
func describe() -> String {
return "I am a circle at \(centerString()) with an area of \(computeArea())"
}
override func area() -> Double {
return Double.pi * radius * radius
}
}
extension Circle: CustomStringConvertible {
var description: String {
return "center = \(centerString) area = \(area())"
}
private var centerString: String {
return "(\(x),\(y))"
}
}The example above demonstrates the following style guidelines:
- Specify types for properties, variables, constants, argument declarations and other statements with a space after the colon but not before, e.g.
x: Int, andCircle: Shape. - Define multiple variables and structures on a single line if they share a common purpose / context.
- Indent getter and setter definitions and property observers.
- Don't add modifiers such as
internalwhen they're already the default. Similarly, don't repeat the access modifier when overriding a method. - Organize extra functionality (e.g. printing) in extensions.
- Hide non-shared, implementation details such as
centerStringinside the extension usingprivateaccess control.
For conciseness, avoid using self since Swift does not require it to access an object's properties or invoke its methods.
Use self only when required by the compiler (in @escaping closures, or in initializers to disambiguate properties from arguments). In other words, if it compiles without self then omit it.
class BoardLocation {
let row: Int, column: Int
init(row: Int, column: Int) {
self.row = row
self.column = column
let closure = {
print(self.row)
}
}
}For conciseness, if a computed property is read-only, omit the get clause. The get clause is required only when a set clause is provided. If the property contains a single expression, use implicit returns.
Preferred:
var diameter: Double {
radius * 2
}Not Preferred:
var diameter: Double {
get {
return radius * 2
}
}Marking classes or members as final in tutorials can distract from the main topic and is not required. Nevertheless, use of final can sometimes clarify your intent and is worth the cost. In the below example, EquilateralTriangle has a particular purpose and customization in a derived class is not intended. Marking it final makes that clear.
// We don't allow subclassing of an equilateral triangle
final class EquilateralTriangle: Triangle {
let sideLength: Int
init(sideLength: Int) {
self.sideLength = sideLength
}
}Keep short function declarations on one line including the opening brace:
func reticulateSplines(spline: [Double]) -> Bool {
// reticulate code goes here
}For functions with long signatures, put each parameter on a new line and add an extra indent on subsequent lines:
func reticulateSplines(
spline: [Double],
adjustmentFactor: Double,
translateConstant: Int,
comment: String
) -> Bool {
// reticulate code goes here
}Don't use (Void) to represent the lack of an input; simply use (). Use Void instead of () for closure and function outputs.
Preferred:
func updateConstraints() -> Void {
// magic happens here
}
typealias CompletionHandler = (result) -> VoidNot Preferred:
func updateConstraints() -> () {
// magic happens here
}
typealias CompletionHandler = (result) -> ()Since SE-0091, where applicable, declare an operator function as a static function within the type it is relevant to.
Preferred:
struct Foo: Equatable {
static func ==(lhs: Foo, rhs: Foo) -> Bool {
// equating code goes here
}
}Not Preferred:
struct Foo: Equatable {}
func ==(lhs: Foo, rhs: Foo) -> Bool {
// equating code goes here
}Mirror the style of function declarations at call sites. Calls that fit on a single line should be written as such:
Not Preferred:
let success = reticulateSplines(splines)If the call site must be wrapped, put each parameter on a new line, indented one additional level:
let success = reticulateSplines(
spline: splines,
adjustmentFactor: 1.3,
translateConstant: 2,
comment: "normalize the display"
)Use trailing closure syntax only if there's a single closure expression parameter at the end of the argument list. Give the closure parameters descriptive names.
Preferred:
UIView.animate(withDuration: 1.0) {
self.myView.alpha = 0
}
UIView.animate(withDuration: 1.0, animations: {
self.myView.alpha = 0
}, completion: { finished in
self.myView.removeFromSuperview()
})Not Preferred:
UIView.animate(withDuration: 1.0, animations: {
self.myView.alpha = 0
})
UIView.animate(withDuration: 1.0, animations: {
self.myView.alpha = 0
}) { f in
self.myView.removeFromSuperview()
}For single-expression closures where the context is clear, use implicit returns:
attendeeList.sort { a, b in
a > b
}Chained methods using trailing closures should be clear and easy to read in context. Decisions on spacing, line breaks, and when to use named versus anonymous arguments is left to the discretion of the author. Examples:
let value = numbers.map { $0 * 2 }.filter { $0 % 3 == 0 }.index(of: 90)
let value = numbers
.map {$0 * 2}
.filter {$0 > 50}
.map {$0 + 10}Empty closures should be defined using the () -> Void declaration.
Preferred:
func performInBackground(activity: @escaping () -> Void)Preferred:
Not Preferred:
func performInBackground(activity: @escaping () -> ())
let widthString = "\(width)" // String
func performInBackground(activity: @escaping Void -> Void)Always use Swift's native types and expressions when available. Swift offers bridging to Objective-C so you can still use the full set of methods as needed.
Preferred:
let width = 120.0 // Double
let widthString = "\(width)" // StringLess Preferred:
let width = 120.0 // Double
let widthString = (width as NSNumber).stringValue // StringNot Preferred:
let width: NSNumber = 120.0 // NSNumber
let widthString: NSString = width.stringValue // NSStringIn drawing code, use CGFloat if it makes the code more succinct by avoiding too many conversions.
Constants are defined using the let keyword and variables with the var keyword. Always use let instead of var if the value of the variable will not change.
Tip: A good technique is to define everything using let and only change it to var if the compiler complains!
You can define constants on a type rather than on an instance of that type using type properties. To declare a type property as a constant simply use static let. Type properties declared in this way are generally preferred over global constants because they are easier to distinguish from instance properties. Example:
Preferred:
enum Math {
static let e = 2.718281828459045235360287
static let root2 = 1.41421356237309504880168872
}
let hypotenuse = side * Math.root2Note: The advantage of using a case-less enumeration is that it can't accidentally be instantiated and works as a pure namespace.
Not Preferred:
let e = 2.718281828459045235360287 // pollutes global namespace
let root2 = 1.41421356237309504880168872
let hypotenuse = side * root2 // what is root2?Static methods and type properties work similarly to global functions and global variables and should be used sparingly. They are useful when functionality is scoped to a particular type or when Objective-C interoperability is required.
Declare variables and function return types as optional with ? where a nil value is acceptable. Use the default nil initializer when declaring optional types.
Preferred
struct Person {
let name: String
let nickname: String?
}Not Preferred
struct Person {
let name: String
let nickname: String? = nil
}Use implicitly unwrapped types declared with ! only for instance variables that you know will be initialized later before use, such as subviews that will be set up in viewDidLoad(). Prefer optional binding to implicitly unwrapped optionals in most other cases.
When accessing an optional value, use optional chaining if the value is only accessed once or if there are many optionals in the chain:
textContainer?.textLabel?.setNeedsDisplay()Use optional binding when it's more convenient to unwrap once and perform multiple operations:
if let textContainer = textContainer {
// do many things with textContainer
}Notes: Swift 5.7 introduced new shorthand syntax for unwrapping optionals into shadowed variables:
if let textContainer {
// do many things with textContainer
}When naming optional variables and properties, avoid naming them like optionalString or maybeView since their optional-ness is already in the type declaration.
For optional binding, shadow the original name whenever possible rather than using names like unwrappedView or actualLabel.
Preferred:
var subview: UIView?
var volume: Double?
// later on...
if let subview = subview, let volume = volume {
// do something with unwrapped subview and volume
}
// another example
resource.request().onComplete { [weak self] response in
guard let self = self else { return }
let model = self.updateModel(response)
self.updateUI(model)
}Not Preferred:
var optionalSubview: UIView?
var volume: Double?
if let unwrappedSubview = optionalSubview {
if let realVolume = volume {
// do something with unwrappedSubview and realVolume
}
}
// another example
UIView.animate(withDuration: 2.0) { [weak self] in
guard let strongSelf = self else { return }
strongSelf.alpha = 1.0
}Consider using lazy initialization for finer grained control over object lifetime. This is especially true for UIViewController that loads views lazily. You can either use a closure that is immediately called { }() or call a private factory method. Example:
lazy var locationManager = makeLocationManager()
private func makeLocationManager() -> CLLocationManager {
let manager = CLLocationManager()
manager.desiredAccuracy = kCLLocationAccuracyBest
manager.delegate = self
manager.requestAlwaysAuthorization()
return manager
}Notes:
[unowned self]is not required here. A retain cycle is not created.- Location manager has a side-effect for popping up UI to ask the user for permission so fine grain control makes sense here.
When using OptionSets, signify the "no" option using an empty array literal rather than the rawValue initializer. For example, given:
struct PackagingOptions : OptionSet {
let rawValue: Int
init(rawValue: Int) { self.rawValue = rawValue }
static let box = PackagingOptions(rawValue: 1)
static let carton = PackagingOptions(rawValue: 2)
}
let options: PackagingOptions = [.box]
func placeOrder(orderId: String, options: PackagingOptions) { ... }Preferred:
placeOrder("1234", options: [])Not Preferred:
placeOrder("1234", options: PackagingOptions(rawValue: 0))Prefer compact code and let the compiler infer the type for constants or variables of single instances. Type inference is also appropriate for small, non-empty arrays and dictionaries. When required, specify the specific type such as CGFloat or Int16.
Preferred:
let message = "Click the button"
let currentBounds = computeViewBounds()
var names = ["Mic", "Sam", "Christine"]
let maximumWidth: CGFloat = 106.5
let options: Colors = [.red, .white, .blue]Not Preferred:
let message: String = "Click the button"
let currentBounds: CGRect = computeViewBounds()
var names = [String]()
let maximumWidth = CGFloat(106.5)
let options = [Colors.red, Colors.white, Colors.blue]For empty arrays and dictionaries, use type annotation. (For an array or dictionary assigned to a large, multi-line literal, use type annotation.)
Preferred:
var names: [String] = []
var lookup: [String: Int] = [:]Not Preferred:
var names = [String]()
var lookup = [String: Int]()NOTE: Following this guideline means picking descriptive names is even more important than before.
Prefer the shortcut versions of type declarations over the full generics syntax.
Preferred:
var deviceModels: [String]
var employees: [Int: String]
var faxNumber: Int?Not Preferred:
var deviceModels: Array<String>
var employees: Dictionary<Int, String>
var faxNumber: Optional<Int>Free functions, which aren't attached to a class or type, should be used sparingly. When possible, prefer to use a method instead of a free function. This aids in readability and discoverability.
Free functions are most appropriate when they aren't associated with any particular type or instance.
Preferred
let sorted = items.mergeSorted() // easily discoverable
rocket.launch() // acts on the modelNot Preferred
let sorted = mergeSort(items) // hard to discover
launch(&rocket)Free Function Exceptions
let tuples = zip(a, b) // feels natural as a free function (symmetry)
let value = max(x, y, z) // another free function that feels naturalCode (even non-production, tutorial demo code) should not create reference cycles. Analyze your object graph and prevent strong cycles with weak and unowned references. Alternatively, use value types (struct, enum) to prevent cycles altogether.
Extend object lifetime using the [weak self] and guard let self else { return } idiom. [weak self] is preferred to [unowned self] where it is not immediately obvious that self outlives the closure. Explicitly extending lifetime is preferred to optional chaining.
Preferred
resource.request().onComplete { [weak self] response in
guard let self else { return }
let model = self.updateModel(response)
self.updateUI(model)
}resource.request().onComplete { [weak self] response in
self?.handleResponse(response)
}Not Preferred
// might crash if self is released before response returns
resource.request().onComplete { [unowned self] response in
let model = self.updateModel(response)
self.updateUI(model)
}Not Preferred
// deallocate could happen between updating the model and updating UI
resource.request().onComplete { [weak self] response in
let model = self?.updateModel(response)
self?.updateUI(model)
}Use private as the leading property specifier. Using private and fileprivate appropriately, however, adds clarity and promotes encapsulation. Prefer private to fileprivate; use fileprivate only when the compiler insists.
Only explicitly use open, public, and internal when you require a full access control specification.
Use access control as the leading property specifier. The only things that should come before access control are the static specifier or attributes such as @IBAction, @IBOutlet and @discardableResult.
Preferred:
private let message = "Great Scott!"
class TimeMachine {
private dynamic lazy var fluxCapacitor = FluxCapacitor()
}Not Preferred:
fileprivate let message = "Great Scott!"
class TimeMachine {
lazy dynamic private var fluxCapacitor = FluxCapacitor()
}Prefer the for-in style of for loop over the while-condition-increment style.
Preferred:
for _ in 0..<3 {
print("Hello three times")
}
for (index, person) in attendeeList.enumerated() {
print("\(person) is at position #\(index)")
}
for index in stride(from: 0, to: items.count, by: 2) {
print(index)
}
for index in (0...3).reversed() {
print(index)
}Not Preferred:
var i = 0
while i < 3 {
print("Hello three times")
i += 1
}
var i = 0
while i < attendeeList.count {
let person = attendeeList[i]
print("\(person) is at position #\(i)")
i += 1
}The Ternary operator, ?: , should only be used when it increases clarity or code neatness. A single condition is usually all that should be evaluated. Evaluating multiple conditions is usually more understandable as an if statement or refactored into instance variables. In general, the best use of the ternary operator is during assignment of a variable and deciding which value to use.
Preferred:
let value = 5
result = value != 0 ? x : y
let isHorizontal = true
result = isHorizontal ? x : yNot Preferred:
result = a > b ? x = c > d ? c : d : yThird party libraries that have yet to annotate their Objective-C headers for nullability will be imported into Swift with implicitly unwrapped optionals. These should be treated as optionals and guarded on use as any other optional parameter. For example, an Objective-C class imported into Swift might look like:
open class ThirdPartyDelegate : NSObject {
open func onError(errorInfo: ErrorInfo!)
}An overriding subclass should instead treat implicitly unwrapped optional parameters as optionals.
open class MyDelegate: ThirdPartyDelegate {
override open func onError(errorInfo: ErrorInfo?) {
logger.error("[ERROR] ThirdPartyDelegate: \(errorInfo?.message)")
}
}Specifically, the imported type ErrorInfo! has been changed to ErrorInfo?
When coding with conditionals, the left-hand margin of the code should be the "golden" or "happy" path. That is, don't nest if statements. Multiple return statements are OK. The guard statement is built for this.
Preferred:
func computeFFT(context: Context?, inputData: InputData?) throws -> Frequencies {
guard let context = context else {
throw FFTError.noContext
}
guard let inputData = inputData else {
throw FFTError.noInputData
}
// use context and input to compute the frequencies
return frequencies
}Not Preferred:
func computeFFT(context: Context?, inputData: InputData?) throws -> Frequencies {
if let context = context {
if let inputData = inputData {
// use context and input to compute the frequencies
return frequencies
} else {
throw FFTError.noInputData
}
} else {
throw FFTError.noContext
}
}When multiple optionals are unwrapped either with guard or if let, minimize nesting by using the compound version when possible. In the compound version, place the guard on its own line, then indent each condition on its own line. The else clause is indented to match the guard itself, as shown below. Example:
Preferred:
guard
let number1 = number1,
let number2 = number2,
let number3 = number3
else {
fatalError("impossible")
}
// do something with numbersNot Preferred:
if let number1 = number1 {
if let number2 = number2 {
if let number3 = number3 {
// do something with numbers
} else {
fatalError("impossible")
}
}
} else {
fatalError("impossible")
}
} else {
fatalError("impossible")
}Guard statements are required to exit in some way. Generally, this should be simple one line statement such as return, throw, break, continue, and fatalError(). Large code blocks should be avoided. If cleanup code is required for multiple exit points, consider using a defer block to avoid cleanup code duplication.
Swift does not require a semicolon after each statement in your code. They are only required if you wish to combine multiple statements on a single line.
Do not write multiple statements on a single line separated with semicolons.
The only exception to this rule is the for-conditional-increment construct, which requires semicolons. However, alternative for-in constructs should be used where possible.
Preferred:
let swift = "not a scripting language"Not Preferred:
let swift = "not a scripting language";Parentheses around conditionals are not required and should be omitted.
Preferred:
if name == "Hello" {
print("World")
}Not Preferred:
if (name == "Hello") {
print("World")
}In larger expressions, optional parentheses can sometimes make code read more clearly.
Preferred:
let playerMark = (player == current ? "X" : "O")When building a long string literal, you're encouraged to use the multi-line string literal syntax. Open the literal on the same line as the assignment but do not include text on that line. Indent the text block one additional level.
Preferred:
let message = """
You cannot charge the flux \
capacitor with a 9V battery.
You must use a super-charger \
which costs 10 credits. You currently \
have \(credits) credits available.
"""Not Preferred:
let message = """You cannot charge the flux \
capacitor with a 9V battery.
You must use a super-charger \
which costs 10 credits. You currently \
have \(credits) credits available.
"""Not Preferred:
let message = "You cannot charge the flux " +
"capacitor with a 9V battery.\n" +
"You must use a super-charger " +
"which costs 10 credits. You currently " +
"have \(credits) credits available."Do not use emoji in your projects. For those readers who actually type in their code, it's an unnecessary source of friction. While it may be cute, it doesn't add to the learning and it interrupts the coding flow for these readers.
Likewise, do not use Xcode's ability to drag a color or an image into a source statement. These turn into #colorLiteral and #imageLiteral, respectively, and present unpleasant challenges for a reader trying to enter them based on tutorial text. Instead, use UIColor(red:green:blue) and UIImage(imageLiteralResourceName:).
Where an Xcode project is involved, the organization should be set to Ray Wenderlich and the Bundle Identifier set to com.raywenderlich.TutorialName where TutorialName is the name of the tutorial project.
When instantiating view controllers from storyboards, it is impossible to use non-optional property types since the view controller's lifecycle is out of the app's direct control.
- For instantiation, consider a factory method, conventionally named
make. - For preparation during segue, consider a method named
prepare. - Both methods should require external names for all parameters, to mimic the feel of initializers.
Example make
class ProfileViewController: UIViewController {
var user: User!
class func make(user user: User) -> ProfileViewController {
let storyboard = UIStoryboard(name: "ProfileViewController", bundle: nil)
let controller = storyboard.instantiateInitialViewController() as! ProfileViewController
controller.user = user
return controller
}
}Example prepare
class ProfileViewController: UIViewController {
var user: User!
func prepare(user user: User) {
self.user = user
}
}Combined Example
Naturally, if a controller could be instantiated directly, or navigated to via a segue, the approaches should be combined:
class ProfileViewController: UIViewController {
var user: User!
class func build(user user: User) -> ProfileViewController {
let storyboard = UIStoryboard(name: "ProfileViewController", bundle: nil)
let controller = storyboard.instantiateInitialViewController() as! ProfileViewController
controller.prepare(user: user)
return controller
}
func prepare(user user: User) {
self.user = user
}
}When writing client software, use their appropriate copyright notice, or the default Xcode copyright.
For open source WillowTree projects, the following copyright statement should be included at the top of every source file and as part of a LICENSE file at the root of the system directory:
/*
* Copyright (c) 2023 WillowTree.
*
* Permission is hereby granted, free of charge, to any person obtaining a copy
* of this software and associated documentation files (the "Software"), to deal
* in the Software without restriction, including without limitation the rights
* to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
* copies of the Software, and to permit persons to whom the Software is
* furnished to do so, subject to the following conditions:
*
* The above copyright notice and this permission notice shall be included in
* all copies or substantial portions of the Software.
*
* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
* IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
* FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
* AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
* LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
* OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
* THE SOFTWARE.
*/
It is a collaborative effort from the most stylish willowtreeapps.com team members and its community:
[willowtreeapps.com]
[raywenderlich.com / kodeco]
- Jawwad Ahmad
- Soheil Moayedi Azarpour
- Scott Berrevoets
- Eric Cerney
- Sam Davies
- Evan Dekhayser
- Jean-Pierre Distler
- Colin Eberhardt
- Ray Fix
- Joshua Greene
- Greg Heo
- Matthijs Hollemans
- Erik Kerber
- Christopher LaPollo
- Ben Morrow
- Andy Pereira
- Ryan Nystrom
- Andy Obusek
- Cesare Rocchi
- Ellen Shapiro
- Marin Todorov
- Chris Wagner
- Ray Wenderlich
- Jack Wu
Hat tip to Nicholas Waynik and the Objective-C Style Guide team!
We also draw inspiration from Apple’s reference material on Swift: