Following this style guide should:
- Make it easier to read and begin understanding unfamiliar code.
- Make code easier to maintain.
- Reduce simple programmer errors.
- Reduce cognitive load while coding.
- Keep discussions on diffs focused on the code's logic rather than its style.
Note that brevity is not a primary goal. Code should be made more concise only if other good code qualities (such as readability, simplicity, and clarity) remain equal or are improved.
- This guide is in addition to the official Swift API Design Guidelines. These rules should not contradict that document.
- These rules should not fight Xcode's ^ + I indentation behavior.
- We strive to make every rule lintable:
- If a rule changes the format of the code, it needs to be able to be reformatted automatically (either using SwiftFormat or SwiftLint autocorrect).
- For rules that don't directly change the format of the code, we should have a lint rule that throws a warning.
- Exceptions to these rules should be rare and heavily justified.
This repo includes a Swift Package Manager command plugin that you can use to automatically reformat or lint your package according to the style guide. To use this command plugin with your package, all you need to do is add this repo as a dependency:
dependencies: [
.package(url: "https://github.com/airbnb/swift", from: "1.0.0"),
]and then run the format command plugin in your package directory:
$ swift package formatUsage guide
# Supported in Xcode 14+. Prompts for permission to write to the package directory.
$ swift package format
# When using the Xcode 13 toolchain, or a noninteractive shell, you must use:
$ swift package --allow-writing-to-package-directory format
# To just lint without reformatting, you can use `--lint`:
$ swift package format --lint
# By default the command plugin runs on the entire package directory.
# You can exclude directories using `exclude`:
$ swift package format --exclude Tests
# Alternatively you can explicitly list the set of paths and/or SPM targets:
$ swift package format --paths Sources Tests Package.swift
$ swift package format --targets AirbnbSwiftFormatTool
# The plugin infers your package's minimum Swift version from the `swift-tools-version`
# in your `Package.swift`, but you can provide a custom value with `--swift-version`:
$ swift package format --swift-version 5.3The package plugin returns a non-zero exit code if there is a lint failure that requires attention.
- In
--lintmode, any lint failure from any tool will result in a non-zero exit code. - In standard autocorrect mode without
--lint, only failures from SwiftLint lint-only rules will result in a non-zero exit code.
- Xcode Formatting
- Naming
- Style
- Patterns
- File Organization
- Objective-C Interoperability
- Contributors
- Amendments
You can enable the following settings in Xcode by running this script, e.g. as part of a "Run Script" build phase.
-
(link) Each line should have a maximum column width of 100 characters.
Due to larger screen sizes, we have opted to choose a page guide greater than 80.
We currently only "strictly enforce" (lint / auto-format) a maximum column width of 130 characters to limit the cases where manual clean up is required for reformatted lines that fall slightly above the threshold.
-
(link) Use 2 spaces to indent lines.
-
(link) Trim trailing whitespace in all lines.
-
(link) Use PascalCase for type and protocol names, and lowerCamelCase for everything else.
protocol SpaceThing { // ... } class SpaceFleet: SpaceThing { enum Formation { // ... } class Spaceship { // ... } var ships: [Spaceship] = [] static let worldName: String = "Earth" func addShip(_ ship: Spaceship) { // ... } } let myFleet = SpaceFleet()
Exception: You may prefix a private property with an underscore if it is backing an identically-named property or method with a higher access level.
There are specific scenarios where a backing property or method that is prefixed with an underscore could be easier to read than using a more descriptive name.
- Type erasure
public final class AnyRequester<ModelType>: Requester { public init<T: Requester>(_ requester: T) where T.ModelType == ModelType { _executeRequest = requester.executeRequest } @discardableResult public func executeRequest( _ request: URLRequest, onSuccess: @escaping (ModelType, Bool) -> Void, onFailure: @escaping (Error) -> Void) -> URLSessionCancellable { return _executeRequest(request, onSuccess, onFailure) } private let _executeRequest: ( URLRequest, @escaping (ModelType, Bool) -> Void, @escaping (Error) -> Void) -> URLSessionCancellable }
- Backing a less specific type with a more specific type
final class ExperiencesViewController: UIViewController { // We can't name this view since UIViewController has a view: UIView property. private lazy var _view = CustomView() loadView() { self.view = _view } }
-
(link) Name booleans like
isSpaceship,hasSpacesuit, etc. This makes it clear that they are booleans and not other types. -
(link) Acronyms in names (e.g.
URL) should be all-caps except when it’s the start of a name that would otherwise be lowerCamelCase, in which case it should be uniformly lower-cased.// WRONG class UrlValidator { func isValidUrl(_ URL: URL) -> Bool { // ... } func isProfileUrl(_ URL: URL, for userId: String) -> Bool { // ... } } let URLValidator = UrlValidator() let isProfile = URLValidator.isProfileUrl(URLToTest, userId: IDOfUser) // RIGHT class URLValidator { func isValidURL(_ url: URL) -> Bool { // ... } func isProfileURL(_ url: URL, for userID: String) -> Bool { // ... } } let urlValidator = URLValidator() let isProfile = urlValidator.isProfileURL(urlToTest, userID: idOfUser)
-
(link) Names should be written with their most general part first and their most specific part last. The meaning of "most general" depends on context, but should roughly mean "that which most helps you narrow down your search for the item you're looking for." Most importantly, be consistent with how you order the parts of your name.
// WRONG let rightTitleMargin: CGFloat let leftTitleMargin: CGFloat let bodyRightMargin: CGFloat let bodyLeftMargin: CGFloat // RIGHT let titleMarginRight: CGFloat let titleMarginLeft: CGFloat let bodyMarginRight: CGFloat let bodyMarginLeft: CGFloat
-
(link) Include a hint about type in a name if it would otherwise be ambiguous.
// WRONG let title: String let cancel: UIButton // RIGHT let titleText: String let cancelButton: UIButton
-
(link) Event-handling functions should be named like past-tense sentences. The subject can be omitted if it's not needed for clarity.
// WRONG class ExperiencesViewController { private func handleBookButtonTap() { // ... } private func modelChanged() { // ... } } // RIGHT class ExperiencesViewController { private func didTapBookButton() { // ... } private func modelDidChange() { // ... } }
-
(link) Avoid Objective-C-style acronym prefixes. This is no longer needed to avoid naming conflicts in Swift.
// WRONG class AIRAccount { // ... } // RIGHT class Account { // ... }
-
(link) Avoid
*Controllerin names of classes that aren't view controllers.
-
(link) Don't include types where they can be easily inferred.
// WRONG let sun: Star = Star(mass: 1.989e30) let earth: Planet = Planet.earth // RIGHT let sun = Star(mass: 1.989e30) let earth = Planet.earth // NOT RECOMMENDED. However, since the linter doesn't have full type information, this is not enforced automatically. let moon: Moon = earth.moon // returns `Moon` // RIGHT let moon = earth.moon let moon: PlanetaryBody? = earth.moon // WRONG: Most literals provide a default type that can be inferred. let enableGravity: Bool = true let numberOfPlanets: Int = 8 let sunMass: Double = 1.989e30 // RIGHT let enableGravity = true let numberOfPlanets = 8 let sunMass = 1.989e30 // WRONG: Types can be inferred from if/switch expressions as well if each branch has the same explicit type. let smallestPlanet: Planet = if treatPlutoAsPlanet { Planet.pluto } else { Planet.mercury } // RIGHT let smallestPlanet = if treatPlutoAsPlanet { Planet.pluto } else { Planet.mercury }
-
(link) Prefer letting the type of a variable or property be inferred from the right-hand-side value rather than writing the type explicitly on the left-hand side.
Prefer using inferred types when the right-hand-side value is a static member with a leading dot (e.g. an
init, astaticproperty / function, or an enum case). This applies to both local variables and property declarations:// WRONG struct SolarSystemBuilder { let sun: Star = .init(mass: 1.989e30) let earth: Planet = .earth func setUp() { let galaxy: Galaxy = .andromeda let system: SolarSystem = .init(sun, earth) galaxy.add(system) } } // RIGHT struct SolarSystemBuilder { let sun = Star(mass: 1.989e30) let earth = Planet.earth func setUp() { let galaxy = Galaxy.andromeda let system = SolarSystem(sun, earth) galaxy.add(system) } }
Explicit types are still permitted in other cases:
// RIGHT: There is no right-hand-side value, so an explicit type is required. let sun: Star // RIGHT: The right-hand-side is not a static member of the left-hand type. let moon: PlantaryBody = earth.moon let sunMass: Float = 1.989e30 let planets: [Planet] = [] let venusMoon: Moon? = nil
There are some rare cases where the inferred type syntax has a different meaning than the explicit type syntax. In these cases, the explicit type syntax is still permitted:
extension String { static let earth = "Earth" } // WRONG: fails with "error: type 'String?' has no member 'earth'" let planetName = String?.earth // RIGHT let planetName: String? = .earth
struct SaturnOutline: ShapeStyle { ... } extension ShapeStyle where Self == SaturnOutline { static var saturnOutline: SaturnOutline { SaturnOutline() } } // WRONG: fails with "error: static member 'saturnOutline' cannot be used on protocol metatype '(any ShapeStyle).Type'" let myShape2 = (any ShapeStyle).myShape // RIGHT: If the property's type is an existential / protocol type, moving the type // to the right-hand side will result in invalid code if the value is defined in an // extension like `extension ShapeStyle where Self == SaturnOutline`. // SwiftFormat autocorrect detects this case by checking for the existential `any` keyword. let myShape1: any ShapeStyle = .saturnOutline
-
(link) Don't use
selfunless it's necessary for disambiguation or required by the language.final class Listing { init(capacity: Int, allowsPets: Bool) { // WRONG self.capacity = capacity self.isFamilyFriendly = !allowsPets // `self.` not required here // RIGHT self.capacity = capacity isFamilyFriendly = !allowsPets } private let isFamilyFriendly: Bool private var capacity: Int private func increaseCapacity(by amount: Int) { // WRONG self.capacity += amount // RIGHT capacity += amount // WRONG self.save() // RIGHT save() } }
-
(link) Bind to
selfwhen upgrading from a weak reference.// WRONG class MyClass { func request(completion: () -> Void) { API.request() { [weak self] response in guard let strongSelf = self else { return } // Do work completion() } } } // RIGHT class MyClass { func request(completion: () -> Void) { API.request() { [weak self] response in guard let self else { return } // Do work completion() } } }
-
(link) Add a trailing comma on the last element of a multi-line array.
// WRONG let rowContent = [ listingUrgencyDatesRowContent(), listingUrgencyBookedRowContent(), listingUrgencyBookedShortRowContent() ] // RIGHT let rowContent = [ listingUrgencyDatesRowContent(), listingUrgencyBookedRowContent(), listingUrgencyBookedShortRowContent(), ]
-
(link) There should be no spaces inside the brackets of collection literals.
// WRONG let innerPlanets = [ mercury, venus, earth, mars ] let largestObjects = [ .star: sun, .planet: jupiter ] // RIGHT let innerPlanets = [mercury, venus, earth, mars] let largestObjects = [.star: sun,
.planet: jupiter] -
(link) Name members of tuples for extra clarity. Rule of thumb: if you've got more than 3 fields, you should probably be using a struct.
// WRONG func whatever() -> (Int, Int) { return (4, 4) } let thing = whatever() print(thing.0) // RIGHT func whatever() -> (x: Int, y: Int) { return (x: 4, y: 4) } // THIS IS ALSO OKAY func whatever2() -> (x: Int, y: Int) { let x = 4 let y = 4 return (x, y) } let coord = whatever() coord.x coord.y
-
(link) Colons should always be followed by a space, but not preceded by a space.
// WRONG let planet:CelestialObject = sun.planets[0] let planet : CelestialObject = sun.planets[0] // RIGHT let planet: CelestialObject = sun.planets[0]
// WRONG class Planet : CelestialObject { // ... } // RIGHT class Planet: CelestialObject { // ... }
// WRONG let moons: [Planet : Moon] = [ mercury : [], venus : [], earth : [theMoon], mars : [phobos,deimos], ] // RIGHT let moons: [Planet: Moon] = [ mercury: [], venus: [], earth: [theMoon], mars: [phobos,deimos], ]
-
(link) Place a space on either side of a return arrow for readability.
// WRONG func doSomething()->String { // ... } // RIGHT func doSomething() -> String { // ... }
// WRONG func doSomething(completion: ()->Void) { // ... } // RIGHT func doSomething(completion: () -> Void) { // ... }
-
(link) Omit unnecessary parentheses.
// WRONG if (userCount > 0) { ... } switch (someValue) { ... } let evens = userCounts.filter { (number) in number.isMultiple(of: 2) } let squares = userCounts.map() { $0 * $0 } // RIGHT if userCount > 0 { ... } switch someValue { ... } let evens = userCounts.filter { number in number.isMultiple(of: 2) } let squares = userCounts.map { $0 * $0 }
-
(link) Omit enum associated values from case statements when all arguments are unlabeled.
// WRONG if case .done(_) = result { ... } switch animal { case .dog(_, _, _): ... } // RIGHT if case .done = result { ... } switch animal { case .dog: ... }
-
(link) When destructuring an enum case or a tuple, place the
letkeyword inline, adjacent to each individual property assignment.// WRONG switch result { case let .success(value): // ... case let .error(errorCode, errorReason): // ... } // WRONG guard let case .success(value) else { return } // RIGHT switch result { case .success(let value): // ... case .error(let errorCode, let errorReason): // ... } // RIGHT guard case .success(let value) else { return }
-
Consistency: We should prefer to either always inline the
letkeyword or never inline theletkeyword. In Airbnb's Swift codebase, we observed that inlineletis used far more often in practice (especially when destructuring enum cases with a single associated value). -
Clarity: Inlining the
letkeyword makes it more clear which identifiers are part of the conditional check and which identifiers are binding new variables, since theletkeyword is always adjacent to the variable identifier.
// `let` is adjacent to the variable identifier, so it is immediately obvious // at a glance that these identifiers represent new variable bindings case .enumCaseWithSingleAssociatedValue(let string): case .enumCaseWithMultipleAssociatedValues(let string, let int): // The `let` keyword is quite far from the variable identifiers, // so it is less obvious that they represent new variable bindings case let .enumCaseWithSingleAssociatedValue(string): case let .enumCaseWithMultipleAssociatedValues(string, int):
-
-
(link) Place attributes for functions, types, and computed properties on the line above the declaration.
// WRONG @objc class Spaceship { @ViewBuilder var controlPanel: some View { // ... } @discardableResult func fly() -> Bool { // ... } } // RIGHT @objc class Spaceship { @ViewBuilder var controlPanel: some View { // ... } @discardableResult func fly() -> Bool { // ... } }
-
(link) Place simple attributes for stored properties on the same line as the rest of the declaration. Complex attributes with named arguments, or more than one unnamed argument, should be placed on the previous line.
// WRONG. These simple property wrappers should be written on the same line as the declaration. struct SpaceshipDashboardView { @State private var warpDriveEnabled: Bool @ObservedObject private var lifeSupportService: LifeSupportService @Environment(\.controlPanelStyle) private var controlPanelStyle } // RIGHT struct SpaceshipDashboardView { @State private var warpDriveEnabled: Bool @ObservedObject private var lifeSupportService: LifeSupportService @Environment(\.controlPanelStyle) private var controlPanelStyle }
// WRONG. These complex attached macros should be written on the previous line. struct SolarSystemView { @Query(sort: \.distance) var allPlanets: [Planet] @Query(sort: \.age, order: .reverse) var moonsByAge: [Moon] } // RIGHT struct SolarSystemView { @Query(sort: \.distance) var allPlanets: [Planet] @Query(sort: \.age, order: .reverse) var oldestMoons: [Moon] }
// WRONG. These long, complex attributes should be written on the previous line. struct RocketFactory { @available(*, unavailable, message: "No longer in production") var saturn5Builder: Saturn5Builder @available(*, deprecated, message: "To be retired by 2030") var atlas5Builder: Atlas5Builder @available(*, iOS 18.0, tvOS 18.0, macOS 15.0, watchOS 11.0) var newGlennBuilder: NewGlennBuilder } // RIGHT struct RocketFactory { @available(*, unavailable, message: "No longer in production") var saturn5Builder: Saturn5Builder @available(*, deprecated, message: "To be retired by 2030") var atlas5Builder: Atlas5Builder @available(*, iOS 18.0, tvOS 18.0, macOS 15.0, watchOS 11.0) var newGlennBuilder: NewGlennBuilder }
Unlike other types of declarations, which have braces and span multiple lines, stored property declarations are often only a single line of code. Stored properties are often written sequentially without any blank lines between them. This makes the code compact without hurting readability, and allows for related properties to be grouped together in blocks:
struct SpaceshipDashboardView { @State private var warpDriveEnabled: Bool @State private var lifeSupportEnabled: Bool @State private var artificialGravityEnabled: Bool @State private var tractorBeamEnabled: Bool @Environment(\.controlPanelStyle) private var controlPanelStyle @Environment(\.toggleButtonStyle) private var toggleButtonStyle }
If stored property attributes were written on the previous line (like other types of attributes), then the properties start to visually bleed together unless you add blank lines between them:
struct SpaceshipDashboardView { @State private var warpDriveEnabled: Bool @State private var lifeSupportEnabled: Bool @State private var artificialGravityEnabled: Bool @State private var tractorBeamEnabled: Bool @Environment(\.controlPanelStyle) private var controlPanelStyle @Environment(\.toggleButtonStyle) private var toggleButtonStyle }
If you add blank lines, the list of properties becomes much longer and you lose the ability to group related properties together:
struct SpaceshipDashboardView { @State private var warpDriveEnabled: Bool @State private var lifeSupportEnabled: Bool @State private var artificialGravityEnabled: Bool @State private var tractorBeamEnabled: Bool @Environment(\.controlPanelStyle) private var controlPanelStyle @Environment(\.toggleButtonStyle) private var toggleButtonStyle }
This doesn't apply to complex attributes with named arguments, or multiple unnamed arguments. These arguments are visually complex and typically encode a lot of information, so feel cramped and difficult to read when written on a single line:
// Despite being less than 100 characters long, these lines are very complex and feel unnecessarily long: @available(*, unavailable, message: "No longer in production") var saturn5Builder: Saturn5Builder @available(*, deprecated, message: "To be retired by 2030") var atlas5Builder: Atlas5Builder @available(*, iOS 18.0, tvOS 18.0, macOS 15.0, watchOS 11.0) var newGlennBuilder: NewGlennBuilder
-
(link) Multi-line arrays should have each bracket on a separate line. Put the opening and closing brackets on separate lines from any of the elements of the array. Also add a trailing comma on the last element.
// WRONG let rowContent = [listingUrgencyDatesRowContent(), listingUrgencyBookedRowContent(), listingUrgencyBookedShortRowContent()] // WRONG let rowContent = [ listingUrgencyDatesRowContent(), listingUrgencyBookedRowContent(), listingUrgencyBookedShortRowContent() ] // RIGHT let rowContent = [ listingUrgencyDatesRowContent(), listingUrgencyBookedRowContent(), listingUrgencyBookedShortRowContent(), ]
-
(link) Long type aliases of protocol compositions should wrap before the
=and before each individual&.// WRONG (too long) public typealias Dependencies = CivilizationServiceProviding & LawsOfPhysicsProviding & PlanetBuilderProviding & UniverseBuilderProviding & UniverseSimulatorServiceProviding // WRONG (naive wrapping) public typealias Dependencies = CivilizationServiceProviding & LawsOfPhysicsProviding & PlanetBuilderProviding & UniverseBuilderProviding & UniverseSimulatorServiceProviding // WRONG (unbalanced) public typealias Dependencies = CivilizationServiceProviding & LawsOfPhysicsProviding & PlanetBuilderProviding & UniverseBuilderProviding & UniverseSimulatorServiceProviding // RIGHT public typealias Dependencies = CivilizationServiceProviding & LawsOfPhysicsProviding & PlanetBuilderProviding & UniverseBuilderProviding & UniverseSimulatorServiceProviding
-
(link) Sort protocol composition type aliases alphabetically.
Protocol composition type aliases are an unordered list with no natural ordering. Sorting alphabetically keeps these lists more organized, which is especially valuable for long protocol compositions.
// WRONG (not sorted) public typealias Dependencies = UniverseBuilderProviding & LawsOfPhysicsProviding & UniverseSimulatorServiceProviding & PlanetBuilderProviding & CivilizationServiceProviding // RIGHT public typealias Dependencies = CivilizationServiceProviding & LawsOfPhysicsProviding & PlanetBuilderProviding & UniverseBuilderProviding & UniverseSimulatorServiceProviding
-
(link) Omit the right-hand side of the expression when unwrapping an optional property to a non-optional property with the same name.
Following the rationale in SE-0345, this shorthand syntax removes unnecessary boilerplate while retaining clarity.
// WRONG if let galaxy = galaxy, galaxy.name == "Milky Way" { … } guard let galaxy = galaxy, galaxy.name == "Milky Way" else { … } // RIGHT if let galaxy, galaxy.name == "Milky Way" { … } guard 10000 let galaxy, galaxy.name == "Milky Way" else { … }
-
(link) Else statements should start on the same line as the previous condition's closing brace, unless the conditions are separated by a blank line or comments.
// WRONG if let galaxy { … } else if let bigBangService { … } else { … } // RIGHT if let galaxy { … } else if let bigBangService { … } else { … } // RIGHT, because there are comments between the conditions if let galaxy { … } // If the galaxy hasn't been created yet, create it using the big bang service else if let bigBangService { … } // If the big bang service doesn't exist, fail gracefully else { … } // RIGHT, because there are blank lines between the conditions if let galaxy { … } else if let bigBangService { // If the galaxy hasn't been created yet, create it using the big bang service … } else { // If the big bang service doesn't exist, fail gracefully … }
-
(link) Multi-line conditional statements should break after the leading keyword. Indent each individual statement by 2 spaces.
Breaking after the leading keyword resets indentation to the standard 2-space grid, which helps avoid fighting Xcode's ^ + I indentation behavior.
// WRONG if let galaxy, galaxy.name == "Milky Way" // Indenting by two spaces fights Xcode's ^+I indentation behavior { … } // WRONG guard let galaxy, galaxy.name == "Milky Way" // Variable width indentation (6 spaces) else { … } // WRONG guard let earth = universe.find( .planet, named: "Earth"), earth.isHabitable // Blends in with previous condition's method arguments else { … } // RIGHT if let galaxy, galaxy.name == "Milky Way" { … } // RIGHT guard let galaxy, galaxy.name == "Milky Way" else { … } // RIGHT guard let earth = universe.find( .planet, named: "Earth"), earth.isHabitable else { … } // RIGHT if let galaxy { … } // RIGHT guard let galaxy else { … }
-
(link) Add a line break after the assignment operator (
=) before a multi-lineiforswitchexpression, and indent the followingif/switchexpression. If the declaration fits on a single line, a line break is not required.This makes it so that
ifandswitchexpressions always have the same "shape" as standardifandswitchstatements, where:- The
if/switchkeyword is always the left-most token on a dedicated line of code. - The conditional branches are always to the right of and below the
if/switchkeyword.
This is most consistent with how the
if/switchkeywords are used for control flow, and thus makes it easier to recognize that the code is using aniforswitchexpression at a glance.// WRONG. Should have a line break after the first `=`. let planetLocation = if let star = planet.star { "The \(star.name) system" } else { "Rogue planet" } // WRONG. The first `=` should be on the line of the variable being assigned. let planetLocation = if let star = planet.star { "The \(star.name) system" } else { "Rogue planet" } // WRONG. `switch` expression should be indented. let planetLocation = switch planet { case .mercury, .venus, .earth, .mars: .terrestrial case .jupiter, .saturn, .uranus, .neptune: .gasGiant } // RIGHT let planetLocation = if let star = planet.star { "The \(star.name) system" } else { "Rogue planet" } // RIGHT let planetType: PlanetType = switch planet { case .mercury, .venus, .earth, .mars: .terrestrial case .jupiter, .saturn, .uranus, .neptune: .gasGiant } // ALSO RIGHT. A line break is not required because the declaration fits on a single line. let moonName = if let moon = planet.moon { moon.name } else { "none" } // ALSO RIGHT. A line break is permitted if it helps with readability. let moonName = if let moon = planet.moon { moon.name } else { "none" }
- The
-
(link) When initializing a new property with the result of a conditional statement (e.g. an
iforswitchstatement), use a singleif/switchexpression where possible rather than defining an uninitialized property and initializing it on every branch of the following conditional statement.There are several benefits to using an
if/switchexpression over simply performing assignment on each branch of the following conditional statement:- In most cases, you no longer need to explicitly write a type annotation for the variable that is being assigned to.
- The compiler will diagnose more cases where using a mutable
varis unnecessary. - The resulting syntax is visually lighter because the property name being assigned doesn't need to be written on each branch.
// BEFORE // 1. An explicit type annotation is required for the uninitialized property. // 2. `var` is unnecessary here because `planetLocation` is never modified after being initialized, but the compiler doesn't diagnose. // 3. The `planetLocation` property name is written on each branch so is redundant and visually noisy. var planetLocation: String if let star = planet.star { planetLocation = "The \(star.name) system" } else { planetLocation = "Rogue planet" } print(planetLocation) // AFTER // 1. No need to write an explicit `: String` type annotation. // 2. The compiler correctly diagnoses that the `var` is unnecessary and emits a warning suggesting to use `let` instead. // 3. Each conditional branch is simply the value being assigned. var planetLocation = if let star = planet.star { "The \(star.name) system" } else { "Rogue planet" } print(planetLocation)
// WRONG let planetLocation: String if let star = planet.star { planetLocation = "The \(star.name) system" } else { planetLocation = "Rogue planet" } let planetType: PlanetType switch planet { case .mercury, .venus, .earth, .mars: planetType = .terrestrial case .jupiter, .saturn, .uranus, .neptune: planetType = .gasGiant } let canBeTerraformed: Bool if let star = planet.star, !planet.isHabitable, planet.isInHabitableZone(of: star) { canBeTerraformed = true } else { canBeTerraformed = false } // RIGHT let planetLocation = if let star = planet.star { "The \(star.name) system" } else { "Rogue planet" } let planetType: PlanetType = switch planet { case .mercury, .venus, .earth, .mars: .terrestrial case .jupiter, .saturn, .uranus, .neptune: .gasGiant } let canBeTerraformed = if let star = planet.star, !planet.isHabitable, planet.isInHabitableZone(of: star) { true } else { false } // ALSO RIGHT. This example cannot be converted to an if/switch expression // because one of the branches is more than just a single expression. let planetLocation: String if let star = planet.star { planetLocation = "The \(star.name) system" } else { let actualLocaton = galaxy.name ?? "the universe" planetLocation = "Rogue planet somewhere in \(actualLocation)" }
-
(link) Insert a blank line following a switch case with a multi-line body. Spacing within an individual switch statement should be consistent. If any case has a multi-line body then all cases should include a trailing blank line. The last switch case doesn't need a blank line, since it is already followed by a closing brace.
Like with declarations in a file, inserting a blank line between scopes makes them easier to visually differentiate.
Complex switch statements are visually busy without blank lines between the cases, making it more difficult to read the code and harder to distinguish between individual cases at a glance. Blank lines between the individual cases make complex switch statements easier to read.
// WRONG. These switch cases should be followed by a blank line. func handle(_ action: SpaceshipAction) { switch action { case .engageWarpDrive: navigationComputer.destination = targetedDestination warpDrive.spinUp() warpDrive.activate() case .enableArtificialGravity: artificialGravityEngine.enable(strength: .oneG) case .scanPlanet(let planet): scanner.target = planet scanner.scanAtmosphere() scanner.scanBiosphere() scanner.scanForArtificialLife() case .handleIncomingEnergyBlast: energyShields.engage() } } // WRONG. While the `.enableArtificialGravity` case isn't multi-line, the other cases are. // For consistency, it should also include a trailing blank line. func handle(_ action: SpaceshipAction) { switch action { case .engageWarpDrive: navigationComputer.destination = targetedDestination warpDrive.spinUp() warpDrive.activate() case .enableArtificialGravity: artificialGravityEngine.enable(strength: .oneG) case .scanPlanet(let planet): scanner.target = planet scanner.scanAtmosphere() scanner.scanBiosphere() scanner.scanForArtificialLife() case .handleIncomingEnergyBlast: energyShields.engage() } } // RIGHT. All of the cases have a trailing blank line. func handle(_ action: SpaceshipAction) { switch action { case .engageWarpDrive: navigationComputer.destination = targetedDestination warpDrive.spinUp() warpDrive.activate() case .enableArtificialGravity: artificialGravityEngine.enable(strength: .oneG) case .scanPlanet(let planet): scanner.target = planet scanner.scanAtmosphere() scanner.scanBiosphere() scanner.scanForArtificialLife() case .handleIncomingEnergyBlast: energyShields.engage() } } // RIGHT. Since none of the cases are multi-line, blank lines are not required. func handle(_ action: SpaceshipAction) { switch action { case .engageWarpDrive: warpDrive.engage() case .enableArtificialGravity: artificialGravityEngine.enable(strength: .oneG) case .scanPlanet(let planet): scanner.scan(planet) case .handleIncomingEnergyBlast: energyShields.engage() } } // ALSO RIGHT. Blank lines are still permitted after single-line switch cases if it helps with readability. func handle(_ action: SpaceshipAction) { switch action { case .engageWarpDrive: warpDrive.engage() case .enableArtificialGravity: artificialGravityEngine.enable(strength: .oneG) case .scanPlanet(let planet): scanner.scan(planet) case .handleIncomingEnergyBlast: energyShields.engage() } } // WRONG. While it's fine to use blank lines to separate cases, spacing within a single switch statement should be consistent. func handle(_ action: SpaceshipAction) { switch action { case .engageWarpDrive: warpDrive.engage() case .enableArtificialGravity: artificialGravityEngine.enable(strength: .oneG) case .scanPlanet(let planet): scanner.scan(planet) case .handleIncomingEnergyBlast: energyShields.engage() } }
-
(link) Add a line break before the
elsekeyword in a multi-line guard statement. For single-line guard statements, keep theelsekeyword on the same line as theguardkeyword. The open brace should immediately follow theelsekeyword.// WRONG (else should be on its own line for multi-line guard statements) guard let galaxy, galaxy.name == "Milky Way" else { … } // WRONG (else should be on the same line for single-line guard statements) guard let galaxy else { … } // RIGHT guard let galaxy, galaxy.name == "Milky Way" else { … } // RIGHT guard let galaxy else { … }
-
(link) Indent the body and closing triple-quote of multiline string literals, unless the string literal begins on its own line in which case the string literal contents and closing triple-quote should have the same indentation as the opening triple-quote.
// WRONG var spaceQuote = """ “Space,” it says, “is big. Really big. You just won’t believe how vastly, hugely, mindbogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.” """ // RIGHT var spaceQuote = """ “Space,” it says, “is big. Really big. You just won’t believe how vastly, hugely, mindbogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.” """ // WRONG var universeQuote: String { """ In the beginning the Universe was created. This has made a lot of people very angry and been widely regarded as a bad move. """ } // RIGHT var universeQuote: String { """ In the beginning the Universe was created. This has made a lot of people very angry and been widely regarded as a bad move. """ }
-
(link) Use constructors instead of Make() functions for NSRange and others.
// WRONG let range = NSMakeRange(10, 5) // RIGHT let range = NSRange(location: 10, length: 5)
-
(link) For standard library types with a canonical shorthand form (
Optional,Array,Dictionary), prefer using the shorthand form over the full generic form.// WRONG let optional: Optional<String> = nil let array: Array<String> = [] let dictionary: Dictionary<String, Any> = [:] // RIGHT let optional: String? = nil let array: [String] = [] let dictionary: [String: Any] = [:]
-
(link) Omit explicit
.initwhen not required.// WRONG let universe = Universe.init() // RIGHT let universe = Universe()
-
(link) The opening brace following a single-line expression should be on the same line as the rest of the statement.
// WRONG if !planet.isHabitable { planet.terraform() } class Planet { func terraform() { generateAtmosphere() generateOceans() } } // RIGHT if !planet.isHabitable { planet.terraform() } class Planet { func terraform() { generateAtmosphere() generateOceans() } }
-
(link) The opening brace following a multi-line expression should wrap to a new line.
// WRONG if let star = planet.nearestStar(), planet.isInHabitableZone(of: star) { planet.terraform() } class Planet { func terraform( atmosphereOptions: AtmosphereOptions = .default, oceanOptions: OceanOptions = .default) { generateAtmosphere(atmosphereOptions) generateOceans(oceanOptions) } } // RIGHT if let star = planet.nearestStar(), planet.isInHabitableZone(of: star) { planet.terraform() } class Planet { func terraform( atmosphereOptions: AtmosphereOptions = .default, oceanOptions: OceanOptions = .default) { generateAtmosphere(atmosphereOptions) generateOceans(oceanOptions) } }
-
(link) Braces should be surrounded by a single whitespace character (either a space, or a newline) on each side.
// WRONG struct Planet{ … } // WRONG if condition{ … }else{ … } // RIGHT struct Planet { … } // RIGHT if condition { … } else { … }
-
(link) For function calls and declarations, there should be no spaces before or inside the parentheses of the argument list.
// WRONG func install ( _ engine: Engine ) { } install ( AntimatterDrive( ) ) // RIGHT func install(_ engine: Engine) { } install(AntimatterDrive())
-
(link) Comment blocks should use single-line comments (
//for code comments and///for documentation comments), rather than multi-line comments (/* ... */and/** ... */).// WRONG /** * A planet that exists somewhere in the universe. * * Planets have many properties. For example, the best planets * have atmospheres and bodies of water to support life. */ class Planet { /** Terraforms the planet, by adding an atmosphere and ocean that is hospitable for life. */ func terraform() { /* Generate the atmosphere first, before generating the ocean. Otherwise, the water will just boil off immediately. */ generateAtmosphere() /* Now that we have an atmosphere, it's safe to generate the ocean */ generateOceans() } } // RIGHT /// A planet that exists somewhere in the universe. /// /// Planets have many properties. For example, the best planets /// have atmospheres and bodies of water to support life. class Planet { /// Terraforms the planet, by adding an atmosphere and ocean that is hospitable for life. func terraform() { // Generate the atmosphere first, before generating the ocean. // Otherwise, the water will just boil off immediately. generateAtmosphere() // Now that we have an atmosphere, it's safe to generate the ocean generateOceans() } }
-
(link) Use doc comments (
///) instead of regular comments (//) before declarations within type bodies or at the top level.// WRONG // A planet that exists somewhere in the universe. class Planet { // Data about the composition and density of the planet's atmosphere if present. var atmosphere: Atmosphere? // Data about the size, location, and composition of large bodies of water on the planet's surface. var oceans: [Ocean] // Terraforms the planet, by adding an atmosphere and ocean that is hospitable for life. func terraform() { // This gas composition has a pretty good track record so far! let composition = AtmosphereComposition(nitrogen: 0.78, oxygen: 0.22) // Generate the atmosphere first, then the oceans. Otherwise, the water will just boil off immediately. generateAtmosphere(using: composition) generateOceans() } } // RIGHT /// A planet that exists somewhere in the universe. class Planet { /// Data about the composition and density of the planet's atmosphere if present. var atmosphere: Atmosphere? /// Data about the size, location, and composition of large bodies of water on the planet's surface. var oceans: [Ocean] /// Terraforms the planet, by adding an atmosphere and ocean that is hospitable for life. func terraform() { // This gas composition has a pretty good track record so far! let composition = AtmosphereComposition(nitrogen: 0.78, oxygen: 0.22) // Generate the atmosphere first, then the oceans. Otherwise, the water will just boil off immediately. generateAtmosphere(using: composition) generateOceans() } } // ALSO RIGHT: func terraform() { /// This gas composition has a pretty good track record so far! /// - Doc comments are not required before local declarations in function scopes, but are permitted. let composition = AtmosphereComposition(nitrogen: 0.78, oxygen: 0.22) /// Generate the `atmosphere` first, **then** the `oceans`. Otherwise, the water will just boil off immediately. /// - Comments not preceding declarations can use doc comments, and will not be autocorrected into regular comments. /// This can be useful because Xcode applies markdown styling to doc comments but not regular comments. generateAtmosphere(using: composition) generateOceans() }
Regular comments are permitted before declarations in some cases.
For example, comment directives like
// swiftformat:,// swiftlint:,// sourcery:,// MARK:and// TODO:are typically required to use regular comments and don't work correctly with doc comments:// RIGHT // swiftformat:sort enum FeatureFlags { case allowFasterThanLightTravel case disableGravity case enableDarkEnergy case enableDarkMatter } // TODO: There are no more production consumers of this legacy model, so we // should detangle the remaining code dependencies and clean it up. struct LegacyGeocentricUniverseModel { ... }
Regular comments are also allowed before a grouped block of declarations, since it's possible that the comment refers to the block as a whole rather than just the following declaration:
// RIGHT enum Planet { // The inner planets case mercury case venus case earth case mars // The outer planets case jupiter case saturn case uranus case neptune } // ALSO RIGHT enum Planet { /// The smallest planet case mercury case venus case earth case mars /// The largest planet case jupiter case saturn case uranus case neptune }
-
(link) Place doc comments for a declaration before any attributes.
// WRONG @MainActor /// A spacecraft with everything you need to explore the universe. struct Spaceship { … } // RIGHT /// A spacecraft with everything you need to explore the universe. @MainActor struct Spaceship { … }
-
(link) Include spaces or newlines before and after comment delimiters (
//,///,/*, and*/)// WRONG ///A spacecraft with incredible performance characteristics struct Spaceship { func travelFasterThanLight() {/*unimplemented*/} func travelBackInTime() { }//TODO: research whether or not this is possible } // RIGHT /// A spacecraft with incredible performance characteristics struct Spaceship { func travelFasterThanLight() { /* unimplemented */ } func travelBackInTime() { } // TODO: research whether or not this is possible }
-
(link) Include a single space in an empty set of braces (
{ }).// WRONG extension Spaceship: Trackable {} extension SpaceshipView { var accessibilityIdentifier: String { get { spaceship.name } set {} } } // RIGHT extension Spaceship: Trackable { } extension SpaceshipView { var accessibilityIdentifier: String { get { spaceship.name } set { } } }
-
(link) Prefer using
forloops over the functionalforEach(…)method, unless usingforEach(…)as the last element in a functional chain.For loops are more idiomatic than the
forEach(…)method, and are typically familiar to all developers who have experience with C-family languages.For loops are also more expressive than the
forEach(…)method. For loops support thereturn,continue, andbreakcontrol flow keywords, whileforEach(…)only supportsreturn(which has the same behavior ascontinuein a for loop).// WRONG planets.forEach { planet in planet.terraform() } // WRONG planets.forEach { $0.terraform() } // RIGHT for planet in planets { planet.terraform() } // ALSO FINE, since forEach is useful when paired with other functional methods in a chain. planets .filter { !$0.isGasGiant } .map { PlanetTerraformer(planet: $0) } .forEach { $0.terraform() }
-
(link) Omit the
internalkeyword when defining types, properties, or functions with an internal access control level.// WRONG internal class Spaceship { internal init() { … } internal func travel(to planet: Planet) { … } } // RIGHT, because internal access control is implied if no other access control level is specified. class Spaceship { init() { … } func travel(to planet: Planet) { … } }
-
(link) Omit
Voidreturn types from function definitions.// WRONG func doSomething() -> Void { ... } // RIGHT func doSomething() { ... }
-
(link) Separate long function declarations with line breaks before each argument label, and before the return signature or any effects (
async,throws). Put the open curly brace on the next line so the first executable line doesn't look like it's another parameter.class Universe { // WRONG func generateStars(at location: Point, count: Int, color: StarColor, withAverageDistance averageDistance: Float) -> String { // This is too long and will probably auto-wrap in a weird way } // WRONG func generateStars(at location: Point, count: Int, color: StarColor, withAverageDistance averageDistance: Float) -> String { // Xcode indents all the arguments } // WRONG func generateStars( at location: Point, count: Int, color: StarColor, withAverageDistance averageDistance: Float) -> String { populateUniverse() // this line blends in with the argument list } // WRONG func generateStars( at location: Point, count: Int, color: StarColor, withAverageDistance averageDistance: Float) throws -> String { populateUniverse() // this line blends in with the argument list } // WRONG func generateStars( at location: Point, count: Int, color: StarColor, withAverageDistance averageDistance: Float) async throws // these effects are easy to miss since they're visually associated with the last parameter -> String { populateUniverse() } // RIGHT func generateStars( at location: Point, count: Int, color: StarColor, withAverageDistance averageDistance: Float) -> String { populateUniverse() } // RIGHT func generateStars( at location: Point, count: Int, color: StarColor, withAverageDistance averageDistance: Float) async throws -> String { populateUniverse() } }
-
(link) Long function invocations should also break on each argument. Put the closing parenthesis on the last line of the invocation.
// WRONG universe.generateStars(at: location, count: 5, color: starColor, withAverageDistance: 4) // WRONG universe.generateStars(at: location, count: 5, color: starColor, withAverageDistance: 4) // WRONG universe.generateStars( at: location, count: 5, color: starColor, withAverageDistance: 4 ) // WRONG universe.generate(5, .stars, at: location) // RIGHT universe.generateStars( at: location, count: 5, color: starColor, withAverageDistance: 4) // RIGHT universe.generate( 5, .stars, at: location)
-
(link) Name unused function parameters as underscores (
_).Naming unused function parameters as underscores makes it more clear when the parameter is unused within the function body. This can make it easier to catch subtle logical errors, and can highlight opportunities to simplify method signatures.
// WRONG // In this method, the `newCondition` parameter is unused. // This is actually a logical error, and is easy to miss, but compiles without warning. func updateWeather(_ newCondition: WeatherCondition) -> Weather { var updatedWeather = self updatedWeather.condition = condition // this mistake inadvertently makes this method unable to change the weather condition return updatedWeather } // In this method, the `color` parameter is unused. // Is this a logical error (e.g. should it be passed through to the `universe.generateStars` method call), // or is this an unused argument that should be removed from the method signature? func generateUniverseWithStars( at location: Point, count: Int, color: StarColor, withAverageDistance averageDistance: Float) { let universe = generateUniverse() universe.generateStars( at: location, count: count, withAverageDistance: averageDistance) }
// RIGHT // Automatically reformatting the unused parameter to be an underscore // makes it more clear that the parameter is unused, which makes it // easier to spot the logical error. func updateWeather(_: WeatherCondition) -> Weather { var updatedWeather = self updatedWeather.condition = condition return updatedWeather } // The underscore makes it more clear that the `color` parameter is unused. // This method argument can either be removed if truly unnecessary, // or passed through to `universe.generateStars` to correct the logical error. func generateUniverseWithStars( at location: Point, count: Int, color _: StarColor, withAverageDistance averageDistance: Float) { let universe = generateUniverse() universe.generateStars( at: location, count: count, withAverageDistance: averageDistance) }
-
(link) Remove blank lines between chained functions.
Improves readability and maintainability, making it easier to see the sequence of functions that are applied to the object.
// WRONG var innerPlanetNames: [String] { planets .filter { $0.isInnerPlanet } .map { $0.name } } // WRONG var innerPlanetNames: [String] { planets .filter { $0.isInnerPlanet } // Gets the name of the inner planet .map { $0.name } } // RIGHT var innerPlanetNames: [String] { planets .filter { $0.isInnerPlanet } .map { $0.name } } // RIGHT var innerPlanetNames: [String] { planets .filter { $0.isInnerPlanet } // Gets the name of the inner planet .map { $0.name } }
-
(link) Favor
Voidreturn types over()in closure declarations. If you must specify aVoidreturn type in a function declaration, useVoidrather than()to improve readability.// WRONG func method(completion: () -> ()) { ... } // RIGHT func method(completion: () -> Void) { ... }
-
(link) Name unused closure parameters as underscores (
_).Naming unused closure parameters as underscores reduces the cognitive overhead required to read closures by making it obvious which parameters are used and which are unused.
// WRONG someAsyncThing() { argument1, argument2, argument3 in print(argument3) } // RIGHT someAsyncThing() { _, _, argument3 in print(argument3) }
-
(link) Closures should have a single space or newline inside each brace. Trailing closures should additionally have a single space or newline outside each brace.
// WRONG let evenSquares = numbers.filter{$0.isMultiple(of: 2)}.map{ $0 * $0 } // RIGHT let evenSquares = numbers.filter { $0.isMultiple(of: 2) }.map { $0 * $0 } // WRONG let evenSquares = numbers.filter( { $0.isMultiple(of: 2) } ).map( { $0 * $0 } ) // RIGHT let evenSquares = numbers.filter({ $0.isMultiple(of: 2) }).map({ $0 * $0 }) // WRONG let evenSquares = numbers .filter{ $0.isMultiple(of: 2) } .map{ $0 * $0 } // RIGHT let evenSquares = numbers .filter { $0.isMultiple(of: 2) } .map { $0 * $0 }
-
(link) Omit
Voidreturn types from closure expressions.// WRONG someAsyncThing() { argument -> Void in ... } // RIGHT someAsyncThing() { argument in ... }
-
(link) Prefer trailing closure syntax for closure arguments with no parameter name.
// WRONG planets.map({ $0.name }) // RIGHT planets.map { $0.name } // ALSO RIGHT, since this closure has a parameter name planets.first(where: { $0.isGasGiant }) // ALSO FINE. Trailing closure syntax is still permitted for closures // with parameter names. However, consider using non-trailing syntax // in cases where the parameter name is semantically meaningful. planets.first { $0.isGasGiant }
-
(link) Avoid using
unownedcaptures. Instead prefer safer alternatives likeweakcaptures, or capturing variables directly.`unowned` captures are unsafe because they will cause the application to crash if the referenced object has been deallocated.// WRONG: Crashes if `self` has been deallocated when closures are called. final class SpaceshipNavigationService { let spaceship: Spaceship let planet: Planet func colonizePlanet() { spaceship.travel(to: planet, onArrival: { [unowned self] in planet.colonize() }) } func exploreSystem() { spaceship.travel(to: planet, nextDestination: { [unowned self] in planet.moons?.first }) } }
weakcaptures are safer because they require the author to explicitly handle the case where the referenced object no longer exists.// RIGHT: Uses a `weak self` capture and explicitly handles the case where `self` has been deallocated final class SpaceshipNavigationService { let spaceship: Spaceship let planet: Planet func colonizePlanet() { spaceship.travel(to: planet, onArrival: { [weak self] in guard let self else { return } planet.colonize() }) } func exploreSystem() { spaceship.travel(to: planet, nextDestination: { [weak self] in guard let self else { return nil } return planet.moons?.first }) } }
Alternatively, consider directly capturing the variables that are used in the closure. This lets you avoid having to handle the case where
selfis nil, since you don't even need to referenceself:// RIGHT: Explicitly captures `planet` instead of capturing `self` final class SpaceshipNavigationService { let spaceship: Spaceship let planet: Planet func colonizePlanet() { spaceship.travel(to: planet, onArrival: { [planet] in planet.colonize() }) } func exploreSystem() { spaceship.travel(to: planet, nextDestination: { [planet] in planet.moons?.first }) } }
-
(link) Infix operators should have a single space on either side. However, in operator definitions, omit the trailing space between the operator and the open parenthesis. This rule does not apply to range operators (e.g.
1...3).// WRONG let capacity = 1+2 let capacity = currentCapacity??0 let capacity=newCapacity let latitude = region.center.latitude-region.span.latitudeDelta/2.0 // RIGHT let capacity = 1 + 2 let capacity = currentCapacity ?? 0 let capacity = newCapacity let latitude = region.center.latitude - region.span.latitudeDelta / 2.0
// WRONG static func == (_ lhs: MyView, _ rhs: MyView) -> Bool { lhs.id == rhs.id } // RIGHT static func ==(_ lhs: MyView, _ rhs: MyView) -> Bool { lhs.id == rhs.id }
-
(link) Long ternary operator expressions should wrap before the
?and before the:, putting each conditional branch on a separate line.// WRONG (too long) let destinationPlanet = solarSystem.hasPlanetsInHabitableZone ? solarSystem.planetsInHabitableZone.first : solarSystem.uninhabitablePlanets.first // WRONG (naive wrapping) let destinationPlanet = solarSystem.hasPlanetsInHabitableZone ? solarSystem.planetsInHabitableZone.first : solarSystem.uninhabitablePlanets.first // WRONG (unbalanced operators) let destinationPlanet = solarSystem.hasPlanetsInHabitableZone ? solarSystem.planetsInHabitableZone.first : solarSystem.uninhabitablePlanets.first // RIGHT let destinationPlanet = solarSystem.hasPlanetsInHabitableZone ? solarSystem.planetsInHabitableZone.first : solarSystem.uninhabitablePlanets.first
-
(link) In conditional statements (
if,guard,while), separate boolean conditions using commas (,) instead of&&operators.// WRONG if let star = planet.star, !planet.isHabitable && planet.isInHabitableZone(of: star) { planet.terraform() } if let star = planet.star, !planet.isHabitable && planet.isInHabitableZone(of: star) { planet.terraform() } // RIGHT if let star = planet.star, !planet.isHabitable, planet.isInHabitableZone(of: star) { planet.terraform() } if let star = planet.star, !planet.isHabitable, planet.isInHabitableZone(of: star) { planet.terraform() }
-
(link) When extending bound generic types, prefer using generic bracket syntax (
extension Collection<Planet>), or sugared syntax for applicable standard library types (extension [Planet]) instead of generic type constraints.// WRONG extension Array where Element == Star { … } extension Optional where Wrapped == Spaceship { … } extension Dictionary where Key == Moon, Element == Planet { … } extension Collection where Element == Universe { … } extension StateStore where State == SpaceshipState, Action == SpaceshipAction { … } // RIGHT extension [Star] { … } extension Spaceship? { … } extension [Moon: Planet] { … } extension Collection<Universe> { … } extension StateStore<SpaceshipState, SpaceshipAction> { … } // ALSO RIGHT. There are multiple types that could satisfy this constraint // (e.g. [Planet], [Moon]), so this is not a "bound generic type" and isn't // eligible for the generic bracket syntax. extension Array where Element: PlanetaryBody { }
-
(link) Avoid using semicolons. Semicolons are not required at the end of a line, so should be omitted. While you can use semicolons to place two statements on the same line, it is more common and preferred to separate them using a newline instead.
// WRONG. Semicolons are not required and can be omitted. let mercury = planets[0]; let venus = planets[1]; let earth = planets[2]; // WRONG. While you can use semicolons to place multiple statements on a single line, // it is more common and preferred to separate them using newlines instead. let mercury = planets[0]; let venus = planets[1]; let earth = planets[2]; // RIGHT let mercury = planets[0] let venus = planets[1] let earth = planets[2] // WRONG guard let moon = planet.moon else { completion(nil); return } // WRONG guard let moon = planet.moon else { completion(nil); return } // RIGHT guard let moon = planet.moon else { completion(nil) return }
-
(link) Prefer initializing properties at
inittime whenever possible, rather than using implicitly unwrapped optionals. A notable exception is UIViewController'sviewproperty.// WRONG class MyClass { init() { super.init() someValue = 5 } var someValue: Int! } // RIGHT class MyClass { init() { someValue = 0 super.init() } var someValue: Int }
-
(link) Avoid performing any meaningful or time-intensive work in
init(). Avoid doing things like opening database connections, making network requests, reading large amounts of data from disk, etc. Create something like astart()method if these things need to be done before an object is ready for use. -
(link) Extract complex property observers into methods. This reduces nestedness, separates side-effects from property declarations, and makes the usage of implicitly-passed parameters like
oldValueexplicit.// WRONG class TextField { var text: String? { didSet { guard oldValue != text else { return } // Do a bunch of text-related side-effects. } } } // RIGHT class TextField { var text: String? { didSet { textDidUpdate(from: oldValue) } } private func textDidUpdate(from oldValue: String?) { guard oldValue != text else { return } // Do a bunch of text-related side-effects. } }
-
(link) Extract complex callback blocks into methods. This limits the complexity introduced by weak-self in blocks and reduces nestedness. If you need to reference self in the method call, make use of
guardto unwrap self for the duration of the callback.// WRONG class MyClass { func request(completion: () -> Void) { API.request() { [weak self] response in if let self { // Processing and side effects } completion() } } } // RIGHT class MyClass { func request(completion: () -> Void) { API.request() { [weak self] response in guard let self else { return } self.doSomething(with: self.property, response: response) completion(
) } } func doSomething(with nonOptionalParameter: SomeClass, response: SomeResponseClass) { // Processing and side effects } } -
(link) Prefer using
guardat the beginning of a scope. -
(link) Access control should be at the strictest level possible. Prefer
publictoopenandprivatetofileprivateunless you need that behavior.// WRONG public struct Spaceship { // WRONG: `engine` is used in `extension Spaceship` below, // but extensions in the same file can access `private` members. fileprivate let engine: AntimatterEngine // WRONG: `hull` is not used by any other type, so `fileprivate` is unnecessary. fileprivate let hull: Hull // RIGHT: `navigation` is used in `extension Pilot` below, // so `fileprivate` is necessary here. fileprivate let navigation: SpecialRelativityNavigationService } extension Spaceship { public func blastOff() { engine.start() } } extension Pilot { public func chartCourse() { spaceship.navigation.course = .andromedaGalaxy spaceship.blastOff() } }
// RIGHT public struct Spaceship { fileprivate let navigation: SpecialRelativityNavigationService private let engine: AntimatterEngine private let hull: Hull } extension Spaceship { public func blastOff() { engine.start() } } extension Pilot { public func chartCourse() { spaceship.navigation.course = .andromedaGalaxy spaceship.blastOff() } }
-
(link) Avoid global functions whenever possible. Prefer methods within type definitions.
// WRONG func age(of person: Person, bornAt: TimeInterval) -> Int { // ... } func jump(person: Person) { // ... } // RIGHT class Person { var bornAt: TimeInterval var age: Int { // ... } func jump() { // ... } }
-
(link) Use caseless
enums for organizingpublicorinternalconstants and functions into namespaces.- Avoid creating non-namespaced global constants and functions.
- Feel free to nest namespaces where it adds clarity.
privateglobals are permitted, since they are scoped to a single file and do not pollute the global namespace. Consider placing private globals in anenumnamespace to match the guidelines for other declaration types.
Caseless
enums work well as namespaces because they cannot be instantiated, which matches their intent.// WRONG struct Environment { static let earthGravity = 9.8 static let moonGravity = 1.6 } // WRONG struct Environment { struct Earth { static let gravity = 9.8 } struct Moon { static let gravity = 1.6 } } // RIGHT enum Environment { enum Earth { static let gravity = 9.8 } enum Moon { static let gravity = 1.6 } }
-
(link) Use Swift's automatic enum values unless they map to an external source. Add a comment explaining why explicit values are defined.
To minimize user error, improve readability, and write code faster, rely on Swift's automatic enum values. If the value maps to an external source (e.g. it's coming from a network request) or is persisted across binaries, however, define the values explicitly, and document what these values are mapping to.
This ensures that if someone adds a new value in the middle, they won't accidentally break things.
// WRONG enum ErrorType: String { case error = "error" case warning = "warning" } // WRONG enum UserType: String { case owner case manager case member } // WRONG enum Planet: Int { case mercury = 0 case venus = 1 case earth = 2 case mars = 3 case jupiter = 4 case saturn = 5 case uranus = 6 case neptune = 7 } // WRONG enum ErrorCode: Int { case notEnoughMemory case invalidResource case timeOut } // RIGHT // Relying on Swift's automatic enum values enum ErrorType: String { case error case warning } // RIGHT /// These are written to a logging service. Explicit values ensure they're consistent across binaries. // swiftformat:disable redundantRawValues enum UserType: String { case owner = "owner" case manager = "manager" case member = "member" } // swiftformat:enable redundantRawValues // RIGHT // Relying on Swift's automatic enum values enum Planet: Int { case mercury case venus case earth case mars case jupiter case saturn case uranus case neptune } // RIGHT /// These values come from the server, so we set them here explicitly to match those values. enum ErrorCode: Int { case notEnoughMemory = 0 case invalidResource = 1 case timeOut = 2 }
-
(link) Use optionals only when they have semantic meaning.
-
(link) Prefer immutable values whenever possible. Use
mapandcompactMapinstead of appending to a new collection. Usefilterinstead of removing elements from a mutable collection.Mutable variables increase complexity, so try to keep them in as narrow a scope as possible.
// WRONG var results = [SomeType]() for element in input { let result = transform(element) results.append(result) } // RIGHT let results = input.map { transform($0) }
// WRONG var results = [SomeType]() for element in input { if let result = transformThatReturnsAnOptional(element) { results.append(result) } } // RIGHT let results = input.compactMap { transformThatReturnsAnOptional($0) }
-
(link) Prefer immutable or computed static properties over mutable ones whenever possible. Use stored
static letproperties or computedstatic varproperties over storedstatic varproperties whenever possible, as storedstatic varproperties are global mutable state.Global mutable state increases complexity and makes it harder to reason about the behavior of applications. It should be avoided when possible.
// WRONG enum Fonts { static var title = UIFont(…) } // RIGHT enum Fonts { static let title = UIFont(…) }
// WRONG struct FeatureState { var count: Int static var initial = FeatureState(count: 0) } // RIGHT struct FeatureState { var count: Int static var initial: FeatureState { // Vend static properties that are cheap to compute FeatureState(count: 0) } }
-
(link) Handle an unexpected but recoverable condition with an
assertmethod combined with the appropriate logging in production. If the unexpected condition is not recoverable, prefer apreconditionmethod orfatalError(). This strikes a balance between crashing and providing insight into unexpected conditions in the wild. Only preferfatalErrorover apreconditionmethod when the failure message is dynamic, since apreconditionmethod won't report the message in the crash report.func didSubmitText(_ text: String) { // It's unclear how this was called with an empty string; our custom text field shouldn't allow this. // This assert is useful for debugging but it's OK if we simply ignore this scenario in production. guard !text.isEmpty else { assertionFailure("Unexpected empty string") return } // ... } func transformedItem(atIndex index: Int, from items: [Item]) -> Item { precondition(index >= 0 && index < items.count) // It's impossible to continue executing if the precondition has failed. // ... } func makeImage(name: String) -> UIImage { guard let image = UIImage(named: name, in: nil, compatibleWith: nil) else { fatalError("Image named \(name) couldn't be loaded.") // We want the error message so we know the name of the missing image. } return image }
-
(link) Default type methods to
static. -
(link) Default classes to
final. -
(link) When switching over an enum, generally prefer enumerating all cases rather than using the
defaultcase.Enumerating every case requires developers and reviewers have to consider the correctness of every switch statement when new cases are added in the future.
// NOT PREFERRED switch trafficLight { case .greenLight: // Move your vehicle default: // Stop your vehicle } // PREFERRED switch trafficLight { case .greenLight: // Move your vehicle case .yellowLight, .redLight: // Stop your vehicle } // COUNTEREXAMPLES enum TaskState { case pending case running case canceling case success(Success) case failure(Error) // We expect that this property will remain valid if additional cases are added to the enumeration. public var isRunning: Bool { switch self { case .running: true default: false } } } extension TaskState: Equatable { // Explicitly listing each state would be too burdensome. Ideally this function could be implemented with a well-tested macro. public static func == (lhs: TaskState, rhs: TaskState) -> Bool { switch (lhs, rhs) { case (.pending, .pending): true case (.running, .running): true case (.canceling, .canceling): true case (.success(let lhs), .success(let rhs)): lhs == rhs case (.failure(let lhs), .failure(let rhs)): lhs == rhs default: false } } }
-
(link) Check for nil rather than using optional binding if you don't need to use the value.
-
(link) Omit the
returnkeyword when not required by the language.// WRONG ["1", "2", "3"].compactMap { return Int($0) } var size: CGSize { return CGSize( width: 100.0, height: 100.0) } func makeInfoAlert(message: String) -> UIAlertController { return UIAlertController( title: "ℹ️ Info", message: message, preferredStyle: .alert) } var alertTitle: String { if issue.severity == .critical { return "💥 Critical Error" } else { return "ℹ️ Info" } } func type(of planet: Planet) -> PlanetType { switch planet { case .mercury, .venus, .earth, .mars: return .terrestrial case .jupiter, .saturn, .uranus, .neptune: return .gasGiant } } // RIGHT ["1", "2", "3"].compactMap { Int($0) } var size: CGSize { CGSize( width: 100.0, height: 100.0) } func makeInfoAlert(message: String) -> UIAlertController { UIAlertController( title: "ℹ️ Info", message: message, preferredStyle: .alert) } var alertTitle: String { if issue.severity == .critical { "💥 Critical Error" } else { "ℹ️ Info" } } func type(of planet: Planet) -> PlanetType { switch planet { case .mercury, .venus, .earth, .mars: .terrestrial case .jupiter, .saturn, .uranus, .neptune: .gasGiant } }
-
(link) Use
AnyObjectinstead ofclassin protocol definitions.SE-0156, which introduced support for using the
AnyObjectkeyword as a protocol constraint, recommends preferringAnyObjectoverclass:This proposal merges the concepts of
classandAnyObject, which now have the same meaning: they represent an existential for classes. To get rid of the duplication, we suggest only keepingAnyObjectaround. To reduce source-breakage to a minimum,classcould be redefined astypealias class = AnyObjectand give a deprecation warning on class for the first version of Swift this proposal is implemented in. Later,classcould be removed in a subsequent version of Swift.// WRONG protocol Foo: class { } // RIGHT protocol Foo: AnyObject { }
-
(link) Specify the access control for each declaration in an extension individually.
Specifying the access control on the declaration itself helps engineers more quickly determine the access control level of an individual declaration.
// WRONG public extension Universe { // This declaration doesn't have an explicit access control level. // In all other scopes, this would be an internal function, // but because this is in a public extension, it's actually a public function. func generateGalaxy() { } } // WRONG private extension Spaceship { func enableHyperdrive() { } } // RIGHT extension Universe { // It is immediately obvious that this is a public function, // even if the start of the `extension Universe` scope is off-screen. public func generateGalaxy() { } } // RIGHT extension Spaceship { // Recall that a private extension actually has fileprivate semantics, // so a declaration in a private extension is fileprivate by default. fileprivate func enableHyperdrive() { } }
-
(link) Prefer dedicated logging systems like
os_logorswift-logover writing directly to standard out usingprint(…),debugPrint(…), ordump(…).All log messages should flow into intermediate logging systems that can direct messages to the correct destination(s) and potentially filter messages based on the app's environment or configuration.
print(…),debugPrint(…), ordump(…)will write all messages directly to standard out in all app configurations and can potentially leak personally identifiable information (PII). -
(link) Don't use
#file. Use#fileIDor#filePathas appropriate.The behavior of the
#fileliteral (or macro as of Swift 5.9) has evolved from evaluating to the full source file path (the behavior as of#filePath) to a human-readable string containing module and file name (the behavior of#fileID). Use the literal (or macro) with the most appropriate behavior for your use case. -
(link) Don't use
#filePathin production code. Use#fileIDinstead.#filePathshould only be used in non-production code where the full path of the source file provides useful information to developers. Because#fileIDdoesn’t embed the full path to the source file, it won't expose your file system and reduces the size of the compiled binary. -
(link) Avoid single-expression closures that are always called immediately. Instead, prefer inlining the expression.
// WRONG lazy var universe: Universe = { Universe() }() lazy var stars = { universe.generateStars( at: location, count: 5, color: starColor, withAverageDistance: 4) }() // RIGHT lazy var universe = Universe() lazy var stars = universe.generateStars( at: location, count: 5, color: starColor, withAverageDistance: 4)
-
(link) Omit the
getclause from a computed property declaration that doesn't also have aset,willSet, ordidSetclause.// WRONG var universe: Universe { get { Universe() } } // RIGHT var universe: Universe { Universe() } // RIGHT var universe: Universe { get { multiverseService.current } set { multiverseService.current = newValue } }
-
(link) Prefer using opaque generic parameters (with
some) over verbose named generic parameter syntax where possible.Opaque generic parameter syntax is significantly less verbose and thus more legible than the full named generic parameter syntax.
// WRONG func spaceshipDashboard<WarpDriveView: View, CaptainsLogView: View>( warpDrive: WarpDriveView, captainsLog: CaptainsLogView) -> some View { … } func generate<Planets>(_ planets: Planets) where Planets: Collection, Planets.Element == Planet { … } // RIGHT func spaceshipDashboard( warpDrive: some View, captainsLog: some View) -> some View { … } func generate(_ planets: some Collection<Planet>) { … } // Also fine, since there isn't an equivalent opaque parameter syntax for expressing // that two parameters in the type signature are of the same type: func terraform<Body: PlanetaryBody>(_ planetaryBody: Body, into terraformedBody: Body) { … } // Also fine, since the generic parameter name is referenced in the function body so can't be removed: func terraform<Body: PlanetaryBody>(_ planetaryBody: Body) { planetaryBody.generateAtmosphere(Body.idealAtmosphere) }
Fully-unconstrained generic parameters are somewhat uncommon, but are equivalent to
some Any. For example:func assertFailure<Value>( _ result: Result<Value, Error>, file: StaticString = #filePath, line: UInt = #line) { if case .failure(let error) = result { XCTFail(error.localizedDescription, file: file, line: line) } } // is equivalent to: func assertFailure( _ result: Result<some Any, Error>, file: StaticString = #filePath, line: UInt = #line) { if case .failure(let error) = result { XCTFail(error.localizedDescription, file: file, line: line) } }
some Anyis somewhat unintuitive, and the named generic parameter is useful in this situation to compensate for the weak type information. Because of this, prefer using named generic parameters instead ofsome Any. -
(link) Prefer to avoid using
@unchecked Sendable. Use a standardSendableconformance instead where possible. If working with a type from a module that has not yet been updated to support Swift Concurrency, suppress concurrency-related errors using@preconcurrency import.@unchecked Sendableprovides no guarantees about the thread safety of a type, and instead unsafely suppresses compiler errors related to concurrency checking.There are typically other, safer methods for suppressing concurrency-related errors:
A
Sendableconformance is the preferred way to declare that a type is thread-safe. The compiler will emit an error if a type conforming toSendableis not thread-safe. For example, simple value types and immutable classes can always safely conform toSendable, but mutable classes cannot:// RIGHT: Simple value types are thread-safe. struct Planet: Sendable { var mass: Double } // RIGHT: Immutable classes are thread-safe. final class Planet: Sendable { let mass: Double } // WRONG: Mutable classes are not thread-safe. final class Planet: Sendable { // ERROR: stored property 'mass' of 'Sendable'-conforming class 'Planet' is mutable var mass: Double } // WRONG: @unchecked is unnecessary because the compiler can prove that the type is thread-safe. struct Planet: @unchecked Sendable { var mass: Double }
Mutable classes can be made
Sendableand thread-safe if they are isolated to a single actor / thread / concurrency domain. Any mutable class can be madeSendableby isolating it to a global actor using an annotation like@MainActor(which isolates it to the main actor):// RIGHT: A mutable class isolated to the main actor is thread-safe. @MainActor final class Planet: Sendable { var mass: Double } // WRONG: @unchecked Sendable is unsafe because mutable classes are not thread-safe. struct Planet: @unchecked Sendable { var mass: Double }
If working with a non-
Sendabletype from a module that hasn't yet adopted Swift concurrency, suppress concurrency-related errors using@preconcurrency import./// Defined in `UniverseKit` module class Planet: PlanetaryBody { var star: Star }
// WRONG: Unsafely marking a non-thread-safe class as Sendable only to suppress errors import PlanetaryBody extension PlanetaryBody: @unchecked Sendable { } // RIGHT @preconcurrency import PlanetaryBody
If possible, restructure code so that the compiler can verify that it is thread safe. This lets you use a
Sendableconformance instead of an unsafe@unchecked Sendableconformance.When conforming to
Sendable, the compiler will emit an error in the future if you attempt to make a change that is not thread-safe. This guarantee is lost when using@unchecked Sendable, which makes it easier to accidentally introduce changes which are not thread-safe.For example, given this set of classes:
class PlanetaryBody { let mass: Double } class Planet: PlanetaryBody { let star: Star } // NOT IDEAL: no compiler-enforced thread safety. extension PlanetaryBody: @unchecked Sendable { }
the compiler can't verify
PlanetaryBodyisSendablebecause it is notfinal. Instead of using@unchecked Sendable, you could restructure the code to not use subclassing:// BETTER: Compiler-enforced thread safety. protocol PlanetaryBody: Sendable { var mass: Double { get } } final class Planet: PlanetaryBody, Sendable { let mass: Double let star: Star }
Sometimes it is truly necessary to use
@unchecked Sendable. In these cases, you can add a// swiftlint:disable:next no_unchecked_sendableannotation with an explanation for how we know the type is thread-safe, and why we have to use@unchecked Sendableinstead ofSendable.A canonical, safe use case of
@unchecked Sendableis a class where the mutable state is protected by some other thread-safe mechanism like a lock. This type is thread-safe, but the compiler cannot verify this.struct Atomic<Value> { /// `value` is thread-safe because it is manually protected by a lock. var value: Value { ... } } // WRONG: disallowed by linter extension Atomic: @unchecked Sendable { } // WRONG: suppressing lint error without an explanation // swiftlint:disable:next no_unchecked_sendable extension Atomic: @unchecked Sendable { } // RIGHT: suppressing the linter with an explanation why the type is thread-safe // Atomic is thread-safe because its underlying mutable state is protected by a lock. // swiftlint:disable:next no_unchecked_sendable extension Atomic: @unchecked Sendable { }
It is also reasonable to use
@unchecked Sendablefor types that are thread-safe in existing usage but can't be refactored to support a properSendableconformance (e.g. due to backwards compatibility constraints):class PlanetaryBody { let mass: Double } class Planet: PlanetaryBody { let star: Star } // WRONG: disallowed by linter extension PlanetaryBody: @unchecked Sendable { } // WRONG: suppressing lint error without an explanation // swiftlint:disable:next no_unchecked_sendable extension PlanetaryBody: @unchecked Sendable { } // RIGHT: suppressing the linter with an explanation why the type is thread-safe // PlanetaryBody cannot conform to Sendable because it is non-final and has subclasses. // PlanetaryBody itself is safely Sendable because it only consists of immutable values. // All subclasses of PlanetaryBody are also simple immutable values, so are safely Sendable as well. // swiftlint:disable:next no_unchecked_sendable extension PlanetaryBody: @unchecked Sendable { }
-
(link) Avoid defining properties that are then returned immediately. Instead, return the value directly.
Property declarations that are immediately returned are typically redundant and unnecessary. Sometimes these are unintentionally created as the byproduct of refactoring. Cleaning them up automatically simplifies the code. In some cases this also results in the
returnkeyword itself being unnecessary, further simplifying the code.// WRONG var spaceship: Spaceship { let spaceship = spaceshipBuilder.build(warpDrive: warpDriveBuilder.build()) return spaceship } // RIGHT var spaceship: Spaceship { spaceshipBuilder.build(warpDrive: warpDriveBuilder.build()) } // WRONG var spaceship: Spaceship { let warpDrive = warpDriveBuilder.build() let spaceship = spaceshipBuilder.build(warpDrive: warpDrive) return spaceship } // RIGHT var spaceship: Spaceship { let warpDrive = warpDriveBuilder.build() return spaceshipBuilder.build(warpDrive: warpDrive) }
-
(link) Avoid using
()as a type. PreferVoid.// WRONG let result: Result<(), Error> // RIGHT let result: Result<Void, Error>
-
(link) Avoid using
Void()as an instance ofVoid. Prefer().let completion: (Result<Void, Error>) -> Void // WRONG completion(.success(Void())) // RIGHT completion(.success(()))
-
(link) Alphabetize and deduplicate module imports within a file. Place all imports at the top of the file below the header comments. Do not add additional line breaks between import statements. Add a single empty line before the first import and afte 341A r the last import.
- A standard organization method helps engineers more quickly determine which modules a file depends on.
- Duplicated import statements have no effect and should be removed for clarity.
// WRONG // Copyright © 2018 Airbnb. All rights reserved. // import DLSPrimitives import Constellation import Constellation import Epoxy import Foundation // RIGHT // Copyright © 2018 Airbnb. All rights reserved. // import Constellation import DLSPrimitives import Epoxy import Foundation
Exception:
@testable importshould be grouped after the regular import and separated by an empty line.// WRONG // Copyright © 2018 Airbnb. All rights reserved. // import DLSPrimitives @testable import Epoxy import Foundation import Nimble import Quick // RIGHT // Copyright © 2018 Airbnb. All rights reserved. // import DLSPrimitives import Foundation import Nimble import Quick @testable import Epoxy
-
(link) Limit consecutive whitespace to one blank line or space (excluding indentation). Favor the following formatting guidelines over whitespace of varying heights or widths.
// WRONG struct Planet { let mass: Double let hasAtmosphere: Bool func distance(to: Planet) { } } // RIGHT struct Planet { let mass: Double let hasAtmosphere: Bool func distance(to: Planet) { } }
-
(link) Files should end in a newline.
-
(link) Declarations that include scopes spanning multiple lines should be separated from adjacent declarations in the same scope by a newline. Insert a single blank line between multi-line scoped declarations (e.g. types, extensions, functions, computed properties, etc.) and other declarations at the same indentation level.
Dividing scoped declarations from other declarations at the same scope visually separates them, making adjacent declarations easier to differentiate from the scoped declaration.
// WRONG struct SolarSystem { var numberOfPlanets: Int { … } func distance(to: SolarSystem) -> AstronomicalUnit { … } } struct Galaxy { func distance(to: Galaxy) -> AstronomicalUnit { … } func contains(_ solarSystem: SolarSystem) -> Bool { … } } // RIGHT struct SolarSystem { var numberOfPlanets: Int { … } func distance(to: SolarSystem) -> AstronomicalUnit { … } } struct Galaxy { func distance(to: Galaxy) -> AstronomicalUnit { … } func contains(_ solarSystem: SolarSystem) -> Bool { … } }
-
(link) Remove blank lines at the top and bottom of scopes, excluding type bodies which can optionally include blank lines.
// WRONG class Planet { func terraform() { generateAtmosphere() generateOceans() } } // RIGHT class Planet { func terraform() { generateAtmosphere() generateOceans() } } // Also fine! class Planet { func terraform() { generateAtmosphere() generateOceans() } }
-
(link) Each type and extension which implements a conformance should be preceded by a
MARKcomment.- Types should be preceded by a
// MARK: - TypeNamecomment. - Extensions that add a conformance should be preceded by a
// MARK: - TypeName + ProtocolNamecomment. - Extensions that immediately follow the type being extended should omit that type's name and instead use
// MARK: ProtocolName. - If there is only one type or extension in a file, the
MARKcomment can be omitted. - If the extension in question is empty (e.g. has no declarations in its body), the
MARKcomment can be omitted. - For extensions that do not add new conformances, consider adding a
MARKwith a descriptive comment.
// MARK: - GalaxyView final class GalaxyView: UIView { … } // MARK: ContentConfigurableView extension GalaxyView: ContentConfigurableView { … } // MARK: - Galaxy + SpaceThing, NamedObject extension Galaxy: SpaceThing, NamedObject { … }
- Types should be preceded by a
-
(link) Use
// MARK:to separate the contents of type definitions and extensions into the sections listed below, in order. All type definitions and extensions should be divided up in this consistent way, allowing a reader of your code to easily jump to what they are interested in.// MARK: Lifecycleforinitanddeinitmethods.// MARK: Openforopenproperties and methods.// MARK: Publicforpublicproperties and methods.// MARK: Packageforpackageproperties and methods.// MARK: Internalforinternalproperties and methods.// MARK: Fileprivateforfileprivateproperties and methods.// MARK: Privateforprivateproperties and methods.- If the type in question is an enum, its cases should go above the first
// MARK:. - Do not subdivide each of these sections into subsections, as it makes the method dropdown more cluttered and therefore less useful. Instead, group methods by functionality and use smart naming to make clear which methods are related. If there are enough methods that sub-sections seem necessary, consider refactoring your code into multiple types.
- If all of the type or extension's definitions belong to the same category (e.g. the type or extension only consists of
internalproperties), it is OK to omit the// MARK:s. - If the type in question is a simple value type (e.g. fewer than 20 lines), it is OK to omit the
// MARK:s, as it would hurt legibility.
-
(link) Within each top-level section, place content in the following order. This allows a new reader of your code to more easily find what they are looking for.
- Nested types and type aliases
- Static properties
- Static property with body
- Class properties with body
- SwiftUI dynamic properties (@State, @Environment, @Binding, etc), grouped by type
- Instance properties
- Instance properties with body
- Static methods
- Class methods
- Instance methods
Computed properties and properties with property observers should appear at the end of the set of declarations of the same kind. (e.g. instance properties.)
// WRONG class PlanetView: UIView { static var startOfTime { -CGFloat.greatestFiniteMagnitude / 0 } var atmosphere: Atmosphere { didSet { print("oh my god, the atmosphere changed") } } override class var layerClass: AnyClass { PlanetLayer.self } var gravity: CGFloat static let speedOfLight: CGFloat = 300_000 } // RIGHT class PlanetView: UIView { static let speedOfLight: CGFloat = 300_000 static var startOfTime { -CGFloat.greatestFiniteMagnitude / 0 } override class var layerClass: AnyClass { PlanetLayer.self } var gravity: CGFloat var atmosphere: Atmosphere { didSet { print("oh my god, the atmosphere changed") } } }
SwiftUI Properties are a special type of property that lives inside SwiftUI views. These views conform to the
DynamicPropertyprotocol and cause the view's body to re-compute. Given this common functionality and also a similar syntax, it is preferred to group them.// WRONG struct CustomSlider: View { // MARK: Internal var body: some View { ... } // MARK: Private @Binding private var value: Value private let range: ClosedRange<Double> @Environment(\.sliderStyle) private var style private let step: Double.Stride @Environment(\.layoutDirection) private var layoutDirection } // RIGHT struct CustomSlider: View { // MARK: Internal var body: some View { ... } // MARK: Private @Environment(\.sliderStyle) private var style @Environment(\.layoutDirection) private var layoutDirection @Binding private var value: Value private let range: ClosedRange<Double> private let step: Double.Stride }
Additionally, within the grouping of SwiftUI properties, it is preferred that the properties are also grouped by their dynamic property type. The group order applied by the formatter is determined by the first time a type appears:
// WRONG struct CustomSlider: View { @Binding private var value: Value @State private var foo = Foo() @Environment(\.sliderStyle) private var style @State private var bar = Bar() @Environment(\.layoutDirection) private var layoutDirection private let range: ClosedRange<Double> private let step: Double.Stride } // RIGHT struct CustomSlider: View { @Binding private var value: Value @State private var foo = Foo() @State private var bar = Bar() @Environment(\.sliderStyle) private var style @Environment(\.layoutDirection) private var layoutDirection private let range: ClosedRange<Double> private let step: Double.Stride }
-
(link) Add empty lines between property declarations of different kinds. (e.g. between static properties and instance properties.)
// WRONG static let gravityEarth: CGFloat = 9.8 static let gravityMoon: CGFloat = 1.6 var gravity: CGFloat // RIGHT static let gravityEarth: CGFloat = 9.8 static let gravityMoon: CGFloat = 1.6 var gravity: CGFloat
-
(link) Remove unused private and fileprivate properties, functions, and typealiases
Improves readability since the code has no effect and should be removed for clarity.
// WRONG: Includes private declarations that are unused struct Planet { var ageInBillionYears: Double { ageInMillionYears / 1000 } private var ageInMillionsOfYears: Double private typealias Dependencies = UniverseBuilderProviding // unused private var mass: Double // unused private func distance(to: Planet) { } // unused } // RIGHT struct Planet { var ageInBillionsOfYears: Double { ageInMillionYears / 1000 } private var ageInMillionYears: Double }
-
(link) Remove empty extensions that define no properties, functions, or conformances.
Improves readability since the code has no effect and should be removed for clarity.
// WRONG: The first extension is empty and redundant. extension Planet {} extension Planet: Equatable {} // RIGHT: Empty extensions that add a protocol conformance aren't redundant. extension Planet: Equatable {}
-
(link) Prefer pure Swift classes over subclasses of NSObject. If your code needs to be used by some Objective-C code, wrap it to expose the desired functionality. Use
@objcon individual methods and variables as necessary rather than exposing all API on a class to Objective-C via@objcMembers.class PriceBreakdownViewController { private let acceptButton = UIButton() private func setUpAcceptButton() { acceptButton.addTarget( self, action: #selector(didTapAcceptButton), forControlEvents: .touchUpInside) } @objc private func didTapAcceptButton() { // ... } }
We encourage you to fork this guide and change the rules to fit your team’s style guide. Below, you may list some amendments to the style guide. This allows you to periodically update your style guide without having to deal with merge conflicts.