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Shortcut Sweden Swift Style Guide

Goals

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.

Guiding Tenets

  • 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 SwiftLint autocorrect or SwiftFormat).
    • For rules that don't directly change the format of the code, we should have a lint rule that throws a warning.
    • We list during all pull requests Danger Swift)
    • Exceptions to these rules should be rare and heavily justified.

Table of Contents

  1. Xcode Formatting
  2. Naming
  3. Style
    1. Functions
    2. Closures
    3. Operators
  4. Patterns
  5. File Organization
  6. Objective-C Interoperability
  7. Documentation Comments

Xcode Formatting

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.

    Why?

    Due to larger screen sizes, we have opted to choose a page guide greater than 80

  • (link) Use 2 spaces to indent lines.

  • (link) Trim trailing whitespace in all lines. SwiftFormat: trailingSpace

⬆ back to top

Naming

  • (link) Use PascalCase for type and protocol names, and lowerCamelCase for everything else. SwiftLint: type_name

    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

    Why?

    There are specific scenarios where a backing a property or method 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, session, parser, onSuccess, onFailure)
      }
    
      private let _executeRequest: (
        URLRequest,
        @escaping (ModelType, Bool) -> Void,
        @escaping (NSError) -> 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 isUrlReachable(_ URL: URL) -> Bool {
        // ...
      }
    }
    
    let URLValidator = UrlValidator().isValidUrl(/* some URL */)
    
    // RIGHT
    class URLValidator {
    
      func isValidURL(_ url: URL) -> Bool {
        // ...
      }
    
      func isURLReachable(_ url: URL) -> Bool {
        // ...
      }
    }
    
    let urlValidator = URLValidator().isValidURL(/* some URL */)
  • (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 *Controller in names of classes that aren't view controllers.

    Why?

    Controller is an overloaded suffix that doesn't provide information about the responsibilities of the class.

⬆ back to top

Style

  • (link) Don't include types where they can be easily inferred.

    // WRONG
    let host: Host = Host()
    
    // RIGHT
    let host = Host()
    enum Direction {
      case left
      case right
    }
    
    func someDirection() -> Direction {
      // WRONG
      return Direction.left
    
      // RIGHT
      return .left
    }
  • (link) Don't use self unless it's necessary for disambiguation or required by the language. SwiftFormat: redundantSelf

    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) Add a trailing comma on the last element of a multi-line array. SwiftFormat: trailingCommas

    // WRONG
    let rowContent = [
      listingUrgencyDatesRowContent(),
      listingUrgencyBookedRowContent(),
      listingUrgencyBookedShortRowContent()
    ]
    
    // RIGHT
    let rowContent = [
      listingUrgencyDatesRowContent(),
      listingUrgencyBookedRowContent(),
      listingUrgencyBookedShortRowContent(),
    ]
  • (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) Use constructors instead of Make() functions for CGRect, CGPoint, NSRange and others. SwiftLint: legacy_constructor

    // WRONG
    let rect = CGRectMake(10, 10, 10, 10)
    
    // RIGHT
    let rect = CGRect(x: 0, y: 0, width: 10, height: 10)
  • (link) Favor modern Swift extension methods over older Objective-C global methods. SwiftLint: legacy_cggeometry_functions SwiftLint: legacy_constant SwiftLint: legacy_nsgeometry_functions

    // WRONG
    var rect = CGRectZero
    var width = CGRectGetWidth(rect)
    
    // RIGHT
    var rect = CGRect.zero
    var width = rect.width
  • (link) Place the colon immediately after an identifier, followed by a space. SwiftLint: colon

    // WRONG
    var something : Double = 0
    
    // RIGHT
    var something: Double = 0
    // WRONG
    class MyClass : SuperClass {
      // ...
    }
    
    // RIGHT
    class MyClass: SuperClass {
      // ...
    }
    // WRONG
    var dict = [KeyType:ValueType]()
    var dict = [KeyType : ValueType]()
    
    // RIGHT
    var dict = [KeyType: ValueType]()
  • (link) Place a space on either side of a return arrow for readability. SwiftLint: return_arrow_whitespace

    // WRONG
    func doSomething()->String {
      // ...
    }
    
    // RIGHT
    func doSomething() -> String {
      // ...
    }
    // WRONG
    func doSomething(completion: ()->Void) {
      // ...
    }
    
    // RIGHT
    func doSomething(completion: () -> Void) {
      // ...
    }
  • (link) Omit unnecessary parentheses. SwiftFormat: redundantParens

    // WRONG
    if (userCount > 0) { ... }
    switch (someValue) { ... }
    let evens = userCounts.filter { (number) in number % 2 == 0 }
    let squares = userCounts.map() { $0 * $0 }
    
    // RIGHT
    if userCount > 0 { ... }
    switch someValue { ... }
    let evens = userCounts.filter { number in number % 2 == 0 }
    let squares = userCounts.map { $0 * $0 }
  • (link) Omit enum associated values from case statements when all arguments are unlabeled. SwiftLint: empty_enum_arguments

    // WRONG
    if case .done(_) = result { ... }
    
    switch animal {
    case .dog(_, _, _):
      ...
    }
    
    // RIGHT
    if case .done = result { ... }
    
    switch animal {
    case .dog:
      ...
    }

Functions

  • (link) Omit Void return types from function definitions. SwiftLint: redundant_void_return

    // WRONG
    func doSomething() -> Void {
      ...
    }
    
    // RIGHT
    func doSomething() {
      ...
    }

Closures

  • (link) Favor Void return types over () in closure declarations. If you must specify a Void return type in a function declaration, use Void rather than () to improve readability. SwiftLint: void_return

    // WRONG
    func method(completion: () -> ()) {
      ...
    }
    
    // RIGHT
    func method(completion: () -> Void) {
      ...
    }
  • (link) Name unused closure parameters as underscores (_). SwiftLint: unused_closure_parameter

    Why?

    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) Single-line closures should have a space inside each brace. SwiftLint: closure_spacing

    // WRONG
    let evenSquares = numbers.filter {$0 % 2 == 0}.map {  $0 * $0  }
    
    // RIGHT
    let evenSquares = numbers.filter { $0 % 2 == 0 }.map { $0 * $0 }

Operators

  • (link) Infix operators should have a single space on either side. Prefer parenthesis to visually group statements with many operators rather than varying widths of whitespace. This rule does not apply to range operators (e.g. 1...3) and postfix or prefix operators (e.g. guest? or -1). SwiftLint: operator_usage_whitespace

    // WRONG
    let capacity = 1+2
    let capacity = currentCapacity   ?? 0
    let mask = (UIAccessibilityTraitButton|UIAccessibilityTraitSelected)
    let capacity=newCapacity
    let latitude = region.center.latitude - region.span.latitudeDelta/2.0
    
    // RIGHT
    let capacity = 1 + 2
    let capacity = currentCapacity ?? 0
    let mask = (UIAccessibilityTraitButton | UIAccessibilityTraitSelected)
    let capacity = newCapacity
    let latitude = region.center.latitude - (region.span.latitudeDelta / 2.0)

⬆ back to top

Patterns

  • (link) Prefer initializing properties at init time whenever possible, rather than using implicitly unwrapped optionals. A notable exception is UIViewController's view property. SwiftLint: implicitly_unwrapped_optional

    // WRONG
    class MyClass: NSObject {
    
      init() {
        super.init()
        someValue = 5
      }
    
      var someValue: Int!
    }
    
    // RIGHT
    class MyClass: NSObject {
    
      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 a start() 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 oldValue explicit.

    // 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 guard to unwrap self for the duration of the callback.

    //WRONG
    class MyClass {
    
      func request(completion: () -> Void) {
        API.request() { [weak self] response in
          if let strongSelf = self {
            // Processing and side effects
          }
          completion()
        }
      }
    }
    
    // RIGHT
    class MyClass {
    
      func request(completion: () -> Void) {
        API.request() { [weak self] response in
          guard let strongSelf = self else { return }
          strongSelf.doSomething(strongSelf.property)
          completion()
        }
      }
    
      func doSomething(nonOptionalParameter: SomeClass) {
        // Processing and side effects
      }
    }
  • (link) Prefer using guard at the beginning of a scope.

    Why?

    It's easier to reason about a block of code when all guard statements are grouped together at the top rather than intermixed with business logic.

  • (link) Access control should be at the strictest level possible. Prefer public to open and private to fileprivate unless you need that behavior.

  • (link) Avoid global functions whenever possible. Prefer methods within type definitions.

    // WRONG
    func age(of person, bornAt timeInterval) -> Int {
      // ...
    }
    
    func jump(person: Person) {
      // ...
    }
    
    // RIGHT
    class Person {
      var bornAt: TimeInterval
    
      var age: Int {
        // ...
      }
    
      func jump() {
        // ...
      }
    }
  • (link) Prefer putting constants in the top level of a file if they are private. If they are public or internal, define them as static properties, for namespacing purposes.

    private let privateValue = "secret"
    
    public class MyClass {
    
      public static let publicValue = "something"
    
      func doSomething() {
        print(privateValue)
        print(MyClass.publicValue)
      }
    }
  • (link) Use caseless enums for organizing public or internal constants and functions into namespaces. Avoid creating non-namespaced global constants and functions. Feel free to nest namespaces where it adds clarity.

    Why?

    Caseless enums work well as namespaces because they cannot be instantiated, which matches their intent.

    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. SwiftLint: redundant_string_enum_value

    Why?

    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 explicity, 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"
    }
    
    enum UserType: String {
      case owner
      case manager
      case member
    }
    
    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
    }
    
    enum ErrorCode: Int {
      case notEnoughMemory
      case invalidResource
      case timeOut
    }
    
    // RIGHT
    enum ErrorType: String {
      case error
      case warning
    }
    
    /// These are written to a logging service. Explicit values ensure they're consistent across binaries.
    // swiftlint:disable redundant_string_enum_value
    enum UserType: String {
      case owner = "owner"
      case manager = "manager"
      case member = "member"
    }
    // swiftlint:enable redundant_string_enum_value
    
    enum Planet: Int {
      case mercury
      case venus
      case earth
      case mars
      case jupiter
      case saturn
      case uranus
      case neptune
    }
    
    /// 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 map and compactMap instead of appending to a new collection. Use filter instead of removing elements from a mutable collection.

    Why?

    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) Handle an unexpected but recoverable condition with an assert method combined with the appropriate logging in production. If the unexpected condition is not recoverable, prefer a precondition method or fatalError(). This strikes a balance between crashing and providing insight into unexpected conditions in the wild. Only prefer fatalError over a precondition method when the failure message is dynamic, since a precondition method won't report the message in the crash report. SwiftLint: fatal_error_message SwiftLint: force_cast SwiftLint: force_try SwiftLint: force_unwrapping

    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.

    Why?

    If a method needs to be overridden, the author should opt into that functionality by using the class keyword instead.

    // WRONG
    class Fruit {
      class func eatFruits(_ fruits: [Fruit]) { ... }
    }
    
    // RIGHT
    class Fruit {
      static func eatFruits(_ fruits: [Fruit]) { ... }
    }
  • (link) Default classes to final.

    Why?

    If a class needs to be overridden, the author should opt into that functionality by omitting the final keyword.

    // WRONG
    class SettingsRepository {
      // ...
    }
    
    // RIGHT
    final class SettingsRepository {
      // ...
    }
  • (link) Never use the default case when switching over an enum.

    Why?

    Enumerating every case requires developers and reviewers have to consider the correctness of every switch statement when new cases are added.

    // WRONG
    switch anEnum {
    case .a:
      // Do something
    default:
      // Do something else.
    }
    
    // RIGHT
    switch anEnum {
    case .a:
      // Do something
    case .b, .c:
      // Do something else.
    }
    
  • (link) Check for nil rather than using optional binding if you don't need to use the value. SwiftLint: unused_optional_binding

    Why?

    Checking for nil makes it immediately clear what the intent of the statement is. Optional binding is less explicit.

    var thing: Thing?
    
    // WRONG
    if let _ = thing {
      doThing()
    }
    
    // RIGHT
    if thing != nil {
      doThing()
    }

⬆ back to top

File Organization

  • (link) Alphabetize module imports at the top of the file a single line below the last line of the header comments. Do not add additional line breaks between import statements. SwiftFormat: sortedImports

    Why?

    A standard organization method helps engineers more quickly determine which modules a file depends on.

    // WRONG
    
    //  Copyright © 2022 Shortcut Scandinavia Apps AB. All rights reserved.
    //
    import DLSPrimitives
    import Constellation
    import Epoxy
    
    import Foundation
    
    //RIGHT
    
    //  Copyright © 2022 Shortcut Scandinavia Apps AB. All rights reserved.
    //
    
    import Constellation
    import DLSPrimitives
    import Epoxy
    import Foundation

    Exception: @testable import should be grouped after the regular import and separated by an empty line.

    // WRONG
    
    //  Copyright © 2022 Shortcut Scandinavia Apps AB. All rights reserved.
    //
    
    import DLSPrimitives
    @testable import Epoxy
    import Foundation
    import Nimble
    import Quick
    
    //RIGHT
    
    //  Copyright © 2022 Shortcut Scandinavia Apps AB. All rights reserved.
    //
    
    import DLSPrimitives
    import Foundation
    import Nimble
    import Quick
    
    @testable import Epoxy
  • (link) Limit empty vertical whitespace to one line. Favor the following formatting guidelines over whitespace of varying heights to divide files into logical groupings. SwiftLint: vertical_whitespace

  • (link) Files should end in a newline. SwiftLint: trailing_newline

⬆ back to top

Objective-C Interoperability

  • (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 @objc on 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
      func didTapAcceptButton() {
        // ...
      }
    }

⬆ back to top

Documentation Comments

General Format Documentation comments are written using the format where each line is preceded by a triple slash (///). Javadoc-style block comments (/** ... */) are not permitted.

/// Returns the numeric value of the given digit represented as a Unicode scalar.
///
/// - Parameters:
///   - digit: The Unicode scalar whose numeric value should be returned.
///   - radix: The radix, between 2 and 36, used to compute the numeric value.
/// - Returns: The numeric value of the scalar.
func numericValue(of digit: UnicodeScalar, radix: Int = 10) -> Int {
// ...
}

Single-Sentence Summary

Documentation comments begin with a brief single-sentence summary that describes the declaration. (This sentence may span multiple lines, but if it spans too many lines, the author should consider whether the summary can be simplified and details moved to a new paragraph.)

If more detail is needed than can be stated in the summary, additional paragraphs (each separated by a blank line) are added after it.

The single-sentence summary is not necessarily a complete sentence; for example, method summaries are generally written as verb phrases without “this method […]” because it is already implied as the subject and writing it out would be redundant. Likewise, properties are often written as noun phrases without “this property is […]”. In any case, however, they are still terminated with a period.

/// The background color of the view.
var backgroundColor: UIColor

/// Returns the sum of the numbers in the given array.
///
/// - Parameter numbers: The numbers to sum.
/// - Returns: The sum of the numbers.
func sum(_ numbers: [Int]) -> Int {
// ...
}

Parameter, Returns, and Throws Tags

Clearly document the parameters, return value, and thrown errors of functions using the Parameter(s), Returns, and Throws tags, in that order. None ever appears with an empty description. When a description does not fit on a single line, continuation lines are indented 2 spaces in from the position of the hyphen starting the tag.

The recommended way to write documentation comments in Xcode is to place the text cursor on the declaration and press Command + Option + /. This will automatically generate the correct format with placeholders to be filled in.

Parameter(s) and Returns tags may be omitted only if the single-sentence brief summary fully describes the meaning of those items and including the tags would only repeat what has already been said.

The content following the Parameter(s), Returns, and Throws tags should be terminated with a period, even when they are phrases instead of complete sentences.

When a method takes a single argument, the singular inline form of the Parameter tag is used. When a method takes multiple arguments, the grouped plural form Parameters is used and each argument is written as an item in a nested list with only its name as the tag.

/// Returns the output generated by executing a command.
///
/// - Parameter command: The command to execute in the shell environment.
/// - Returns: A string containing the contents of the invoked process's
///   standard output.
func execute(command: String) -> String {
// ...
}

/// Returns the output generated by executing a command with the given string
/// used as standard input.
///
/// - Parameters:
///   - command: The command to execute in the shell environment.
///   - stdin: The string to use as standard input.
/// - Returns: A string containing the contents of the invoked process's
///   standard output.
func execute(command: String, stdin: String) -> String {
// ...
}

Apple’s Markup Format

Use of Apple’s markup format is strongly encouraged to add rich formatting to documentation. Such markup helps to differentiate symbolic references (like parameter names) from descriptive text in comments and is rendered by Xcode and other documentation generation tools. Some examples of frequently used directives are listed below.

Paragraphs are separated using a single line that starts with /// and is otherwise blank. Single asterisks and single underscores surround text that should be rendered in italic/oblique type. Double asterisks and double underscores surround text that should be rendered in boldface. Names of symbols or inline code are surrounded in backticks. Multi-line code (such as example usage) is denoted by placing three backticks ( `` `) on the lines before and after the code block.

Where to Document

At a minimum, documentation comments are present for every open or public declaration, and every open or public member of such a declaration, with specific exceptions noted below:

  • Individual cases of an enum often are not documented if their meaning is self-explanatory from their name. Cases with associated values, however, should document what those values mean if it is not obvious.

  • A documentation comment is not always present on a declaration that overrides a supertype declaration or implements a protocol requirement, or on a declaration that provides the default implementation of a protocol requirement in an extension.

It is acceptable to document an overridden declaration to describe new behavior from the declaration that it overrides. In no case should the documentation for the override be a mere copy of the base declaration’s documentation.

  • A documentation comment is not always present on test classes and test methods. However, they can be useful for functional test classes and for helper classes/methods shared by multiple tests.

  • A documentation comment is not always present on an extension declaration (that is, the extension itself). You may choose to add one if it help clarify the purpose of the extension, but avoid meaningless or misleading comments.

In the following example, the comment is just repetition of what is already obvious from the source code:

/// WRONG
/// Add `Equatable` conformance.
extension MyType: Equatable {
// ...
}

The next example is more subtle, but it is an example of documentation that is not scalable because the extension or the conformance could be updated in the future. Consider that the type may be made Comparable at the time of that writing in order to sort the values, but that is not the only possible use of that conformance and client code could use it for other purposes in the future.

/// WRONG
/// Make `Candidate` comparable so that they can be sorted.
extension Candidate: Comparable {
// ...
}

In general, if you find yourself writing documentation that simply repeats information that is obvious from the source and sugaring it with words like “a representation of,” then leave the comment out entirely.

However, it is not appropriate to cite this exception to justify omitting relevant information that a typical reader might need to know. For example, for a property named canonicalName, don’t omit its documentation (with the rationale that it would only say /// The canonical name.) if a typical reader may have no idea what the term “canonical name” means in that context. Use the documentation as an opportunity to define the term.

Jazzy

Utilize Jazzy to generate our documentation. To install:

[sudo] gem install jazzy

After getting it installed:

Run jazzy from your command line. Run jazzy -h for a list of additional options.

If your Swift module is the first thing to build, and it builds fine when running xcodebuild without any arguments from the root of your project, then just running jazzy (without any arguments) from the root of your project should succeed too!

You can set options for your project’s documentation in a configuration file, .jazzy.yaml by default. For a detailed explanation and an exhaustive list of all available options, run jazzy --help config.

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