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The Official Mobify Swift Style Guide.

The Mobify style guide for Swift is largely embodied in the Astro Swift code. Think of that codebase as "canonical" Swift code.

We also follow the Swift API design guidelines in all of the code we write.

Our overarching goals are conciseness, readability, and simplicity.

Linting

Included is a default Mobify Swift Style Guide SwiftLint configuration,.swiftlint.yml.

You can install SwiftLint by command brew install swiftlint

Integration

Integrate SwiftLint into Xcode to get warnings and errors displayed. You can add a new "Run Script" with:

if which swiftlint >/dev/null; then
  swiftlint
else
  echo "warning: SwiftLint not installed, install using 'brew install swiftlint'"
fi

Autocorrect is available by command swiftlint autocorrect

For more information on this linter you can visit this page

Table of Contents

Correctness

Consider warnings to be errors. This rule informs many stylistic decisions such as not to use deprecated methods, C-style for loops, or strings as selectors.

Naming

Use descriptive names with camel case for classes, methods, variables, etc. Type names (classes, structures, enumerations and protocols) should be capitalized, while method names and variables should start with a lower case letter.

Preferred:

private let maximumWidgetCount = 100

class WidgetContainer {
  var widgetButton: UIButton
  let widgetHeightPercentage = 0.85
}

Not Preferred:

let MAX_WIDGET_COUNT = 100

class app_widgetContainer {
  var wBut: UIButton
  let wHeightPct = 0.85
}

Abbreviations and acronyms should generally be avoided. Following the API Design Guidelines, abbreviations and initialisms that appear in all uppercase should be uniformly uppercase or lowercase. Examples:

Preferred

let utf8Text: String
let targetURL: URL
let userID: UserID

Not Preferred

let UTF8Text: String
let uRLText: UrlString
let userId: UserId

For functions and init methods, prefer named parameters for all arguments unless the context is very clear. Include external parameter names if it makes function calls more readable.

func dateFromString(dateString: String) -> NSDate
func convertPointAt(column column: Int, row: Int) -> CGPoint
func timedAction(afterDelay delay: NSTimeInterval, perform action: SKAction) -> SKAction!

// would be called like this:
dateFromString("2014-03-14")
convertPointAt(column: 42, row: 13)
timedAction(afterDelay: 1.0, perform: someOtherAction)

For methods, follow the standard Apple convention of referring to the first parameter in the method name:

class Counter {
  func combineWith(otherCounter: Counter, options: Dictionary?) { ... }
  func incrementBy(amount: Int) { ... }
}

Protocols

Following Apple's API Design Guidelines, protocols names that describe what something is should be a noun. Examples: Collection, WidgetFactory. Protocols names that describe an ability should end in -ing, -able, or -ible. Examples: Equatable, Resizing.

Enumerations

Following Apple's API Design Guidelines for Swift 3, use lowerCamelCase for enumeration values.

enum Shape {
  case rectangle
  case square
  case rightTriangle
  case equilateralTriangle
}

Comments

When referring to functions in comments include the required parameter names from the caller's perspective or _ for unnamed parameters. Examples:

Call convertPointAt(column:row:) from your own init implementation.

If you call dateFromString(_:) make sure that you provide a string with the format "yyyy-MM-dd".

If you call timedAction(afterDelay:perform:) from viewDidLoad() remember to provide an adjusted delay value and an action to perform.

You shouldn't call the data source method tableView(_:cellForRowAtIndexPath:) directly.

This is the same as the #selector syntax. When in doubt, look at how Xcode lists the method in the jump bar – our style here matches that.

Selectors

Selectors are Obj-C methods that act as handlers for many Cocoa and Cocoa Touch APIs. Prior to Swift 2.2, they were specified using type unsafe strings. This now causes a compiler warning. The "Fix it" button replaces these strings with the fully qualified type safe selector. Often, however, you can use context to shorten the expression. This is the preferred style.

Preferred:

let sel = #selector(viewDidLoad)

Not Preferred:

let sel = #selector(ViewController.viewDidLoad)

Generics

Generic type parameters should be descriptive, upper camel case names. When a type name doesn't have a meaningful relationship or role, use a traditional single uppercase letter such as T, U, or V.

Preferred:

struct Stack<Element> { ... }
func writeTo<Target: OutputStream>(inout target: Target)
func max<T: Comparable>(x: T, _ y: T) -> T

Not Preferred:

struct Stack<T> { ... }
func writeTo<target: OutputStream>(inout t: target)
func max<Thing: Comparable>(x: Thing, _ y: Thing) -> Thing

Language

Use US English spelling to match Apple's API.

Preferred:

let color = "red"

Not Preferred:

let colour = "red"

Code Organization

Use extensions to organize your code into logical blocks of functionality. Each extension should be set off with a // MARK: - comment to keep things well-organized.

Protocol Conformance

Protocols are awesome. Use them to restrict the API surface area of an object being passed around. In particular, when adding protocol conformance to a model, prefer adding a separate extension for the protocol methods. This keeps the related methods grouped together with the protocol and can simplify instructions to add a protocol to a class with its associated methods.

Preferred:

class MyViewController: UIViewController {
  // class stuff here
}

// MARK: - UITableViewDataSource
extension MyViewController: UITableViewDataSource {
  // table view data source methods
}

// MARK: - UIScrollViewDelegate
extension MyViewController: UIScrollViewDelegate {
  // scroll view delegate methods
}

Not Preferred:

class MyViewController: UIViewController, UITableViewDataSource, UIScrollViewDelegate {
  // all methods
}

Since the compiler does not allow you to re-declare protocol conformance in a derived class, it is not always required to replicate the extension groups of the base class. This is especially true if the derived class is a terminal class and a small number of methods are being overriden. When to preserve the extension groups is left to the discretion of the author.

For UIKit view controllers, consider grouping lifecycle, custom accessors, and IBAction in separate class extensions.

Unused Code

Unused (dead) code, including Xcode template code and placeholder comments should be removed. Aspirational methods whose implementation simply calls the super class should also be removed.

Preferred:

override func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
  return Database.contacts.count
}

Not Preferred:

override func didReceiveMemoryWarning() {
   super.didReceiveMemoryWarning()
  // Dispose of any resources that can be recreated.
}

override func numberOfSectionsInTableView(tableView: UITableView) -> Int {
   // #warning Incomplete implementation, return the number of sections
   return 1
}

override func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
  // #warning Incomplete implementation, return the number of rows
  return Database.contacts.count
}

Minimal Imports

Keep imports minimal. For example, don't import UIKit when importing Foundation will suffice.

Spacing

  • Indent using 4 spaces rather than tabs to conserve space and help prevent line wrapping. Be sure to set this preference in Xcode and in the Project settings.
  • Method braces and other braces (if/else/switch/while etc.) always open on the same line as the statement but close on a new line.
  • Tip: You can re-indent by selecting some code (or ⌘A to select all) and then Control-I (or Editor\Structure\Re-Indent in the menu).

Preferred:

if user.isHappy {
  // Do something
} else {
  // Do something else
}

Not Preferred:

if user.isHappy
{
  // Do something
}
else {
  // Do something else
}

  • There should be exactly one blank line between methods to aid in visual clarity and organization. Whitespace within methods should separate functionality, but having too many sections in a method often means you should refactor into several methods.

  • Colons always have no space on the left and one space on the right. Exceptions are the ternary operator ? :, empty dictionary [:] and #selector syntax for unnamed parameters (_:).

Preferred:

class TestDatabase: Database {
  var data: [String: CGFloat] = ["A": 1.2, "B": 3.2]
}

Not Preferred:

class TestDatabase : Database {
  var data :[String:CGFloat] = ["A" : 1.2, "B":3.2]
}

For Dictionary and Array literals, use python-style indentation:

let nameMap = [
    "George": "Jetson",
    "Astro": "Boy",
    // etc.
]

Comments

When they are needed, use comments to explain why a particular piece of code does something. Comments must be kept up-to-date or deleted.

Avoid block comments inline with code, as the code should be as self-documenting as possible. Exception: This does not apply to those comments used to generate documentation.

Classes and Structures

Which one to use?

Remember, structs have value semantics. Use structs for things that do not have an identity. An array that contains [a, b, c] is really the same as another array that contains [a, b, c] and they are completely interchangeable. It doesn't matter whether you use the first array or the second, because they represent the exact same thing. That's why arrays are structs.

Classes have reference semantics. Use classes for things that do have an identity or a specific life cycle. You would model a person as a class because two person objects are two different things. Just because two people have the same name and birthdate, doesn't mean they are the same person. But the person's birthdate would be a struct because a date of 3 March 1950 is the same as any other date object for 3 March 1950. The date itself doesn't have an identity.

Sometimes, things should be structs but need to conform to AnyObject or are historically modeled as classes already (NSDate, NSSet). Try to follow these guidelines as closely as possible.

Use of Self

For conciseness, avoid using self since Swift does not require it to access an object's properties or invoke its methods.

Use self when required to differentiate between property names and arguments in initializers, and when referencing properties or methods in closure expressions (as required by the compiler):

class BoardLocation {
  let row: Int, column: Int

  init(row: Int, column: Int) {
    self.row = row
    self.column = column

    let closure = {
      print(self.row)
    }
  }
}

Computed Properties

For conciseness, if a computed property is read-only, omit the get clause. The get clause is required only when a set clause is provided.

Preferred:

var diameter: Double {
  return radius * 2
}

Not Preferred:

var diameter: Double {
  get {
    return radius * 2
  }
}

Use computed properties instead of getter and setter functions:

Preferred

class Circle {
    var diameter: Double {
        get {
            return radius * 2
        }
        set {
            radius = newValue/2
        }
    }
}

Not Preferred

class Circle {
    func getDiameter {
        return radius * 2
    }

    func setDiameter(_ diameter: Double) {
        radius = diameter/2
    }
}

Final

Mark classes final when inheritance is not intended. Example:

// Turn any generic type into a reference type using this Box class.
final class Box<T> {
  let value: T
  init(_ value: T) {
    self.value = value
  }
}

Alternatively, consider using a struct instead since they do not allow inheritance by default.

Function Declarations

Keep short function declarations on one line including the opening brace:

func reticulateSplines(spline: [Double]) -> Bool {
  // reticulate code goes here
}

For functions with long signatures, add line breaks after each parameter and indent them on the same level as the first parameter.

func reticulateSplines(spline: [Double],
                       adjustmentFactor: Double,
                       translateConstant: Int,
                       comment: String) -> Bool {
  // reticulate code goes here
}

Closure Expressions

Use trailing closure syntax only if there's a single closure expression parameter at the end of the argument list. Give the closure parameters descriptive names.

Preferred:

UIView.animateWithDuration(1.0) {
  self.myView.alpha = 0
}

UIView.animateWithDuration(1.0,
  animations: {
    self.myView.alpha = 0
  },
  completion: { finished in
    self.myView.removeFromSuperview()
  }
)

Not Preferred:

UIView.animateWithDuration(1.0, animations: {
  self.myView.alpha = 0
})

UIView.animateWithDuration(1.0,
  animations: {
    self.myView.alpha = 0
  }) { f in
    self.myView.removeFromSuperview()
}

For single-expression closures where the context is clear, use implicit returns:

attendeeList.sort { a, b in
  a > b
}

Chained methods using trailing closures should be clear and easy to read in context. Decisions on spacing, line breaks, and when to use named versus anonymous arguments is left to the discretion of the author. Examples:

let value = numbers.map { $0 * 2 }.filter { $0 % 3 == 0 }.indexOf(90)

let value = numbers
   .map {$0 * 2}
   .filter {$0 > 50}
   .map {$0 + 10}

Place the closing curly brace (}) on the same line if the body of the block is empty:

let callback: RpcMethodCallback = { result in }  // Do nothing

Types

Always use Swift's native types when available. Swift offers bridging to Objective-C so you can still use the full set of methods as needed.

Preferred:

let width = 120.0                                  // Double
let labelText = (width as NSNumber).stringValue    // String

Not Preferred:

let width: NSNumber = 120.0                        // NSNumber
let labelText: NSString = width.stringValue        // NSString

Constants

Constants are defined using the let keyword, and variables with the var keyword. Always use let instead of var if the value of the variable will not change.

Tip: A good technique is to define everything using let and only change it to var if the compiler complains!

You can define constants on a type rather than an instance of that type using type properties. To declare a type property as a constant simply use static let. Type properties declared in this way are generally preferred over global constants because they are easier to distinguish from instance properties. Example:

Preferred:

enum Math {
  static let e  = 2.718281828459045235360287
  static let pi = 3.141592653589793238462643
}

radius * Math.pi * 2 // circumference

Note: The advantage of using a case-less enumeration is that it can't accidentally be instantiated and works as a pure namespace.

Not Preferred:

let e  = 2.718281828459045235360287  // pollutes global namespace
let pi = 3.141592653589793238462643

radius * pi * 2 // is pi instance data or a global constant?

When using let assign the value in the variable declaration if possible:

class Component {
    let viewController = UIViewController() // Assign here instead of the in the initializer
}

Static Methods and Variable Type Properties

Static methods and type properties work similarly to free functions and global variables and should be used sparingly. They are useful when functionality is scoped to a particular type or when Objective-C interoperability is required.

Optionals

Declare variables and function return types as optional with ? where a nil value is acceptable.

Use implicitly unwrapped types declared with ! only for instance variables that you know will be initialized later before use, such as subviews that will be set up in viewDidLoad. Other cases you should use ! are:

  • You just assigned to an object where you know initialization can not fail:

    let numberSeven = Int("7")! //Don't do this.

    In this case, wrap this into a static function and write a test for it:

        extension Int {
        static func seven() -> Int {
            return Int("7")!
        }
    }
        import XCTest

    class IntegerTests: XCTestCase {
        func testSeven() {
            XCTAssertEqual(7, Int.seven())
        }
    }

    Although the test might seem redundant, because seven() returns a non-optional, this ensures that even if in the future someone changes the implementation of seven(), it won't get past CI if the explicit unwrapping would fail.

  • You are creating an object during initialization but need to pass self to that object's initializer:

    class Component: ParentComponent {
    var controller: CustomViewController! // We want to use it as a non-optional but have to initialize after super.init()

    override init() {
        // controller = CustomViewController(component: self) <-- compiler error
        super.init()
        controller = CustomViewController(component: self)
    }
}

When accessing an optional value, use optional chaining if the value is only accessed once or if there are many optionals in the chain:

self.textContainer?.textLabel?.setNeedsDisplay()

Use optional binding when it's more convenient to unwrap once and perform multiple operations:

if let textContainer = self.textContainer {
  // do many things with textContainer
}

When naming optional variables and properties, avoid naming them like optionalString or maybeView since their optional-ness is already in the type declaration.

For optional binding, shadow the original name when appropriate rather than using names like unwrappedView or actualLabel.

Use as for type coercion if possible (this is enforced statically). Otherwise use as?.

Preferred:

var subview: UIView?
var volume: Double?

// later on...
if let subview = subview, volume = volume {
  // do something with unwrapped subview and volume
}

Not Preferred:

var optionalSubview: UIView?
var volume: Double?

if let unwrappedSubview = optionalSubview {
  if let realVolume = volume {
    // do something with unwrappedSubview and realVolume
  }
}

Struct Initializers

Use the native Swift struct initializers rather than the legacy CGGeometry constructors.

Preferred:

let bounds = CGRect(x: 40, y: 20, width: 120, height: 80)
let centerPoint = CGPoint(x: 96, y: 42)

Not Preferred:

let bounds = CGRectMake(40, 20, 120, 80)
let centerPoint = CGPointMake(96, 42)

Prefer the struct-scope constants CGRect.infinite, CGRect.null, etc. over global constants CGRectInfinite, CGRectNull, etc. For existing variables, you can use the shorter .zero.

Lazy Initialization

Consider using lazy initialization for finer grain control over object lifetime. This is especially true for UIViewController that loads views lazily. Use a closure that is immediately called { }(). Example:

lazy var locationManager: CLLocationManager = {
  let manager = CLLocationManager()
  manager.desiredAccuracy = kCLLocationAccuracyBest
  manager.delegate = self
  manager.requestAlwaysAuthorization()
  return manager
}()

Notes:

  • [unowned self] is not required here. A retain cycle is not created.
  • Location manager has a side-effect for popping up UI to ask the user for permission so fine grain control makes sense here.

Type Inference

Prefer compact code and let the compiler infer the type for constants or variables of single instances. Type inference is also appropriate for small (non-empty) arrays and dictionaries. When required, specify the specific type such as CGFloat or Int16.

Preferred:

let message = "Click the button"
let currentBounds = computeViewBounds()
var names = ["Mic", "Sam", "Christine"]
let maximumWidth: CGFloat = 106.5

Not Preferred:

let message: String = "Click the button"
let currentBounds: CGRect = computeViewBounds()
let names: [String] = ["Mic", "Sam", "Christine"]

Type Annotation for Empty Arrays and Dictionaries

For empty arrays and dictionaries, use type annotation. (For an array or dictionary assigned to a large, multi-line literal, use type annotation to make it clear what type(s) the array/dictionary contains)

Preferred:

var names = [String]()
var lookup = [String: Int]()

Not Preferred:

var names: [String] = []
var lookup: [String: Int] = [:]

NOTE: Following this guideline means picking descriptive names is even more important than before.

Syntactic Sugar

Prefer the shortcut versions of type declarations over the full generics syntax.

Preferred:

var deviceModels: [String]
var employees: [Int: String]
var faxNumber: Int?

Not Preferred:

var deviceModels: Array<String>
var employees: Dictionary<Int, String>
var faxNumber: Optional<Int>

Functions vs Methods

Free functions, which aren't attached to a class or type, should be used sparingly. When possible, prefer to use a method instead of a free function. This aids in readability and discoverability.

Free functions are most appropriate when they aren't associated with any particular type or instance.

Preferred

let sorted = items.mergeSort()  // easily discoverable
rocket.launch()  // clearly acts on the model

Not Preferred

let sorted = mergeSort(items)  // hard to discover
launch(&rocket)

Free Function Exceptions

let tuples = zip(a, b)  // feels natural as a free function (symmetry)
let value = max(x,y,z)  // another free function that feels natural

Memory Management

Code (even non-production) should not create reference cycles. Analyze your object graph and prevent strong cycles with weak and unowned references. Alternatively, use value types (struct, enum) to prevent cycles altogether.

Variables that are delegates should be weak.

Extending object lifetime

Use [weak self] to avoid retain cycles. Within a closure, extend object lifetime using the [weak self] and guard let strongSelf = self else { return } idiom. For example:

func doSomething() {
   self.test.asyncTask() { [weak self] bar in
     guard let strongSelf = self else { return } // gives self local scope, self will be retained for as long as 'strongSelf' is in scope
     let x = bar.doThis()
     strongSelf.foo = x // safe
   }
}

[weak self] is preferred to [unowned self] where it is not immediately obvious that self outlives the closure. Explicitly extending lifetime is preferred to optional unwrapping.

Preferred

resource.request().onComplete { [weak self] response in
  guard let strongSelf = self else { return }
  let model = strongSelf.updateModel(response)
  strongSelf.updateUI(model)
}

Not Preferred

// might crash if self is released before response returns
resource.request().onComplete { [unowned self] response in
  let model = self.updateModel(response)
  self.updateUI(model)
}

Not Preferred

// deallocate could happen between updating the model and updating UI
resource.request().onComplete { [weak self] response in
  let model = self?.updateModel(response)
  self?.updateUI(model)
}

Access Control

Using private appropriately adds clarity and promotes encapsulation. Use private as the leading property specifier. The only things that should come before access control are the static specifier or attributes such as @IBAction and @IBOutlet.

An exception to this is RPC methods within Astro, which should be marked as internal.

Preferred:

class TimeMachine {  
  private dynamic lazy var fluxCapacitor = FluxCapacitor()
}

Not Preferred:

class TimeMachine {  
  lazy dynamic private var fluxCapacitor = FluxCapacitor()
}

Control Flow

Prefer the for-in style of for loop over the while-condition-increment style.

Preferred:

for _ in 0..<3 {
  print("Hello three times")
}

for (index, person) in attendeeList.enumerate() {
  print("\(person) is at position #\(index)")
}

for index in 0.stride(to: items.count, by: 2) {
  print(index)
}

for index in (0...3).reverse() {
  print(index)
}

Not Preferred:

var i = 0
while i < 3 {
  print("Hello three times")
  i += 1
}


var i = 0
while i < attendeeList.count {
  let person = attendeeList[i]
  print("\(person) is at position #\(i)")
  i += 1
}

Error Handling

As of Swift 2.0 exceptions have been introduced into the language and the APIs that formerly used someMethod(..., error: *NSError) make use of them. We have decided to stick with our existing pattern for returning errors.

We typically use an enumeration type:

public enum ParseResult {
    case Result(AST)
    case Error(String)
}
func parse(string: String) -> ParseResult

Golden Path

When coding with conditionals, the left hand margin of the code should be the "golden" or "happy" path. That is, don't nest if statements. Multiple return statements are OK. The guard statement is built for this.

Preferred:

func computeFFT(context: Context?, inputData: InputData?) throws -> Frequencies {

  guard let context = context else {
      throw FFTError.noContext
  }
  guard let inputData = inputData else {
      throw FFTError.noInputData
  }

  // use context and input to compute the frequencies

  return frequencies
}

Not Preferred:

func computeFFT(context: Context?, inputData: InputData?) throws -> Frequencies {

  if let context = context {
    if let inputData = inputData {
      // use context and input to compute the frequencies

      return frequencies
    }
    else {
      throw FFTError.noInputData
    }
  }
  else {
    throw FFTError.noContext
  }
}

When multiple optionals are unwrapped either with guard or if let, minimize nesting by using the compound version when possible. Example:

Preferred:

guard let number1 = number1, number2 = number2, number3 = number3 else { fatalError("impossible") }
// do something with numbers

Not Preferred:

if let number1 = number1 {
  if let number2 = number2 {
    if let number3 = number3 {
      // do something with numbers
    }
    else {
      fatalError("impossible")
    }
  }
  else {
    fatalError("impossible")
  }
}
else {
  fatalError("impossible")
}

Failing Guards

Guard statements are required to exit in some way. Generally, this should be simple one line statement such as return, throw, break, continue, and fatalError(). Large code blocks should be avoided. If cleanup code is required for multiple exit points, consider using a defer block to avoid cleanup code duplication.

Semicolons

Swift does not require a semicolon after each statement in your code. They are only required if you wish to combine multiple statements on a single line.

Do not write multiple statements on a single line separated with semicolons.

Preferred:

let swift = "not a scripting language"

Not Preferred:

let swift = "not a scripting language";

Parentheses

Parentheses around conditionals are not required and should be omitted.

Preferred:

if name == "Hello" {
  print("World")
}

Not Preferred:

if (name == "Hello") {
  print("World")
}

String Interpolation

Prefer interpolation over concatenation:

let name = "John"
let messsage = "Hello \(name)" //YES
let greeting = "Hello " + name //NO

Objective-C Interoperability

Do not use @objc or inherit from NSObject unless absolutely necessary.

Typealiasing

typealias standard types if used in specific contexts.

typealias MessageAddress = String

Also, you will often find it useful to typealias function types:

typealias RpcMethodCallback = (RpcMethodResult) -> Void

Tests

Order the values in a test assertion in the following way:

XCTAssertEqual(actualValue, expectedValue)

Autolayout

Prefer Autolayout over springs+struts (autoresizing mask). Autolayout automatically handles many things that springs+struts doesn't (status bar hiding/showing, device rotation)

Credits

Heavily based on The Official raywenderlich.com Swift Style Guide