The Official MIL Swift Style Guide

This style guide is different from others you may see, because the focus is centered on readability for print and the web. We created this style guide to keep the code in our books, tutorials, and starter kits nice and consistent — even though we have many different authors working on the books.

Our overarching goals are conciseness, readability, and simplicity.

Writing Objective-C? Check out our Objective-C Style Guide too.

Naming

Use descriptive names with camel case for classes, methods, variables, etc. Class names 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
}

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: Int, #row: Int) -> CGPoint
func timedAction(#delay: NSTimeInterval, perform action: SKAction) -> SKAction!

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

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

class Guideline {
  func combineWithString(incoming: String, options: Dictionary?) { ... }
  func upvoteBy(amount: Int) { ... }
}

Enumerations

Use UpperCamelCase for enumeration values:

enum Shape {
  case Rectangle
  case Square
  case Triangle
  case Circle
}

Prose

When referring to functions in prose (tutorials, books, comments) include the required parameter names from the caller's perspective or _ for unnamed parameters.

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(delay: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.

When in doubt, look at how Xcode lists the method in the jump bar – our style here matches that.

Class Prefixes

Swift types are automatically namespaced by the module that contains them and you should not add a class prefix. If two names from different modules collide you can disambiguate by prefixing the type name with the module name.

import SomeModule

let myClass = MyModule.UsefulClass()

Spacing

Indent using 4 spaces or tabs to help with readability. Be sure to set this preference in Xcode as shown below:
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.
Space out braces from conditional statements as to avoid if{ or }else{ for readability sake.
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). Some of the Xcode template code will have 4-space tabs hard coded, so this is a good way to fix that.

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.

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.

Example definition

Here's an example of a well-styled class definition:

class Circle: Shape {

  var x: Int, y: Int
  var radius: Double
  var diameter: Double {
    get {
      return radius * 2
    }
    set {
      radius = newValue / 2
    }
  }

  init(x: Int, y: Int, radius: Double) {
    self.x = x
    self.y = y
    self.radius = radius
  }

  convenience init(x: Int, y: Int, diameter: Double) {
    self.init(x: x, y: y, radius: diameter / 2)
  }

  func describe() -> String {
    return "I am a circle at \(centerString()) with an area of \(computeArea())"
  }

  override func computeArea() -> Double {
    return M_PI * radius * radius
  }

  private func centerString() -> String {
    return "(\(x),\(y))"
  }
}

The example above demonstrates the following style guidelines:

Specify types for properties, variables, constants, argument declarations and other statements with a space after the colon but not before, e.g. x: Int, and Circle: Shape.
Define multiple variables and structures on a single line if they share a common purpose / context.
Indent getter and setter definitions and property observers.
Don't add modifiers such as internal when they're already the default. Similarly, don't repeat the access modifier when overriding a method.

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 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 = {
      println(self.row)
    }
  }
}

Protocol Conformance

When adding protocol conformance to a class, prefer adding a separate class 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.

Also, don't forget the // MARK: - comment to keep things well-organized!

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
}

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
  }
}

Function Declarations

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

func reticulateSplines(spline: [Double]) -> 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
}

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 widthString = (width as NSNumber).stringValue    // String

Not Preferred:

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

In Sprite Kit code, use CGFloat if it makes the code more succinct by avoiding too many conversions.

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!

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.

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.

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.infiniteRect, CGRect.nullRect, etc. over global constants CGRectInfinite, CGRectNull, etc. For existing variables, you can use the shorter .zeroRect.

Type Inference

Prefer compact code and let the compiler infer the type for a constant or variable, unless you need a specific type other than the default such as CGFloat or Int16.

Preferred:

let message = "Click the button"
let currentBounds = computeViewBounds()
var names = [String]()
let maximumWidth: CGFloat = 106.5

Not Preferred:

let message: String = "Click the button"
let currentBounds: CGRect = computeViewBounds()
var names: [String] = []

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>

Control Flow

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

Preferred:

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

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

Not Preferred:

for var i = 0; i < 3; i++ {
  println("Hello three times")
}

for var i = 0; i < attendeeList.count; i++ {
  let person = attendeeList[i]
  println("\(person) is at position #\(i)")
}

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.

The only exception to this rule is the for-conditional-increment construct, which requires semicolons. However, alternative for-in constructs should be used where possible.

Preferred:

let swift = "not a scripting language"

Not Preferred:

let swift = "not a scripting language";

NOTE: Swift is very different to JavaScript, where omitting semicolons is generally considered unsafe

Copyright Statement

The following copyright statement should be included at the top of every source file:

/*
Licensed Materials - Property of IBM
© Copyright IBM Corporation 2015. All Rights Reserved.
*/

Credits

This style guide is a collaborative effort from the most stylish raywenderlich.com team members:

Hat tip to Nicholas Waynik and the Objective-C Style Guide team!

We also drew inspiration from Apple’s reference material on Swift:

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The Official MIL Swift Style Guide

Table of Contents

Naming

Enumerations

Prose

Class Prefixes

Spacing

Comments

Classes and Structures

Which one to use?

Example definition

Use of Self

Protocol Conformance

Computed Properties

Function Declarations

Closure Expressions

Types

Constants

Optionals

Struct Initializers

Type Inference

Syntactic Sugar

Control Flow

Semicolons

Copyright Statement

Credits

About

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The Official MIL Swift Style Guide

Table of Contents

Naming

Enumerations

Prose

Class Prefixes

Spacing

Comments

Classes and Structures

Which one to use?

Example definition

Use of Self

Protocol Conformance

Computed Properties

Function Declarations

Closure Expressions

Types

Constants

Optionals

Struct Initializers

Type Inference

Syntactic Sugar

Control Flow

Semicolons

Copyright Statement

Credits

About

Resources

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