The Official zooplus Swift Style Guide.

Updated for Swift 4

Our top goals are clarity, consistency and brevity, in that order. Other references:

Correctness

Strive to make your code compile without warnings.

Naming

Descriptive and consistent naming makes software easier to read and understand. Use the Swift naming conventions described in the API Design Guidelines. Some key takeaways include:

striving for clarity at the call site
prioritizing clarity over brevity
using camel case (not snake case)
using uppercase for types (and protocols), lowercase for everything else
including all needed words while omitting needless words
using names based on roles, not types
sometimes compensating for weak type information
striving for fluent usage
beginning factory methods with make
naming methods for their side effects
- verb methods follow the -ed, -ing rule for the non-mutating version
- noun methods follow the formX rule for the mutating version
- boolean types should read like assertions
- protocols that describe what something is should read as nouns
- protocols that describe a capability should end in -able or -ible
using terms that don't surprise experts or confuse beginners
generally avoiding abbreviations
using precedent for names
preferring methods and properties to free functions
casing acronyms and initialisms uniformly up or down
giving the same base name to methods that share the same meaning
avoiding overloads on return type
choosing good parameter names that serve as documentation
labeling closure and tuple parameters
taking advantage of default parameters

Class Prefixes

Swift types are automatically namespaced by the module that contains them and you should not add a class prefix such as ZP. If two names from different modules collide you can disambiguate by prefixing the type name with the module name. However, only specify the module name when there is possibility for confusion (which should be rare).

import SomeModule

let myClass = MyModule.UsefulClass()

Delegates

When creating custom delegate methods, an unnamed first parameter should be the delegate source. (UIKit contains numerous examples of this).

Preferred:

func namePickerView(_ namePickerView: NamePickerView, didSelectName name: String)
func namePickerViewShouldReload(_ namePickerView: NamePickerView) -> Bool

Not Preferred:

func didSelectName(namePicker: NamePickerViewController, name: String)
func namePickerShouldReload() -> Bool

Use Type Inferred Context

Use compiler inferred context to write shorter, clear code. (Also see Type Inference.)

Preferred:

let selector = #selector(viewDidLoad)
view.backgroundColor = .red
let toView = context.view(forKey: .to)
let view = UIView(frame: .zero)

Not Preferred:

let selector = #selector(ViewController.viewDidLoad)
view.backgroundColor = UIColor.red
let toView = context.view(forKey: UITransitionContextViewKey.to)
let view = UIView(frame: CGRect.zero)

On the other hand, when declaring class properties, always specify the type. It makes it easier to read and faster to compile.

Preferred:

let selector = #selector(viewDidLoad)
view.backgroundColor = .red
let toView = context.view(forKey: .to)
let view = UIView(frame: .zero)

Not Preferred:

let selector = #selector(ViewController.viewDidLoad)
view.backgroundColor = UIColor.red
let toView = context.view(forKey: UITransitionContextViewKey.to)
let view = UIView(frame: CGRect.zero)

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 Person {
	var firstName: String = ""
	var lastName: String = ""
}

Not Preferred:

struct Person {
	var firstName = ""
	var lastName = ""
}

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

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
}

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. This includes any empty/unused UIApplicationDelegate methods.

Preferred:

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

Not Preferred:

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

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

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

Minimal Imports

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

Spacing

Indent using tabs
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). 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.
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]
}

Long lines should be wrapped at around 70 characters. A hard limit is intentionally not specified.
Remove trailing whitespaces at the ends of lines.
Add a single newline character at the end of each file.

Comments

When they are needed, use comments to explain why a particular piece of code does something, especially for public methods. 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.

Example definition

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

class Circle: Shape {
  	var x: Int 
	var 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)
  	}

 	override func area() -> Double {
		return Double.pi * radius * radius
	}
}

extension Circle: CustomStringConvertible {
	var description: String {
		return "center = \(centerString) area = \(area())"
 	}
	private var centerString: String {
		return "(\(x),\(y))"
	}
}

The example above demonstrates the following style guidelines:

Specify types for properties, variables, constants, argument declarations and other statements with a space after the colon but not before, e.g. x: Int, and Circle: Shape.
Indent getter and setter definitions and property observers. Keep the code in the same line of the getter and setter definitions if there's only one line of code.
Don't add modifiers such as internal when they're already the default. Similarly, don't repeat the access modifier when overriding a method.
Organize extra functionality (e.g. printing) in extensions.
Hide non-shared, implementation details such as centerString inside the extension using private access control.

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 only when required by the compiler (in @escaping closures, or in initializers to disambiguate properties from arguments). In other words, if it compiles without self then omit it.

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
}

For functions with long signatures, add line breaks at appropriate points and add an extra indent on subsequent lines:

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.animate(withDuration: 1.0) {
  self.myView.alpha = 0
}

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

Not Preferred:

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

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

For single-expression closures where the context is clear, use implicit returns and keep it in the same line:

attendeeList.sort { $0 > $1 }

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 developer. Examples:

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

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

Always include exactly one white space after and before curly braces in closures.

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

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

Preferred:

enum Math {
  static let e = 2.718281828459045235360287
  static let root2 = 1.41421356237309504880168872
}

let hypotenuse = side * Math.root2

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 root2 = 1.41421356237309504880168872

let hypotenuse = side * root2 // what is root2?

Static Methods and Variable Type Properties

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

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, let 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
  }
}

Lazy Initialization

Consider using lazy initialization for finer grain control over object lifetime. This is especially true for UIViewController that loads views lazily. You can either use a closure that is immediately called { }() or call a private factory method. Example:

lazy var locationManager: CLLocationManager = self.makeLocationManager()

private func makeLocationManager() -> CLLocationManager {
  let manager = CLLocationManager()
  manager.desiredAccuracy = kCLLocationAccuracyBest
  manager.delegate = self
  manager.requestAlwaysAuthorization()
  return manager
}

Notes:

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

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]()

Property declarations are an exception to this rule and should have type annotation.

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.)

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.mergeSorted()  // easily discoverable
rocket.launch()  // 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 should not create reference cycles. Analyze your object graph and prevent strong cycles with weak and unowned references.

Extending object lifetime

Extend object lifetime using the [weak self] and guard let strongSelf = self else { return } idiom. [weak self] is preferred to [unowned self] where it is not immediately obvious that self outlives the closure. Explicitly extending lifetime is preferred to optional 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 and fileprivate appropriately adds clarity and promotes encapsulation. Prefer private to fileprivate when possible. Using extensions may require you to use fileprivate.

Only explicitly use open, public, and internal when you require a full access control specification.

Use access control as the leading property specifier. The only things that should come before access control are the static specifier or attributes such as @IBAction, @IBOutlet and @discardableResult.

Preferred:

private let message = "Great Scott!"

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

Not Preferred:

fileprivate let message = "Great Scott!"

class TimeMachine {  
	lazy dynamic fileprivate 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.enumerated() {
	print("\(person) is at position #\(index)")
}

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

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

Not Preferred:

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


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

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,
      let number2 = number2,
      let 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";

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

Parentheses

Parentheses around conditionals are not required and should be omitted.

Preferred:

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

Not Preferred:

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

In larger expressions, optional parentheses can sometimes make code read more clearly.

Preferred:

let playerMark = (player == current ? "X" : "O")

Name		Name	Last commit message	Last commit date
Latest commit History 5 Commits
screens		screens
CONTRIBUTING.markdown		CONTRIBUTING.markdown
CREDITS.markdown		CREDITS.markdown
LICENSE.txt		LICENSE.txt
README.markdown		README.markdown

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The Official zooplus Swift Style Guide.

Updated for Swift 4

Table of Contents

Correctness

Naming

Class Prefixes

Delegates

Use Type Inferred Context

Generics

Language

Code Organization

Protocol Conformance

Unused Code

Minimal Imports

Spacing

Comments

Classes and Structures

Which one to use?

Example definition

Use of Self

Computed Properties

Function Declarations

Closure Expressions

Types

Constants

Static Methods and Variable Type Properties

Optionals

Lazy Initialization

Type Inference

Type Annotation for Empty Arrays and Dictionaries

Syntactic Sugar

Functions vs Methods

Memory Management

Extending object lifetime

Access Control

Control Flow

Golden Path

Failing Guards

Semicolons

Parentheses

References

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