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All Projects → rosberry → swift-style-guide

rosberry / swift-style-guide

Licence: MIT license
Swift style guide used at Rosberry

Rosberry Swift Style Guide

Based on Ray Wenderlich Swift Style Guide with changes and additions. Make sure to read Swift API Design Guidelines.

Table of Contents

Correctness

Strive to make your code compile without warnings. This rule informs many style decisions such as using #selector types instead of string literals.

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

Prose

When referring to methods in prose, being unambiguous is critical. To refer to a method name, use the simplest form possible.

  1. Write the method name with no parameters. Example: Next, you need to call the method addTarget.
  2. Write the method name with argument labels. Example: Next, you need to call the method addTarget(_:action:).
  3. Write the full method name with argument labels and types. Example: Next, you need to call the method addTarget(_: Any?, action: Selector?).

For the above example using UIGestureRecognizer, 1 is unambiguous and preferred.

Pro Tip: You can use Xcode's jump bar to lookup methods with argument labels.

Methods in Xcode jump bar

Class Prefixes

Swift types are automatically namespaced by the module that contains them and you should not add a class prefix such as RSB. 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.). You can explicitly add types for closure parameters.

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 Stack<Element> { ... }
func write<Target: OutputStream>(to target: inout Target)
func swap<T>(_ a: inout T, _ b: inout T)

Not Preferred:

struct Stack<T> { ... }
func write<target: OutputStream>(to target: inout target)
func swap<Thing>(_ a: inout Thing, _ b: inout Thing)

Actions

Names of actions in view controllers should describe action element and action.

Preferred:

@objc private func nameButtonPressed() {
    // event logic
}

Not Preferred:

@objc private func nameButtonAction() {
    // event logic
}

Names of event in view models/presenters should describe event and end with EventTriggered.

Preferred:

func loginEventTriggered() {
    // event logic
}

Not Preferred:

func didTriggerLoginEvent() {
    // event logic
}

Closure Naming

Use success and failure names for closures in functions contains two closures for success and failure cases. Use completion closure name for once called closure. Use handler closure name in other cases.

Preferred:

func test(success: (() -> Void), failure: (() -> Void)) {
    if true {
        success()
    }
    else {
        failure()
    }
}

Preferred:

func test(completion: (() -> Void)) {
    completion()
}

Not Preferred:

func test(completionHandler: (() -> Void)) {
    completionHandler()
}

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.

Lifecycle

Deinit should be declared below last init:

init() {
    // initialization logic
}
	
deinit {
    // deinitialization logic
}

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
}

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 overridden. 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 not directly associated with the app whose implementation simply calls the superclass should also 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 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 as shown below:

Xcode indent 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). Some of the Xcode template code will have 4-space tabs hardcoded, 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]
}

  • Avoid trailing whitespaces at the ends of lines.

  • Add a single newline character at the end of each file.

Newline

Else statement should be on newline except guard-else statement. Enum cases should be on newline:

Preferred:

enum Colors {
    case red
    case green
    case blue
}

Not Preferred:

enum Colors {
    case red, green, blue
}

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)
    }

    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 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.
  • 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
    }
}

Final

Marking classes or members as final is necessary because it increases performance of the application. It is better to mark all classes or members as final initially and change it later if inheritance is needed.

// 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
    }
}

Nested types

Nested types and type aliases should be at the top of classes.

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, indent all parameters so they appear under the first one in a column:

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:

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

Chained methods using trailing closures should be clear and easy to read in context. Decisions on spacing and line breaks are left to the discretion of the author, but don't use anonymous arguments. Examples:

let value = numbers
    .map { number in
        number * 2 
    }
    .filter { number in 
        number > 50
    }
    .map { number in
        number + 10
    }

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

If variable makes sense only as an Obj-C type, use Obj-C type from the get-go to avoid unnecessary casting.

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 is not intended to be changed or changes to this variable will not be properly handled.

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.

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. Examples:

lazy var locationManager: CLLocationManager = self.makeLocationManager()

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

Notes:

  • [unowned self] is not required here. A retain cycle is not created.
  • Location manager has a side-effect of 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. Optionally add the type if it is not obvious in context of the declaration. 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
let people = fetchAttendeeList()
//or let people: [Person] = fetchAttendeeList()

Not Preferred:

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

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 (even non-production, demo code) 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.

Access Control

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 higher order functions like map, flatMap, forEach and the for-in style over and the while-condition-increment style.

Preferred:

(0...3).forEach { number in
    print(number)
}

Not Preferred:

for number in 0...3 {
    print(number)
}

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
}

NOTE: All higher order functions check every element in sequence. If it's needed to immediately interrupt execution of the loop, use the for-in style.

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 {
    return
}
// do something with numbers

Not Preferred:

if let number1 = number1 {
    if let number2 = number2 {
        if let number3 = number3 {
            // do something with numbers
        } 
        else {
            return
    else {
        return
    }
} 
else {
    return
}

Failing Guards

Guard statements are required to exit in some way. Statement such as return, throw, break and continue should be on newline. 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")

Fatal errors

Avoid fatal errors if possible.

Preferred:

guard views.count > 0 else {
  	return
}

Not Preferred:

guard views.count > 0 else {
    fatalError("Views must contain at least one view")
  	return
}

Swiftlint

Use this Swiftlint configuration to detect common issues in projects.

About

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License

Rosberry Swift Style Guide is available under the MIT license. See the LICENSE file for more info.

Note that the project description data, including the texts, logos, images, and/or trademarks, for each open source project belongs to its rightful owner. If you wish to add or remove any projects, please contact us at [email protected].