File structure

• 1 Properties: @IBOutlets -> line -> public -> line -> private
• 2 Static properties
• 3 Static functions
• 4 Init
• 6 Deinit
• 7 Lifecycle
• 8 Overrides
• 9 ‘MARK: - User Actions’ (Could be Extension)
• 10 ‘MARK: - Public’ (Could be Extension)
• 11 Extension ‘MARK: - Notifications’
• 12 Extension ‘MARK: - Delegates’
• 13 Extension ‘MARK: - Private’

When dealing with an implicit array or dictionary large enough to warrant splitting it into multiple lines, treat the [ and ] as if they were braces in a method, if statement, etc. Closures in a method should be treated similarly.

someFunctionWithABunchOfArguments(
    someStringArgument: "hello I am a string",
    someArrayArgument: [
        "dadada daaaa daaaa dadada daaaa daaaa dadada daaaa daaaa",
        "string one is crazy - what is it thinking?"
    ],
    someDictionaryArgument: [
        "dictionary key 1": "some value 1, but also some more text here",
        "dictionary key 2": "some value 2"
    ],
    someClosure: { parameter1 in
        print(parameter1)
    })

Naming

• 1 Swift types are automatically namespaced by the module that contains them and you should not add a class prefix such as RW. 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()

• 2 Use PascalCase for type names (e.g. struct, enum, class, typedef, associatedtype, etc.).

• 3 Use camelCase (initial lowercase letter) for function, method, property, constant, variable, argument names, enum cases, etc.).

• 4 When dealing with an acronym or other name that is usually written in all caps, actually use all caps in any names that use this in code. The exception is if this word is at the start of a name that needs to start with lowercase - in this case, use all lowercase for the acronym.

// "HTML" is at the start of a constant name, so we use lowercase "html"
let htmlBodyContent: String = "<p>Hello, World!</p>"
// Prefer using ID to Id
let profileID: Int = 1
// Prefer URLFinder to UrlFinder
class URLFinder {
    /* ... */
}

• 5 All constants other than singletons that are instance-independent should be static. All such static constants should be placed in a container enum type. The naming of this container should be singular (e.g. Constant and not Constants) and it should be named such that it is relatively obvious that it is a constant container. If this is not obvious, you can add a Constant suffix to the name. You should use these containers to group constants that have similar or the same prefixes, suffixes and/or use cases.

class MyClassName {
    // PREFERRED
    enum AccessibilityIdentifier {
        static let pirateButton = "pirate_button"
    }
    enum SillyMathConstant {
        static let indianaPi = 3
    }
    static let shared = MyClassName()

    // NOT PREFERRED
    static let kPirateButtonAccessibilityIdentifier = "pirate_button"
    enum SillyMath {
        static let indianaPi = 3
    }
    enum Singleton {
        static let shared = MyClassName()
    }
}

• 6 For generics and associated types, use a PascalCase word that describes the generic. If this word clashes with a protocol that it conforms to or a superclass that it subclasses, you can append a Type suffix to the associated type or generic name.

class SomeClass<Model> { /* ... */ }
protocol Modelable {
    associatedtype Model
}
protocol Sequence {
    associatedtype IteratorType: Iterator
}

• 7 Names should be descriptive and unambiguous.

// PREFERRED
class RoundAnimatingButton: UIButton { /* ... */ }

// NOT PREFERRED
class CustomButton: UIButton { /* ... */ }

• 8 Do not abbreviate, use shortened names, or single letter names.

// PREFERRED
class RoundAnimatingButton: UIButton {
    let animationDuration: NSTimeInterval

    func startAnimating() {
        let firstSubview = subviews.first
    }

}

// NOT PREFERRED
class RoundAnimating: UIButton {
    let aniDur: NSTimeInterval

    func srtAnmating() {
        let v = subviews.first
    }
}

• 9 Include type information in constant or variable names when it is not obvious otherwise.

// PREFERRED
class ConnectionTableViewCell: UITableViewCell {
    let personImageView: UIImageView

    let animationDuration: TimeInterval

    // it is ok not to include string in the ivar name here because it's obvious
    // that it's a string from the property name
    let firstName: String

    // though not preferred, it is OK to use `Controller` instead of `ViewController`
    let popupController: UIViewController
    let popupViewController: UIViewController

    // when working with a subclass of `UIViewController` such as a table view
    // controller, collection view controller, split view controller, etc.,
    // fully indicate the type in the name.
    let popupTableViewController: UITableViewController

    // when working with outlets, make sure to specify the outlet type in the
    // property name.
    @IBOutlet weak var submitButton: UIButton!
    @IBOutlet weak var emailTextField: UITextField!
    @IBOutlet weak var nameLabel: UILabel!

}

// NOT PREFERRED
class ConnectionTableViewCell: UITableViewCell {
    // this isn't a `UIImage`, so shouldn't be called image
    // use personImageView instead
    let personImage: UIImageView

    // this isn't a `String`, so it should be `textLabel`
    let text: UILabel

    // `animation` is not clearly a time interval
    // use `animationDuration` or `animationTimeInterval` instead
    let animation: TimeInterval

    // this is not obviously a `String`
    // use `transitionText` or `transitionString` instead
    let transition: String

    // this is a view controller - not a view
    let popupView: UIViewController

    // as mentioned previously, we don't want to use abbreviations, so don't use
    // `VC` instead of `ViewController`
    let popupVC: UIViewController

    // even though this is still technically a `UIViewController`, this property
    // should indicate that we are working with a *Table* View Controller
    let popupViewController: UITableViewController

    // for the sake of consistency, we should put the type name at the end of the
    // property name and not at the start
    @IBOutlet weak var btnSubmit: UIButton!
    @IBOutlet weak var buttonSubmit: UIButton!

    // we should always have a type in the property name when dealing with outlets
    // for example, here, we should have `firstNameLabel` instead
    @IBOutlet weak var firstName: UILabel!
}

• 10 When naming function arguments, make sure that the function can be read easily to understand the purpose of each argument.

• 11 As per Apple's API Design Guidelines, a protocol should be named as nouns if they describe what something is doing (e.g. Collection) and using the suffixes able, ible, or ing if it describes a capability (e.g. Equatable, ProgressReporting). If neither of those options makes sense for your use case, you can add a Protocol suffix to the protocol's name as well. Some example protocols are below.

// here, the name is a noun that describes what the protocol does
protocol TableViewSectionProvider {
    func rowHeight(at row: Int) -> CGFloat
    var numberOfRows: Int { get }
    /* ... */
}

// here, the protocol is a capability, and we name it appropriately
protocol Loggable {
    func logCurrentState()
    /* ... */
}

// suppose we have an `InputTextView` class, but we also want a protocol
// to generalize some of the functionality - it might be appropriate to
// use the `Protocol` suffix here
protocol InputTextViewProtocol {
    func sendTrackingEvent()
    func inputText() -> String
    /* ... */
}

Code Formatting

• 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). 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.

• Make liberal use of vertical whitespace to divide code into logical chunks.

• 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.

• Avoid trailing whitespaces at the ends of lines. (Xcode->Preferences->Text Editing->Automatically trim trailing whitespace + Including whitespace-only lines)

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

• In general, there should be a space following a comma.

let myArray = [1, 2, 3, 4, 5]

Prefer let-bindings over var-bindings wherever possible

Use let foo = … over var foo = … wherever possible (and when in doubt). Only use var if you absolutely have to (i.e. you know that the value might change, e.g. when using the weak storage modifier).

Rationale: The intent and meaning of both keywords are clear, but let-by-default results in safer and clearer code.

A let-binding guarantees and clearly signals to the programmer that its value will never change. Subsequent code can thus make stronger assumptions about its usage.

It becomes easier to reason about code. Had you used var while still making the assumption that the value never changed, you would have to manually check that.

Accordingly, whenever you see a var identifier being used, assume that it will change and ask yourself why.

Return and break early

When you have to meet certain criteria to continue execution, try to exit early. So, instead of this:

if n.isNumber {
    // Use n here
} else {
    return
}

use this:

guard n.isNumber else {
    return
}
// Use n here

You can also do it with if statement, but using guard is preferred, because guard statement without return, break or continue produces a compile-time error, so exit is guaranteed.

Avoid Using Force-Unwrapping of Optionals

If you have an identifier foo of type FooType? or FooType!, don't force-unwrap it to get to the underlying value (foo!) if possible. Don't use as! or try!.

Instead, prefer this:

if let foo = foo {
    // Use unwrapped `foo` value in here
} else {
    // If appropriate, handle the case where the optional is nil
}

Alternatively, you might want to use Swift's Optional Chaining in some of these cases, such as:

// Call the function if `foo` is not nil. If `foo` is nil, ignore we ever tried to make the call
foo?.callSomethingIfFooIsNotNil()

Rationale: Explicit if let-binding of optionals results in safer code. Force unwrapping is more prone to lead to runtime crashes.

Avoid Using Implicitly Unwrapped Optionals

Where possible, use let foo: FooType? instead of let foo: FooType! if foo may be nil (Note that in general, ? can be used instead of !).

The only time you should be using implicitly unwrapped optionals is with @IBOutlets. In every other case, it is better to use a non-optional or regular optional property. Yes, there are cases in which you can probably "guarantee" that the property will never be nil when used, but it is better to be safe and consistent. Similarly, don't use force unwraps.

Rationale: Explicit optionals result in safer code. Implicitly unwrapped optionals have the potential of crashing at runtime.

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
  }
}

If you don't plan on actually using the value stored in an optional, but need to determine whether or not this value is nil, explicitly check this value against nil as opposed to using if let syntax.

// PREFERERED
if someOptional != nil {
    // do something
}

// NOT PREFERRED
if let _ = someOptional {
    // do something
}

Don't use unowned. You can think of unowned as somewhat of an equivalent of a weak property that is implicitly unwrapped (though unowned has slight performance improvements on account of completely ignoring reference counting). Since we don't ever want to have implicit unwraps, we similarly don't want unowned properties.

// PREFERRED
weak var parentViewController: UIViewController?

// NOT PREFERRED
weak var parentViewController: UIViewController!
unowned var parentViewController: UIViewController

When unwrapping optionals, use the same name for the unwrapped constant or variable where appropriate.

guard let myValue = myValue else {
    return
}

Prefer implicit getters on read-only properties and subscripts

When possible, omit the get keyword on read-only computed properties and read-only subscripts.

So, write these:

var myGreatProperty: Int {
	return 4
}

subscript(index: Int) -> T {
    return objects[index]
}

… not these:

var myGreatProperty: Int {
	get {
		return 4
	}
}

subscript(index: Int) -> T {
    get {
        return objects[index]
    }
}

Rationale: The intent and meaning of the first version are clear, and results in less code.

Always specify access control explicitly for top-level definitions

Top-level functions, types, and variables should always have explicit access control specifiers:

public var whoopsGlobalState: Int
internal struct TheFez {}
private func doTheThings(things: [Thing]) {}

However, definitions within those can leave access control implicit, where appropriate:

internal struct TheFez {
	var owner: Person = Joshaber()
}

Rationale: It's rarely appropriate for top-level definitions to be specifically internal, and being explicit ensures that careful thought goes into that decision. Within a definition, reusing the same access control specifier is just duplicative, and the default is usually reasonable.

Access Control

Prefer private to fileprivate when possible. Using extensions may require you to use fileprivate.

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

Preferred:

private let message = "Great Scott!"
private static let myPrivateNumber: Int

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

Not Preferred:

fileprivate let message = "Great Scott!"

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

When specifying a type, always associate the colon with the identifier

When specifying the type of an identifier, always put the colon immediately after the identifier, followed by a space and then the type name.

class SmallBatchSustainableFairtrade: Coffee { ... }

let timeToCoffee: NSTimeInterval = 2

func makeCoffee(type: CoffeeType) -> Coffee { ... }

Rationale: The type specifier is saying something about the identifier so it should be positioned with it.

Also, when specifying the type of a dictionary, always put the colon immediately after the key type, followed by a space and then the value type.

let capitals: [Country: City] = [sweden: stockholm]

Only explicitly refer to self when required

When accessing properties or methods on self, leave the reference to self implicit by default:

private class History {
	var events: [Event]

	func rewrite() {
		events = []
	}
}

Only include the explicit keyword when required by the language—for example, in @escaping closures, in initializers, or when parameter names conflict:

extension History {
	init(events: [Event]) {
		self.events = events
	}

	var whenVictorious: () -> () {
		return {
			self.rewrite()
		}
	}
}

Rationale: This makes the capturing semantics of self stand out more in closures, and avoids verbosity elsewhere.

Classes and Structures

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.

Note that inheritance is (by itself) usually not a good reason to use classes, because polymorphism can be provided by protocols, and implementation reuse can be provided through composition.

For example, this class hierarchy:

class Vehicle {
    let numberOfWheels: Int

    init(numberOfWheels: Int) {
        self.numberOfWheels = numberOfWheels
    }

    func maximumTotalTirePressure(pressurePerWheel: Float) -> Float {
        return pressurePerWheel * Float(numberOfWheels)
    }
}

class Bicycle: Vehicle {
    init() {
        super.init(numberOfWheels: 2)
    }
}

class Car: Vehicle {
    init() {
        super.init(numberOfWheels: 4)
    }
}

could be refactored into these definitions:

protocol Vehicle {
    var numberOfWheels: Int { get }
}

func maximumTotalTirePressure(vehicle: Vehicle, pressurePerWheel: Float) -> Float {
    return pressurePerWheel * Float(vehicle.numberOfWheels)
}

struct Bicycle: Vehicle {
    let numberOfWheels = 2
}

struct Car: Vehicle {
    let numberOfWheels = 4
}

Rationale: Value types are simpler, easier to reason about, and behave as expected with the let keyword.

Make classes final by default

Classes should start as final, and only be changed to allow subclassing if a valid need for inheritance has been identified. Even in that case, as many definitions as possible within the class should be final as well, following the same rules.

Rationale: Composition is usually preferable to inheritance, and opting in to inheritance hopefully means that more thought will be put into the decision.

// 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
  }
}

Omit type parameters where possible

Methods of parameterized types can omit type parameters on the receiving type when they’re identical to the receiver’s. For example:

struct Composite<T> {
	…
	func compose(other: Composite<T>) -> Composite<T> {
		return Composite<T>(self, other)
	}
}

could be rendered as:

struct Composite<T> {
	…
	func compose(other: Composite) -> Composite {
		return Composite(self, other)
	}
}

Rationale: Omitting redundant type parameters clarifies the intent, and makes it obvious by contrast when the returned type takes different type parameters.

Use whitespace around operator definitions

Use whitespace around operators when defining them. Instead of:

func <|(lhs: Int, rhs: Int) -> Int
func <|<<A>(lhs: A, rhs: A) -> A

write:

func <| (lhs: Int, rhs: Int) -> Int
func <|< <A>(lhs: A, rhs: A) -> A

Rationale: Operators consist of punctuation characters, which can make them difficult to read when immediately followed by the punctuation for a type or value parameter list. Adding whitespace separates the two more clearly.

• There should be a space before and after a binary operator such as +, ==, or ->. There should also not be a space after a ( and before a ).

let myValue = 20 + (30 / 2) * 3
if 1 + 1 == 3 {
    fatalError("The universe is broken.")
}
func pancake(with syrup: Syrup) -> Pancake {
    /* ... */
}

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

#### 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:**
```swift
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)

#### 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:**
```swift
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
}

Unused Code

Unused (dead) code should be removed or commented with description why it left in code. Xcode template code and placeholder comments should be removed.

Minimal Imports

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

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.

Function Declarations

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

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

We follow Xcode's recommended indentation style (i.e. your code should not change if CTRL-I is pressed). When declaring a function that spans multiple lines, prefer using that syntax to which Xcode, as of version 7.3, defaults.

// Xcode indentation for a function declaration that spans multiple lines
func myFunctionWithManyParameters(parameterOne: String,
                                  parameterTwo: String,
                                  parameterThree: String) {
    // Xcode indents to here for this kind of statement
    print("\(parameterOne) \(parameterTwo) \(parameterThree)")
}

// Xcode indentation for a multi-line `if` statement
if myFirstValue > (mySecondValue + myThirdValue)
    && myFourthValue == .someEnumValue {

    // Xcode indents to here for this kind of statement
    print("Hello, World!")
}

When calling a function that has many parameters, put each argument on a separate line with a single extra indentation.

someFunctionWithManyArguments(
    firstArgument: "Hello, I am a string",
    secondArgument: resultFromSomeFunction(),
    thirdArgument: someOtherLocalProperty)

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, 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 }.index(of: 90)

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

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!

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.

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]()

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, tutorial 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. Be wary of retain cycles when creating delegates/protocols for your classes; typically, these properties should be declared weak.

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)
}

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";

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")

Prefer using local constants or other mitigation techniques to avoid multi-line predicates where possible.

// PREFERRED
let firstCondition = x == firstReallyReallyLongPredicateFunction()
let secondCondition = y == secondReallyReallyLongPredicateFunction()
let thirdCondition = z == thirdReallyReallyLongPredicateFunction()
if firstCondition && secondCondition && thirdCondition {
    // do something
}

// NOT PREFERRED
if x == firstReallyReallyLongPredicateFunction()
    && y == secondReallyReallyLongPredicateFunction()
    && z == thirdReallyReallyLongPredicateFunction() {
    // do something
}

Prefer the composition of map, filter, reduce, etc. over iterating when transforming from one collection to another. Make sure to avoid using closures that have side effects when using these methods.

// PREFERRED
let stringOfInts = [1, 2, 3].flatMap { String($0) }
// ["1", "2", "3"]

// NOT PREFERRED
var stringOfInts: [String] = []
for integer in [1, 2, 3] {
    stringOfInts.append(String(integer))
}

// PREFERRED
let evenNumbers = [4, 8, 15, 16, 23, 42].filter { $0 % 2 == 0 }
// [4, 8, 16, 42]

// NOT PREFERRED
var evenNumbers: [Int] = []
for integer in [4, 8, 15, 16, 23, 42] {
    if integer % 2 == 0 {
        evenNumbers.append(integer)
    }
}

Prefer not declaring types for constants or variables if they can be inferred anyway.

If a function returns multiple values, prefer returning a tuple to using inout arguments (it’s best to use labeled tuples for clarity on what you’re returning if it is not otherwise obvious). If you use a certain tuple more than once, consider using a typealias. If you’re returning 3 or more items in a tuple, consider using a struct or class instead.

func pirateName() -> (firstName: String, lastName: String) {
    return ("Guybrush", "Threepwood")
}

let name = pirateName()
let firstName = name.firstName
let lastName = name.lastName

Don't use labeled breaks.

Don't place parentheses around control flow predicates.

// PREFERRED
if x == y {
    /* ... */
}

// NOT PREFERRED
if (x == y) {
    /* ... */
}

Avoid writing out an enum type where possible - use shorthand.

// PREFERRED
imageView.setImageWithURL(url, type: .person)

// NOT PREFERRED
imageView.setImageWithURL(url, type: AsyncImageView.Type.person)

Don’t use shorthand for class methods since it is generally more difficult to infer the context from class methods as opposed to enums.

// PREFERRED
imageView.backgroundColor = UIColor.white

// NOT PREFERRED
imageView.backgroundColor = .white

Prefer not writing self. unless it is required.

• 3.1.12 When writing methods, keep in mind whether the method is intended to be overridden or not. If not, mark it as final, though keep in mind that this will prevent the method from being overwritten for testing purposes. In general, final methods result in improved compilation times, so it is good to use this when applicable. Be particularly careful, however, when applying the final keyword in a library since it is non-trivial to change something to be non-final in a library as opposed to have changing something to be non-final in your local project.

When using a statement such as else, catch, etc. that follows a block, put this keyword on the same line as the block. Again, we are following the 1TBS style here. Example if/else and do/catch code is below.

if someBoolean {
    // do something
} else {
    // do something else
}

do {
    let fileContents = try readFile("filename.txt")
} catch {
    print(error)
}

Prefer static to class when declaring a function or property that is associated with a class as opposed to an instance of that class. Only use class if you specifically need the functionality of overriding that function or property in a subclass, though consider using a protocol to achieve this instead.

If you have a function that takes no arguments, has no side effects, and returns some object or value, prefer using a computed property instead.

For the purpose of namespacing a set of static functions and/or static properties, prefer using a caseless enum over a class or a struct. This way, you don't have to add a private init() { } to the container.

If a property needs to be accessed by unit tests, you will have to make it internal to use @testable import ModuleName. If a property should be private, but you declare it to be internal for the purposes of unit testing, make sure you add an appropriate bit of documentation commenting that explains this. You can make use of the - warning: markup syntax for clarity as shown below.

/**
 This property defines the pirate's name.
 - warning: Not `private` for `@testable`.
 */
let pirateName = "LeChuck"

Prefer private to fileprivate where possible.

When choosing between public and open, prefer open if you intend for something to be subclassable outside of a given module and public otherwise. Note that anything internal and above can be subclassed in tests by using @testable import, so this shouldn't be a reason to use open. In general, lean towards being a bit more liberal with using open when it comes to libraries, but a bit more conservative when it comes to modules in a codebase such as an app where it is easy to change things in multiple modules simultaneously.

Custom Operators

Prefer creating named functions to custom operators.

If you want to introduce a custom operator, make sure that you have a very good reason why you want to introduce a new operator into global scope as opposed to using some other construct.

You can override existing operators to support new types (especially ==). However, your new definitions must preserve the semantics of the operator. For example, == must always test equality and return a boolean.

Switch Statements and enums

• 1 When using a switch statement that has a finite set of possibilities (enum), do NOT include a default case. Instead, place unused cases at the bottom and use the break keyword to prevent execution.

• 2 Since switch cases in Swift break by default, do not include the break keyword if it is not needed.

• 3 The case statements should line up with the switch statement itself as per default Swift standards.

• 4 When defining a case that has an associated value, make sure that this value is appropriately labeled as opposed to just types (e.g. case Hunger(hungerLevel: Int) instead of case Hunger(Int)).

enum Problem {
    case attitude
    case hair
    case hunger(hungerLevel: Int)
}

func handleProblem(problem: Problem) {
    switch problem {
    case .attitude:
        print("At least I don't have a hair problem.")
    case .hair:
        print("Your barber didn't know when to stop.")
    case .hunger(let hungerLevel):
        print("The hunger level is \(hungerLevel).")
    }
}

• 5 Prefer lists of possibilities (e.g. case 1, 2, 3:) to using the fallthrough keyword where possible).

• 6 If you have a default case that shouldn't be reached, preferably throw an error (or handle it some other similar way such as asserting).

func handleDigit(_ digit: Int) throws {
    switch digit {
    case 0, 1, 2, 3, 4, 5, 6, 7, 8, 9:
        print("Yes, \(digit) is a digit!")
    default:
        throw Error(message: "The given number was not a digit.")
    }
}

When implementing protocols, there are two ways of organizing your code:

1. Using // MARK: comments to separate your protocol implementation from the rest of your code
2. Using an extension outside your class/struct implementation code, but in the same source file

Keep in mind that when using an extension, however, the methods in the extension can't be overridden by a subclass, which can make testing difficult. If this is a common use case, it might be better to stick with method #1 for consistency. Otherwise, method #2 allows for cleaner separation of concerns.

Even when using method #2, add // MARK: statements anyway for easier readability in Xcode's method/property/class/etc. list UI.

Though you can create a custom name for the new or old value for willSet/didSet and set, use the standard newValue/oldValue identifiers that are provided by default.

var storedProperty: String = "I'm selling these fine leather jackets." {
    willSet {
        print("will set to \(newValue)")
    }
    didSet {
        print("did set from \(oldValue) to \(storedProperty)")
    }
}

var computedProperty: String  {
    get {
        if someBool {
            return "I'm a mighty pirate!"
        }
        return storedProperty
    }
    set {
        storedProperty = newValue
    }
}

You can declare a singleton property as follows:

class PirateManager {
    static let shared = PirateManager()

    /* ... */
}

If the types of the parameters are obvious, it is OK to omit the type name, but being explicit is also OK. Sometimes readability is enhanced by adding clarifying detail and sometimes by taking repetitive parts away - use your best judgment and be consistent.

// omitting the type
doSomethingWithClosure() { response in
    print(response)
}

// explicit type
doSomethingWithClosure() { response: NSURLResponse in
    print(response)
}

// using shorthand in a map statement
[1, 2, 3].flatMap { String($0) }

If specifying a closure as a type, you don’t need to wrap it in parentheses unless it is required (e.g. if the type is optional or the closure is within another closure). Always wrap the arguments in the closure in a set of parentheses - use () to indicate no arguments and use Void to indicate that nothing is returned.

let completionBlock: (Bool) -> Void = { (success) in
    print("Success? \(success)")
}

let completionBlock: () -> Void = {
    print("Completed!")
}

let completionBlock: (() -> Void)? = nil

Keep parameter names on same line as the opening brace for closures when possible without too much horizontal overflow (i.e. ensure lines are less than 160 characters).

Use trailing closure syntax unless the meaning of the closure is not obvious without the parameter name (an example of this could be if a method has parameters for success and failure closures).

// trailing closure
doSomething(1.0) { (parameter1) in
    print("Parameter 1 is \(parameter1)")
}

// no trailing closure
doSomething(1.0, success: { (parameter1) in
    print("Success with \(parameter1)")
}, failure: { (parameter1) in
    print("Failure with \(parameter1)")
})

In general, avoid accessing an array directly with subscripts. When possible, use accessors such as .first or .last, which are optional and won’t crash. Prefer using a for item in items syntax when possible as opposed to something like for i in 0 ..< items.count. If you need to access an array subscript directly, make sure to do proper bounds checking. You can use for (index, value) in items.enumerated() to get both the index and the value.

Never use the += or + operator to append/concatenate to arrays. Instead, use .append() or .append(contentsOf:) as these are far more performant (at least with respect to compilation) in Swift's current state. If you are declaring an array that is based on other arrays and want to keep it immutable, instead of let myNewArray = arr1 + arr2, use let myNewArray = [arr1, arr2].joined().

Error Handling

Suppose a function myFunction is supposed to return a String, however, at some point it can run into an error. A common approach is to have this function return an optional String? where we return nil if something went wrong.

Example:

func readFile(named filename: String) -> String? {
    guard let file = openFile(named: filename) else {
        return nil
    }

    let fileContents = file.read()
    file.close()
    return fileContents
}

func printSomeFile() {
    let filename = "somefile.txt"
    guard let fileContents = readFile(named: filename) else {
        print("Unable to open file \(filename).")
        return
    }
    print(fileContents)
}

Instead, we should be using Swift's try/catch behavior when it is appropriate to know the reason for the failure.

You can use a struct such as the following:

struct Error: Swift.Error {
    public let file: StaticString
    public let function: StaticString
    public let line: UInt
    public let message: String

    public init(message: String, file: StaticString = #file, function: StaticString = #function, line: UInt = #line) {
        self.file = file
        self.function = function
        self.line = line
        self.message = message
    }
}

Example usage:

func readFile(named filename: String) throws -> String {
    guard let file = openFile(named: filename) else {
        throw Error(message: "Unable to open file named \(filename).")
    }

    let fileContents = file.read()
    file.close()
    return fileContents
}

func printSomeFile() {
    do {
        let fileContents = try readFile(named: filename)
        print(fileContents)
    } catch {
        print(error)
    }
}

There are some exceptions in which it does make sense to use an optional as opposed to error handling. When the result should semantically potentially be nil as opposed to something going wrong while retrieving the result, it makes sense to return an optional instead of using error handling.

In general, if a method can "fail", and the reason for the failure is not immediately obvious if using an optional return type, it probably makes sense for the method to throw an error.

Using guard Statements

In general, we prefer to use an "early return" strategy where applicable as opposed to nesting code in if statements. Using guard statements for this use-case is often helpful and can improve the readability of the code.

// PREFERRED
func eatDoughnut(at index: Int) {
    guard index >= 0 && index < doughnuts.count else {
        // return early because the index is out of bounds
        return
    }

    let doughnut = doughnuts[index]
    eat(doughnut)
}

// NOT PREFERRED
func eatDoughnut(at index: Int) {
    if index >= 0 && index < doughnuts.count {
        let doughnut = doughnuts[index]
        eat(doughnut)
    }
}

When unwrapping optionals, prefer guard statements as opposed to if statements to decrease the amount of nested indentation in your code.

// PREFERRED
guard let monkeyIsland = monkeyIsland else {
    return
}
bookVacation(on: monkeyIsland)
bragAboutVacation(at: monkeyIsland)

// NOT PREFERRED
if let monkeyIsland = monkeyIsland {
    bookVacation(on: monkeyIsland)
    bragAboutVacation(at: monkeyIsland)
}

// EVEN LESS PREFERRED
if monkeyIsland == nil {
    return
}
bookVacation(on: monkeyIsland!)
bragAboutVacation(at: monkeyIsland!)

When deciding between using an if statement or a guard statement when unwrapping optionals is not involved, the most important thing to keep in mind is the readability of the code. There are many possible cases here, such as depending on two different booleans, a complicated logical statement involving multiple comparisons, etc., so in general, use your best judgement to write code that is readable and consistent. If you are unsure whether guard or if is more readable or they seem equally readable, prefer using guard.

// an `if` statement is readable here
if operationFailed {
    return
}

// a `guard` statement is readable here
guard isSuccessful else {
    return
}

// double negative logic like this can get hard to read - i.e. don't do this
guard !operationFailed else {
    return
}

If choosing between two different states, it makes more sense to use an if statement as opposed to a guard statement.

// PREFERRED
if isFriendly {
    print("Hello, nice to meet you!")
} else {
    print("You have the manners of a beggar.")
}

// NOT PREFERRED
guard isFriendly else {
    print("You have the manners of a beggar.")
    return
}

print("Hello, nice to meet you!")

You should also use guard only if a failure should result in exiting the current context. Below is an example in which it makes more sense to use two if statements instead of using two guards - we have two unrelated conditions that should not block one another.

if let monkeyIsland = monkeyIsland {
    bookVacation(onIsland: monkeyIsland)
}

if let woodchuck = woodchuck, canChuckWood(woodchuck) {
    woodchuck.chuckWood()
}

Often, we can run into a situation in which we need to unwrap multiple optionals using guard statements. In general, combine unwraps into a single guard statement if handling the failure of each unwrap is identical (e.g. just a return, break, continue, throw, or some other @noescape).

// combined because we just return
guard let thingOne = thingOne,
    let thingTwo = thingTwo,
    let thingThree = thingThree else {
    return
}

// separate statements because we handle a specific error in each case
guard let thingOne = thingOne else {
    throw Error(message: "Unwrapping thingOne failed.")
}

guard let thingTwo = thingTwo else {
    throw Error(message: "Unwrapping thingTwo failed.")
}

guard let thingThree = thingThree else {
    throw Error(message: "Unwrapping thingThree failed.")
}

Don’t use one-liners for guard statements.

// PREFERRED
guard let thingOne = thingOne else {
    return
}

// NOT PREFERRED
guard let thingOne = thingOne else { return }

Documentation

If a function is more complicated than a simple O(1) operation, you should generally consider adding a doc comment for the function since there could be some information that the method signature does not make immediately obvious. If there are any quirks to the way that something was implemented, whether technically interesting, tricky, not obvious, etc., this should be documented. Documentation should be added for complex classes/structs/enums/protocols and properties. All public functions/classes/properties/constants/structs/enums/protocols/etc. should be documented as well (provided, again, that their signature/name does not make their meaning/functionality immediately obvious).

After writing a doc comment, you should option click the function/property/class/etc. to make sure that everything is formatted correctly.

Be sure to check out the full set of features available in Swift's comment markup described in Apple's Documentation.

Guidelines:

• 160 character column limit (like the rest of the code).

• Even if the doc comment takes up one line, use block (/** */).

• Do not prefix each additional line with a *.

• Use the new - parameter syntax as opposed to the old :param: syntax (make sure to use lower case parameter and not Parameter). Option-click on a method you wrote to make sure the quick help looks correct.

class Human {
    /**
     This method feeds a certain food to a person.

     - parameter food: The food you want to be eaten.
     - parameter person: The person who should eat the food.
     - returns: True if the food was eaten by the person; false otherwise.
    */
    func feed(_ food: Food, to person: Human) -> Bool {
        // ...
    }
}

• If you’re going to be documenting the parameters/returns/throws of a method, document all of them, even if some of the documentation ends up being somewhat repetitive (this is preferable to having the documentation look incomplete). Sometimes, if only a single parameter warrants documentation, it might be better to just mention it in the description instead.

• For complicated classes, describe the usage of the class with some potential examples as seems appropriate. Remember that markdown syntax is valid in Swift's comment docs. Newlines, lists, etc. are therefore appropriate.

/**
 ## Feature Support

 This class does some awesome things. It supports:

 - Feature 1
 - Feature 2
 - Feature 3

 ## Examples

 Here is an example use case indented by four spaces because that indicates a
 code block:

     let myAwesomeThing = MyAwesomeClass()
     myAwesomeThing.makeMoney()

 ## Warnings

 There are some things you should be careful of:

 1. Thing one
 2. Thing two
 3. Thing three
 */
class MyAwesomeClass {
    /* ... */
}

• When mentioning code, use code ticks - `

/**
 This does something with a `UIViewController`, perchance.
 - warning: Make sure that `someValue` is `true` before running this function.
 */
func myFunction() {
    /* ... */
}

• When writing doc comments, prefer brevity where possible.

Other Commenting Guidelines

• 4.2.1 Always leave a space after //.
• 4.2.2 Always leave comments on their own line.
• 4.2.3 When using // MARK: - whatever, leave a newline after the comment.

class Pirate {

    // MARK: - instance properties

    private let pirateName: String

    // MARK: - initialization

    init() {
        /* ... */
    }

}