Protocols

Version: Swift 5.6 Source: swift-book: Protocols Digest Date: February 3, 2022

A protocol defines a blueprint of methods, properties, and other requirements that suit a particular task or piece of functionality. The protocol can then be adopted by a class, structure, or enumeration to provide an actual implementation of those requirements. Any type that satisfies the requirements of a protocol is said to conform to that protocol.

In addition to specifying requirements that conforming types must implement, you can extend a protocol to implement some of these requirements or to implement additional functionality that conforming types can take advantage of.

• Protocols

  • Protocol Syntax

  • Property Requirements

  • Method Requirements

  • Mutating Method Requirements

  • Initializer Requirements

    • Class Implementations of Protocol Initializer Requirements

    • Failable Initializer Requirements

  • Protocols as Types

  • Delegation

  • Adding Protocol Conformance with an Extension

    • Conditionally Conforming to a Protocol

    • Declaring Protocol Adoption with an Extension

  • Adopting a Protocol Using a Synthesized Implementation

    • Equatable

    • Hashable

    • Comparable

  • Collections of Protocol Types

  • Protocol Inheritance

  • Class-Only Protocols

  • Protocol Composition

  • Checking for Protocol Conformance

  • Optional Protocol Requirements

  • Protocol Extensions

    • Providing Default Implementations

    • Adding Constraints to Protocol Extensions

Protocol Syntax

You define protocols in a very similar way to classes, structures, and enumerations:

protocol SomeProtocol {
    // protocol definition goes here
}

Custom types state that they adopt a particular protocol by placing the protocol’s name after the type’s name, separated by a colon, as part of their definition. Multiple protocols can be listed, and are separated by commas:

struct SomeStructure: FirstProtocol, AnotherProtocol {
    // structure definition goes here
}

If a class has a superclass, list the superclass name before any protocols it adopts, followed by a comma:

class SomeClass: SomeSuperclass, FirstProtocol, AnotherProtocol {
    // class definition goes here
}

Property Requirements

A protocol can require any conforming type to provide an instance property or type property with a particular name and type. The protocol doesn’t specify whether the property should be a stored property or a computed property, it only specifies the required property name and type. The protocol also specifies whether each property must be gettable or gettable and settable.

• If a protocol requires a property to be gettable and settable, that property requirement can’t be fulfilled by a constant stored property or a read-only computed property.

• If the protocol only requires a property to be gettable, the requirement can be satisfied by any kind of property, and it’s valid for the property to be also settable if this is useful for your own code.

Property requirements are always declared as variable properties, prefixed with the var keyword. Gettable and settable properties are indicated by writing { get set } after their type declaration, and gettable properties are indicated by writing { get }.

protocol SomeProtocol {
    var mustBeSettable: Int { get set }
    var doesNotNeedToBeSettable: Int { get }
}

Always prefix type property requirements with the static keyword when you define them in a protocol. This rule pertains even though type property requirements can be prefixed with the class or static keyword when implemented by a class:

protocol AnotherProtocol {
    static var someTypeProperty: Int { get set }
}

Here’s an example of a protocol with a single instance property requirement:

protocol FullyNamed {
    var fullName: String { get }
}

Here’s an example of a simple structure that adopts and conforms to the FullyNamed protocol:

struct Person: FullyNamed {
    var fullName: String
}
let john = Person(fullName: "John Appleseed")
// john.fullName is "John Appleseed"

Here’s a more complex class, which also adopts and conforms to the FullyNamed protocol:

class Starship: FullyNamed {
    var prefix: String?
    var name: String
    init(name: String, prefix: String? = nil) {
        self.name = name
        self.prefix = prefix
    }
    var fullName: String {
        return (prefix != nil ? prefix! + " " : "") + name
    }
}
var ncc1701 = Starship(name: "Enterprise", prefix: "USS")
// ncc1701.fullName is "USS Enterprise"

This class implements the fullName property requirement as a computed read-only property for a starship. Each Starship class instance stores a mandatory name and an optional prefix. The fullName property uses the prefix value if it exists, and prepends it to the beginning of name to create a full name for the starship.

Method Requirements

Protocols can require specific instance methods and type methods to be implemented by conforming types. These methods are written as part of the protocol’s definition in exactly the same way as for normal instance and type methods, but without curly braces or a method body. Variadic parameters are allowed, subject to the same rules as for normal methods. Default values, however, can’t be specified for method parameters within a protocol’s definition.

As with type property requirements, you always prefix type method requirements with the static keyword when they’re defined in a protocol. This is true even though type method requirements are prefixed with the class or static keyword when implemented by a class:

protocol SomeProtocol {
    static func someTypeMethod()
}

The following example defines a protocol with a single instance method requirement:

protocol RandomNumberGenerator {
    func random() -> Double
}

This protocol, RandomNumberGenerator, requires any conforming type to have an instance method called random, which returns a Double value whenever it’s called. Although it’s not specified as part of the protocol, it’s assumed that this value will be a number from 0.0 up to (but not including) 1.0.

Here’s an implementation of a class that adopts and conforms to the RandomNumberGenerator protocol. This class implements a pseudorandom number generator algorithm known as a linear congruential generator:

class LinearCongruentialGenerator: RandomNumberGenerator {
    var lastRandom = 42.0
    let m = 139968.0
    let a = 3877.0
    let c = 29573.0
    func random() -> Double {
        lastRandom = ((lastRandom * a + c)
            .truncatingRemainder(dividingBy:m))
        return lastRandom / m
    }
}
let generator = LinearCongruentialGenerator()
print("Here's a random number: \(generator.random())")
// Prints "Here's a random number: 0.3746499199817101"
print("And another one: \(generator.random())")
// Prints "And another one: 0.729023776863283"

Mutating Method Requirements

It’s sometimes necessary for a method to modify (or mutate) the instance it belongs to. For instance methods on value types (that is, structures and enumerations) you place the mutating keyword before a method’s func keyword to indicate that the method is allowed to modify the instance it belongs to and any properties of that instance. This process is described in Modifying Value Types from Within Instance Methods.

NOTE: If you mark a protocol instance method requirement as mutating, you don’t need to write the mutating keyword when writing an implementation of that method for a class. The mutating keyword is only used by structures and enumerations.

The example below defines a protocol called Togglable, which defines a single instance method requirement called toggle. As its name suggests, the toggle() method is intended to toggle or invert the state of any conforming type, typically by modifying a property of that type.

protocol Togglable {
    mutating func toggle()
}

The example below defines an enumeration called OnOffSwitch. This enumeration toggles between two states, indicated by the enumeration cases on and off. The enumeration’s toggle implementation is marked as mutating, to match the Togglable protocol’s requirements:

enum OnOffSwitch: Togglable {
    case off, on
    mutating func toggle() {
        switch self {
        case .off:
            self = .on
        case .on:
            self = .off
        }
    }
}
var lightSwitch = OnOffSwitch.off
lightSwitch.toggle()
// lightSwitch is now equal to .on

Initializer Requirements

Protocols can require specific initializers to be implemented by conforming types. You write these initializers as part of the protocol’s definition in exactly the same way as for normal initializers, but without curly braces or an initializer body:

protocol SomeProtocol {
    init(someParameter: Int)
}

Class Implementations of Protocol Initializer Requirements

You can implement a protocol initializer requirement on a conforming class as either a designated initializer or a convenience initializer. In both cases, you must mark the initializer implementation with the required modifier:

class SomeClass: SomeProtocol {
    required init(someParameter: Int) {
        // initializer implementation goes here
    }
}

The use of the required modifier ensures that you provide an explicit or inherited implementation of the initializer requirement on all subclasses of the conforming class, such that they also conform to the protocol.

For more information on required initializers, see Required Initializers.

NOTE: You don’t need to mark protocol initializer implementations with the required modifier on classes that are marked with the final modifier, because final classes can’t subclassed. For more about the final modifier, see Preventing Overrides.

If a subclass overrides a designated initializer from a superclass, and also implements a matching initializer requirement from a protocol, mark the initializer implementation with both the required and override modifiers:

protocol SomeProtocol {
    init()
}

class SomeSuperClass {
    init() {
        // initializer implementation goes here
    }
}

class SomeSubClass: SomeSuperClass, SomeProtocol {
    // "required" from SomeProtocol conformance; "override" from SomeSuperClass
    required override init() {
        // initializer implementation goes here
    }
}

Failable Initializer Requirements

Protocols can define failable initializer requirements for conforming types, as defined in Failable Initializers.

A failable initializer requirement can be satisfied by a failable or nonfailable initializer on a conforming type. A nonfailable initializer requirement can be satisfied by a nonfailable initializer or an implicitly unwrapped failable initializer.

Protocols as Types

Protocols don’t actually implement any functionality themselves. Nonetheless, you can use protocols as a fully fledged types in your code. Using a protocol as a type is sometimes called an existential type, which comes from the phrase “there exists a type T such that T conforms to the protocol”.

You can use a protocol in many places where other types are allowed, including:

• As a parameter type or return type in a function, method, or initializer

• As the type of a constant, variable, or property

• As the type of items in an array, dictionary, or other container

Here’s an example of a protocol used as a type:

class Dice {
    let sides: Int
    let generator: RandomNumberGenerator
    init(sides: Int, generator: RandomNumberGenerator) {
        self.sides = sides
        self.generator = generator
    }
    func roll() -> Int {
        return Int(generator.random() * Double(sides)) + 1
    }
}

This example defines a new class called Dice, which represents an n-sided dice for use in a board game. Dice instances have an integer property called sides, which represents how many sides they have, and a property called generator, which provides a random number generator from which to create dice roll values.

Dice provides one instance method, roll, which returns an integer value between 1 and the number of sides on the dice. This method calls the generator’s random() method to create a new random number between 0.0 and 1.0, and uses this random number to create a dice roll value within the correct range.

Here’s how the Dice class can be used to create a six-sided dice with a LinearCongruentialGenerator instance as its random number generator:

var d6 = Dice(sides: 6, generator: LinearCongruentialGenerator())
for _ in 1...5 {
    print("Random dice roll is \(d6.roll())")
}
// Random dice roll is 3
// Random dice roll is 5
// Random dice roll is 4
// Random dice roll is 5
// Random dice roll is 4

Delegation

Delegation is a design pattern that enables a class or structure to hand off (or delegate) some of its responsibilities to an instance of another type.

The example below defines two protocols for use with dice-based board games:

protocol DiceGame {
    var dice: Dice { get }
    func play()
}
protocol DiceGameDelegate: AnyObject {
    func gameDidStart(_ game: DiceGame)
    func game(_ game: DiceGame, didStartNewTurnWithDiceRoll diceRoll: Int)
    func gameDidEnd(_ game: DiceGame)
}

The DiceGame protocol is a protocol that can be adopted by any game that involves dice.

The DiceGameDelegate protocol can be adopted to track the progress of a DiceGame. To prevent strong reference cycles, delegates are declared as weak references. For information about weak references, see Strong Reference Cycles Between Class Instances.

Marking the protocol as class-only lets the SnakesAndLadders class later in this chapter declare that its delegate must use a weak reference. A class-only protocol is marked by its inheritance from AnyObject, as discussed in Class-Only Protocols.

Here’s a version of the Snakes and Ladders game originally introduced in Control Flow. This version is adapted to use a Dice instance for its dice-rolls; to adopt the DiceGame protocol; and to notify a DiceGameDelegate about its progress:

class SnakesAndLadders: DiceGame {
    let finalSquare = 25
    let dice = Dice(sides: 6, generator: LinearCongruentialGenerator())
    var square = 0
    var board: [Int]
    init() {
        board = Array(repeating: 0, count: finalSquare + 1)
        board[03] = +08; board[06] = +11; board[09] = +09; board[10] = +02
        board[14] = -10; board[19] = -11; board[22] = -02; board[24] = -08
    }
    weak var delegate: DiceGameDelegate?
    func play() {
        square = 0
        delegate?.gameDidStart(self)
        gameLoop: while square != finalSquare {
            let diceRoll = dice.roll()
            delegate?.game(self, didStartNewTurnWithDiceRoll: diceRoll)
            switch square + diceRoll {
            case finalSquare:
                break gameLoop
            case let newSquare where newSquare > finalSquare:
                continue gameLoop
            default:
                square += diceRoll
                square += board[square]
            }
        }
        delegate?.gameDidEnd(self)
    }
}

For a description of the Snakes and Ladders gameplay, see Break.

This version of the game is wrapped up as a class called SnakesAndLadders, which adopts the DiceGame protocol. It provides a gettable dice property and a play() method in order to conform to the protocol. (The dice property is declared as a constant property because it doesn’t need to change after initialization, and the protocol only requires that it must be gettable.)

Note that the delegate property is defined as an optional DiceGameDelegate, because a delegate isn’t required in order to play the game. Because it’s of an optional type, the delegate property is automatically set to an initial value of nil. Thereafter, the game instantiator has the option to set the property to a suitable delegate. Because the DiceGameDelegate protocol is class-only, you can declare the delegate to be weak to prevent reference cycles.

This next example shows a class called DiceGameTracker, which adopts the DiceGameDelegate protocol:

class DiceGameTracker: DiceGameDelegate {
    var numberOfTurns = 0
    func gameDidStart(_ game: DiceGame) {
        numberOfTurns = 0
        if game is SnakesAndLadders {
            print("Started a new game of Snakes and Ladders")
        }
        print("The game is using a \(game.dice.sides)-sided dice")
    }
    func game(_ game: DiceGame, didStartNewTurnWithDiceRoll diceRoll: Int) {
        numberOfTurns += 1
        print("Rolled a \(diceRoll)")
    }
    func gameDidEnd(_ game: DiceGame) {
        print("The game lasted for \(numberOfTurns) turns")
    }
}

Here’s how DiceGameTracker looks in action:

let tracker = DiceGameTracker()
let game = SnakesAndLadders()
game.delegate = tracker
game.play()
// Started a new game of Snakes and Ladders
// The game is using a 6-sided dice
// Rolled a 3
// Rolled a 5
// Rolled a 4
// Rolled a 5
// The game lasted for 4 turns

Adding Protocol Conformance with an Extension

You can extend an existing type to adopt and conform to a new protocol, even if you don’t have access to the source code for the existing type. Extensions can add new properties, methods, and subscripts to an existing type, and are therefore able to add any requirements that a protocol may demand. For more about extensions, see Extensions.

For example, this protocol, called TextRepresentable, can be implemented by any type that has a way to be represented as text. This might be a description of itself, or a text version of its current state:

protocol TextRepresentable {
    var textualDescription: String { get }
}

The Dice class from above can be extended to adopt and conform to TextRepresentable:

extension Dice: TextRepresentable {
    var textualDescription: String {
        return "A \(sides)-sided dice"
    }
}

Any Dice instance can now be treated as TextRepresentable:

let d12 = Dice(sides: 12, generator: LinearCongruentialGenerator())
print(d12.textualDescription)
// Prints "A 12-sided dice"

Similarly, the SnakesAndLadders game class can be extended to adopt and conform to the TextRepresentable protocol:

extension SnakesAndLadders: TextRepresentable {
    var textualDescription: String {
        return "A game of Snakes and Ladders with \(finalSquare) squares"
    }
}
print(game.textualDescription)
// Prints "A game of Snakes and Ladders with 25 squares"

Conditionally Conforming to a Protocol

A generic type may be able to satisfy the requirements of a protocol only under certain conditions, such as when the type’s generic parameter conforms to the protocol. You can make a generic type conditionally conform to a protocol by listing constraints when extending the type. Write these constraints after the name of the protocol you’re adopting by writing a generic where clause. For more about generic where clauses, see Generic Where Clauses.

The following extension makes Array instances conform to the TextRepresentable protocol whenever they store elements of a type that conforms to TextRepresentable.

extension Array: TextRepresentable where Element: TextRepresentable {
    var textualDescription: String {
        let itemsAsText = self.map { $0.textualDescription }
        return "[" + itemsAsText.joined(separator: ", ") + "]"
    }
}
let myDice = [d6, d12]
print(myDice.textualDescription)
// Prints "[A 6-sided dice, A 12-sided dice]"

Declaring Protocol Adoption with an Extension

If a type already conforms to all of the requirements of a protocol, but hasn’t yet stated that it adopts that protocol, you can make it adopt the protocol with an empty extension:

struct Hamster {
    var name: String
    var textualDescription: String {
        return "A hamster named \(name)"
    }
}
extension Hamster: TextRepresentable {}

Instances of Hamster can now be used wherever TextRepresentable is the required type:

let simonTheHamster = Hamster(name: "Simon")
let somethingTextRepresentable: TextRepresentable = simonTheHamster
print(somethingTextRepresentable.textualDescription)
// Prints "A hamster named Simon"

NOTE: Types don’t automatically adopt a protocol just by satisfying its requirements. They must always explicitly declare their adoption of the protocol.

Adopting a Protocol Using a Synthesized Implementation

Swift can automatically provide the protocol conformance for Equatable, Hashable, and Comparable in many simple cases. Using this synthesized implementation means you don’t have to write repetitive boilerplate code to implement the protocol requirements yourself.

Equatable

Swift provides a synthesized implementation of Equatable for the following kinds of custom types:

• Structures that have only stored properties that conform to the Equatable protocol

• Enumerations that have only associated types that conform to the Equatable protocol

• Enumerations that have no associated types

To receive a synthesized implementation of ==, declare conformance to Equatable in the file that contains the original declaration, without implementing an == operator yourself. The Equatable protocol provides a default implementation of !=.

The example below defines a Vector3D structure for a three-dimensional position vector (x, y, z), similar to the Vector2D structure. Because the x, y, and z properties are all of an Equatable type, Vector3D receives synthesized implementations of the equivalence operators.

struct Vector3D: Equatable {
    var x = 0.0, y = 0.0, z = 0.0
}

let twoThreeFour = Vector3D(x: 2.0, y: 3.0, z: 4.0)
let anotherTwoThreeFour = Vector3D(x: 2.0, y: 3.0, z: 4.0)
if twoThreeFour == anotherTwoThreeFour {
    print("These two vectors are also equivalent.")
}
// Prints "These two vectors are also equivalent."

Hashable

Swift provides a synthesized implementation of Hashable for the following kinds of custom types:

• Structures that have only stored properties that conform to the Hashable protocol

• Enumerations that have only associated types that conform to the Hashable protocol

• Enumerations that have no associated types

To receive a synthesized implementation of hash(into:), declare conformance to Hashable in the file that contains the original declaration, without implementing a hash(into:) method yourself.

Comparable

Swift provides a synthesized implementation of Comparable for enumerations that don’t have a raw value. If the enumeration has associated types, they must all conform to the Comparable protocol.

To receive a synthesized implementation of <, declare conformance to Comparable in the file that contains the original enumeration declaration, without implementing a < operator yourself. The Comparable protocol’s default implementation of <=, >, and >= provides the remaining comparison operators.

The example below defines a SkillLevel enumeration with cases for beginners, intermediates, and experts. Experts are additionally ranked by the number of stars they have.

enum SkillLevel: Comparable {
    case beginner
    case intermediate
    case expert(stars: Int)
}
var levels = [SkillLevel.intermediate, SkillLevel.beginner,
              SkillLevel.expert(stars: 5), SkillLevel.expert(stars: 3)]
for level in levels.sorted() {
    print(level)
}
// Prints "beginner"
// Prints "intermediate"
// Prints "expert(stars: 3)"
// Prints "expert(stars: 5)"

Collections of Protocol Types

A protocol can be used as the type to be stored in a collection such as an array or a dictionary, as mentioned in Protocols as Types. This example creates an array of TextRepresentable things:

let things: [TextRepresentable] = [game, d12, simonTheHamster]

It’s now possible to iterate over the items in the array, and print each item’s textual description:

for thing in things {
    print(thing.textualDescription)
}
// A game of Snakes and Ladders with 25 squares
// A 12-sided dice
// A hamster named Simon

Protocol Inheritance

A protocol can inherit one or more other protocols and can add further requirements on top of the requirements it inherits. The syntax for protocol inheritance is similar to the syntax for class inheritance, but with the option to list multiple inherited protocols, separated by commas:

protocol InheritingProtocol: SomeProtocol, AnotherProtocol {
    // protocol definition goes here
}

Here’s an example of a protocol that inherits the TextRepresentable protocol from above:

protocol PrettyTextRepresentable: TextRepresentable {
    var prettyTextualDescription: String { get }
}

The SnakesAndLadders class can be extended to adopt and conform to PrettyTextRepresentable:

extension SnakesAndLadders: PrettyTextRepresentable {
    var prettyTextualDescription: String {
        var output = textualDescription + ":\n"
        for index in 1...finalSquare {
            switch board[index] {
            case let ladder where ladder > 0:
                output += "▲ "
            case let snake where snake < 0:
                output += "▼ "
            default:
                output += "○ "
            }
        }
        return output
    }
}

The prettyTextualDescription property can now be used to print a pretty text description of any SnakesAndLadders instance:

print(game.prettyTextualDescription)
// A game of Snakes and Ladders with 25 squares:
// ○ ○ ▲ ○ ○ ▲ ○ ○ ▲ ▲ ○ ○ ○ ▼ ○ ○ ○ ○ ▼ ○ ○ ▼ ○ ▼ ○

Class-Only Protocols

You can limit protocol adoption to class types (and not structures or enumerations) by adding the AnyObject protocol to a protocol’s inheritance list.

protocol SomeClassOnlyProtocol: AnyObject, SomeInheritedProtocol {
    // class-only protocol definition goes here
}

Protocol Composition

It can be useful to require a type to conform to multiple protocols at the same time. You can combine multiple protocols into a single requirement with a protocol composition.

Protocol compositions have the form SomeProtocol & AnotherProtocol. You can list as many protocols as you need, separating them with ampersands (&). In addition to its list of protocols, a protocol composition can also contain one class type, which you can use to specify a required superclass.

Here’s an example that combines two protocols called Named and Aged into a single protocol composition requirement on a function parameter:

protocol Named {
    var name: String { get }
}
protocol Aged {
    var age: Int { get }
}
struct Person: Named, Aged {
    var name: String
    var age: Int
}
func wishHappyBirthday(to celebrator: Named & Aged) {
    print("Happy birthday, \(celebrator.name), you're \(celebrator.age)!")
}
let birthdayPerson = Person(name: "Malcolm", age: 21)
wishHappyBirthday(to: birthdayPerson)
// Prints "Happy birthday, Malcolm, you're 21!"

The type of the celebrator parameter is Named & Aged, which means “any type that conforms to both the Named and Aged protocols.” It doesn’t matter which specific type is passed to the function, as long as it conforms to both of the required protocols.

Here’s an example that combines the Named protocol from the previous example with a Location class:

class Location {
    var latitude: Double
    var longitude: Double
    init(latitude: Double, longitude: Double) {
        self.latitude = latitude
        self.longitude = longitude
    }
}
class City: Location, Named {
    var name: String
    init(name: String, latitude: Double, longitude: Double) {
        self.name = name
        super.init(latitude: latitude, longitude: longitude)
    }
}
func beginConcert(in location: Location & Named) {
    print("Hello, \(location.name)!")
}

let seattle = City(name: "Seattle", latitude: 47.6, longitude: -122.3)
beginConcert(in: seattle)
// Prints "Hello, Seattle!"

The beginConcert(in:) function takes a parameter of type Location & Named, which means “any type that’s a subclass of Location and that conforms to the Named protocol.” In this case, City satisfies both requirements.

• Passing birthdayPerson to the beginConcert(in:) function is invalid because Person isn’t a subclass of Location.

• Likewise, if you made a subclass of Location that didn’t conform to the Named protocol, calling beginConcert(in:) with an instance of that type is also invalid.

Checking for Protocol Conformance

You can use the is and as operators described in Type Casting to check for protocol conformance, and to cast to a specific protocol. Checking for and casting to a protocol follows exactly the same syntax as checking for and casting to a type:

• The is operator returns true if an instance conforms to a protocol and returns false if it doesn’t.

• The as? version of the downcast operator returns an optional value of the protocol’s type, and this value is nil if the instance doesn’t conform to that protocol.

• The as! version of the downcast operator forces the downcast to the protocol type and triggers a runtime error if the downcast doesn’t succeed.

This example defines a protocol called HasArea, with a single property requirement of a gettable Double property called area:

protocol HasArea {
    var area: Double { get }
}

Here are two classes, Circle and Country, both of which conform to the HasArea protocol:

class Circle: HasArea {
    let pi = 3.1415927
    var radius: Double
    var area: Double { return pi * radius * radius }
    init(radius: Double) { self.radius = radius }
}
class Country: HasArea {
    var area: Double
    init(area: Double) { self.area = area }
}

Both classes correctly conform to the HasArea protocol:

• The Circle class implements the area property requirement as a computed property, based on a stored radius property.

• The Country class implements the area requirement directly as a stored property.

Here’s a class called Animal, which doesn’t conform to the HasArea protocol:

class Animal {
    var legs: Int
    init(legs: Int) { self.legs = legs }
}

The Circle, Country and Animal classes don’t have a shared base class. Nonetheless, they’re all classes, and so instances of all three types can be used to initialize an array that stores values of type AnyObject:

let objects: [AnyObject] = [
    Circle(radius: 2.0),
    Country(area: 243_610),
    Animal(legs: 4)
]

The objects array can now be iterated, and each object in the array can be checked to see if it conforms to the HasArea protocol:

for object in objects {
    if let objectWithArea = object as? HasArea {
        print("Area is \(objectWithArea.area)")
    } else {
        print("Something that doesn't have an area")
    }
}
// Area is 12.5663708
// Area is 243610.0
// Something that doesn't have an area

Whenever an object in the array conforms to the HasArea protocol, the optional value returned by the as? operator is unwrapped with optional binding into a constant called objectWithArea. The objectWithArea constant is known to be of type HasArea, and so its area property can be accessed and printed in a type-safe way.

Optional Protocol Requirements

You can define optional requirements for protocols. These requirements don’t have to be implemented by types that conform to the protocol. Optional requirements are prefixed by the optional modifier as part of the protocol’s definition.

Optional requirements are available so that you can write code that interoperates with Objective-C. Both the protocol and the optional requirement must be marked with the @objc attribute. Note that @objc protocols can be adopted only by classes that inherit from Objective-C classes or other @objc classes. They can’t be adopted by structures or enumerations.

When you use a method or property in an optional requirement, its type automatically becomes an optional. For example, a method of type (Int) -> String becomes ((Int) -> String)?. Note that the entire function type is wrapped in the optional, not the method’s return value.

An optional protocol requirement can be called with optional chaining, to account for the possibility that the requirement was not implemented by a type that conforms to the protocol. You check for an implementation of an optional method by writing a question mark after the name of the method when it’s called, such as someOptionalMethod?(someArgument). For information on optional chaining, see Optional Chaining.

The following example defines an integer-counting class called Counter, which uses an external data source to provide its increment amount. This data source is defined by the CounterDataSource protocol, which has two optional requirements:

@objc protocol CounterDataSource {
    @objc optional func increment(forCount count: Int) -> Int
    @objc optional var fixedIncrement: Int { get }
}

The CounterDataSource protocol defines an optional method requirement called increment(forCount:) and an optional property requirement called fixedIncrement. These requirements define two different ways for data sources to provide an appropriate increment amount for a Counter instance.

The Counter class, defined below, has an optional dataSource property of type CounterDataSource?:

class Counter {
    var count = 0
    var dataSource: CounterDataSource?
    func increment() {
        if let amount = dataSource?.increment?(forCount: count) {
            count += amount
        } else if let amount = dataSource?.fixedIncrement {
            count += amount
        }
    }
}

The Counter class stores its current value in a variable property called count. The Counter class also defines a method called increment, which increments the count property every time the method is called.

Here’s a simple CounterDataSource implementation where the data source returns a constant value of 3 every time it’s queried. It does this by implementing the optional fixedIncrement property requirement:

class ThreeSource: NSObject, CounterDataSource {
    let fixedIncrement = 3
}

You can use an instance of ThreeSource as the data source for a new Counter instance:

var counter = Counter()
counter.dataSource = ThreeSource()
for _ in 1...4 {
    counter.increment()
    print(counter.count)
}
// 3
// 6
// 9
// 12

Here’s a more complex data source called TowardsZeroSource, which makes a Counter instance count up or down towards zero from its current count value:

class TowardsZeroSource: NSObject, CounterDataSource {
    func increment(forCount count: Int) -> Int {
        if count == 0 {
            return 0
        } else if count < 0 {
            return 1
        } else {
            return -1
        }
    }
}

You can use an instance of TowardsZeroSource with the existing Counter instance to count from -4 to zero. Once the counter reaches zero, no more counting takes place:

counter.count = -4
counter.dataSource = TowardsZeroSource()
for _ in 1...5 {
    counter.increment()
    print(counter.count)
}
// -3
// -2
// -1
// 0
// 0

Protocol Extensions

Protocols can be extended to provide method, initializer, subscript, and computed property implementations to conforming types. This allows you to define behavior on protocols themselves, rather than in each type’s individual conformance or in a global function.

For example, the RandomNumberGenerator protocol can be extended to provide a randomBool() method, which uses the result of the required random() method to return a random Bool value:

extension RandomNumberGenerator {
    func randomBool() -> Bool {
        return random() > 0.5
    }
}

By creating an extension on the protocol, all conforming types automatically gain this method implementation without any additional modification.

let generator = LinearCongruentialGenerator()
print("Here's a random number: \(generator.random())")
// Prints "Here's a random number: 0.3746499199817101"
print("And here's a random Boolean: \(generator.randomBool())")
// Prints "And here's a random Boolean: true"

Protocol extensions can add implementations to conforming types but can’t make a protocol extend or inherit from another protocol. Protocol inheritance is always specified in the protocol declaration itself.

Providing Default Implementations

You can use protocol extensions to provide a default implementation to any method or computed property requirement of that protocol. If a conforming type provides its own implementation of a required method or property, that implementation will be used instead of the one provided by the extension.

NOTE: Protocol requirements with default implementations provided by extensions are distinct from optional protocol requirements. Although conforming types don’t have to provide their own implementation of either, requirements with default implementations can be called without optional chaining.

For example, the PrettyTextRepresentable protocol, which inherits the TextRepresentable protocol can provide a default implementation of its required prettyTextualDescription property to simply return the result of accessing the textualDescription property:

extension PrettyTextRepresentable  {
    var prettyTextualDescription: String {
        return textualDescription
    }
}

Adding Constraints to Protocol Extensions

When you define a protocol extension, you can specify constraints that conforming types must satisfy before the methods and properties of the extension are available. You write these constraints after the name of the protocol you’re extending by writing a generic where clause. For more about generic where clauses, see Generic Where Clauses.

For example, you can define an extension to the Collection protocol that applies to any collection whose elements conform to the Equatable protocol. By constraining a collection’s elements to the Equatable protocol, a part of the standard library, you can use the == and != operators to check for equality and inequality between two elements.

extension Collection where Element: Equatable {
    func allEqual() -> Bool {
        for element in self {
            if element != self.first {
                return false
            }
        }
        return true
    }
}

The allEqual() method returns true only if all the elements in the collection are equal.

Consider two arrays of integers, one where all the elements are the same, and one where they aren’t:

let equalNumbers = [100, 100, 100, 100, 100]
let differentNumbers = [100, 100, 200, 100, 200]

Because arrays conform to Collection and integers conform to Equatable, equalNumbers and differentNumbers can use the allEqual() method:

print(equalNumbers.allEqual())
// Prints "true"
print(differentNumbers.allEqual())
// Prints "false"

NOTE: If a conforming type satisfies the requirements for multiple constrained extensions that provide implementations for the same method or property, Swift uses the implementation corresponding to the most specialized constraints.