《Swift从入门到精通》（十八）：协议

协议（学习笔记）

环境Xcode 11.0 beta4 swift 5.1

协议语法

示例

protocol SomeProtocol {
    // protocol definition goes here
}
struct SomeStructure: FirstProtocol, AnotherProtocol {
    // structure definition goes here
}
class SomeClass: SomeSuperclass, FirstProtocol, AnotherProtocol {
    // class definition goes here
}

属性要求

协议不需指定属性是存储或计算属性，只需指定所需的属性名称和类型；协议指定了每个属性是可读或可读可写，通常在类型后跟 {get set} 或 { get } 标识
```
protocol SomeProtocol {
    var mustBeSettable: Int { get set }
    var doesNotNeedToBeSettable: Int { get }
}
```

协议中类型属性仍然用关键字 static 或 class 修饰

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

// 只有一个属性的协议
protocol FullyNamed {
    var fullName: String { get }
}
// 遵守协议必需要提供一个一个名为 fullName 的可获取实例属性
struct Person: FullyNamed {
    var fullName: String
}
let john = Person(fullName: "John Appleseed")
// john.fullName is "John Appleseed"
// 
// 遵守 FullNamed 协议，此处用的是一个只可读的计算属性
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"

方法要求

协议定义中可定义实例和类型方法，与正常的方法定义类似，方法参数可以是可变参数，但不能有方法体，且参数不能设置默认值

protocol SomeProtocol {
    static func someTypeMethod()
}
protocol RandomNumberGenerator {
    func random() -> Double
}
// 示例如下
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"

可变方法要求

协议中方法可用 mutatuing 修饰，这位遵守此协议的方法可以修改属性值（结构体、枚举也可以），且可在实现方法时可省略此关键字

protocol Togglable {
    mutating func toggle()
}
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

初始化器要求

协议中可定义初始化器让遵守的类型实现，但协议中的初始化器没有 {} 或函数体，实现类实现初始化器（便捷或指定初始化器）必需加 required 修饰符
```
protocol SomeProtocol {
    init(someParameter: Int)
}
class SomeClass: SomeProtocol {
    required init(someParameter: Int) {
        // initializer implementation goes here
    }
}
```

如果一个子类重写父类的指定初始化器同时遵守协议的初始化器的要求，必需同时用 required 和 override 修饰符

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
    }
}

协议作为类型

可以做参数类型、函数返回值、方法返回值、初始化器返回值类型
可以做常量、变量、属性的类型

可以用在字典、数组、其它容器中元素的类型

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
    }
}
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

代理

代理是一种重要的设计模式，示例代码

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)
}
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? // 一般用 weak
    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)
    }
}
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")
    }
}
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

用扩展添加协议的一致性

扩展可以为现有的类添加属性、方法、下标，因此可以添加协议可能需要的任何要求

protocol TextRepresentable {
    var textualDescription: String { get }
}
extension Dice: TextRepresentable {
    var textualDescription: String {
        return "A \(sides)-sided dice"
    }
}
let d12 = Dice(sides: 12, generator: LinearCongruentialGenerator())
print(d12.textualDescription)
// Prints "A 12-sided dice"
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"

有条件的遵守协议

// Array 中的元素要遵守  TextRepresentable 协议
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]"

采用扩展申明协议

struct Hamster {
    var name: String
    var textualDescription: String {
        return "A hamster named \(name)"
    }
}
extension Hamster: TextRepresentable {}
let simonTheHamster = Hamster(name: "Simon")
// 这里要显示申明类型
let somethingTextRepresentable: TextRepresentable = simonTheHamster
print(somethingTextRepresentable.textualDescription)
// Prints "A hamster named Simon"

协议类型集合

示例

let things: [TextRepresentable] = [game, d12, simonTheHamster]
for thing in things {
    print(thing.textualDescription)
}
// A game of Snakes and Ladders with 25 squares
// A 12-sided dice
// A hamster named Simon

协议继承

示例

protocol InheritingProtocol: SomeProtocol, AnotherProtocol {
    // protocol definition goes here
}
protocol PrettyTextRepresentable: TextRepresentable {
    var prettyTextualDescription: String { get }
}
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
    }
}
print(game.prettyTextualDescription)
// A game of Snakes and Ladders with 25 squares:
// ○ ○ ▲ ○ ○ ▲ ○ ○ ▲ ▲ ○ ○ ○ ▼ ○ ○ ○ ○ ▼ ○ ○ ▼ ○ ▼ ○

Class-Only 协议
- 可以限制协议只能类来遵守，通过添加 AnyObject 协议在继承列表中，如果结构体或枚举遵守会报运行时错误
```
protocol SomeClassOnlyProtocol: AnyObject, SomeInheritedProtocol {
    // class-only protocol definition goes here
}
```

协议合成

用 & 符号将多个协议连接起来

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!"
//
//
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!"

协议一致性检查

is 实例遵守协议返回 true，否则 false
as? 返回一个协议类型的可选值，如果不遵守协议则是 nil

as! 强制解包，如果失败触发运行时错误

protocol HasArea {
    var area: Double { get }
}
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 }
}
class Animal {
    var legs: Int
    init(legs: Int) { self.legs = legs }
}
let objects: [AnyObject] = [
    Circle(radius: 2.0),
    Country(area: 243_610),
    Animal(legs: 4)
]
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

可选协议要求

用 optional 修饰，协议和可选需求都必需用 @objc 标记，被标记的协议只能是OC类或其它 @objc类可用，枚举结构体不可用

@objc protocol CounterDataSource {
    @objc optional func increment(forCount count: Int) -> Int
    @objc optional var fixedIncrement: Int { get }
}
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
        }
    }
}
class ThreeSource: NSObject, CounterDataSource {
    let fixedIncrement = 3
}
var counter = Counter()
counter.dataSource = ThreeSource()
for _ in 1...4 {
    counter.increment()
    print(counter.count)
}
// 3
// 6
// 9
// 12
class TowardsZeroSource: NSObject, CounterDataSource {
    func increment(forCount count: Int) -> Int {
        if count == 0 {
            return 0
        } else if count < 0 {
            return 1
        } else {
            return -1
        }
    }
}
counter.count = -4
counter.dataSource = TowardsZeroSource()
for _ in 1...5 {
    counter.increment()
    print(counter.count)
}
// -3
// -2
// -1
// 0
// 0

协议扩展

可以为协议添加扩展，扩展里可以添加方法、计算属性、下标、初始化器等，这样类型的一致性或全局函数的行为可在此实现

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

提供默认的实现

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

协议扩展添加约束

extension Collection where Element: Equatable {
    func allEqual() -> Bool {
        for element in self {
            if element != self.first {
                return false
            }
        }
        return true
    }
}
let equalNumbers = [100, 100, 100, 100, 100]
let differentNumbers = [100, 100, 200, 100, 200]
print(equalNumbers.allEqual())
// Prints "true"
print(differentNumbers.allEqual())
// Prints "false"

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《Swift从入门到精通》（十八）：协议

协议（学习笔记）

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