21.协议 Protocols Swift官方文档——版纳的笔记

//: Playground - noun: a place where people can play

import UIKit

// # 协议的语法

// # 属性要求
// 必须定义为变量属性,因为在定义时看上去是个计算属性.
protocol AProtocol {
    var mustBeSettable: Int { get set } // 只能是get-set计算或var存储属性
    var canBeSettable: Int { get } // 可以是各类属性.
    static var someTypeProperty: Int { get set }
}
protocol FullyNamed {
    var fullName: String { get }
}
struct Person: FullyNamed {
    var fullName: String
}
let john = Person(fullName: "John Appleseed")
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

// # 方法要求
// 方法参数不能定义默认值
protocol RandomNumberGenerator {
    func random() -> Double
}
// 线性同余法生成伪随机数
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(generator.random())
print(generator.random())

// # 异变方法要求
protocol Togglable {
    mutating func toggle()
}
enum OnOffSwitch: Togglable {
    case off, on
    mutating func toggle() {
        switch self {
        case .off:
            self = .on
        default:
            self = .off
        }
    }
}
var lightSwitch = OnOffSwitch.off
lightSwitch.toggle()

// # 初始化器要求
// 可以使用required, final关键字
protocol SomeProtocol {
    init()
}
class SomeSuperClass {
    init() {}
}
class SomeSubClass: SomeSuperClass, SomeProtocol {
    required override init() {}
}
// 遵循协议的类型可以使用一个可失败的或不可失败的初始化器满足一个可失败的初始化器要求。不可失败初始化器要求可以使用一个不可失败初始化器或隐式展开的可失败初始化器满足。

// # 将协议作为类型***
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())")
}

// # 委托
protocol DiceGame {
    var dice: Dice { get }
    func play()
}
protocol DiceGameDelegate {
    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
    }
    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)
    }
}
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()

// # 在扩展里添加协议遵循
// 类型已经存在的实例会在给它的类型扩展中添加遵循协议时自动地采纳和遵循这个协议。
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)
extension SnakesAndLadders: TextRepresentable {
    var textualDescription: String {
        return "A game of Snakes and Ladders with \(finalSquare) squares"
    }
}
print(game.textualDescription)
// 如果一个类型已经遵循了协议的所有需求，但是还没有声明它采纳了这个协议，可以让通过一个空的扩展来让它采纳这个协议
struct Hamster {
    var name: String
    var textualDescription: String {
        return "A hamster named \(name)"
    }
}
extension Hamster: TextRepresentable {}
let simonTheHamster = Hamster(name: "Simon")
let somethingTextRepresentable: TextRepresentable = simonTheHamster
print(somethingTextRepresentable.textualDescription)

// # 协议类型的集合
let things: [TextRepresentable] = [game, d12, simonTheHamster]
for thing in things {
    print(thing.textualDescription)
}

// # 协议继承
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)

// # 类专用的协议
protocol SomeClassOnlyProtocol: AnyObject {
    // class-only protocol definition goes here.
}
// AnyObject is a protocol to which all classes implicitly conform.

// # 协议组合
// 用&连接
protocol Named {
    var name: String { get }
}
protocol Aged {
    var age: Int { get }
}
struct OnePerson: 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 = OnePerson(name: "Malcolm", age: 21)
wishHappyBirthday(to: birthdayPerson)
// 也可以包含一个父类
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 beginConert(in location: Location & Named) {
    print("Hello, \(location.name)!")
}
let seattle = City(name: "Seattle", latitude: 47.6, longitude: -122.3)
beginConert(in: seattle)

// # 协议遵循的检查
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 }
}
// Here is a class does not conform to HasArea protocol.
class Animal {
    var legs: Int
    init(legs: Int) {
        self.legs = legs
    }
}
// But we can put all three of they into an array...
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")
    }
}

// # 可选协议要求
// 允许与Objective-C协作.@objc 协议只能被继承自 Objective-C 类或其他 @objc 类采纳。它们不能被结构体或者枚举采纳。
@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 // there may or may not be an increment func
        } else if let amount = dataSource?.fixedIncrement {
            count += amount
        }
    }
}
// NSObject means this is an Obj-C class
class ThreeSource: NSObject, CounterDataSource {
    let fixedIncrement: Int = 3
}
var counter = Counter()
counter.dataSource = ThreeSource()
for _ in 1...4 {
    counter.increment()
    print(counter.count)
}
// more complicated one
@objc 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)
}

// # 协议扩展
// 为所有遵循协议的实现添加扩展方法和属性.
extension RandomNumberGenerator {
    func randomBool() -> Bool {
        return random() > 0.5
    }
}
let generatorA = LinearCongruentialGenerator()
print("Here's a random number: \(generatorA.random())")
print("And here's a random Boolean: \(generatorA.randomBool())")
// 注意:提供的只是默认实现.如果遵循类型给这个协议的要求提供了它自己的实现，那么它就会替代扩展中提供的默认实现。
extension PrettyTextRepresentable {
    var prettyTextualDescription: String {
        return textualDescription
    }
}
// 给协议扩展添加限制.
// trick: where's implement.
extension Collection where Iterator.Element: TextRepresentable {
    var textualDescription: String {
        // trick: map: returns an array containing the results of mapping the given closure over the sequence’s elements.
        let itemsAsText = self.map { $0.textualDescription }
        // trick: .joined: make them a String
        return "[" + itemsAsText.joined(separator: ", ") + "]"
    }
}
let murrayTheHamster = Hamster(name: "Murray")
let morganTheHamster = Hamster(name: "Morgan")
let mauriceTheHamster = Hamster(name: "Maurice")
let hamsters = [murrayTheHamster, morganTheHamster, mauriceTheHamster]
print(hamsters.textualDescription)

// ***父类服从某协议，则子类也服从该协议，可以用is探测