如何理解swift中的delegate

Delegation翻译为代理或者委托，是一种设计模式。顾名思义，使class或struct能够将某些职责移交给其他类型的实例。
该设计模式通过定义一个封装（包含）delegate的protocol（协议）来实现，从而保证这个代理囊括所定义的功能。Delegation可用于响应特定操作，或者从外部源检索数据，而不需要知道该源的基础类型。

（一）这里举一个dice-based（摇骰子）的游戏作为例子：
定义两个protocol：
DiceGame：被所有需要骰子的游戏采用；
DiceGameDelegate：被所有需要track（跟踪）游戏过程的类型所采用；

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)
}

（二）这里有一个Snakes and Ladders（蛇与梯子）的游戏，使用了筛子所以采用DiceGame协议（protocol），并且设置了一个DiceGameDelegate的代理来track游戏的过程。想具体了解Snakes and ladders这个游戏过程的可以参考[Control flow 中的break]。(https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/ControlFlow.html#//apple_ref/doc/uid/TP40014097-CH9-ID137)

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)
    }
}

（三）

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")
    }
}

DiceGameTracker实现了所有DiceGameDelegate中所要求的三个方法，从而达到记录整个游戏过程的目的。

（四）下面是整个游戏实际运行的过程：

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

分别定义了一个实现DiceGameDelegate的tracker和一个游戏game，将game的代理设置为tracker，当game调用play函数的时候，track会代理地记录下游戏的整个过程。

参考：
The swift programming language

https://developer.apple.com/library/content/documentation/Swift/Conceptual/Swift_Programming_Language/Protocols.html#//apple_ref/doc/uid/TP40014097-CH25-ID267