如何编写一个关于如何在 Swift 5 中使用委托的简单示例

Question

我想写一个简单的例子来说明委托在 Swift 中是如何工作的。我已经设置了一个类、一个协议和另一个遵守该协议的类。但是我在两个不同的地方遇到了同样的错误。我在 xcode 中创建了一个简单的 swift 命令行工具，我的所有代码都在 main.swift 中。该代码除了学习委托的工作方式之外没有任何实际功能。

以下是错误消息：

一行上的连续声明必须用';'分隔
插入';'
函数声明的参数列表中应为“（”
函数声明体中应为“{”
实例方法声明中的预期“func”关键字
插入 'func'
预期声明
'delegate()' //或其他错误中的任何东西（）的重新声明无效

代码如下：

class MainClass {

    var delegate: MyProtocol? = nil

    delegate.doAnything() //getting 1st error here
}


protocol MyProtocol {
    func doAnything()
}


class OtherClass: MyProtocol {

    let anything = MainClass()

    anything?.delegate = self //getting 2nd error here

    func doAnything() {
        print("text")
    }
}

Answer 1

所以第一个错误是因为您试图在可以执行的任何地方之外调用委托函数。您需要创建一个调用该函数的函数，或者在 init 中调用它。在制作示例时，请尝试使用现实世界的概念来为您的示例建模。你可以做一些像指挥班和火车班这样的事情。售票员可以实现一些控制协议来控制火车的速度。

无论如何，您的第二个错误是因为self 尚未初始化。要将变量分配给self，您必须先初始化类，这样您就可以这样做

init() {
   anything?.delegate = self
}

欢迎 DM 进一步了解这个概念，我稍后会在这里发布一个完整的示例。

编辑：完整示例，请随时提问

import Foundation

enum Direction {
    case north
    case east
    case south
    case west
}

protocol VehicleControls {
    var speed: Float {get set}
    var direction: Direction {get set}
    var numPassengers: Int {get}

    func change(newSpeed: Float)

    func change(newDirection: Direction)

    func createNoise()
}

class Conductor {
    var vehicle: VehicleControls

    init() {
        vehicle = Train(s: 1.5, d: .west, nP: 50)
    }

    func controlVehicle() {
        vehicle.change(newSpeed: 2.5)
        vehicle.change(newDirection: .east)
        vehicle.createNoise()
        print("\n")
    }
}

class Train: VehicleControls {
    var speed: Float
    var direction: Direction
    var numPassengers: Int

    init() {
        self.speed = 0
        self.direction = .north
        self.numPassengers = 0
    }

    init(s: Float, d: Direction, nP: Int) {
        self.speed = s
        self.direction = d
        self.numPassengers = nP
    }

    func change(newSpeed: Float) {
        print("changing speed from \(speed), to \(newSpeed)")
        self.speed = newSpeed
    }

    func change(newDirection: Direction) {
        print("changing direction from \(direction) to \(newDirection)")
        self.direction = newDirection
    }

    func createNoise() {
        print("Chugga, Chugga... Chugga, Chugga... CHOO CHOO")
    }
}

class Car: VehicleControls {
    var speed: Float
    var direction: Direction
    var numPassengers: Int

    init() {
        self.speed = 0
        self.direction = .north
        self.numPassengers = 0
    }

    init(s: Float, d: Direction, nP: Int) {
        self.speed = s
        self.direction = d
        self.numPassengers = nP
    }

    func change(newSpeed: Float) {
        print("changing speed from \(speed), to \(newSpeed)")
        self.speed = newSpeed
    }

    func change(newDirection: Direction) {
        print("changing direction from \(direction) to \(newDirection)")
        self.direction = newDirection
    }

    func createNoise() {
        print("HONK HONK, BEEP BEEP")
    }
}

let newConductor = Conductor()

newConductor.controlVehicle()

newConductor.vehicle = Car(s: 60.56, d: .north, nP: 2)

newConductor.controlVehicle()

Answer 2

在 Playground 文件中编辑您的代码：

protocol MyProtocol {
    func doAnything()
}

class MainClass {

    var delegate: MyProtocol? = nil

    func callMeForGetCallBackWithDelegate() {
        // send call back inside any function after call or inside in init
        if let delegate = self.delegate {
            delegate.doAnything()
        }
    }
}


class OtherClass: MyProtocol {

    let anything = MainClass()

    init() {
        self.anything.delegate = self // set delegate in init or set inside any function
        self.anything.callMeForGetCallBackWithDelegate()
    }

    func doAnything() {
        print("text")
    }
}

let otherClass = OtherClass()