方法（学习笔记）

环境Xcode 11.0 beta4 swift 5.1

• 方法
  • 结构体、枚举、类都可以定义方法（实例方法、类型方法）
• 实例方法（Instance Methods）

  • 实例方法只能用实例来调用，不能单独使用

    class Counter {
        var count = 0
        func increment() {
            count += 1
        }
        func increment(by amount: Int) {
            count += amount
        }
        func reset() {
            count = 0
        }
    }
    let counter = Counter()
    // the initial counter value is 0
    counter.increment()
    // the counter's value is now 1
    counter.increment(by: 5)
    // the counter's value is now 6
    counter.reset()
    // the counter's value is now 0

  • self 属性，每个类型的实例都有的隐式的属性

    // 上面的increment()可写成如下形式
    func increment() {
        self.count += 1
    }
    // 如果方法的参数与实例相同时，就要显示的调用 self
    struct Point {
        var x = 0.0, y = 0.0
        func isToTheRightOf(x: Double) -> Bool {
            return self.x > x
        }
    }
    let somePoint = Point(x: 4.0, y: 5.0)
    if somePoint.isToTheRightOf(x: 1.0) {
        print("This point is to the right of the line where x == 1.0")
    }
    // Prints "This point is to the right of the line where x == 1.0"

  • 实例方法内修改值类型用 mutating 关键字修饰，该方法分配一个新的实例给隐藏的 self ,方法结束这个新的实例将会替换掉已经存在的实例

    // 枚举、结构体都是值类型
    struct Point {
        var x = 0.0, y = 0.0
        mutating func moveBy(x deltaX: Double, y deltaY: Double) {
            x += deltaX
            y += deltaY
        }
    }
    var somePoint = Point(x: 1.0, y: 1.0)
    somePoint.moveBy(x: 2.0, y: 3.0)
    print("The point is now at (\(somePoint.x), \(somePoint.y))")
    // Prints "The point is now at (3.0, 4.0)"
    // 不能用常量来调用 `mutating` 修饰的方法，因为常量不可修改
    let fixedPoint = Point(x: 3.0, y: 3.0)
    fixedPoint.moveBy(x: 2.0, y: 3.0)
    // this will report an error

  • 可变方法内给 self 赋值（Assigning to self Within a Mutating Method）

    // 上面的Point可以写成下面的形式
    struct Point {
        var x = 0.0, y = 0.0
        mutating func moveBy(x deltaX: Double, y deltaY: Double) {
            self = Point(x: x + deltaX, y: y + deltaY)
        }
    }
    // 枚举的可变方法
    enum TriStateSwitch {
        case off, low, high
        mutating func next() {
            switch self {
            case .off:
                self = .low
            case .low:
                self = .high
            case .high:
                self = .off
            }
        }
    }
    var ovenLight = TriStateSwitch.low
    ovenLight.next()
    // ovenLight is now equal to .high
    ovenLight.next()
    // ovenLight is now equal to .off

• 类型方法

  • 在 func 前用关键字 static 或者 class (只能在类中使用，且子类可以重写父类的实现)修饰，类型方法中 self 指向类型本身

    class SomeClass {
        class func someTypeMethod() {
            // type method implementation goes here
        }
    }
    SomeClass.someTypeMethod()

    struct LevelTracker {
        static var highestUnlockedLevel = 1
        var currentLevel = 1
        //
        static func unlock(_ level: Int) {
            if level > highestUnlockedLevel { highestUnlockedLevel = level }
        }
        // 
        static func isUnlocked(_ level: Int) -> Bool {
            return level <= highestUnlockedLevel
        }
        // 
        @discardableResult
        mutating func advance(to level: Int) -> Bool {
            if LevelTracker.isUnlocked(level) {
                currentLevel = level
                return true
            } else {
                return false
            }
        }
    }
    class Player {
        var tracker = LevelTracker()
        let playerName: String
        func complete(level: Int) {
            LevelTracker.unlock(level + 1)
            tracker.advance(to: level + 1)
        }
        init(name: String) {
            playerName = name
        }
    }
    var player = Player(name: "Argyrios")
    player.complete(level: 1)
    print("highest unlocked level is now \(LevelTracker.highestUnlockedLevel)")
    // Prints "highest unlocked level is now 2"
    // 创建一个新的player
    player = Player(name: "Beto")
    if player.tracker.advance(to: 6) {
        print("player is now on level 6")
    } else {
        print("level 6 has not yet been unlocked")
    }
    // Prints "level 6 has not yet been unlocked"

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