Swift 5.x 关键字 mutating 的作用

1. 在实例方法中修改属性

• 结构体和枚举是值类型. 默认情况下, 值类型属性不能被自身的实例方法修改.

• 可以选择在func关键字前加上mutating关键字来指定实例方法内可以修改属性

  e.g.

    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: 2.0)
    print("The point is now at (\(somePoint.x), \(somePoint.y))")
  

  输出结果:

    The point is now at (3.0, 3.0)
  

  如果在方法前没有加mutating修饰, 那么在修改属性的位置就会报错: Left side of mutating operator isn't mutable: 'self' is immutable

2. 在mutating方法中赋值给self

• mutating 方法可以指定整个实例给隐含的self属性

  e.g.

    struct Point {
        var x = 0.0, y = 0.0
        mutating func moveBy(x deltaX: Double, y deLtaY: Double) {
            self = Point(x: 3.0, y: 3.0)
        }
    }
  

3. 枚举的mutating 方法

• 枚举的mutating方法可以设置隐含的self属性为相同枚举里的不同成员, 与结构体类似

  e.g.

    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.next()
  
    print(ovenLight)
  

  输出结果:

    off