Swift5.1学习随笔之mutating

结构体和枚举是值类型，默认情况下，值类型的属性不能被自身的实例方法修改
在func关键字前面加mutating可以允许这种修改行为

struct Point {
    var x = 0.0, y = 0.0
    func moveBy(deltaX: Double, deltaY: Double) {
        x += deltaX // Left side of mutating operator isn't mutable: 'self' is immutable
        y += deltaY // Left side of mutating operator isn't mutable: 'self' is immutable
    }
}

struct Point {
    var x = 0.0, y = 0.0
    mutating func moveBy(deltaX: Double, deltaY: Double) {
        x += deltaX 
        y += deltaY 
    }
}

enum StateSwitch {
    case low, middle, high
    func next() {
        switch self {
        case .low:
            self = .middle // Cannot assign to value: 'self' is immutable
        case .middle:
            self = .high // Cannot assign to value: 'self' is immutable
        case .high:
            self = .low // Cannot assign to value: 'self' is immutable
        }
     }
}

enum StateSwitch {
    case low, middle, high
    mutating func next() {
        switch self {
        case .low:
            self = .middle 
        case .middle:
            self = .high
        case .high:
            self = .low
        }
     }
}