Swift-----属性观察器（didSet、willSet）

应用场景一：保证对属性所赋动值是合法的，可以对不合法的数据进行处理。

class LightBulb {
    
    static let maxCurrent = 30
    var current: Int = 0 {
        
        // 如果不设置newCurrent，默认可以使用newValue
        willSet(newCurrent) {
            print("Current value changed, the change is \(abs(newCurrent - current)).")
        }
    
        // 如果不定义oldCurrent，默认可以使用oldValue
        didSet(oldCurrent) {
            if current == LightBulb.maxCurrent {
                print("Pay attention, the current value get to the maxinum point.")
            } else if current > LightBulb.maxCurrent {
                print("Current too high, failing back to previous setting.")
                current = oldCurrent
            }
            print("The current is \(current).")
        }
    } 
}

let bulb = LightBulb()
bulb.current = 20
bulb.current = 30
bulb.current = 40

应用场景二：一个属性的值发生改变，其他会属性的值会跟着改变。

enum Theme {
　　case DayMode
　　case NightMode
}

class UI {
    
    var fontColor: UIColor!
    var backColor: UIColor!
    var theme: Theme {
        
        didSet {
            switch (theme) {
            case .DayMode:
                fontColor = UIColor.black
                backColor = UIColor.white
            case .NightMode:
                fontColor = UIColor.white
                backColor = UIColor.black
            }
        }
    }
    
    init(theme: Theme) {
        self.theme = theme
    }    
}

let ui = UI(theme: .DayMode)
ui.fontColor   // nil
ui.backColor   // nil

　　执行完上面的代码会发现，fontColor和backColor都为nil。这是因为，在初始化函数init中不会调用didSet、willSet方法。故可以将上面的代码改为下面的代码。

class UI {
    
    var fontColor: UIColor!
    var backColor: UIColor!
    var theme: Theme {        
        didSet {
            self.changeMode(theme: theme)
        }
    }
    
    init(theme: Theme) {
        self.theme = theme
        self.changeMode(theme: theme)
    }
    
    func changeMode(theme: Theme) {
        switch (theme) {
        case .DayMode:
            fontColor = UIColor.black
            backColor = UIColor.white
        case .NightMode:
            fontColor = UIColor.white
            backColor = UIColor.black
        }
    }    
}

let ui = UI(theme: .DayMode)
ui.fontColor
ui.backColor