属性观察器可让我们在使用属性时候，在希望一个属性发生改变时候进行某些操作。也就是一些小的代码块，可在一个属性值即可发生改变之前 willSet 或者之后 didSet 运行。要创建一个属性观察器，可在属性后面添加大括号（类似对计算属性的做法）

class PropertyObserverExmple {
    var number: Int = 0 {
        willSet(newNumber) {
            print("About to change to \(newNumber)")
        }
        didSet(oldNumber) {
            print("Just changed from \(oldNumber) to \(self.number)")
        }
    }
}

var observer = PropertyObserverExmple()
observer.number = 4
// About to change to 4 ,Just changed from 0 to 4

使用例子-比如更换UI主题模式

//属性观察器 Property Observer
//注意：didSet 和 willSet 不会再初始化阶段init时候调用
enum Theme {
    case DayMode
    case NightMode
}

class UI{
    
    private var fontColor: UIColor!
    private var backgroundColor: UIColor!
    var themeMode: Theme = .DayMode {
        didSet {
            self.changeTheme(themeMode)
        }
    }
    
    init(themeMode: Theme) {
        self.themeMode = themeMode
        self.changeTheme(themeMode)
    }
    
    private func changeTheme(themeMode: Theme) {
        switch themeMode {
        case .DayMode:
            fontColor = UIColor.blackColor()
            backgroundColor = UIColor.whiteColor()
        case .NightMode:
            fontColor = UIColor.whiteColor()
            backgroundColor = UIColor.blackColor()
        }
    }
}

themeMode