屬性觀察器可讓我們在使用屬性時(shí)候，在希望一個(gè)屬性發(fā)生改變時(shí)候進(jìn)行某些操作。也就是一些小的代碼塊，可在一個(gè)屬性值即可發(fā)生改變之前 willSet 或者之后 didSet 運(yùn)行。要?jiǎng)?chuàng)建一個(gè)屬性觀察器，可在屬性后面添加大括號（類似對計(jì)算屬性的做法）

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-比如更換UI主題模式

//屬性觀察器 Property Observer
//注意：didSet 和 willSet 不會再初始化階段init時(shí)候調(diào)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