簡單例子：

class TestClass{
var totalNumber:Int = 0{
      willSet{
          print(newValue)//newValue(自帶屬性)是新傳進(jìn)來的值竖般，這時(shí)totalNumber還沒有變成剛傳進(jìn)來的值
        }
      didSet {
         print(oldValue)//oldValue（自帶屬性）艰亮，這時(shí)totalNumber已經(jīng)改變迄埃，變?yōu)閭鬟M(jìn)來的值
     }

}

為了保存右邊中間那個(gè)價(jià)格數(shù)自己還想了很久怎么寫程序，因?yàn)槟莻€(gè)文本是一個(gè)字符串流译，它夾帶有一個(gè)幣種符號(hào)叠赦，剛開始我是這樣保存的,后來發(fā)現(xiàn)每次刷新就要截取出symbol這個(gè)不是數(shù)字的字符除秀，好麻煩

self.oldCurrentPrice = self.labelCurrentPrice.text!
self.labelCurrentPrice.text = "\(symbol)\(curentPrice)"

后來上司教我使用屬性觀察設(shè)計(jì)模式來對currentPrice 進(jìn)行觀察的值

var currentPrice: Double = 0 {               //當(dāng)前價(jià)格
        willSet {
            var marketPriceTrend: MarketPriceTrend
            let newPrice = newValue
            let trend = newPrice - currentPrice
            self.trendImage.hidden = false
            if trend > 0 {
                //走勢向上
                marketPriceTrend = MarketPriceTrend.Up
            } else if trend < 0 {
                //走勢向下
                marketPriceTrend = MarketPriceTrend.Down
            } else {
                //相等
                self.trendImage.hidden = true
                marketPriceTrend = MarketPriceTrend.Equal
            }
            
            let currentCurrencyType = self.selectedExchangeType.currencyType
            let currencyData = CurrencyData.getCurrencyData(currentCurrencyType)!
            let symbol = currencyData.symbol
            self.labelCurrentPrice.text = "\(symbol)\(newValue.toString())"
          //加載圖片升降，這里用的是枚舉返回UIImage
            self.trendImage.image = marketPriceTrend.imageTrend
          //  設(shè)置字體顏色棍好，也是用枚舉
            self.labelCurrentPrice.textColor = marketPriceTrend.trendColor
        }

marketPriceTrend枚舉：

/// 市場價(jià)格趨勢
enum MarketPriceTrend {
    case Up, Down, Equal
    
    var imageTrend: UIImage {
        switch self {
        case Up:
            return UIImage(named: "btn_market_BZSZ_arrow2")!
        case Down:
            return UIImage(named: "btn_market_BZSZ_arrow1")!
        case Equal:
            return UIImage(named: "btn_market_BZSZ_arrow2")!
        }
    }
    
    var trendColor: UIColor {
        switch self {
        case Up:
            return UIColor(hex: 0xFC461E)
        case Down:
            return UIColor(hex: 0x149073)
        case Equal:
            return UIColor(hex: 0xFC461E)
        }
    }
    

觀察模式很好用