简单例子：

class TestClass{
var totalNumber:Int = 0{
      willSet{
          print(newValue)//newValue(自带属性)是新传进来的值，这时totalNumber还没有变成刚传进来的值
        }
      didSet {
         print(oldValue)//oldValue（自带属性），这时totalNumber已经改变，变为传进来的值
     }

}

为了保存右边中间那个价格数自己还想了很久怎么写程序，因为那个文本是一个字符串，它夹带有一个币种符号，刚开始我是这样保存的,后来发现每次刷新就要截取出symbol这个不是数字的字符，好麻烦

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

后来上司教我使用属性观察设计模式来对currentPrice 进行观察的值

var currentPrice: Double = 0 {               //当前价格
        willSet {
            var marketPriceTrend: MarketPriceTrend
            let newPrice = newValue
            let trend = newPrice - currentPrice
            self.trendImage.hidden = false
            if trend > 0 {
                //走势向上
                marketPriceTrend = MarketPriceTrend.Up
            } else if trend < 0 {
                //走势向下
                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())"
          //加载图片升降，这里用的是枚举返回UIImage
            self.trendImage.image = marketPriceTrend.imageTrend
          //  设置字体颜色，也是用枚举
            self.labelCurrentPrice.textColor = marketPriceTrend.trendColor
        }

marketPriceTrend枚举：

/// 市场价格趋势
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)
        }
    }
    

观察模式很好用