class Dog: NSObject {    var name:String?{//        属性监听器//        属性即将改变监听        willSet{//            print("111")//            print(name)            print(newValue)        }//        属性已经改变监听//        开发中常用这个监听属性改变        didSet{//            print("222")//            print(name)            print(oldValue)        }    }    }let d = Dog()d.name = "why"//d.name = "ee"print(d.name)