Swift中的willSet与didSet

import Foundationclass People : NSObject{    //普通属性    var firstName:String = ""    var lastName:String  = ""    var nickName:String  = ""        //计算属性    var fullName:String    {        get        {            return nickName + " " + firstName + " " + lastName        }    }        //带属性监视器的普通属性    var age:Int = 0    {        //我们需要在age属性变化前做点什么        willSet        {            PRintln("Will set an new value /(newValue) to age")        }        //我们需要在age属性发生变化后，更新一下nickName这个属性        didSet        {            println("age filed changed form /(oldValue) to /(age)")            if age<10            {                nickName = "Little"            }else            {                nickName = "Big"            }        }    }        func toString() -> String    {        return "Full Name: /(fullName) " + ", Age: /(age) "    }    }var me = People()me.firstName = "Zhang"me.lastName  = "San"me.age = 20println(me.toString())/*程序输出Will set an new value 20 to ageage filed changed form 0 to 20Full Name: Big Zhang San , Age: 20*/