Swift中的willSet与didSet

在Swift语言中用了willSet和didSet这两个特性来监视属性的除初始化之外的属性值变化

无需说太多，看看下面的代码你就能很快明白的

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*/

//监听类的属性改变class Person{    var name : String = ""{        //属性监听器：选中其中之一即可        //监听属性即将发生改变，还没有改变        willSet(newValue){            print(newValue)//"why"            print(name) //""            print("属性即将改变")        }        //监听属性已经发生改变，已经改变        didSet(oldValue){            print(oldValue) //""            print(name) //"why"            print("属性发生改变")        }    }}let p = Person()p.name = "why"