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

复制代码
import Foundation

class 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 = 20

println(me.toString())

/*程序输出 Will set an new value 20 to age age filed changed form 0 to 20 Full Name: Big Zhang San,Age: 20 */