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 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
*/