属性观察者willSet与didSet

在Swift中使用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。

willSet里面是新值，即属性即将要变为的值；didSet里面是旧值，即属性变化之前的值。

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
*/