willSet & didSet

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"

  }

  }

}