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 = {
//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<{
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 =
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
*/