swift - set/get/willSet/didSet

swift的类似set/get方法的例子：

//public公开的，private私有的，只有在本类中访问，internal本类和子类中访问

  private var _backGroundColor:String?
    var backGroundColor:String?{
        get{
            return _backGroundColor
        }
        set{
            _backGroundColor = newValue
        }
    }
    
    private var _alpha:Float?
    var alpha:Float?{
        get{
            return _alpha
        }
        set{
            _alpha = newValue
        }
    }

注意willSet方法里的newValue属性和didSet里面的oldValue属性。

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
*/