swift的willSet和didSet

var age: Int = 0{
        willSet{
            print("Will set an new value \(newValue) for \(age)")
        }
        didSet{
            print("age filed changed form \(oldValue) to \(age)")
        }
    }

当输入 age = 10 时
willSet： Will set an new value 10 for 0
didSet： age filed changed form 0 to 10