Swift 中willSet 和didSet

代碼以下： 

class People: NSObject {
    
    // 普通屬性
    var firstName : String = ""
    
    var lastName : String = ""
    
    var nickName : String = ""
    
    
    // 計算屬性
    var fullName : String  {
        
        get{
            
            return nickName + "" + firstName + "" + lastName
        }
        
    }
    
    // 帶屬性監視器的普通屬性
    var age: Int = 0 {
        
        // 咱們須要在get 屬性變化前作點什麼
        willSet {
            
            print("給age從新賦值\(newValue) to age")
            
        }
        
        // 咱們須要在age 屬性發生變化後，更新一下nickName 這個屬性
        didSet {
            
            print("age 從 \(oldValue) 改變成 \(age)")
            if age < 10 {
                
                nickName = "看我多小"
                
            }else{
                
                nickName = "看我多大"
            }
            
        }
        
    }
    
    func toSting() -> String {
        
        return "fullName:\(fullName) + Age:\(age)"
    }
    
}


var me = People()

me.firstName = "張"
me.lastName = "四"

me.age = 20

print(me.toSting())