Swift 中willSet 和didSet

代码以下： 

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 {
        
        // 咱们须要在get 属性变化前作点什么
        willSet {
            
            print("给age从新赋值\(newValue) to age")
            
        }
        
        // 咱们须要在age 属性发生变化后，更新一下nickName 这个属性
        didSet {
            
            print("age 从 \(oldValue) 改变成 \(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 = "张"
me.lastName = "四"

me.age = 20

print(me.toSting())