@interface AppDelegate()  
      
    @property UILabel* show;  
    @end  
      
    @implementation AppDelegate  
      
    - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions  
    {  
        self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];  
        // Override point for customization after application launch.  
        self.window.backgroundColor = [UIColor whiteColor];  
          
        //创建一个UIViewController 对象  
        UIViewController* controller = [[UIViewController alloc] init];  
        //让该程序的窗口加载并显示 viewController 视图控制器关联的用户界面  
        self.window.rootViewController = controller;  
        //创建一个UIView 对象  
        UIView* rootView = [[UIView alloc] initWithFrame:[[UIScreen mainScreen] bounds]];  
        //设置 controller 显示 rootView 控件  
        controller.view = rootView;  
        //创建一个圆角按钮  
        UIButton* button = [UIButton buttonWithType:UIButtonTypeRoundedRect];  
        //设置按钮的大小  
        button.frame = CGRectMake(120, 100, 80, 40);  
        //为按钮设置文本  
        [button setTitle:@"确定" forState:UIControlStateNormal];  
        //将按钮添加到 rootView 控件中  
        [rootView addSubview:button];  
        //创建一个 UILabel 对象  
        self.show = [[UILabel alloc] initWithFrame:CGRectMake(60, 40, 180, 30)];  
    //    UILabel* show = [[UILabel alloc] initWithFrame:CGRectMake(60, 40, 180, 30)];  
        //将UILabel 添加到 rootView 控件中  
        [rootView addSubview: self.show];  
        //设置 UILabel 默认显示的文本  
        self.show.text = @"初始文本";  
        self.show.backgroundColor = [UIColor grayColor];  
        //为圆角按钮的触碰事件绑定事件处理方法  
        [button addTarget:self action:@selector(clickHandler:) forControlEvents:UIControlEventTouchUpInside];  
          
        //将该 UIWindow 对象设为主窗口并显示出来  
        [self.window makeKeyAndVisible];  
        return YES;  
    }  

    - (void)clickHandler:(id)sender {  
        self.show.text = @"开始学习 IOS 吧！";  
    }