- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
    self.window=[[UIWindow alloc]initWithFrame:[[UIScreen mainScreen]bounds] ];
    
    self.viewController=[[ViewController alloc] initWithNibName:@"ViewController" bundle:nil];
    
    
    // 创建导航控制器（并且设置 ViewController是第一个控制器）
    UINavigationController * nav=[[UINavigationController alloc] initWithRootViewController:self.viewController];
    self.window.rootViewController=nav;
    [self.window makeKeyAndVisible];
    return YES;
}
    

self.navigationItem.title=@"第一个";

   // 这一行代码会使顶层控制器左上角显示的内容。
    self.navigationItem.rightBarButtonItem =[[UIBarButtonItem alloc]
                                           initWithBarButtonSystemItem:UIBarButtonSystemItemCamera target:self action:@selector(clickSetting)];}

self.navigationItem.backBarButtonItem=[[UIBarButtonItem alloc]initWithTitle:@"我是一个界面" style:UIBarButtonItemStyleDone target:nil action:nil];

-(void)clickSetting
{
    ThirdViewController *third=[[ThirdViewController alloc]init ];
    [self.navigationController pushViewController:third animated:YES];
}