UI: 使用 UIViewController 展现和管理视图

- (BOOL) application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{ 
self.window = [[UIWindow alloc]initWithFrame:[[UIScreen mainScreen] bounds]];
[self.window makeKeyAndVisible];
self.rootViewController = [[RootViewController alloc] initWithNibName:nil bundle:NULL];
[self.window addSubview:self.rootViewController.view];
return YES;
}