iOS代码初始化window的rootViewController

现在iOS新建项目工程时，window 默认情况下是以 Main.storyboard 中 initial view controller, 在项目中，如果需要通过代码控制 window 的 rootViewController ， 可以通过如下方法:

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
    self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];
    UIStoryboard *storyboard = [UIStoryboard storyboardWithName:@"Main" bundle:nil];
    RootViewController *rootViewController = [storyboard instantiateViewControllerWithIdentifier:@"RootViewController"];
    self.window.rootViewController = rootViewController;
    [self.window makeKeyAndVisible];
    return YES;
}