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:

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- (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;
}