1.找window *****UIWindow *window = [UIApplication sharedApplication].delegate.window;
2.找storyboard *****UIStoryboard *storyBord = [UIStoryboard storyboardWithName:@"Main" bundle:nil];
3.貌似用于只有一个vc？或者找初始vc *****UIViewController *VC = storyBord.instantiateInitialViewController;
4.通过识别符，在storybord中找vc *****UIViewController *vc2 = [storyBord instantiateViewControllerWithIdentifier:@"xxx"];
window.rootViewController = VC;
