获得ViewController对象

iOS5中用storyboard，在AppDelegate中获得ViewController对象的方法记录：

UIStoryboard *mainStoryboard = [UIStoryboard storyboardWithName:@"MainStoryboard_iPhone" bundle: nil];
self.viewController = (ViewController* )[mainStoryboard instantiateViewControllerWithIdentifier: @"LoginViewController"];

然后在viewController中设置identifier就可以了。