presentMoviePlayerViewController动画全屏通知

Question

要显示视频，请使用此功能：

[self presentMoviePlayerViewControllerAnimated:videoPlayerViewController];

并自动输入全屏，但我无法使用此处理通知：

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(moviePlayerWillEnterFullscreenNotification:) name:MPMoviePlayerWillEnterFullscreenNotification object:nil];

未调用方法moviePlayerWillEnterFullscreenNotification。如何检测视频何时开始全屏播放？