Ios7 电影播放器-播放结束后出现黑屏

我遇到的问题是，当我按下完成按钮或电影完成播放时，会出现一个黑屏。我想回到我放置UIImage的开始屏幕

-(void)playMovie:(id)sender
{

    NSString * str=[[NSBundle mainBundle]pathForResource:@"yo2" ofType:@"mov"];
    NSURL * url=[NSURL fileURLWithPath:str];
    MPMoviePlayerController * movieController=[[MPMoviePlayerController alloc]initWithContentURL:url];
    movieController.controlStyle=MPMovieControlStyleFullscreen;
    [movieController.view setFrame:self.view.bounds];
    [self.view addSubview:movieController.view];
    [movieController prepareToPlay];
    [movieController play];
    _moviePlayer =  [[MPMoviePlayerController alloc]
                     initWithContentURL:url];


    _moviePlayer.controlStyle = MPMovieControlStyleDefault;

    _moviePlayer.shouldAutoplay = YES;
    [self.view addSubview:_moviePlayer.view];
    [_moviePlayer setFullscreen:YES animated:YES];

    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(moviePlayBackDidFinish:)
                                                 name:MPMoviePlayerPlaybackDidFinishNotification
                                               object:_moviePlayer];


}



- (void) moviePlayBackDidFinish:(NSNotification*)notification {

    // Remove observer
    [[NSNotificationCenter defaultCenter] removeObserver:self
                                                    name:MPMoviePlayerPlaybackDidFinishNotification
                                                  object:nil];
    [self dismissViewControllerAnimated:YES
                             completion:^{
                                 [self performSegueWithIdentifier:@"back" sender:self];
                             }];

}

@end

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这听起来好像当用户按下“完成”按钮时，通知处理程序根本没有被调用，对吗？是的，类似这样。当我按下“完成”按钮时，电影就消失了。黑屏上显示的是。有什么想法吗？