Objective c MPMoviePlayerController启动时出现问题

我对MPMoviePlayerController有一个问题，因为当我全屏启动它时，它是存在的，但歌曲仍在播放。我添加了MPMoviePlayerDidExitFullscreenNotification通知，它说当视频开始播放时，它会出现全屏。这是我的密码：

_multimediaPlayer = [[MPMoviePlayerController alloc] init];
_multimediaPlayer.controlStyle = MPMovieControlStyleDefault;
_multimediaPlayer.initialPlaybackTime = 0;

    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(videoPlayBackDidFinish:) 
                                                 name:MPMoviePlayerPlaybackDidFinishNotification 
                                               object:nil];

    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(videoPlayerDidExitFullscreen:) 
                                                 name:MPMoviePlayerDidExitFullscreenNotification
                                               object:nil];

    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(videoPlayerPlaybackStateChanged:) 
                                                 name:MPMoviePlayerPlaybackStateDidChangeNotification
                                               object:nil];

    NSString *path = [NSString stringWithFormat:@"%@/mag_%d/%@", [FMUtils documentsFolderPathWithFile:nil], _magID, _pageObject.fileName];
    if ([FMUtils fileExistsAtPath:path]) {
        _multimediaPlayer.contentURL = [NSURL fileURLWithPath:path];
    } else {
        _multimediaPlayer.contentURL = [NSURL URLWithString:[NSString stringWithFormat:@"%@%@", self.dataURL, _pageObject.fileName]];
    }

    CGSize objectViewSize = self.frame.size;
    _multimediaPlayer.view.frame = CGRectMake(0, 0, objectViewSize.width, objectViewSize.height);
    [self addSubview:_multimediaPlayer.view];

    if (_pageObject.blink)  {
        [_multimediaPlayer setFullscreen:YES animated:YES];
    }

    [_multimediaPlayer play];

大多数情况下，它发生在视频未下载且需要流式传输时

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你知道为什么会发生这种情况吗？

问题解决了，而我没有听MPMoviePlayerPlayerBackstateDidChangeNotification