MPNowPlayingInfoCenter无法在锁定屏幕上播放

Question

我使用MPMoviePlayerController播放音频/视频，并希望在应用背景和锁定屏幕亮起时继续播放。我在viewDidLoad中设置了MPNowPlayingInfoCenter的nowPlayingInfo，并在UIApplicationWillResignActiveNotification 1通知中设置了以下内容：

- (void)appWillResignActive {
  NSMutableDictionary *nowPlayingInfo = [[NSMutableDictionary alloc] init];
  [nowPlayingInfo setObject:_session[@"session"][@"title"] forKey:MPMediaItemPropertyTitle];
  [nowPlayingInfo setObject:_session[@"session"][@"author"] forKey:MPMediaItemPropertyArtist];
  [nowPlayingInfo setObject:_session[@"session"][@"album"] forKey:MPMediaItemPropertyAlbumTitle];
  [nowPlayingInfo setObject:[NSNumber numberWithDouble:_videoController.playableDuration] forKey:MPMediaItemPropertyPlaybackDuration];
  [nowPlayingInfo setObject:@(1.0f) forKey:MPNowPlayingInfoPropertyPlaybackRate];
  [nowPlayingInfo setObject:[NSNumber numberWithDouble:_videoController.currentPlaybackTime] forKey:MPNowPlayingInfoPropertyElapsedPlaybackTime];

  if (_session[@"session"][@"album_art"] != nil) {
    UIImage *albumArtImage = [UIImage imageNamed:_session[@"session"][@"album_art"]];
    MPMediaItemArtwork *albumArt = [[MPMediaItemArtwork alloc] initWithImage:albumArtImage];
    [nowPlayingInfo setObject:albumArt forKey:MPMediaItemPropertyArtwork];
  }

  [[MPNowPlayingInfoCenter defaultCenter] setNowPlayingInfo:nowPlayingInfo];
}

锁定屏幕显示当前信息，但它根本没有播放。经过的时间似乎是正确的，但总的可玩时间是不正确的。按下播放也不会启动它。我不确定是什么错，有什么想法吗？