Xamarin AVFoundation，为视频添加字幕

Question

我正在尝试为视频添加字幕。但是没有视频不能播放。这是我的代码；

AVPlayer avp;
        AVPlayerViewController avpvc;
        AVMutableComposition videoPlusSubtitles;

        var subtitleEn = "SOME_VTT";
        var videoURL = "SOME_MP4_URL";
        var url = NSUrl.FromString(videoURL);

        var nsVideoUrl = new NSUrl(videoURL);
        var localVideoAsset = new AVUrlAsset(nsVideoUrl);
        videoPlusSubtitles = new AVMutableComposition();
        var videoTrack = videoPlusSubtitles.AddMutableTrack(AVMediaType.Video, 0);

        NSError error = new NSError();
        videoTrack.InsertTimeRange(CoreMedia.CMTimeRange.Zero, videoPlusSubtitles.TracksWithMediaType(AVMediaType.Video)[0], localVideoAsset.Duration, out error);

        var subtitle1 = new NSUrl(subtitleEn);
        var subtitleAsset = new AVUrlAsset(subtitle1);

        var subttileTrack = videoPlusSubtitles.AddMutableTrack(AVMediaType.Text, 0);
        subttileTrack.InsertTimeRange(CoreMedia.CMTimeRange.Zero, videoPlusSubtitles.TracksWithMediaType(AVMediaType.Text)[0], subtitleAsset.Duration, out error);

        var playerItems = new AVPlayerItem(videoPlusSubtitles);

        avpvc = new AVPlayerViewController();
        avp = new AVPlayer(playerItems);
        avpvc.Player = avp;
        AddChildViewController(avpvc);
        View.AddSubview(avpvc.View);
        avpvc.View.Frame = View.Frame;
        avpvc.ShowsPlaybackControls = true;


        avp.Play();

当我输入 avp = new AVPlayer(url);而不是 avp = new AVPlayer(playerItems);一切正常，所以视频没有问题。有什么想法吗？