C# MediaElement -- 为什么 Play() 在切换源后有时会静默失败？

Question

我在自定义 UserControl 中为我一直在制作的视频播放器控件设置了 MediaElement——播放/暂停按钮、滑块、剩余时间等。我将 ScrubbingEnabled 设置为 True这样我就可以根据 SO 帖子 here 向用户显示视频的第一帧，还可以使用 Slider 元素来允许用户清理视频。

问题：我使用绑定来切换视频播放器的来源。有时，如果我在播放视频时切换视频，MediaElement 会停止响应Play() 命令。即使在MediaFailed 事件中，也不会给出错误。每次调用Play()（或Pause() 然后Play()）都会失败。我可以在MediaElement失败后切换视频源，然后它会重新开始工作。

XAML：

<MediaElement LoadedBehavior="Manual" ScrubbingEnabled="True" 
              UnloadedBehavior="Stop"
              MediaOpened="VideoPlayer_MediaOpened" x:Name="VideoPlayer"/>

相关控制代码：

public static DependencyProperty VideoSourceProperty =
            DependencyProperty.Register("VideoSource", typeof(string), typeof(MediaElementVideoPlayer),
                new FrameworkPropertyMetadata(null,
                    FrameworkPropertyMetadataOptions.AffectsRender | FrameworkPropertyMetadataOptions.AffectsMeasure,
                    new PropertyChangedCallback(OnSourceChanged)));

private static void OnSourceChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    MediaElementVideoPlayer player = d as MediaElementVideoPlayer;
    if (player != null)
    {
        player.Dispatcher.Invoke(() => {
            if (player.VideoSource != null && player.VideoSource != "")
            {
                if (player._isPlaying)
                {
                    player.VideoPlayer.Stop();
                    var uriSource = new Uri(@"/ImageResources/vid-play.png", UriKind.Relative);
                    player.PlayPauseImage.Source = new BitmapImage(uriSource);
                    player._isPlaying = false;
                } 
                player.VideoPlayer.Source = new Uri(player.VideoSource);
            }
        });
    }
}

private void VideoPlayer_MediaOpened(object sender, RoutedEventArgs e)
{
    Dispatcher.Invoke(() =>
    {
        VideoPlayer.Pause();
        VideoPlayer.Position = TimeSpan.FromTicks(0);
        Player.IsMuted = false;
        TimeSlider.Minimum = 0;
        // Set the time slider values & time label
        if (VideoPlayer.NaturalDuration != null && VideoPlayer.NaturalDuration != Duration.Automatic)
        {
            TimeSlider.Maximum = VideoPlayer.NaturalDuration.TimeSpan.TotalSeconds;
            TimeSlider.Value = 0;
            double totalSeconds = VideoPlayer.NaturalDuration.TimeSpan.TotalSeconds;
            _durationString = Utilities.numberSecondsToString((int)totalSeconds, true, true);
            TimeLabel.Content = "- / " + _durationString;
        }
    });
}

如果我让视频一直自动播放，那么播放器会 100% 正常工作，这很奇怪。不幸的是，我需要在设置源时暂停视频。 有谁知道如何避免MediaElement 的随机隐藏故障，同时仍然交换视频、显示加载视频的第一帧等？

here 和 here 有非常相似的问题，但我的问题有不同的症状，因为我只使用 1 个MediaElement。

Answer 1

奇怪的是，如果将ScrubbingEnabled 设置为False，MediaElement 将不再随机停止响应。禁用ScrubbingEnabled 会中断显示第一帧并使Slider 元素无法正常工作，因此这里介绍了如何在不损坏MediaElement 的情况下保留这些功能。

显示第一帧：

private static void OnSourceChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    MediaElementVideoPlayer player = d as MediaElementVideoPlayer;
    if (player != null)
    {
        player.Dispatcher.Invoke(() => {
            if (player.VideoSource != null && player.VideoSource != "")
            {
                if (player._isPlaying)
                {
                    player.VideoPlayer.Stop();
                    var uriSource = new Uri(@"/ImageResources/vid-play.png", UriKind.Relative);
                    player.PlayPauseImage.Source = new BitmapImage(uriSource);
                    player._isPlaying = false;
                } 
                player.VideoPlayer.Source = new Uri(player.VideoSource);
                // Start the video playing so that it will show the first frame. 
                // We're going to pause it immediately so that it doesn't keep playing.
                player.VideoPlayer.IsMuted = true; // so sound won't be heard
                player.VideoPlayer.Play();
            }
        });
    }
}

private void VideoPlayer_MediaOpened(object sender, RoutedEventArgs e)
{
    Dispatcher.Invoke(() =>
    {
        // the video thumbnail is now showing!
        VideoPlayer.Pause();
        // reset video to initial position
        VideoPlayer.Position = TimeSpan.FromTicks(0);
        Player.IsMuted = false; // unmute video
        TimeSlider.Minimum = 0;
        // Set the time slider values & time label
        if (VideoPlayer.NaturalDuration != null && VideoPlayer.NaturalDuration != Duration.Automatic)
        {
            TimeSlider.Maximum = VideoPlayer.NaturalDuration.TimeSpan.TotalSeconds;
            TimeSlider.Value = 0;
            double totalSeconds = VideoPlayer.NaturalDuration.TimeSpan.TotalSeconds;
            _durationString = Utilities.numberSecondsToString((int)totalSeconds, true, true);
            TimeLabel.Content = "- / " + _durationString;
        }
    });
}

在拖动滑块时启用视频拖拽：

// ValueChanged for Slider
private void TimeSlider_ValueChanged(object sender, RoutedPropertyChangedEventArgs<double> e)
{
    Dispatcher.Invoke(() => {
        TimeSpan videoPosition = TimeSpan.FromSeconds(TimeSlider.Value);
        VideoPlayer.Position = videoPosition;
        TimeLabel.Content = Utilities.numberSecondsToString((int)VideoPlayer.Position.TotalSeconds, true, true) + " / " + _durationString;
    });
}

// DragStarted event
private void slider_DragStarted(object sender, System.Windows.Controls.Primitives.DragStartedEventArgs e)
{
    Dispatcher.Invoke(() => {
        VideoPlayer.ScrubbingEnabled = true;
        VideoPlayer.Pause();
    });
}

// DragCompleted event
private void slider_DragCompleted(object sender, System.Windows.Controls.Primitives.DragCompletedEventArgs e)
{
    Dispatcher.Invoke(() => {
        VideoPlayer.ScrubbingEnabled = false;
        TimeSpan videoPosition = TimeSpan.FromSeconds(TimeSlider.Value);
        VideoPlayer.Position = videoPosition;
        if (_isPlaying) // if was playing when drag started, resume playing
            VideoPlayer.Play();
        else
            VideoPlayer.Pause();
    });
}