C# MediaElement -- 为什么 Play() 有时会在切换源后无提示地失败？

Question

我在自定义用户控件中设置了一个 MediaElement，用于我一直在制作的视频播放器控件——播放/暂停按钮、 slider 、剩余时间等。我有 ScrubbingEnabled 设置为 True 以便我可以根据 SO 帖子 here 向用户显示视频的第一帧，并使用 Slider 元素允许用户滑动视频。

问题:我使用绑定(bind)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。没有给出错误，即使在 MediaFailed 事件中也是如此。调用 Play()(或 Pause() 然后 Play())每次都失败。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，播放器会在 100% 的时间内正常工作，这很奇怪。不幸的是，我需要在设置源时暂停视频。 有谁知道如何在交换视频、显示加载视频的第一帧等时避免 MediaElement 的随机、隐藏故障？

有奇怪的相似问题here和 here ，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 元素无法很好地运行，因此这里介绍了如何在没有，呃，损坏的 的情况下保留这些功能媒体元素。

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 slider 时启用视频擦洗:

// 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();
    });
}