如何监听MediaPlayerElement的TimedTextSource变更事件？

Hey there! I’ve spent a fair amount of time working with MediaPlayerElement in UWP, so I can walk you through how to detect when a TimedTextSource changes—since there’s no out-of-the-box event for this, we’ll use a couple of reliable workarounds:

• Option 1: Create a wrapped property with a custom event
  If you’re setting the TimedTextSource directly in your code, you can wrap it in a property that triggers a custom event whenever the value updates. This gives you full control over when the change is detected:

  private TimedTextSource _activeTimedTextSource;
  public TimedTextSource ActiveTimedTextSource
  {
      get => _activeTimedTextSource;
      set
      {
          if (_activeTimedTextSource != value)
          {
              _activeTimedTextSource = value;
              // Raise your custom change event
              TimedTextSourceChanged?.Invoke(this, EventArgs.Empty);
          }
      }
  }
  
  // Define the custom event to subscribe to
  public event EventHandler TimedTextSourceChanged;
  

  Then, whenever you assign ActiveTimedTextSource = newTimedSource, you can listen to TimedTextSourceChanged to run your logic.

• Option 2: Monitor MediaSource’s ExternalTimedTextSources collection
  If your TimedTextSource is added to a MediaSource via ExternalTimedTextSources, this collection is an ObservableCollection—so you can listen to its CollectionChanged event to detect when sources are added or removed:

  // Assume you have a MediaSource set up for your media
  var mediaSource = MediaSource.CreateFromUri(new Uri("your-media-file-path-or-url"));
  
  // Subscribe to the collection change event
  mediaSource.ExternalTimedTextSources.CollectionChanged += (sender, e) =>
  {
      switch (e.Action)
      {
          case NotifyCollectionChangedAction.Add:
              foreach (var item in e.NewItems)
              {
                  if (item is TimedTextSource newSource)
                  {
                      // Handle new TimedTextSource being added
                  }
              }
              break;
          case NotifyCollectionChangedAction.Remove:
              // Handle TimedTextSource being removed
              break;
      }
  };
  
  // Assign the MediaSource to your MediaPlayerElement
  mediaPlayerElement.Source = MediaPlaybackItem.CreateFromMediaSource(mediaSource);
  

• Bonus: Track subtitle track changes
  If you need to react when the user switches between subtitle tracks (which are part of a TimedTextSource), you can listen to the MediaPlayer.TimedMetadataTracksChanged event:

  mediaPlayerElement.MediaPlayer.TimedMetadataTracksChanged += (sender, e) =>
  {
      // Check which track is now active and respond accordingly
      var activeTrack = mediaPlayerElement.MediaPlayer.TimedMetadataTracks.FirstOrDefault(t => t.IsActive);
      if (activeTrack != null)
      {
          // Do something with the active subtitle track
      }
  };
  

The reason there’s no built-in TimedTextSourceChanged event is that the framework expects developers to handle source assignment logic directly, or use the MediaSource collection events for dynamic updates. These workarounds cover most use cases you’ll run into!

内容的提问来源于stack exchange，提问作者Vincent