When I originally created marker support I had given this some thought and was just waiting for the opportunity to add this feature.  I am excited someone really wants it.  Back in July I created a post called Communicating between two Silverlight objects on a single page. In this post I demonstrate how one can use a [ScriptableMember] to expose a method to JavaScript. To add this feature to the Video Player I can use this technique.

The first step was to create a method in my mediaControls control, which would allow a time (TimeSpan) to be passed during playback.  This call will jump the slider to that position in time and begin or continue playback.

Once we have a method in mediaControl we can expose a scriptable method in Page.xaml. This method can be called from any page hosting out Silverlight XAP.

To demonstrate how to use this method I created a sample html page that presents a list of bookmarks that target the video player.  Each link calls a JavaScript method, that in turn calls my [ScritableMember]

In this first version I made a decision to display closed captioning when the volume is muted, but with the right visualization we should be able to find a nice place for a ToggleButton.  Here is a list of a few other things I did to the project.

• Updated Xml parsing to support markers with multiple types (Marker, CC, etc)
• Converted speaker button control to ToggleButton to display a different visualization when the player is muted.
• Linked volumeSlider to speaker button so slider drops to zero on mute and back to previous volume when mute is turned off.
• Added a new event from mediaControl that is fired when the volume is changed.  This allows the player to control closed captioning display and not the media controller.
• Wrote closedCaptioning UserControl that visualizes the scrolling captions.

The first step in implementing this functionality was to download the marker Xml.  In the event that this marker file was quite large in size I decided to make the download asynchronous.  Downloading a file asynchronously required that the path to the file is absolute.  Since I have no real control over my initParams I decided to support both an absolute or relative Uri.  In the event that the path to the Xml document is relative, I make a reasonable assumption that it is located in the same location as the XAP.  As a result ,I can define my Uri with “Host.Source” as the baseUri of the Xml document.  To Download the file, I simply make a WebRequest and define an AsyncCallback to be fired when the file has completed downloading. 

Threading Considerations

In Silverlight 2 Beta 2 there was a significant change in the concurrency model used for asynchronous communications.  In Beta 1 these type of requests returned on the UI thread.  In Beta 2, when you choose to use the BeginGetResponse of the WebRequest you are telling Silverlight to use a worker thread that comes from a thread pool.  As a result, you can NOT update any user interface elements on the UI thread.  Initially I thought I would be able to use “Application.Current.RootVisual.Dispatcher” as a way to execute a delegate on the UIThread.  Unfortunately this throws a Cross Thread exception.  After explored a few options I realized that I could simply call Dispatcher.BeginInvoke(MyMethod) from within my AsyncCallback.  here is what that looked like.

void OpenStreamCompleted(IAsyncResult ar)
{
    HttpWebRequest request = ar.AsyncState as HttpWebRequest;
    WebResponse response = request.EndGetResponse(ar);
 
    // Read the response into a Stream object.
    Stream responseStream = response.GetResponseStream();
 
    // Create a new StreamReader instance using the specified Stream object.
    using (StreamReader streamreader = new StreamReader(responseStream))
    {
        XElement document = XElement.Load(streamreader); 
        _externalMarkerData = (from el in document.Elements() select GetMarkerData(el)).ToList();
    }
    Dispatcher.BeginInvoke(ProcessParsedMarkerCollection);
}

As you can see above, once I have the Xml downloaded I parse it from Xml into a collection. For this I thought it would be cool to use Linq for XML.  I am always amazed at how few a lines of code I need to write to parse an Xml document with Linq.  The end product of the parse is a List<> containing a very simple custom object called “MarkerData”.  If I had chosen to move this parsing back to the UIThread I could have just created a collection of TimelineMarkers, but I thought there was no harm in having my own object to expand over time.

To get the latest source visit the project at http://www.codeplex.com/sl2videoplayer