Silverlight Event Tokens

Events are easy to add and remove when using the CLR and C#. Just sprinkle some += and -= in the right places and the runtime does the rest. Silverlight programming with JavaScript is a bit different, however, and although the addEventListener and removeEventListener appear similar to a DOM element's event registration APIs, these two methods have a twist.

As an example, let's say we wanted to listen to the MediaElement's BufferingProgressChanged event, but we want to stop listening once buffering has exceeded 50%. The following code does work:

.. but this code will never unsubscribe from the event:

When not using a quoted string to specify the event handler, you have to save the value returned from addEventListener. The return value is a token that is "just an integer value to track the order in which handlers were added for each object-event combination" (read MSDN for other subtleties in the event registration methods).

A proper unsubscribe looks like the following: