Playing around with System.Reactive in Silverlight

I’ve just been checking out some of the asynchronous programming stuff that will be shipped with .Net 4.0.  I managed to obtain a preview copy of the System.Reactive.dll by downloading the lates Silverlight Toolkit source.  This seems like very exciting stuff indeed.  Basically turning the whole .Net event infrastructure on its head.  This is based on the observation that we can think of the observer pattern as being the dual of the enumerable pattern.  The power of this is that we can use Linq to enumerate over events just as we do over objects in an enumerable.  Or, in other words, Linq of pushed rather than pulled data.

Check this out for more information:

http://themechanicalbride.blogspot.com/2009/07/introducing-rx-linq-to-events.html

My one criticism of this so far is:

Say you want to create an observer on the MouseMove event on a control.  You’d do something like this:

var mouseEventSource = Observable.FromEvent<MouseEventArgs>(this, “MouseMove”);
var mouseMoveSubscription = mouseEventSource.Subscribe(mea => Debug.WriteLine(“Mouse Moved”));

And this works perfectly.  The only problem… that pesky “MouseMove” parameter when converting the event into and IObservable.  I noticed that there’s an overload of the same method that takes Actions to add and remove the EventHandler.  This would look something like this:

var mouseEventSource = Observable.FromEvent<MouseEventArgs>(eh=>this.MouseMove += eh, eh=>this.MouseMove -= eh);

This looks much nicer as I’m not passing in that ‘orrible string and therefore have compile-time checking of the event.  Only problem with this is that it requires the MouseMove event handler to be a generic EventHandler<MouseEventArgs>.  Sadly, the MouseEventHandler is a delegate in its own right rather than deriving from the generic EventHandler.

This leads me to the following conclusion.  This looks like really interesting technology, but it’ll require a massive overhauling of the .Net base class libraries to be truely useful.

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