Events vs. Yield

Question

I have a multithreaded application that spawns threads for several hardware instruments. Each thread is basically an infinite loop (for the lifetime of the application) that polls the hardware for new data, and activates an event (which passes the data) each time it collects something new. There is a single listener class that consolidates all these instruments, performs some calculations, and fires a new event with this calculation.

However, I'm wondering if, since there is a single listener, it would be better to expose an IEnumerable<> method off these instruments, and use a yield return to return the data, instead of firing events.

I'd like to see if anybody knows of differences in these two methods. In particular, I'm looking for the best reliability, best ability to pause/cancel operation, best for threading purposes, general safety, etc.

Also, with the second method is it possible to still run the IEnumerable loop on a separate thread? Many of these instruments are somewhat CPU-bound, so ensuring each one is on a different thread is vital.

Answer 1

This sounds like a very good use case for the Reactive Extensions. There's a little bit of a learning curve to it but in a nutshell, IObservable is the dual of IEnumerable. Where IEnumerable requires you to pull from it, IObservable pushes its values to the observer. Pretty much any time you need to block in your enumerator, it's a good sign you should reverse the pattern and use a push model. Events are one way to go but IObservable has much more flexibility since it's composable and thread-aware.

instrument.DataEvents
          .Where(x => x.SomeProperty == something)
          .BufferWithTime( TimeSpan.FromSeconds(1) )
          .Subscribe( x => DoSomethingWith(x) );

In the above example, DoSomethingWith(x) will be called whenever the subject (instrument) produces a DataEvent that has a matching SomeProperty and it buffers the events into batches of 1 second duration.

There's plenty more you could do such as merging in the events produced by other subjects or directing the notifications onto the UI thread, etc. Unfortunately documentation is currently pretty weak but there's some good information on Matthew Podwysocki's blog. (Although his posts almost exclusively mention Reactive Extensions for JavaScript, it's pretty much all applicable to Reactive Extensions for .NET as well.)