Reactive Extensions for .NET

Mark Allan

So what is Rx?

“Rx is a library for composing

asynchronous and event-based programs

using observable collections”

.NET 3.5+, Silverlight 3+, WP7

And Reactive Extensions for Javascript!

Pull collections

public interface IEnumerable<T>{

IEnumerator<T> GetEnumerator();}

public interface IEnumerator<T>{

bool MoveNext();T Current { get; }void Reset();

}

Push collections

public interface IObservable<T>{

IDisposable Subscribe(IObserver<T> observer);}

public interface IObserver<T>{

void OnNext(T value);void OnError(Exception exception);void OnCompleted();

}

A simple example

IObservable<int> source = Observable.GenerateWithTime(1,x => x < 100,x => x,x => TimeSpan.FromMilliseconds(x),x => x + 1);

using (source.Subscribe(x => Console.WriteLine("OnNext: {0}", x),ex => Console.WriteLine("OnError: {0}", ex),() => Console.WriteLine("OnCompleted")))

{Console.WriteLine("Press ENTER to unsubscribe...");Console.ReadLine();

}

LINQ to events

Filter an observable collection with Where,

Take etc just as with IEnumerable.

textBoxValues.Where(str => str.Length > 3).DistinctUntilChanged().TakeUntil(submitClicked).Subscribe(AutoComplete);

Working with .NET events

So how do we get “textBoxValues”?

IObservable<string> textBoxValues =Observable.FromEvent<EventArgs>(txt, "TextChanged").Select(evt => ((TextBox)evt.Sender).Text);

Asynchronous pattern

A very similar method is used to wrap

BeginXXX/EndXXX.

var request = WebRequest.Create("http://w3c.org");

IObservable<WebResponse> resp =Observable.FromAsyncPattern<WebResponse>(

request.BeginGetResponse,request.EndGetResponse)();

resp.Subscribe(r => Console.WriteLine(r.ContentType));

Concurrency and synchronisation

Want to run your OnNext code on the UI

thread?

events.ObserveOnDispatcher().Subscribe(methodToRunOnUI)

Want to spawn a new thread for each

event? Or use the thread pool?

events.SubscribeOn(Scheduler.NewThread).Subscribe(…)

events.SubscribeOn(Scheduler.ThreadPool).Subscribe(…)

Another example

To run multiple network requests, update the UI as they come in and stop the progress bar when they’re all complete.

requests.SubscribeOn(Scheduler.NewThread).ObserveOnDispatcher().Take(count).Finally(stopProgressBar).Subscribe(updateUI);

Finally, test it

Testing traditional event-driven code can

be a bit ugly.

Not so with Rx:

var testEvents = new[] { event1, event2, event3 }.ToObservable();

Questions?