Sergiy Grytsenko, Senior Software Engineer “Reactive Extensions: classic Observer in .NET” • Why should we use Rx when we have events? • Key types & methods • Lifetime management, flow control • Combining several streams • Tests, I need unit tests!

1. Reactive Extensions
classic observer in .NET
presenter: Sergiy Grytsenko

2. About presenter
 Senior Developer @ EPAM Systems
 17 years of programing with Basic, Pascal, Assembler, C/C++, C#
 12 years of professional development
 Developed several projects using Rx

3. Target auditory
 UI developers that deals with asynchronous code/want responsive UI
 Back-end developers that wish to make server async and responsive
 You just curious about the IObservable<T> and IObserver<T>

4. Agenda
 Why use Rx when we have events?
 Key types & methods
 Lifetime management & flow control
 Combining several streams
 Scheduling and Threading
 Tests, I need unit tests!

5. Why use Rx when we have events?
“To be or not to be” ©

6. Why Rx?
 You demand fire-and-forget messaging
 You want to have the result pushed to you when it is ready
 You do not want wait for entire set to be processed before you see first one
 Developers have tools to push data
 Developers need tools to react to push data
 Rx enables developers to solve problems in an elegant, familiar and
declarative style with less code
 It is easily to manage events and subscriptions

7. Why Rx?
 Integrated
 Unitive
 Extensible
 Declarative
 Composable
 Transformative

8. Why Rx?
Useful for
 UI events
 Domain events
 Infrastructure events
 Integration events

9. Key types & methods
The key to the door that you want to open

10. Key types & methods
 IObservable<T>
 IObserver<T>
 Subjects
 ObservableExtensions
 Observable
 Disposables

11. Key types & methods
Disposables
 BooleanDisposable
 CancellationDisposable
 CompositeDisposable
 ContextDisposable
 Disposable
 MultipleAssignmentDisposable
 RefCountDisposable
 ScheduledDisposable
 SerialDisposable
 SingleAssignmentDisposable

12. Key types & methods
Subscription overrides
public static class ObservableExtensions
{
public static IDisposable Subscribe<T>(this IObservable<T> source);
public static IDisposable Subscribe<T>(this IObservable<T> source, Action<T> onNext);
public static IDisposable Subscribe<T>(this IObservable<T> source, Action<T> onNext, Action<Exception> onError);
public static IDisposable Subscribe<T>(this IObservable<T> source, Action<T> onNext, Action onCompleted);
public static IDisposable Subscribe<T>(this IObservable<T> source, Action<T> onNext, Action<Exception> onError,
Action onCompleted);
public static void Subscribe<T>(this IObservable<T> source, IObserver<T> observer, CancellationToken token);
public static void Subscribe<T>(this IObservable<T> source, CancellationToken token);
public static void Subscribe<T>(this IObservable<T> source, Action<T> onNext, CancellationToken token);
public static void Subscribe<T>(this IObservable<T> source, Action<T> onNext, Action<Exception> onError,
CancellationToken token);
public static void Subscribe<T>(this IObservable<T> source, Action<T> onNext, Action onCompleted, CancellationToken token);
public static void Subscribe<T>(this IObservable<T> source, Action<T> onNext, Action<Exception> onError,
Action onCompleted, CancellationToken token);
public static IDisposable SubscribeSafe<T>(this IObservable<T> source, IObserver<T> observer);
}

13. Key types & methods
Creating observables
 Observable.Empty<T>()
 Observable.Return<T>(T)
 Observable.Never<T>()
 Observable.Throw<T>(Exception)
 Observable.Create<T>(Func<IObserver<T>, IDisposable>)
 IObservable<int> Observable.Range(int, int)
 IObservalbe<long> Observable.Interval(TimeSpan);
 Observable.Start<T/Unit>(Func<T>/Action)
 ToObservable<T>(IEnumerable<T>)
 Observalble.Generate(initialState, condition, iterate, resultSelector)

14. Key types & methods
Subjects
 Subject.Create(observer, observable)
 Subject<T>
 AsyncSubject<T>
 BehaviorSubject<T>
 ReplaySubject<T>

15. Lifetime management & flow
control
Build your own pipe & valve system

16. Lifetime management & flow control
Subscription lifetime
 There is many Subscribe overloads but no Unsubscribe method
 Rx returns IDisposable whenever subscription takes place
 Or you can provide CancellationToken to unsubscribe
 Think of IDisposable as subscription token
 You can call Subscribe many times on single IObservable

17. Lifetime management & flow control
Observable lifetime
 Both OnError & OnCompleted signify the completion of a stream
 No futher calls to OnNext can be performed
 When stream completes or errors, you should still dispose subscription

18. Lifetime management & flow control
Flow control
 OnError publishes exception, not thows
 Subscribe w/o OnError handler causes exception to throw

19. Lifetime management & flow control
Visualization
 Stream that publishes 3 values and then completes
 Stream that publishes 4 values and then errors

20. Lifetime management & flow control
Constructs
 Retry<T>(this IObservable<T> source)
 OnErrorResumeNext<T>(this IObservable<T> source, IObservable<T> next)
 Catch(this IObservable<T> source, Func<TException, IObservalbe<T>> next)

21. Lifetime management & flow control
Constructs
 Materialize/Dematerialize
 Do/Run

22. Combining several streams
So many combinations…

23. Combining several streams
Transfer from one stream
 Concat
 Amb (short for Abmiguous)

24. Combining several streams
 Merge
 SelectMany(other, selector)
 Zip(other, selector)

25. Combining several streams
 CombineLatest
 ForkJoin

26. Scheduling and Threading
effectively remove the need for WaitHandles, and any explicit calls to
using Threads, the ThreadPool and the new shiny Task type

27. Scheduling and Threading
 The invocation of the subscription
 The publishing of notifications
public static class Observable
{
public static IObservable<TSource> ObserveOn<TSource>(
this IObservable<TSource> source, IScheduler scheduler)
{...}
public static IObservable<TSource> SubscribeOn<TSource>(
this IObservable<TSource> source, IScheduler scheduler)
{...}
}

28. Scheduling and Threading
public interface IScheduler
{
IDisposable Schedule(Action action);
IDisposable Schedule(Action action, TimeSpan dueTime);
DateTimeOffset Now { get; }
}

29. Scheduling and Threading
 Scheduler.Dispatcher
 Scheduler.NewThread
 Scheduler.ThreadPool
 Scheduler.TaskPool
 Scheduler.Immediate
 Scheduler.CurrentThread

30. Tests, I need unit tests!
Mastering time…

31. Tests, I need unit tests!
 Scheduling and therefore Threading are generally avoided in test scenarios
as they can introduce race conditions which may lead to non-deterministic
tests.
 Tests should run as fast as possible.
 Rx is a new technology/library so naturally as we master it, we will refactor
our code. We want to use to tests to ensure our refactoring have not
altered the internal behavior of our code base.

32. Tests, I need unit tests!
TestScheduler
 A virtual scheduler to allow us emulate and control time.
var scheduler = new TestScheduler();
var wasExecuted = false;
scheduler.Schedule(() => wasExecuted = true);
Assert.IsFalse(wasExecuted);
scheduler.AdvanceTo(1); //execute 1 tick of queued actions
Assert.IsTrue(wasExecuted);

33. Tests, I need unit tests!
TestScheduler
var scheduler = new TestScheduler();
var dueTime = TimeSpan.FromMilliseconds(400);
var delta = TimeSpan.FromMilliseconds(100);
scheduler.Schedule(dueTime, () => Console.WriteLine("1"));
scheduler.Schedule(dueTime, () => Console.WriteLine("2"));
scheduler.Schedule(dueTime.Add(delta), () => Console.WriteLine("3"));
scheduler.Schedule(dueTime.Add(delta), () => Console.WriteLine("4"));
Console.WriteLine("RunTo(dueTime)");
scheduler.AdvanceTo(dueTime.Ticks);
Console.WriteLine("Run()");
scheduler.Start();
/* Output:
RunTo(dueTime)
1
2
Run()
3
4
*/

34. Useful links
 IObservable<T> interface – MSDN
 IObserver<T> interface – MSDN
 Observer Design pattern - MSDN
 Rx Home http://msdn.microsoft.com/en-us/devlabs/gg577609
 Exploring the Major Interfaces in Rx – MSDN
 ObservableExtensions class - MSDN
 Using Rx Subjects - MSDN
 System.Reactive.Subjects Namespace - MSDN
 Subject<T> - MSDN
 AsyncSubject<T> - MSDN
 BehaviorSubject<T> - MSDN
 ReplaySubject<T> - MSDN
 Subject static class - MSDN
 ISubject<TSource, TResult> - MSDN
 ISubject<T> - MSDN

35. Useful links
 Observable class - MSDN
 Observer class - MSDN
 Qservable class - MSDN
 The Rx Wiki site 101 Samples http://rxwiki.wikidot.com/101samples
 http://channel9.msdn.com/shows/Going+Deep/Wes-Dyer-and-Jeffrey-
Van-Gogh-Inside-Rx-Virtual-Time/
 http://channel9.msdn.com/posts/J.Van.Gogh/Rx-API-in-depth-Hot-and-
Cold-observables/
 http://blogs.microsoft.co.il/blogs/bnaya/archive/2010/03/13/rx-for-beginners-
part-9-hot-vs-cold-observable.aspx

36. Useful links
 http://channel9.msdn.com/Shows/Going+Deep/Bart-De-Smet-MinLINQ-The-
Essence-of-LINQ
 http://leecampbell.blogspot.com/2010/08/reactive-extensions-for-net.html
 http://channel9.msdn.com/Shows/This+Week+On+Channel+9/TWC9-
August-17-2012
 http://channel9.msdn.com/Series/Rx-Workshop
 http://channel9.msdn.com/Blogs/J.Van.Gogh
 http://channel9.msdn.com/Blogs/Charles/Erik-Meijer-Rx-in-15-Minutes
 http://channel9.msdn.com/Blogs/Peli/Testing-Rx-with-Pex