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Introduction to Reactive Extensions
Peter Goodman
An Introduction to Reactive
Extensions

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Introduction to Reactive Extensions
What are Reactive Extensions?

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Introduction to Reactive Extensions
What are Reactive Extensions?

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Introduction to Reactive Extensions
What are Reactive Extensions?

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Introduction to Reactive Extensions
What are Reactive Extensions?

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Introduction to Reactive Extensions
Events in .Net
form1.MouseMove += (sender, args) => {
if (args.Location.X == args.Location.Y)
// I’d like to raise another event
};
form1.MouseMove -= /* what goes here? */

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Introduction to Reactive Extensions
Collections are Enumerables
interface IEnumerable<out T>
{
IEnumerator<T> GetEnumerator();
}
interface IEnumerator<out T> : IDisposable
{
bool MoveNext();
T Current { get; }
void Reset();
}

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Introduction to Reactive Extensions
What if events were collections?
 Collection
Move Next Move Next Move Next Move Next Move Next Move Next
 Event Stream
TIME
OnNext OnNext OnNext OnNext OnNext OnNext

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Introduction to Reactive Extensions
Observables
interface IObservable<out T>
{
IDisposable Subscribe(IObserver<T> observer);
}
interface IObserver<in T>
{
void OnNext(T value);
void OnError(Exception ex);
void OnCompleted();
}

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Introduction to Reactive Extensions
Summary Push vs Pull
Application
Interactive
MoveNext
Reactive
Got next?
OnNext
Have next!
IEnumerable<T> IObservable<T>
IEnumerator<T> IObserver<T>
Environment

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Introduction to Reactive Extensions
Creating Observables - Primitive
OnCompleted
.Empty<int>() new int[0]
OnNext
.Return(42) new[] { 42 }
OnError
.Throw<int>(ex) Throwing iterator
.Never<int>()
Iterator that got stuck
Notion of time

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Introduction to Reactive Extensions
Creating Observables - Range
OnNext(0) OnNext(1) OnNext(2)
.Range(0, 3)
yield 0 yield 1 yield 2
.Range(0, 3)

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Introduction to Reactive Extensions
Generating values and Subscribing
A variant with time notion Hypothetical anonymous
exists (GenerateWithTime) iterator syntax in C#
o = Observable.Generate( e = new IEnumerable<int> {
0, for (int i = 0;
i => i < 10, i < 10;
i => i + 1, i++)
i => i * i yield return i * i;
);
};
Asynchronous Synchronous
o.Subscribe(x => { foreach (var x in e) {
Console.WriteLine(x); Console.WriteLine(x);
}); }

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Introduction to Reactive Extensions
Subscribing
IObservable<int> o = Observable.Create<int>(observer => {
// Assume we introduce concurrency (see later)…
observer.OnNext(42);
observer.OnCompleted();
});
IDisposable subscription = o.Subscribe(
onNext: x => { Console.WriteLine("Next: " + x); },
onError: ex => { Console.WriteLine("Oops: " + ex); },
onCompleted: () => { Console.WriteLine("Done"); }
);

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Introduction to Reactive Extensions
DEMO
Generating events and subscribing to them

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Introduction to Reactive Extensions
Observable Querying
 Observables are sources of data
 Data is sent to you (push based)
 Extra (optional) notion of time.
 Hence we can query over them
// Producing an IObservable<Point> using Select
var from in Observable MouseEventArgs
select .Location
// Filtering for the first bisector using Where
var from in
where
select

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Introduction to Reactive Extensions
DEMO
Querying, Composition and introducing Concurrency

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Introduction to Reactive Extensions
Introducing Asynchrony
 An Asynchronous operation can be thought of as an
Observable that returns a single value and completes.
 FromAsync
 Takes an APM method pair (BeginExecute, EndExecute) and
creates an Observable
 ToAsync
 Takes a method and creates an Observable (Like TPL)

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Introduction to Reactive Extensions
Introducing Concurrency
 Many operators in Rx introduce Concurrency
 Throttle
 Interval
 Delay
 BufferWithTime
 Generally they provide an overload to supply a Scheduler
 ImmediateScheduler – Static Immediate
 CurrentThreadScheduler – Placed on a queue for the current thread
 NewThreadScheduler – Spawn a new Thread
 DispatcherScheduler - Silverlight
 TaskPoolScheduler - TPL
 ThreadPoolScheduler

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Introduction to Reactive Extensions
Concurrency and Synchronization
•
var Observable.Return Scheduler.ThreadPool
" "
•
.ObserveOnDispatcher()
" "

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Introduction to Reactive Extensions
DEMO
Synchronization

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Introduction to Reactive Extensions
Rx for JavaScript (RxJS)
 Parity with Rx for .NET
 Set of operators
 Taming asynchronous JS
 JavaScript-specific bindings
 jQuery
 ExtJS
 Dojo
 Prototype
 YUI3
 MooTools
 Bing APIs

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Introduction to Reactive Extensions
DEMO
Rx for JavaScript

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Introduction to Reactive Extensions
Reactive Extensions
Questions?

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Introduction to Reactive Extensions
Resources
 MSDN
 Bart de Smet / Rx (Channel 9)
 Reactive UI
 Pushqa / SignalR

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Introduction to Reactive Extensions
Contact
 pete@petegoo.com
 http://blog.petegoo.com
 Twitter: @petegoo