Rx: Curing Your Asynchronous Programming Blues | QCon London

1. Rx: Curing your Asynchronous Programming Blues Bart J.F. De Smet Microso' Corpora,on bartde@microso'.com

2. Why Should I Care? GPS RSS feeds Stock ,ckers Social media Server management

3. Mission Statement Too hard today! (! ◦ ")(#) = !("(#)) Rx is a library for composing asynchronous and event-based programs using observable sequences. Queries? LINQ? •  .NET 3.5 SP1 and 4.0 •  Silverlight 3 and 4 •  XNA 3.1 for XBOX and Zune •  Windows Phone 7 •  JavaScript (RxJS) MSDN Data Developer Center NuGet

4. Essen,al Interfaces interface IEnumerable<out T> { IEnumerator<T> GetEnumerator(); } C# 4.0 covariance interface IEnumerator<out T> : IDisposable { You could get bool MoveNext(); stuck T Current { get; } void Reset(); }

5. Essen,al Interfaces (WaiHng to move next) C# 4.0 covariance moving on You could get stuck

6. Mathema,cal Duality Because the Dutch are (said to be) cheap –  Electricity: inductor and capacitor –  Logic: De Morgan’s Law ¬(%∧&)≡¬%∨¬& ¬(%∨&)≡¬%∧¬& –  Programming?

7. What’s the dual of IEnumerable? The recipe to dualiza,on h^p://en.wikipedia.org/wiki/Dual_(category_theory) Reversing arrows… Input becomes output and vice versa Making a U-‐turn in synchrony

8. What’s the dual of IEnumerable? interface IEnumerable<out T> { IEnumerator<T> GetEnumerator(); } interface IEnumerator<out T> : IDisposable { bool MoveNext(); T Current { get; } Properties void Reset(); are methods }

9. What’s the dual of IEnumerable? interface IEnumerable<out T> { IEnumerator<T> GetEnumerator(); } interface IEnumerator<out T> : IDisposable { bool MoveNext(); T GetCurrent(); void Reset(); A historical mishap }

10. What’s the dual of IEnumerable? interface IEnumerable<out T> { IEnumerator<T> GetEnumerator(); } interface IEnumerator<out T> : IDisposable { bool MoveNext(); No checked T GetCurrent(); exceptions in .NET / C# }

11. What’s the dual of IEnumerable? interface IEnumerable<out T> { IEnumerator<T> GetEnumerator(); } interface IEnumerator<out T> : IDisposable { bool MoveNext() throws Exception; T GetCurrent(); }

12. What’s the dual of IEnumerable? interface IEnumerable<out T> { IEnumerator<T> GetEnumerator(); } interface IEnumerator<out T> : IDisposable { bool MoveNext() throws Exception; T GetCurrent(); This is an } output too!

13. What’s the dual of IEnumerable? interface IEnumerable<out T> { IEnumerator<T> GetEnumerator(); } interface IEnumerator<out T> : IDisposable { (bool | Exception) MoveNext(); T GetCurrent(); Discriminated union } Really only two type (“or”) values

14. What’s the dual of IEnumerable? interface IEnumerable<out T> { IEnumerator<T> GetEnumerator(); } interface IEnumerator<out T> : IDisposable { (true | false | Exception) MoveNext(); T GetCurrent(); } Got true? Really got T!

15. What’s the dual of IEnumerable? interface IEnumerable<out T> { IEnumerator<T> GetEnumerator(); } interface IEnumerator<out T> : IDisposable { (T | false | Exception) MoveNext(); } Got false? Really got void!

16. What’s the dual of IEnumerable? interface IEnumerable<out T> { IEnumerator<T> GetEnumerator(); } Every enumerator is disposable interface IEnumerator<out T> : IDisposable { (T | void | Exception) MoveNext(); }

17. What’s the dual of IEnumerable? interface IEnumerable<out T> { (IEnumerator<T> & IDisposable) GetEnumerator(); } Hypothe4cal syntax (“and”) interface IEnumerator<out T> { (T | void | Exception) MoveNext(); }

18. What’s the dual of IEnumerable? interface IEnumerable<out T> { (IEnumerator<T> & IDisposable) GetEnumerator(); } Variance will flip! interface IEnumerator<out T> { (T | void | Exception) MoveNext(); } Will rename too This is really void

19. What’s the dual of IEnumerable? interface IEnumerable<out T> { (IEnumerator<T> & IDisposable) GetEnumerator(); } interface IEnumeratorDual<in T> { void OnNext(T | void | Exception); } Can encode as three methods

20. What’s the dual of IEnumerable? interface IEnumerable<out T> { (IEnumerator<T> & IDisposable) GetEnumerator(); } interface IEnumeratorDual<in T> { Color of the void OnNext(T value); bikeshed (*) void OnCompleted(); void OnError(Exception exception); } (*) Visit h^p://en.wikipedia.org/wiki/Color_of_the_bikeshed

21. What’s the dual of IEnumerable? Will leave this interface IEnumerable<out T> part alone { (IEnumerator<T> & IDisposable) GetEnumerator(); } This is the data source Need to patch interface IObserver<in T> this one up too { void OnNext(T value); void OnCompleted(); void OnError(Exception exception); }

22. What’s the dual of IEnumerable? Color of the interface IEnumerableDual<out T> bikeshed (*) { IDisposable SetObserver(IObserver<T> observer); } interface IObserver<in T> { void OnNext(T value); void OnCompleted(); void OnError(Exception exception); } (*) Visit h^p://en.wikipedia.org/wiki/Color_of_the_bikeshed

23. What’s the dual of IEnumerable? interface IObservable<out T> { IDisposable Subscribe(IObserver<T> observer); } interface IObserver<in T> { void OnNext(T value); void OnCompleted(); void OnError(Exception exception); }

24. Essen,al Interfaces interface IObservable<out T> { IDisposable Subscribe(IObserver<T> observer); } C# 4.0 contravariance interface IObserver<in T> { You could get void OnNext(T value); flooded void OnError(Exception ex); void OnCompleted(); }

25. Essen,al Interfaces ApplicaHon InteracHve MoveNext Got next? OnNext ReacHve Have next! IEnumerable<T> IObservable<T> IEnumerator<T> IObserver<T> Environment

26. Demo

27. Geeng Your Observables 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

28. Geeng Your Observables OnNext OnError OnCompleted OnNext(0) OnNext(1) OnNext(2) .Range(0, 3) yield 0 yield 1 yield 2 .Range(0, 3)

29. Geeng Your Observables A variant with time notion Hypothetical anonymous exists (GenerateWithTime) iterator syntax in C# o = Observable.Generate( new IEnumerable int 0, for int i => i < 10, i => i + 1, i => i * i yield return ); Asynchronou Synchronous s o.Subscribe(x => { foreach var in Console.WriteLine(x); Console });

30. Geeng Your Observables IObservable<int> o = Observable.Create<int>(observer => { // Assume we introduce concurrency (see later)… observer.OnNext(42); observer.OnCompleted(); return () => { /* unsubscribe action */ }; }); IDisposable subscription = o.Subscribe( onNext: x => { Console.WriteLine("Next: " + x); }, onError: ex => { Console.WriteLine("Oops: " + ex); }, onCompleted: () => { Console.WriteLine("Done"); } ); C# doesn’t have anonymous interface C# 4.0 named implementation, so we provide various parameter syntax extension methods that take lambdas.

31. Geeng Your Observables IObservable<int> o = Observable.Create<int>(observer => { // Assume we introduce concurrency (see later)… observer.OnNext(42); observer.OnCompleted(); return () => { /* unsubscribe action */ }; }); IDisposable subscription = o.Subscribe( onNext: x => { Console.WriteLine("Next: " + x); }, onError: ex => { Console.WriteLine("Oops: " + ex); }, onCompleted: () => { Console.WriteLine("Done"); } ); Thread.Sleep(30000); // Main thread is blocked… F10

34. Geeng Your Observables IObservable<int> o = Observable.Create<int>(observer => { // Assume we introduce concurrency (see later)… observer.OnNext(42); observer.OnCompleted(); return () => { /* unsubscribe action */ }; }); Breakpoint got hit IDisposable subscription = o.Subscribe( onNext: x => { Console.WriteLine("Next: " + x); }, onError: ex => { Console.WriteLine("Oops: " + ex); }, onCompleted: () => { Console.WriteLine("Done"); } ); Thread.Sleep(30000); // Main thread is blocked…

35. Demo

36. Bridging Rx with the World How to pass around? Hidden data source form1.MouseMove += (sender, args) => { if (args.Location.X == args.Location.Y) // I’d like to raise another event }; Lack of composition form1.MouseMove -= /* what goes here? */ Resource maintenance?

37. Bridging Rx with the World Source of Point Objects can be passed values IObservable<Point> mouseMoves = Observable.FromEvent(frm, "MouseMove"); var filtered = mouseMoves .Where(pos => pos.X == pos.Y); Can define operators var subscription = filtered.Subscribe(…); subscription.Dispose(); Resource maintenance!

38. Bridging Rx with the World Exceptions? Hidden data source FileStream fs = File.OpenRead("data.txt"); byte[] bs = new byte[1024]; fs.BeginRead(bs, 0, bs.Length, Really a method pair new AsyncCallback(iar => { int bytesRead = fs.EndRead(iar); Cancel? // Do something with bs[0..bytesRead-‐1] }), null Lack of composition ); Synchronous State? completion?

39. Bridging Rx with the World FileStream fs = File.OpenRead("data.txt"); Func<byte[], int, int, IObservable<int>> read = Observable.FromAsyncPattern<byte[], int, int, int>( fs.BeginRead, fs.EndRead); byte[] bs = new byte[1024]; read(bs, 0, bs.Length).Subscribe(bytesRead => { // Do something with bs[0..bytesRead-‐1] }); Tip: a nicer wrapper can easily be made using various operators

40. Bridging Rx with the World don’t replace unify composiHonality generic operators build bridges!

41. Bridging Rx with the World •  Cold observables var Observable.Return Triggered by subscription .Subscribe( ) // Prints 42 .Subscribe( ) // Prints 42 again •  Hot observables var Observable.FromEvent MouseEventArgs "MouseMove" Mouse events going before subscription Console

42. Demo

43. Composi,on and Querying Parameterization IScheduler of operators var .Return Scheduler.ThreadPool Console Will run on the source’s scheduler

44. Composi,on and Querying duality –  Convert between both worlds // Introduces concurrency to enumerate and signal… var .ToObservable(); // Removes concurrency by observing and yielding… var .ToEnumerable() •  “Time-‐centric” reacHve operators: Race! .Amb( )

45. Composi,on and Querying synchronize .Return Scheduler.ThreadPool "Answer = " IScheduler interface •  WPF dispatcher •  WinForms control •  SynchronizationContext .ObserveOn(frm) "Answer = "

46. Composi,on and Querying •  Observables are sources of data –  Data is sent to you (push based) –  Extra (op,onal) noHon of Hme •  Hence we can query over them // Producing an IObservable<Point> using Select var mme = from mm in Observable.FromEvent<MouseEventArgs>( form, “MouseMove”) select mm.EventArgs.Location; // Filtering for the first bisector using Where var res = from mm in mme where mm.X == mm.Y select mm;

47. Composi,on and Querying Asynchronou s request IObservable<string> TextChanged React Dictionary web service Reaction Reactive Reactor IObservable<DictionaryWord[]> Data binding on UI thread

48. Composi,on and Querying // IObservable<string> from TextChanged events var changed = Observable.FromEvent<EventArgs>(txt, "TextChanged"); var input = (from text in changed select ((TextBox)text.Sender).Text); .DistinctUntilChanged() .Throttle(TimeSpan.FromSeconds(1)); // Bridge with the dictionary web service var svc = new DictServiceSoapClient(); var lookup = Observable.FromAsyncPattern<string, DictionaryWord[]> (svc.BeginLookup, svc.EndLookup); // Compose both sources using SelectMany var res = from term in input input.SelectMany(term => from words in lookup(term) lookup(term)) select words;

49. Demo

52. Composi,on and Querying // IObservable<string> from TextChanged events var changed = Observable.FromEvent<EventArgs>(txt, "TextChanged"); var input = (from text in changed select ((TextBox)text.Sender).Text); .DistinctUntilChanged() .Throttle(TimeSpan.FromSeconds(1)); // Bridge with the dictionary web service var svc = new DictServiceSoapClient(); var lookup = Observable.FromAsyncPattern<string, DictionaryWord[]> (svc.BeginLookup, svc.EndLookup); Very local fix // Compose both sources using SelectMany J var res = from term in input from words in lookup(term).TakeUntil(input) select words;

53. Composi,on and Querying // IObservable<string> from TextChanged events var changed = Observable.FromEvent<EventArgs>(txt, "TextChanged"); var input = (from text in changed select ((TextBox)text.Sender).Text); .DistinctUntilChanged() .Throttle(TimeSpan.FromSeconds(1)); // Bridge with the dictionary web service var svc = new DictServiceSoapClient(); var lookup = Observable.FromAsyncPattern<string, DictionaryWord[]> (svc.BeginLookup, svc.EndLookup); // Alternative approach for composition using: // IObservable<T> Switch<T>(IObservable<IObservable<T>> sources) var res = (from term in input Hops from source select lookup(term)).Switch(); to source

54. Demo

55. Mission Accomplished Way simpler with Rx (! ◦ ")(#) = !("(#)) Rx is a library for composing asynchronous and event-based programs using observable sequences. Queries! LINQ! •  .NET 3.5 SP1 and 4.0 •  Silverlight 3 and 4 •  XNA 3.1 for XBOX and Zune •  Windows Phone 7 •  JavaScript (RxJS) MSDN Data Developer Center NuGet

56. Related Content •  Hands-‐on Labs –  Two ﬂavors: •  Curing the asynchronous blues with the Reac,ve Extensions (Rx) for .NET •  Curing the asynchronous blues with the Reac,ve Extensions (Rx) for JavaScript –  Both can be found via the Rx forums •  Rx team web presence –  Rx team blog – h^p://blogs.msdn.com/rxteam –  DevLabs – h^p://msdn.microso'.com/en-‐us/devlabs/ee794896.aspx –  MSDN forums – h^p://social.msdn.microso'.com/Forums/en-‐US/rx –  Channel9 – h^p://channel9.msdn.com/Tags/Rx

57. Thanks! Bart J.F. De Smet

Rx: Curing Your Asynchronous Programming Blues | QCon London

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