Pieter Nijs gives a presentation on reactive programming. He defines reactive programming as being event-driven, scalable, and responsive. It involves programming that reacts to data pushed from the environment rather than pulling data. Observables allow pushing asynchronous data streams via OnNext, OnError, and OnCompleted notifications. Subscribers can listen for these notifications. Reactive Extensions (Rx) brings the benefits of functional reactive programming to .NET via LINQ-like operators on observables. Async/await also integrates well with Rx. ReactiveUI allows building MVVM apps reactively by observing view model properties and commands.

1. REACTIVE
PROGRAMMING
TAKING THE RED PILL
Pieter Nijs

2. Welcome
Pieter Nijs
Senior .NET Consultant
Competence Lead Mobile
Microsoft Extended Experts Team member
Realm MVP
E pieternijs@live.be
T @nijspieter
B blog.pieeatingninjas.be
@ Ordina Belgium

3. Welcome
Pieter Nijs
Senior .NET Consultant
Competence Lead Mobile
Microsoft Extended Experts Team member
Realm MVP
E pieternijs@live.be
T @nijspieter
B blog.pieeatingninjas.be
@ Ordina Belgium

5. Don’t worry about the
difference of opinion

6. What is
Reactive Programming?

7. Reactive programming
Helps with today’s challenges
Nothingnew

8. Reactive programming
World around us has changed
LOB / internal
business applications
Consumer-facing apps
CRUD Real-time data consumption
PCs / terminals
Smart phones, tablets, TVs,
wearables, coffee machines,
mirrors,…

9. Reactive programming
Event-driven
Scalable Resilient
Responsive

10. Reactive programming
Responsive
…and lazy

11. Interactive programming
Program asks the enviroment for something
Programs PULLS data from the environment
var input = Console.ReadLine();
…
foreach(var item in items) { }

12. Reactive programming
Environment sends data to the program
Program may need to react to that
Environment PUSHES data to the program
okButton.Click += OnButtonClicked;
DoStuffAsync(callbackWhenItsDone);

13. What is
IObservable<T>?

14. “Events are just lists... backwards.”
- Paul Betts

15. IObservable<T>
OnCompleted OnNext OnNext OnNext OnNext
OnError

16. IEnumerable<T>
IEnumerable<T>
IObservable<T>
MyApp
MoveNext() MoveNext()
IObservable<T>
MyApp
OnNext() OnNext() OnCompleted()

17. Example Observables
Observable.FromRange(0, 5);
OnNext OnNext OnNext OnNext OnNext
OnCompleted
0 1 2 3 4

18. Example Observables
Observable.FromEventPattern<PropertyChangedEventHandler,
PropertyChangedEventArgs>(
x => PropertyChanged += x,
x => PropertyChanged -= x);
OnNext OnNext OnNext
EventPattern<PropertyChangedEventArgs> EventPattern<PropertyChangedEventArgs>

19. What is
an Observer?

20. IObservable<T>
T T T T T

22. Subscribing
Listening to the incoming messages
• Get the values of IObservable
• Process each value
• Filter on values
• Multiple subscriptions possible
• Listen for error
• IDisposable

23. Subscribing
var rangeObservable = Observable.Range(0, 5);
var rangeObserver = rangeObservable.Subscribe(i =>
{
Debug.WriteLine(i);
});
rangeObserver.Dispose();

24. Subscribing
var propChangedObservable =
Observable.FromEventPattern
<PropertyChangedEventHandler,
PropertyChangedEventArgs>(
x => PropertyChanged += x,
x => PropertyChanged -= x);
var propChangedObserver =
propChangedObservable.Subscribe(propchange =>
{
Debug.WriteLine(
$"Prop'{propchange.EventArgs.PropertyName}' changed.");
});

25. What does it look
like in code?

26. Convinced?

27. “Honestly,
I don’t know…”

28. Who loves
functional programming?

29. LINQ

30. Why do we love LINQ so much?
var names = new string[] { "Pieter", "Stefanie", "Bruce" };
var query = names.Where(filterOnName).OrderBy(n => n).Select(n => n.ToUpper());
foreach (var item in query)
{
Console.WriteLine(item);
}
Immutability
Function pipeline
Lazy evaluation
Abstraction

31. IEnumerable => LINQ
IObservable => Rx

32. Reactive Extensions (Rx)
Created by Microsoft in 2009
RxJS (2010)
RxJava (2012)
RxCpp (2012)
RxRuby (2012)
RxScala (2013)
RxPHP (2013)
RxSwift (2015)

33. Reactive Extensions (Rx)
Take
Where
Select
Delay
Throttle
DistinctUntilChangedCombineLatest
Buffer
Sample
Retry
Aggregate

34. What does it look
like in code?

35. What about
async/await?

36. What if I told you
async/await is
compatible with Rx

37. Async/await
Observable.FromAsync(() => DoRequestAsync(url))
.Timeout(TimeSpan.FromMilliseconds(200))
.Retry(3)
.OnErrorResumeNext(
Observable.FromAsync(
() => DoRequestAsync2(url)))
• Async/await ++

38. Async/await
You can await an Observable!
var result = await observable;
string result = await observable.FirstAsync();
string result = await observable.FirstOrDefaultAsync();
string result = await observable.LastAsync();

39. What does it look
like in code?

40. But…
I like MVVM…

41. What if I told you
MVVM and Rx can work
perfectly together?

42. DIY
INotifyPropertyChanged

43. INotifyPropertyChanged
var propChangedObservable =
Observable.FromEventPattern
<PropertyChangedEventHandler,
PropertyChangedEventArgs>(
x => PropertyChanged += x,
x => PropertyChanged -= x);
var propChangedObserver =
propChangedObservable.Subscribe(propchange =>
{
...
});

44. ReactiveUI
public class MyViewModel : ReactiveObject
• Reactive Bindings
• ReactiveCommands
• PropertyChanged observation with WhenAnyValue()

45. What does it look
like in code?

46. Conclusion

47. Reactive programming
• Event-driven architecture
• Resilient
• Scalable
• Responsive
• Events
• User interaction
• Data flowing-in through streams
• Not the silver bullet
• Use the right tool for the job!

48. Thank you
Pieter Nijs
Senior .NET Consultant
Competence Lead Mobile
Microsoft Extended Experts Team member
Realm MVP
E pieternijs@live.be
T @nijspieter
B blog.pieeatingninjas.be
@ Ordina Belgium

49. Thank you
Pieter Nijs
Senior .NET Consultant
Competence Lead Mobile
Microsoft Extended Experts Team member
Realm MVP
E pieternijs@live.be
T @nijspieter
B blog.pieeatingninjas.be
@ Ordina Belgium