Reactive Programming is a relatively new kid on the block: concepts like functional, async and non-blocking have been around for a while but are only now going mainstream. The server side is a front runner, with seminal works as the Reactive Manifesto and many frameworks available. But how about the mobile front-end? In this talk we will experiment with mixing the principles of the Reactive Manifesto to the Android development environment.

1. Reactive Android: RxJava & beyond
Fabio Tiriticco / @ticofab
AMSTERDAM 11-12 MAY 2016

2. A little info about myself
Android Engineer
Scala Engineer

3. The Reactive Amsterdam meetup
Reactive Amsterdam
Because nobody knows what Reactive means
To explore the concept in all of its forms.

4. “Reactive” in the vocabulary
Reactive (adjective):
Tending to act in response to an agent or influence

5. Reactive Confusion
“I was thinking of using Node.js, but
maybe I can build it with Reactive.”
“Thanks for running a meetup about React.js”

6. Reactive Confusion
• The concept of “Reactive” has common
basis but slightly different meanings in
each domain
• Most of us think that Reactive == React.js
Two main observations:

7. Reactive in (Android) frontend VS backend
ANDROID
Using RxJava
WHAT DO PEOPLE THINK THAT
“BEING REACTIVE” MEANS?
BACKEND
?

8. Hello, RxJava!
• Asynchronous programming
• Observable streams
Open source libraries for

9. Hello, RxJava!
Mobile engineering is hard and there are high expectations.
RxJava is cool because
• it makes it super easy to switch between threads
• lets us deal with data as a stream
• brings us some degree of functional programming.

10. RxJava goes hand in hand with Java8’s Lambdas
new Func1<String, Integer>() {
@Override
public Integer call(String s) {
return s.length();
}
}
(String s) -> {
return s.length();
}
s -> s.length();
Retrolamba plugin for
Android < N

11. RxJava: dealing with a stream of items
class Cat {
...
public Collar getCollar() {…}
public Picture fetchPicture() {…}
...
}

12. RxJava: dealing with a stream of items
Observable.from(myCats);
(cat -> Log.d(TAG, “got cat”));
List<Cat> myCats;
Observable obs =
obs.subscribe

13. RxJava: dealing with a stream of items
Observable.from(myCats)
.subscribe(cat -> Log.d(TAG, “got cat”));
List<Cat> myCats;

14. RxJava: work on the stream
Observable.from(myCats)
.map(cat -> cat.getCollar())
.subscribe(collar -> Log.d(TAG, “got collar”));
map: T -> R
(this map: Cat -> Collar)

15. RxJava: operators to manipulate the stream
Observable.from(myCats)
.distinct()
.delay(2, TimeUnit.SECONDS)
.filter(cat -> cat.isWhite())
.subscribe(cat -> Log.d(TAG, “got white cat”));

16. Observable.from(myCats)
.subscribe(
cat -> Log.d(TAG, “cat”),
error -> error.printStackTrace(),
() -> Log.d(TAG, “done”)
);
RxJava: subscriber interface
Observable<T>.from(myCats)
.subscribe(
onNext<T>, // next item T
onError, // throwable
onComplete // void
);

17. Observable.from(myCats)
.subscribe(cat -> Log.d(TAG, “cat”));
RxJava: unsubscribe
Subscription subs =
subs.unsubscribe();

18. RxJava: threading
Observable.from(myCats)
.map(cat -> cat.fetchPicture())
.map(picture -> Filter.applyFilter(picture))
.subscribe(
filteredPicture -> display(filteredPicture)
);
.subscribeOn(Schedulers.newThread())
.observeOn(AndroidSchedulers.mainThread())

19. RxJava: other ways of creating Observables
// emits on single item and completes
Observable.just
// emits one item after a certain delay
Observable.timer
.. plus many others, and you can create your own!

20. RxJava: demo with Retrofit & Meetup Streams
http://stream.meetup.com/2/rsvp
Meetup Streaming API

21. RxJava: demo with Meetup Streams

22. RxJava: demo with Retrofit & Meetup Streams
interface MeetupAPI {
@GET("http://stream.meetup.com/2/rsvp")
@Streaming
Observable<ResponseBody> meetupStream();
}

23. RxJava: demo with RSVP Meetup Streams
meetupAPI.meetupStream()
...
.flatMap(responseBody -> events(responseBody.source()))
.map(string -> gson.fromJson(string, RSVP.class))
...
.subscribe(rsvp -> ...);
map: T -> R
flatMap: T -> Observable<R>

24. RxJava: demo with Meetup Streams

25. RxJava: demo with Meetup Streams
https://github.com/ticofab/android-meetup-streams

26. Reactive in (Android) frontend VS backend
ANDROID
Using RxJava
WHAT DO PEOPLE THINK THAT
“BEING REACTIVE” MEANS?
BACKEND
?

27. Evolution of server applications & user expectations
2006 2016
Servers ~10 The sky is the limit.
Response time seconds milliseconds
Offline maintenance hours what?
Data amount Gigabytes Petabytes
Machines Single core, little distribution
Must work across async
boundaries (location, threads)
Kind of data Request - response Streams (endless)

28. Evolution of internet usage

29. The Reactive traits
A reactive computer system must Trait
React to its users Responsive
React to failure and stay available Resilient
React to varying load conditions Elastic
Its components must react to inputs Message-driven

30. The Reactive Manifesto
Responsive
ResilientElastic
Message Driven

31. Reactive traits: Responsive
• A human who visits a website
• A client which makes a request to a server
• A consumer which contacts a provider
• . . .
A Reactive system responds to inputs and usage from its user.

32. Reactive traits: Elastic
• Scale OUT and IN: use just the right amount
• Elasticity relies on distribution
• Ensure replication in case of failure
Scale on demand to react to varying load

33. Reactive traits: Resilient
It doesn't matter how great your application is if it doesn't work.
FAULT TOLERANCE RESILIENCEVS

34. Reactive traits: Message Driven
Reactive systems design concentrates on Messages.
Asynchronous messaging enables:
• Separation between components
• Error containment - avoid chain failures
• Domain mapping closer to reality

35. The Reactive Manifesto, take 2
Responsive
ResilientElastic
Message Driven

36. Why Functional Programming?
More supportive of reasoning about problems in concurrent and
parallelised applications.
• encourages use of pure functions - minimise side effects
• encourages immutability - state doesn’t get passed around
• higher-order functions - reusability and composability

37. Reactive Patterns
Design patterns to achieve Reactive principles.
Toolkits exist to implement these patterns.

38. Reactive Pattern: Simple Component Pattern
“One component should do only one thing but do it in full. The aim is to
maximise cohesion and minimise coupling between components.”

39. Reactive Pattern: Simple Component Pattern
Actor 1
Actor 3
Actor 2
• contains state
• has a mailbox to receive
and send messages
• contains behaviour logic
• has a supervisor
Actor model Supervisor

40. Reactive Patterns: Let it crash!
"Prefer a full component restart to complex internal failure handling".
• failure conditions WILL occur
• they might be rare and hard to reproduce
• it is much better to start clean than to try to recover
• …which might be expensive and difficult!

42. Reactive Patterns: Let it crash!
• Components should be isolated - state is not shared
• Components should have a supervisor and delegate to it
some or all error handling
• The supervisor can transparently restart the component
• Message-passing architectures enforce state
confinement, error containment and transparency
In practice…

43. Reactive Patterns: Let it crash!
Actor supervision example
Actor 1
Actor 2
Supervisor
WhateverException!
X
Fix or
Restart

44. Reactive Patterns: Let it crash!
• the machine only
accepts exact change

45. Reactive Patterns: Let it crash!
Scenario 1:
The user inserts wrong
amount of coins
X
Error!

46. Reactive Patterns: Let it crash!
Scenario 2:
The machine is out of
coffee beans
Failure!
( != error)

47. Reactive Patterns: Let it crash!

48. Reactive Patterns: Let it crash!
Randomly kills instances
of their AWS system to
ensure that no failure is
propagated.

49. BACKEND
?
BACKEND
Elasticity
Asyncrhonicity
Resilience
Supervision
Message passing
State encapsulation
Streams
Backpressure
…
Reactive in (Android) frontend VS backend
ANDROID
Using RxJava
WHAT DO PEOPLE THINK THAT
“BEING REACTIVE” MEANS?

50. Reactive traits in Android?
Can we apply some of the Reactive Manifesto
principles to our Android development?

51. Reactive traits in Android?
Reactive trait In Android?
Responsive Execute as much as possible asynchronously
Elastic —
Resilient Delegate risky stuff to (Intent)Services or isolated components
Message passing
Communication via ResultReceiver
Use some event Bus

52. Reactive traits in Android?
…but I am sure that we can take this further.

53. Resources
https://github.com/ReactiveX/RxJava/wiki RxJava documentation & wiki
http://rxmarbles.com RxJava operators explained visually
http://www.reactivemanifesto.org The reactive manifesto
https://www.youtube.com/watch?v=fNEZtx1VVAk
https://www.youtube.com/watch?v=ryIAibBibQI
https://www.youtube.com/watch?v=JvbUF33sKf8
The Reactive Revealed series: awesome webinars by the
creators of the Reactive Manifesto.
https://www.manning.com/books/reactive-design-patterns
https://www.youtube.com/watch?v=nSfXcSWq0ug
Reactive design patterns, book and webinar.

54. Thanks!
@ticofab
All pictures belong
to their respective authors
AMSTERDAM 11-12 MAY 2016