Slides for a workshop about RxJava & RxAndroid delivered in Codemotion 2015.

1. MADRID · NOV 27-28 · 2015
RxJava in practice
Javier Gamarra

2. MADRID · NOV 27-28 · 2015
http://kcy.me/29crj (slides)
&
http://kcy.me/296bu (commits)

3. MADRID · NOV 27-28 · 2015
Environment
Eclipse | Android Studio (RxAndroid)
RxJava.jar
[Java 8] || [Retrolambda] :P

4. MADRID · NOV 27-28 · 2015
Environment
Android Studio:
compile 'io.reactivex:rxandroid:1.0.1'
compile 'io.reactivex:rxjava:1.0.16'
Eclipse:
● Copy jars to lib/ and add jar in project

5. MADRID · NOV 27-28 · 2015
Github
● http://kcy.me/296bu
● koans
● código
● commit a commit
● Podéis preguntarme en cualquier
momento

6. MADRID · NOV 27-28 · 2015
Who?
Javier Gamarra / @nhpatt
@liferay
@cyliconvalley / @agilespain

7. MADRID · NOV 27-28 · 2015
Ask!
Please? Pretty please? Please pretty please with sugar on top?

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Background?

9. MADRID · NOV 27-28 · 2015
Why?

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Why? (in java)
multithreading is hard
futures are complicated
callbacks are hell

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Rx comes to the rescue!

12. MADRID · NOV 27-28 · 2015
a library
to represent any operation as an
asynchronous data stream
on any thread,
declaratively composed,
and consumed by multiple objects

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a library

14. MADRID · NOV 27-28 · 2015
A library?
Extensions to .NET framework
Netflix

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Libraries are tools
If all you have is a hammer, everything looks like a nail

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a library to model
Observables & Subscribers

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Observables and Subscribers
An Observable emits items.
A Subscriber consumes those items.

18. MADRID · NOV 27-28 · 2015
Observer pattern?

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My very first observable
Observable<String> myObs = Observable. create(new Observable.
OnSubscribe<String>() {
@Override
public void call(Subscriber<? super String> subscriber) {
subscriber.onNext( "Hi!");
subscriber.onCompleted();
}
});

20. MADRID · NOV 27-28 · 2015
My very first subscriber
Subscriber<String> mySubs = new Subscriber<String>() {
@Override
public void onCompleted() {
}
@Override
public void onError(Throwable throwable) {
}
@Override
public void onNext(String s) {
System. out.println(s);
}
};

21. MADRID · NOV 27-28 · 2015
My very first subscription
myObs.subscribe(mySubs);

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A bit verbose...
Let’s simplify with Java8 and RxJava
● lamdbas
● Observable.just
● .subscribe()

23. MADRID · NOV 27-28 · 2015
That’s better...
Observable
.just("Hi!")
.subscribe(System.out::println);

24. MADRID · NOV 27-28 · 2015
Why?
More than the observer pattern
● Observables only emit when someone
listening
● OnFinished
● OnError

25. MADRID · NOV 27-28 · 2015
Let’s use those differences
subscribe admits 3 parameters
● OnNext
● OnFinished
● OnError

26. MADRID · NOV 27-28 · 2015
You said “items”
Observable.from admits a list of elements

27. MADRID · NOV 27-28 · 2015
Ok, let’s recap…
Iterators?

28. MADRID · NOV 27-28 · 2015
Iterators?
● Iterators are pull and synchronous
● Subscribers are push and asynchronous

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to represent any operation as an
“asynchronous data stream”

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Everything is a stream

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Everything is a stream
We can model everything as a
stream

32. MADRID · NOV 27-28 · 2015
Everything is a stream
A network call is also a stream
Like using retrofit with this API

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“declaratively composed”

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Operators

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Operators
Transform a stream
rxmarbles and android app

36. MADRID · NOV 27-28 · 2015
Operators
Map

37. MADRID · NOV 27-28 · 2015
Map
List<String> severalThings = Arrays.asList("1", "2");
Observable.from(severalThings)
.map((s) -> “Element “ + s)
.subscribe(System.out::println);

38. MADRID · NOV 27-28 · 2015
Map
We can return another type:
List<String> severalThings = Arrays.asList("1", "2");
Observable.from(severalThings)
.map(Integer::valueOf)
.subscribe(System.out::println);

39. MADRID · NOV 27-28 · 2015
Map
Let’s do the same with our repos…
And… I can’t because we return a List
And using Observable.from… NOP

40. MADRID · NOV 27-28 · 2015
Flatmap
Flatmap
Obs -> elements

41. MADRID · NOV 27-28 · 2015
Flatmap
service.listRepos("nhpatt")
.flatMap(Observable::from)
.map(Repo::getName)
.map((s) -> s.replace("-", " "))
.subscribe(System.out::println);

42. MADRID · NOV 27-28 · 2015
Flatmap
Concatmap -> ordered flatmap

43. MADRID · NOV 27-28 · 2015
Filter
service.listRepos("nhpatt")
.flatMap(Observable::from)
.map(Repo::getName)
.map((s) -> s.replace("-", " "))
.filter((s) -> s.startsWith("Android"))
.subscribe(System.out::println);

44. MADRID · NOV 27-28 · 2015
Scan & old code
service.listRepos("nhpatt")
.flatMap(Observable::from)
.map(Repo::getName)
.map((s) -> s.replace("-", " "))
.filter((s) -> s.startsWith("Android"))
.take(2)
.map(String::length)
.scan((x, y) -> x * y)
.subscribe(System.out::println);

45. MADRID · NOV 27-28 · 2015
Scan & old code
(l) -> {
int result = 0;
int i = 0;
int oldLength = 1;
for (Repo repo : l) {
String name = repo.getName();
String replacedName = name.replace( "-", " ");
if (replacedName.startsWith( "Android") && i < 2) {
result = replacedName.length() * oldLength;
oldLength = result;
System.out.println(result);
i++;
}
}
return result;

46. MADRID · NOV 27-28 · 2015
Operators
● merge
● flatMap
● zip
Nice things™ : parallel jobs!

47. MADRID · NOV 27-28 · 2015
Zipping requests
Observable<Repo> repo = service.listRepos("nhpatt")
.flatMap(Observable::from)
.take(1);
Observable<Commit> commit = service.listCommits("nhpatt",
"Android")
.flatMap(Observable::from)
.take(1);
Observable.zip(repo, commit, this::updateCommit).subscribe
(repo1 -> {
System.out.println(repo1.getCommit().getUrl());
});

48. MADRID · NOV 27-28 · 2015
Operators
Amazing documentation
● Async
● Blocking
● Combining
● Conditional
● Error handling
● Filtering
● Mathematical
● Creation
● String
● Transforming
● Utility

49. MADRID · NOV 27-28 · 2015
Subscribers are asynchronous?
No, not really

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“on any thread”

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Schedulers!

52. MADRID · NOV 27-28 · 2015
Schedulers
I define an API declaratively
and later in the implementation run it:
asynchronously
or in a separate Thread
or in a Threadpool
or synchronously
...

53. MADRID · NOV 27-28 · 2015
Schedulers
.subscribeOn(Schedulers...)
.observeOn(Schedulers...)
● io
● computation
● newThread
● trampoline
● test

54. MADRID · NOV 27-28 · 2015
Schedulers
service.listRepos("nhpatt")
.subscribeOn(Schedulers.immediate())
.observeOn(Schedulers.immediate())
.subscribe(System.out::println);

55. MADRID · NOV 27-28 · 2015
Schedulers
RxJava operators have default threads
Interval?

56. MADRID · NOV 27-28 · 2015
Use cases?

57. MADRID · NOV 27-28 · 2015
Use cases
● Autocomplete with debounce | sample…
● Accumulate calls with buffer...
● Polling with timeout | window…
● Form Validation with combineLatest…
● Retrieve data fast from cache | network call with
concat | merge…
● Button click with delay, listen on other thread

58. MADRID · NOV 27-28 · 2015
Android?

59. MADRID · NOV 27-28 · 2015
Why?

60. MADRID · NOV 27-28 · 2015
Why? (in android)
Main/UI thread and background problem
● Can’t do heavy tasks on UI
● Can’t update view on background
● AsyncTasks are bad
● Handlers can leak

61. MADRID · NOV 27-28 · 2015
How RxAndroid helps?

62. MADRID · NOV 27-28 · 2015
Android schedulers
service.listRepos("nhpatt")
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(newRepos -> {
repos.addAll(newRepos);
adapter.notifyDataSetChanged();
});

63. MADRID · NOV 27-28 · 2015
Orientation Problem
Orientation Problem
● onCreate gets called again
● state is lost

64. MADRID · NOV 27-28 · 2015
Orientation Problem
Programmer’s job:
● Store/Restore state
● Restore view
● Call again the user task?
● Wait until it finishes?

65. MADRID · NOV 27-28 · 2015
How RxAndroid helps?

66. MADRID · NOV 27-28 · 2015
Orientation Problem
Observable is easy to persist
(retain | singleton)
Observable can continue (cache and retry)
RxLifecycle

67. MADRID · NOV 27-28 · 2015
Orientation Problem
Observable<List<Repo>> github = RetrofitService.
getGithub()
.listRepos("nhpatt")
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.cache();

68. MADRID · NOV 27-28 · 2015
More Android
Lifecycle (RxLifecyle)
Views (RxBinding)

69. MADRID · NOV 27-28 · 2015
But...

70. MADRID · NOV 27-28 · 2015
But...
Subscriptions leak memory :’(
We have to call to unsubscribe
(CompositeSubscription helps)
And don’t include references to the
object/activity

71. MADRID · NOV 27-28 · 2015
Easy to unsubscribe
@Override
protected void onStop() {
super.onStop();
subscription.unsubscribe();
}
.isUnsubscribed() :(

72. MADRID · NOV 27-28 · 2015
Let’s recap

73. MADRID · NOV 27-28 · 2015
a library
to represent any operation as an
asynchronous data stream
on any thread,
declaratively composed,
and consumed by multiple objects

74. MADRID · NOV 27-28 · 2015
a library
to represent any operation as an
observable
on any thread,
declaratively composed,
and consumed by multiple objects

75. MADRID · NOV 27-28 · 2015
a library
to represent any operation as an
observable
with schedulers
declaratively composed,
and consumed by multiple objects

76. MADRID · NOV 27-28 · 2015
a library
to represent any operation as an
observable
with schedulers
and operators
and consumed by multiple objects

77. MADRID · NOV 27-28 · 2015
a library
to represent any operation as an
observable
with schedulers
and operators
listened by subscribers

78. MADRID · NOV 27-28 · 2015
But...

79. MADRID · NOV 27-28 · 2015
But...
● Learning curve
● Hard to debug (Frodo library)
● Backpressure

80. MADRID · NOV 27-28 · 2015
Questions?

81. MADRID · NOV 27-28 · 2015
Where to know more...
● reactivex.io (amazing docs)
● RxJava wiki
● Dan Lew
● Use cases

82. MADRID · NOV 27-28 · 2015
Where to know more...
nhpatt (twitter | github | email | slack)
I’ll do anything for good votes

83. MADRID · NOV 27-28 · 2015
Feedback
Click here :P

84. MADRID · NOV 27-28 · 2015
RxJava in practice
Javier Gamarra

85. MADRID · NOV 27-28 · 2015
Other interesting operators...

86. MADRID · NOV 27-28 · 2015
Take
service.listRepos("nhpatt")
.flatMap(Observable::from)
.map(Repo::getName)
.map((s) -> s.replace("-", " "))
.filter((s) -> s.startsWith("Android"))
.take(2)
.subscribe(System.out::println);

87. MADRID · NOV 27-28 · 2015
Operators
distinctUntilChanged
compose

88. MADRID · NOV 27-28 · 2015
Errors?

89. MADRID · NOV 27-28 · 2015
onError() is called if an Exception is thrown at
any time (this is cool)
The operators don't have to handle the
Exception
No more callbacks
Errors

90. MADRID · NOV 27-28 · 2015
Errors
And we can customize the flow:
onErrorResumeNext
onErrorReturn
retry
...

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Why this is useful?

92. MADRID · NOV 27-28 · 2015
Why this is useful?
Observables and subscribers can do
anything

93. MADRID · NOV 27-28 · 2015
Why this is useful?
Observable a database query
Subscriber displaying results on the screen

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Why this is useful?
Observable a click on the screen
Subscriber reacting to it

95. MADRID · NOV 27-28 · 2015
Why this is useful?
Observable a stream of bytes from the
internet
Subscriber write them to disk

96. MADRID · NOV 27-28 · 2015
Why this is useful?
Observable and Subscriber are independent
of the transformations

97. MADRID · NOV 27-28 · 2015
Why this is useful?
I can compose/chain any map/filter calls I
want
Only matters the return type

98. MADRID · NOV 27-28 · 2015
Side effects?

99. MADRID · NOV 27-28 · 2015
Side effects
doOn methods:
● doOnNext() for debugging
● doOnError() for error handling
● doOnNext() to save/cache results

100. MADRID · NOV 27-28 · 2015
Side effects
Observable someObservable = Observable
.from(Arrays.asList(new Integer[]{2, 7, 11}))
.doOnNext(System.out::println)
.filter(prime -> prime % 2 == 0)
.doOnNext(System.out::println)
.count()
.doOnNext(System.out::println)
.map(
number -> String.format(“Contains %d
elements”, number)
);

101. MADRID · NOV 27-28 · 2015
Side effects
flatMap(id -> service.get()
.doOnError(t -> {
// report problem to UI
})
.onErrorResumeNext(Observable.empty()))

102. MADRID · NOV 27-28 · 2015
Cold & Hot

103. MADRID · NOV 27-28 · 2015
Cold & Hot
Observables only emit when someone
listening?
Cold observables only emit when subscribed
But hot observables emit instantly

104. MADRID · NOV 27-28 · 2015
Cold & Hot
You can convert between both states
● “Hot -> cold” using defer() or merge(),
zip()...
● “Cold -> Hot” using publish() & connect()

105. MADRID · NOV 27-28 · 2015
Cold & Hot
publish & connect?
“like transactions”
Assume everything is cold

106. MADRID · NOV 27-28 · 2015
Testing?

107. MADRID · NOV 27-28 · 2015
More cool things?
● testscheduler
● toIterator()
● Obs.error(), Obs.empty()

108. MADRID · NOV 27-28 · 2015
Subjects

109. MADRID · NOV 27-28 · 2015
Subjects
Both observable & observer

110. MADRID · NOV 27-28 · 2015
Subjects
AsyncSubject -> takeLast(1)
PublishSubject -> publish()
ReplaySubject -> replay()/cache()
BehaviorSubject -> replay(1)/cache(1)

111. MADRID · NOV 27-28 · 2015
RxJava in practice
Javier Gamarra