Lecture on Reactive programming on Android. DevFest Prague 2014.

1. Reactive programming on
Android
Tomáš Kypta

2. What’s reactive programming?
reactive programming or functional reactive
programming (FRP)

3. What’s reactive programming?
functional
– lambdas
– closures
– pure
– composable
reactive
– data flow
– asynchronous
– values
– events
– push

4. Reactive?
• observer pattern

5. Typical App
Event Source
Views Network DB Other
Listener Listener Listener Listener
logic logic logic logic
State

6. Views Network DB
Transformation
Reactive
Event Source
Other
Observable Observable Observable Observable
Observer Observer

7. Reactive
• abstract away from concerns about
– low-level threading
– side effects
– synchronization
– encapsulation
– resource management

8. Java 8
Observable.toObservable(“a”, “b”, “c”)
.take(2)
.subscribe((arg) -> {
System.out.println(arg);
});
Reactive Code

9. Reactive on Android
• evading callback hell
• How to execute heavy tasks on
background threads?
• And deliver results on the main (UI)
thread?

10. Async on Android

11. Handler handler = new Handler();
new Thread(){
@Override
public void run() {
final String result = somethingDemanding();
handler.post(new Runnable() {
@Override
public void run() {
showResult(result);
}
});
}
}.start();
Thread + Handler

12. Handler handler = new Handler();
new Thread(){
@Override
public void run() {
final String result = somethingDemanding();
handler.post(new Runnable() {
@Override
public void run() {
showResult(result);
}
});
}
}.start();
Thread + Handler

13. Thread + Handler
• simple
• difficult to deliver on the main thread
• broken data flow

14. new AsyncTask<Void, Integer, String>(){
@Override
protected String doInBackground(Void...
params) {
return somethingDemanding();
}
@Override
protected void onPostExecute(String s) {
showResult(s);
}
}.execute();
AsyncTask

15. AsyncTask
• deals with the main thread
• error-prone
• difficult error propagation
• difficult to bound to activity/fragment
lifecycle
• difficult composition

16. class MyFragment extends Fragment implements
LoaderManager.LoaderCallbacks<String> {
@Override public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
getLoaderManager().initLoader(42, null, this);
}
@Override public Loader<String> onCreateLoader(int id, Bundle args) {
return new AsyncTaskLoader<String>(getActivity()) {
@Override public String loadInBackground() {
return doHeavyTask();
}
};
}
@Override public void onLoadFinished(Loader<String> loader, String data) {
showResult(data);
}
@Override public void onLoaderReset(Loader<String> loader) {}
}
Loaders

17. Loaders

18. Loaders
• boilerplate
• good for cursors
• deals with activity/fragment lifecycle
• difficult composition
• difficult to adjust logic

19. Async on Android
• many other async libraries
– some are better
– some are worse

20. RxJava

21. RxJava
• Java VM implementation of Reactive
Extensions
• a library for composing flows and
sequences of asynchronous data
• open-source
• https://github.com/ReactiveX/RxJava
• by Netflix

22. RxJava
• DSL for creating computation flows from
async sources
• flows called Observables
• push semantics of Observables

23. RxJava … not sure if …

24. Future
proxy or wrapper around an object that is not
yet there
future.get()
future.isDone()
• non-trivial complexity when nested
• difficult to compose conditional
asynchronous execution flows

25. Iterable vs. Observable

26. Iterable vs. Observable
// Iterable<String>
getData()
.skip(10)
.take(5)
.map({s ->
return s + “1”})
.forEach({
println it
});
// Observable<String>
getData()
.skip(10)
.take(5)
.map({s ->
return s + “1”})
.subscribe({
println it
});

27. Iterable vs. Observable
single items multiple items
synchronous T getData() Iterable<T> getData()
asynchronous Future<T> getData() Observable<T> getData()

28. Iterable vs. Observable
event Iterable (pull) Observable (push)
retrieve data T next() onNext(T)
discover error throws Exception onError(Exception)
complete !hasNext() onCompleted()

29. Observable
Calling Thread
public Observable<String> getData();
Callback Thread
synchronous on calling thread

30. Observable
Calling Thread
public Observable<String> getData();
Callback Thread
Thread Pool
asynchronous on a separate thread

31. Observable
Calling Thread
public Observable<String> getData();
Callback Threads
Thread Pool
asynchronous on multiple threads

32. Observable
• not biased toward some particular source
of asynchronicity
• the implementation chooses if the code will
be blocking or non-blocking

33. public interface Observer <T> {
void onCompleted();
void onError(Throwable throwable);
void onNext(T t);
}
Observable

34. public Observable<String> getStrings() {
return Observable.create(new Observable.OnSubscribe<String>() {
@Override
public void call(Subscriber<? super String> subscriber) {
try {
subscriber.onNext("Hello");
subscriber.onNext("World");
subscriber.onCompleted();
} catch (Exception ex) {
subscriber.onError(ex);
}
}
});
}
Observable

35. Observable<String> strings = getStrings();
strings.subscribe(
new Observer<String>() {
@Override
public void onCompleted() {
Log.d("rx", "no more data");
}
@Override
public void onError(Throwable throwable) {
Log.e(“rx”, “error”, throwable);
}
@Override
public void onNext(String s) {
showResult(s);
}
});
Observer

36. Observable<String> strings = getStrings();
strings.subscribe(
new Observer<String>() {
@Override
public void onCompleted() {
Log.d("rx", "no more data");
}
@Override
public void onError(Throwable throwable) {
Log.e(“rx”, “error”, throwable);
}
@Override
public void onNext(String s) {
showResult(s);
}
});
Observer

37. Observable<String> strings = getStrings();
strings.subscribe(
new Observer<String>() {
@Override
public void onCompleted() {
Log.d("rx", "no more data");
}
@Override
public void onError(Throwable throwable) {
Log.e(“rx”, “error”, throwable);
}
@Override
public void onNext(String s) {
showResult(s);
}
});
Observer

38. Observable<String> strings = getStrings();
strings.subscribe(
new Observer<String>() {
@Override
public void onCompleted() {
Log.d("rx", "no more data");
}
@Override
public void onError(Throwable throwable) {
Log.e(“rx”, “error”, throwable);
}
@Override
public void onNext(String s) {
showResult(s);
}
});
Observer

39. Observable<String> strings = getStrings();
Subscription subsctiption = strings.subscribe(
new Observer<String>() {
@Override
public void onCompleted() {
Log.d("rx", "no more data");
}
@Override
public void onError(Throwable throwable) {
Log.e(“rx”, “error”, throwable);
}
@Override
public void onNext(String s) {
showResult(s);
}
});
Observer

40. Subscription
Subscription subsctiption;
subscription.unsubscribe();
subscription.isUnsubscribed();

41. Android & RxJava

42. RxAndroid
Android specific things for Rx
• Scheduler
• reliable and thread-safe with regarding
Fragment/Activity life-cycle
• reactive components for Android use cases
and UI

43. RxAndroid
• https://github.com/ReactiveX/RxAndroid
• 'io.reactivex:rxandroid:0.23.0'
• formerly rxjava-android

44. Image Loading
Observable<Bitmap> imageObservable =
Observable.create(observer -> {
return downloadBitmap();
});
imageObservable
.subscribe(image -> loadToImageView(image));

45. Image Loading
Observable<Bitmap> imageObservable =
Observable.create(observer -> {
return downloadBitmap();
});
imageObservable
.subscribe(image -> loadToImageView(image));

46. Image Loading
imageObservable
.subscribeOn(Schedulers.newThread())
.subscribe(image -> loadToImageView(image));

47. Image Loading
imageObservable
.subscribeOn(Schedulers.newThread())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(image -> loadToImageView(image));

48. Image Loading
imageObservable
.subscribeOn(Schedulers.newThread())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(image -> loadToImageView(image));

49. public void onDestroy() {
super.onDestroy();
subscription.unsubscribe();
}
Lifecycle & Memory Leaks

50. Fragment/Activity Lifecycle
ContentObservable
.bindFragment(fragment, observable);
ContentObservable
.bindActivity(activity, observable);

51. Broadcast Receiver
Observable<Intent> observable =
ContentObservable
.fromBroadcast(context, intentFilter);
Observable<Intent> observable =
ContentObservable
.fromLocalBroadcast(context, intentFilter);

52. Views
Observable<OnClickEvent> observable =
ViewObservable.clicks(view);
ViewObservable
.bindView(view, observable);

53. Creating Observables

54. Creating Observables
• manually from scratch
create()

55. Creating Observables
• convert existing data structure into an
Observable
from()
– Iterable, Future, Array
repeat()
– emit source Observable repeatedly

56. Creating Observables
• convert existing data structure into an
Observable
timer()
– emits a single item after a given time

57. Creating Observables
• convert existing data structure into an
Observable
empty()
– emits nothing and completes
error()
– emits nothing and signals an error
never()
– emits nothing at all

58. Obervables are composable
transform - map, flatMap, reduce, scan
filter - take, skip, sample, filter
combine - concat, merge, zip, cache
concurrency - observeOn, subcribeOn
error handling - onErrorReturn

59. Transforming Observables

60. Transforming Observables
map()
– apply a function to each item

61. Transforming Observables
flatMap()
– transform items into Observables and
flatten

62. Transforming Observables
scan()
– apply a function to each item
sequentially and emit each successive
value

63. Transforming Observables
groupBy()
– divide into a set of Observables

64. Transforming Observables
buffer()
– periodically gather items into bundles
and emits these bundles

65. Filtering Observables

66. Filtering Observable
filter()
– filter emitted items

67. Filtering Observable
take()
– emit only first n items

68. Filtering Observable
timeout()
– emit items, but issue an exception if
nothing emitted in a timespan

69. Filtering Observable
distinct()
– suppress duplicate items

70. Filtering Observable
takeLastBuffer()
first()
elementAt()
– emit only n-th element
ignoreElements()
– pass only error or completed

71. Combining Observables

72. Combining Observables
merge()
– combine multiple Observables

73. Combining Observables
zip()
– combine multiple Observables via a
function, emit results of the function

74. Error Handling

75. Error Handling
recovering from onError()
onErrorResumeNext()
– emit a sequence if error happens

76. Error Handling
onErrorReturn()
– emit an item if error happens

77. Error Handling
retry()
– resubscribe to a source

78. Support for RxJava in other
libraries

79. Retrofit
@GET("/user/{id}/photo")
Observable<Photo> getUserPhoto(@Path("id") int id);

80. private Object simpleMethod() { ... }
public Observable<Object> newMethod() {
return Observable.just(simpleMethod());
}
Plain old library

81. private Object simpleMethod() { ... }
public Observable<Object> newMethod() {
return Observable.defer(() ->
Observable.just(simpleMethod())
);
}
Plain old library

82. Java 8 & Android
No lambdas on Android
– use different language on Android
– or rather not

83. Retrolambda
transforms Java 8 compiled bytecode for
older JVM
• gradle-retrolambda
– https://github.com/evant/gradle-retrolambda
– 'me.tatarka:gradle-retrolambda:2.4.1'
• retrolambda
– https://github.com/orfjackal/retrolambda

84. strings.map(new Func1<String, Integer>() {
@Override
public Integer call(String s) {
return s.length();
}
});
Without Retrolambda

85. strings.map(
(String s) -> { return s.length(); }
);
With Retrolambda

86. Problems
• RxAndroid APIs not yet stabilized even
though RxJava hitting version 1.0
– RxAndroid version 0.23
• backpressure

87. Questions?

88. THE END