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The Mayans Lost Guide To RxJava on Android

@fernando_cejas

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#Reactive

1- of, relating to, or marked by reaction or reactance.

2- readily responsive to a stimulus.

RxJava is a Java VM implementation of Reactive X (Reactive Extensions): ˝

a library for composing asynchronous and event-based programs by using

observable sequences.

What is RxJava?

Why should we go

reactive?

Multithreading is always complex˝

Concurrency˝Java Futures are Expensive to Compose˝

Java Futures˝Callbacks Have Their Own Problems˝

Callbacks˝@benjchristensen from @netflix

public class FuturesA {

public static void run() throws Exception { ExecutorService executor = new ThreadPoolExecutor(4, 4, 1, TimeUnit.MINUTES, new LinkedBlockingQueue<Runnable>());

Future<String> f1 = executor.submit(new CallToRemoteServiceA()); Future<String> f2 = executor.submit(new CallToRemoteServiceB());

System.out.println(f1.get() + " - " + f2.get()); }

}

https://gist.github.com/benjchristensen/4670979

#Java Futures

#Java Futures

https://gist.github.com/benjchristensen/4671081

public static void run() throws Exception { ExecutorService executor = new ThreadPoolExecutor(4, 4, 1, TimeUnit.MINUTES, new LinkedBlockingQueue<Runnable>()); try { // get f3 with dependent result from f1 Future<String> f1 = executor.submit(new CallToRemoteServiceA()); Future<String> f3 = executor.submit(new CallToRemoteServiceC(f1.get()));

/* The work below can not proceed until f1.get() completes even though there is no dependency */

// also get f4/f5 after dependency f2 completes Future<Integer> f2 = executor.submit(new CallToRemoteServiceB()); Future<Integer> f4 = executor.submit(new CallToRemoteServiceD(f2.get())); Future<Integer> f5 = executor.submit(new CallToRemoteServiceE(f2.get()));

System.out.println(f3.get() + " => " + (f4.get() * f5.get())); } finally { executor.shutdownNow(); }}

#Java Futures

https://gist.github.com/benjchristensen/4671081

public static void run4() throws Exception { ExecutorService executor = new ThreadPoolExecutor(4, 4, 1, TimeUnit.MINUTES, new LinkedBlockingQueue<Runnable>()); try { List<Future<?>> futures = new ArrayList<Future<?>>(); // kick off several async tasks futures.add(executor.submit(new CallToRemoteServiceA())); futures.add(executor.submit(new CallToRemoteServiceB())); futures.add(executor.submit(new CallToRemoteServiceC("A"))); futures.add(executor.submit(new CallToRemoteServiceC("B"))); futures.add(executor.submit(new CallToRemoteServiceD(1))); futures.add(executor.submit(new CallToRemoteServiceE(2))); futures.add(executor.submit(new CallToRemoteServiceE(3)));

// as each completes do further work for (Future<?> f : futures) { /* this blocks so even if other futures in the list complete earlier they will wait until this one is done */ doMoreWork(f.get()); } } finally { executor.shutdownNow(); } }

Multithreading is always complex˝

Concurrency˝Java Futures are Expensive to Compose˝

Java Futures˝Callbacks Have Their Own Problems˝

Callbacks˝@benjchristensen from @netflix

#Callbacks

https://gist.github.com/benjchristensen/4677544

...// get f3 with dependent result from f1executor.execute(new CallToRemoteServiceA(new Callback<String>() {

@Override public void call(String f1) { executor.execute(new CallToRemoteServiceC(new Callback<String>() {

@Override public void call(String f3) { // we have f1 and f3 now need to compose with others System.out.println("intermediate callback: " + f3 + " => " + ("f4 * f5")); // set to thread-safe variable accessible by external scope f3Value.set(f3); latch.countDown(); }

}, f1)); }

}));...

Observables

Subscribers

Subscriptions

Schedulers

Operators

#RxJava Guys:

Observables

The Observable object is who does the job.

Represents an object that sends notifications (Provider) to a

Subscriptor (Observer).

ObservablesAdd 2 missing semantics to the

Observer pattern:

#1: Emits a signal to the consumer when there is no more data available.

#2: Emits a signal to the consumer when an error has occurred.

Observables

Observables

Subscribers

Subscribers provides a mechanism for receiving push-

based notifications from Observables, and permits

manual unsubscribing from these Observables.

SubscribersNot an observer pattern:

Observables often don't start emitting items until

someone explicitly subscribes to them.

Subscribers

Observables often don't start emitting items until

someone explicitly subscribes to them.

Subscriberspublic class DefaultSubscriber<T> extends rx.Subscriber<T> { @Override public void onCompleted() { }

@Override public void onError(Throwable e) { }

@Override public void onNext(T t) { }}

Subscriptions

Subscriptions represents the link between your Observable and

your Subscriber.

#1: Subscriptions

#2: CompositeSubscriptions

#1: Schedulers.io()

#2: Schedulers.computation()

#3: Schedulers.from()

SchedulersIf you want to introduce multithreading into your cascade of Observable operators, you

can do so by instructing those operators (or particular Observables) to operate on

particular Schedulers.

Operators

Operators can be used in between the source Observable and the ultimate Subscriber to

manipulate emitted items.

You can even write your own custom operators.

map()Transform the items emitted by an Observable by applying a function to each item.

flatMap()Transforms the items emitted by an Observable into Observables, then flatten the emissions from those into a single Observable (no order)

concatMap()Transforms the items emitted by an Observable into Observables, then flatten the emissions from those into a single Observable (keeps order)

flatMap() vs concatMap()

http://fernandocejas.com/2015/01/11/rxjava-observable-tranformation-concatmap-vs-flatmap/

filter()Emits the same item it received, but only if it passes the boolean check (predicate).

take()Emit only the first n items emitted by an Observable

doOnNext()Allows us to add extra behavior each time an item is emitted.

onError() is called if an exception is thrown at any time.

Error handling

The operators do not have to handle the exception.

onErrorResumeNext()Instructs an Observable to emit a sequence of items if it encounters an error.

onErrorReturn()Instructs an Observable to emit a particular item when it encounters an error.

onExceptionResumeNext()Instructs an Observable to continue emitting items after it encounters an exception.

retry()If a source Observable emits an error, resubscribe to it in the hopes that it will complete without error.

retryWhen()If a source Observable emits an error, pass that error to another Observable to determine whether to resubscribe to the source.

Error handling Operators

#1: Observable and Subscriber can do anything

#2: The Observable and Subscriber are independent of the transformational steps in between them.

#3: Operators let you do anything to the stream of data.

Key ideas behind RxJava

http://blog.danlew.net/2014/09/15/grokking-rxjava-part-1/

#1: Learning curve

#2: Too many anonymous classes generated (OutOfMemory?)

#3: Verbosity (retrolambda to the rescue?)

But there are some pitfalls…

#Example: Clean Architecture

#Example: Clean Architecture

@PerActivitypublic class UserListPresenter extends DefaultSubscriber<List<User>> implements Presenter {

private UserListView viewListView; private final UseCase getUserListUseCase;

@Inject public UserListPresenter(@Named("userList") UseCase getUserListUserCase, UserModelDataMapper userModelDataMapper) { this.getUserListUseCase = getUserListUserCase; }

@Override public void destroy() { this.getUserListUseCase.unsubscribe(); }

private void getUserList() { this.getUserListUseCase.execute(this); }

...}

#Example: Reactive Presenter

#Example: Reactive Presenter

@PerActivitypublic class UserListPresenter extends DefaultSubscriber<List<User>> implements Presenter {

...

@Override public void onCompleted() { this.hideViewLoading(); }

@Override public void onError(Throwable e) { this.hideViewLoading(); this.showErrorMessage(new DefaultErrorBundle((Exception) e)); this.showViewRetry(); }

@Override public void onNext(List<User> users) { this.showUsersCollectionInView(users); }}

#Example: Clean Architecture

#Example: UseCasepublic abstract class UseCase {

private final ThreadExecutor threadExecutor; private final PostExecutionThread postExecutionThread;

private Subscription subscription = Subscriptions.empty();

protected UseCase(ThreadExecutor threadExecutor, PostExecutionThread postExecutionThread) { this.threadExecutor = threadExecutor; this.postExecutionThread = postExecutionThread; }

protected abstract Observable buildUseCaseObservable();

public void execute(Subscriber UseCaseSubscriber) { this.subscription = this.buildUseCaseObservable() .subscribeOn(Schedulers.from(threadExecutor)) .observeOn(postExecutionThread.getScheduler()) .subscribe(UseCaseSubscriber); }

public void unsubscribe() { if (!subscription.isUnsubscribed()) subscription.unsubscribe(); }}

#Example: Execution Thread

/** * MainThread (UI Thread) implementation based on a * {@link rx.Scheduler} which will execute actions on * the Android UI thread */@Singletonpublic class UIThread implements PostExecutionThread {

@Inject public UIThread() {}

@Override public Scheduler getScheduler() { return AndroidSchedulers.mainThread(); }}

#Example: UseCase

/** * This class is an implementation of {@link UseCase} that represents a * use case for retrieving a collection of all {@link User}. */public class GetUserListUseCase extends UseCase {

private final UserRepository userRepository;

@Inject public GetUserListUseCase(UserRepository userRepository, ThreadExecutor threadExecutor, PostExecutionThread postExecutionThread) { super(threadExecutor, postExecutionThread); this.userRepository = userRepository; }

@Override public Observable buildUseCaseObservable() { return this.userRepository.getUsers(); }}

#Example: Clean Architecture

#Example: Operator and Actionpublic class CloudUserDataStore implements UserDataStore { private final RestApi restApi; private final UserCache userCache;

private final Action1<UserEntity> saveToCacheAction = new Action1<UserEntity>() { @Override public void call(UserEntity userEntity) { if (userEntity != null) { CloudUserDataStore.this.userCache.put(userEntity); } } };

public CloudUserDataStore(RestApi restApi, UserCache userCache) { this.restApi = restApi; this.userCache = userCache; }

@Override public Observable<List<UserEntity>> getUserEntityList() { return this.restApi.getUserEntityList(); }

@Override public Observable<UserEntity> getUserEntityDetails(final int userId) { return this.restApi.getUserEntityById(userId).doOnNext(saveToCacheAction); }}

#Example: Data transformation@Singletonpublic class UserDataRepository implements UserRepository {

private final UserDataStoreFactory userDataStoreFactory; private final UserEntityDataMapper userEntityDataMapper;

private final Func1<List<UserEntity>, List<User>> userListEntityMapper = new Func1<List<UserEntity>, List<User>>() { @Override public List<User> call(List<UserEntity> userEntities) { return UserDataRepository.this.userEntityDataMapper.transform(userEntities); } };

private final Func1<UserEntity, User> userDetailsEntityMapper = new Func1<UserEntity, User>() { @Override public User call(UserEntity userEntity) { return UserDataRepository.this.userEntityDataMapper.transform(userEntity); } }; @Override public Observable<List<User>> getUsers() { final UserDataStore userDataStore = this.userDataStoreFactory.createCloudDataStore(); return userDataStore.getUserEntityList().map(userListEntityMapper); }}

#Example: Observable creation

@Override public Observable<List<UserEntity>> getUserEntityList() {return Observable.create(new Observable.OnSubscribe<List<UserEntity>>() { @Override public void call(Subscriber<? super List<UserEntity>> subscriber) {

if (isThereInternetConnection()) { try { String responseUserEntities = getUserEntitiesFromApi(); subscriber.onNext(userEntityJsonMapper.transformUserEntityCollection( responseUserEntities)); subscriber.onCompleted(); } catch (Exception e) { subscriber.onError(new NetworkConnectionException(e.getCause())); } } else { subscriber.onError(new NetworkConnectionException()); } }});}

#1: Good starting point to switch to Rx Observables.

#2: No need to deal with threading an synchronization.

#3: Very simple to wrap an http connection in an Observable

How do I start with RxJava?

Rx at data level

#1: We can convert our events into Rx Observables

How do I start with RxJava?

Rx at view level

Observable input = Observable.FromEventPattern(textView, "TextChanged") .Select(_ => textbox.Text) .Throttle(TimeSpan.FromSeconds(0.5)) .DistinctUntilChanged();

#1: You will return Rx Observables in domain layer.

#2: Be careful with side effects (Rx Schedulers other than UI Thread)

How do I start with RxJava?

Rx at domain level

#1: By default, RxJava is synchronous.

#2: onSubscribe() is executed separately for every new subscriber.

#3: Subscriptions leak memory.

Tips and Tricks

#4: Read the official documentation

References

Reactive Programming on Android With RxJavahttps://mttkay.github.io/blog/2013/08/25/functional-reactive-programming-on-android-with-rxjava/

Grokking RxJavahttp://blog.danlew.net/2014/09/15/grokking-rxjava-part-1/

Reactive Programming in the Netflix API with RxJavahttp://techblog.netflix.com/2013/02/rxjava-netflix-api.html

Rx for .NET and RxJava for Androidhttp://futurice.com/blog/tech-pick-of-the-week-rx-for-net-and-rxjava-for-android

https://github.com/android10/Android-CleanArchitecture

Official Documentationhttps://github.com/ReactiveX/RxJava/wiki

https://github.com/android10/Android-ReactiveProgramming

?˝

Source: US Census Bureau

Questions

@fernando_cejas

http://soundcloud.com/jobs

Thanks!