RxJava for the rest of us Hugo Visser @botteaap [email protected] 

// Everything is awesome and simple! (pulling the result) public Article getArticle() throws Exception {} // Asynchronous callback (awaiting the result) public void getArticle(ArticleCallback callback) {} // Asynchronous execution w/ callback on main thread public AsyncTask fetchArticle() {} Typical code 

rx.Observable ● Nothing is executed when you call this method! Pass it around! ● Synchronous or asynchronous (& cancelable) ● Error handling public Observable article() {} 

rx.Observable - “The stream” ● Value (onNext) ● Error + termination (onError) ● Completion + termination (onComplete) public Observable article() {} 

Subscription Subscription s = article().subscribe(new Subscriber() { @Override public void onNext(Article article) { // this can be called multiple times } @Override public void onError(Throwable e) { // no more events, and automatically unsubscribed } @Override public void onCompleted() { // no more events, and automatically unsubscribed } }); 

Subscription Subscription s = article().subscribe(new Action1() { @Override public void call(Article article) { } }, new Action1() { @Override public void call(Throwable throwable) { } }, new Action0() { @Override public void call() { } }); 

Subscription (lambda expressions) Subscription s = article().subscribe( article -> { ... }, error -> { ... }, () -> { ... }); 

No longer interested s.unsubscribe(); ● Always unsubscribe when done! (e.g. life cycle) ● Memory leaks may occur otherwise ● Don’t “lose” subscriptions 

Steep learning curve? 

Architectural change A different way requires different thinking 

Architectural change Stream of events vs a single value 

Architectural change Does this stream ever complete? 

Architectural change Am I handling errors correctly? 

Functional style Immutable types, chaining of operations 

Operators Lots of them! 

Documentation Might be daunting (generics, marble diagrams) 

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Gentle start ● Don’t try to go all out “reactive” ● Apply where it makes sense ○ Domain layer / business logic ○ REST calls (Retrofit) ○ When coping with life cycle ○ More specialised event bus ● UI, maybe, but save it for later and/or specific cases. 

Convert from Article to rfidTag string public Observable rfIdTags(Observable observable) { // map = convert an object to another object return observable.map(article -> article.rfIdTag); } 

Convert from Article to rfidTag string public Observable rfIdTags(Observable observable) { // map = convert an object to another object return observable.map(article -> article.rfIdTag); } 

Convert from Article to rfidTag string public Observable rfIdTags(Observable observable) { // map = convert an object to another object return observable.map(article -> article.rfIdTag); } 

Convert from Article to rfidTag string public Observable rfIdTags(Observable observable) { // map = convert an object to another object return observable.map(article -> article.rfIdTag); } 

Convert from Article to rfidTag string public Observable rfIdTags(Observable observable) { // map = convert an object to another object return observable.map(article -> article.rfIdTag); } 

Count (group) same articles 1. Only proces articles that are supposed to be in stock 2. Group all articles by the article id, derived from the rfid tag 3. Count all articles in the group 4. Emit one ArticleQuantity object per group 

Count (group) same articles public Observable quantity(Observable observable) { return observable.filter(article -> article.isInStock). groupBy(this::articleIdFromTag). // convert each group to a new observable flatMap(group -> // each item in the group is converted to ArticleQuantity group.map(articleInfo -> new ArticleQuantity(group.getKey(), 1)). // then these items are reduced to a single item per group reduce((q1, q2) -> new ArticleQuantity(q1.articleId, q1.quantity + q2.quantity)) ); } 

Count (group) same articles public Observable quantity(Observable observable) { return observable.filter(article -> article.isInStock). groupBy(this::articleIdFromTag). // convert each group to a new observable flatMap(group -> // each item in the group is converted to ArticleQuantity group.map(article -> new ArticleQuantity(group.getKey(), 1)). // then these items are reduced to a single item per group reduce((q1, q2) -> new ArticleQuantity(q1.articleId, q1.quantity + q2.quantity)) ); } 

Count (group) same articles public Observable quantity(Observable observable) { return observable.filter(article -> article.isInStock). groupBy(this::articleIdFromTag). // convert each group to a new observable flatMap(group -> // each item in the group is converted to ArticleQuantity group.map(article -> new ArticleQuantity(group.getKey(), 1)). // then these items are reduced to a single item per group reduce((q1, q2) -> new ArticleQuantity(q1.articleId, q1.quantity + q2.quantity)) ); } 

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Count (group) same articles public Observable quantity(Observable observable) { return observable.filter(article -> article.isInStock). groupBy(this::articleIdFromTag). // convert each group to a new observable // group type: Observable> flatMap(group -> // GroupObservable // each item in the group is converted to ArticleQuantity group.map(article -> new ArticleQuantity(group.getKey(), 1)). // then these items are reduced to a single item per group reduce((q1, q2) -> new ArticleQuantity(q1.articleId, q1.quantity + q2.quantity)) ); } 

Count (group) same articles public Observable quantity(Observable observable) { return observable.filter(article -> article.isInStock). groupBy(this::articleIdFromTag). // convert each group to a new observable // group type: Observable> flatMap(group -> // GroupObservable // each item in the group is converted to ArticleQuantity group.map(article -> new ArticleQuantity(group.getKey(), 1)). // then these items are reduced to a single item per group reduce((q1, q2) -> new ArticleQuantity(q1.articleId, q1.quantity + q2.quantity)) ); } 

Count (group) same articles public Observable quantity(Observable observable) { return observable.filter(article -> article.isInStock). groupBy(this::articleIdFromTag). // convert each group to a new observable flatMap(group -> // each item in the group is converted to ArticleQuantity group.map(articleInfo -> new ArticleQuantity(group.getKey(), 1)). // then these items are reduced to a single item per group reduce((q1, q2) -> new ArticleQuantity(q1.articleId, q1.quantity + q2.quantity)) ); } 

Combine the list of all rfidTags and the expected ArticleQuantities Expected stock Observable expectedStock(Observable articles) { Observable observable = articles.publish().autoConnect(2); return Observable.combineLatest(rfIdTags(observable).toList(), quantity(observable).toList(), (rfidTags, quantities) -> new ExpectedStock(rfidTags, quantities)); } 

Combine the list of all rfidTags and the expected ArticleQuantities Expected stock Observable expectedStock(Observable articles) { Observable observable = articles.publish().autoConnect(2); return Observable.combineLatest(rfIdTags(observable).toList(), quantity(observable).toList(), (rfidTags, quantities) -> new ExpectedStock(rfidTags, quantities)); } 

Postpone subscription to the source until connect() is called ConnectableObservable ConnectableObservable connectable = observable.publish(); Subscription s1 = connectable.subscribe(); Subscription s2 = connectable.subscribe(); connectable.connect(); 

Connect only after a specific number of subscriptions AutoConnect ConnectableObservable connectable = observable.publish().autoConnect(2); Subscription s1 = connectable.subscribe(); Subscription s2 = connectable.subscribe(); // happens automatically now // connectable.connect(); 

Combine the list of all rfidTags and the expected ArticleQuantities Expected stock Observable expectedStock(Observable articles) { Observable observable = articles.publish().autoConnect(2); return Observable.combineLatest(rfIdTags(observable).toList(), quantity(observable).toList(), (rfidTags, quantities) -> new ExpectedStock(rfidTags, quantities)); } 

Schedulers (threading) ● Schedulers.io(), Schedulers.computation(), Schedulers.from(executor) … ● AndroidSchedulers.mainThread() from rxandroid ● subscribeOn() and observeOn() operators 

Schedulers (threading) ● subscribeOn controls the thread when subscribed (apply once!) ● observeOn controls the thread for events ● Operators with a default scheduler, like timer() observable.map(...).subscribeOn(io()).observeOn(mainThread()).flatMap(...); 

Common mistakes 

Subscriptions ● Not keeping references → unable to cancel + possible memory leaks public class MyActivity extends Activity { private CompositeSubscription mSubscriptions = new CompositeSubscription(); @Override protected void onStart() { super.onStart(); mSubscriptions.add(createObservable().subscribe(...)); } @Override protected void onStop() { super.onStop(); // use clear(), not unsubscribe() mSubscriptions.clear(); } } 

Subscriptions ● Error handling (or lack thereof) ● Always log to prevent swallowing unexpected errors ● rxlint lint check can help and flag missing error handlers ● // Oops, no error handling! createObservable().subscribe(value -> { … }) // errors are handled here createObservable().subscribe(value -> { … }, error -> { … }) 

Subscriptions ● Rock it like it’s AsyncTask public void fetchValueFromTheNetwork() { createObservable().subscribeOn(Schedulers.io()). observeOn(AndroidSchedulers.mainThread()). subscribe(value -> { EventBus.getDefault().post(new ValueEvent(value)); }, error -> { EventBus.getDefault().post(new ValueErrorEvent(error)); }); } 

Subscriptions ● Rock it like it’s AsyncTask public Observable fetchValueFromTheNetwork() { return createObservable().subscribeOn(Schedulers.io()). observeOn(AndroidSchedulers.mainThread()); } mSubscriptions.add(fetchValueFromTheNetwork().subscribe(value -> { … }, error -> { … })); 

Breaking the stream ● For side effects use doOnXXX() ● Don’t subscribe() in an operator, but flatMap createObservable().map(value -> { // this is not the place to subscribe or call // other functions. createAnotherObservable().subscribe( … ); return value; }); 

Tips ● Compose observables (aid testing, prevent huge chains) ● Apply schedulers late as possible for better API // this gets boring and cumbersome retrieveArticle(). subscribeOn(Schedulers.io()). observeOn(AndroidSchedulers.mainThread()). subscribe(article -> {}); 

Tips ● Compose observables (aid testing, prevent huge chains) ● Apply schedulers late as possible for better API /** * Retrieve a single article. Will return the result on the main thread * @return the observable */ public Observable retrieveArticle() { return articleHttpCall(). subscribeOn(Schedulers.io()). observeOn(AndroidSchedulers.mainThread()); } // does the right thing retrieveArticle().subscribe(article -> { … }, … ); 

Conclusion ● RxJava is a different way of doing things ● Very powerful, but takes time and practice to learn ● Take small steps learning it 

onCompleted() @botteaap [email protected]