Ron Shapiro · @rdshapiro DroidconNYC 2014 Easy wins to help get you in the groove RxJava + Android

Quick Survey Who has played around with RxJava? Who uses RxJava in production? compile 'io.reactivex:rxjava:1.+' formerly: compile 'com.netflix.rxjava:rxjava-core:+'

What is Rx? Let's take a step back. Imperative programming:

What is Rx? int a = 1; int b = a + 2; a = 5; assertTrue(3, b); Let's take a step back. Imperative programming:

A "real-world" example of Reactive Programming

A "real-world" example of Reactive Programming Excel spreadsheet:

A "real-world" example of Reactive Programming Excel spreadsheet: A 1 2 2 4 3 =A1 + A2

A "real-world" example of Reactive Programming Excel spreadsheet: A 1 2 2 4 3 =A1 + A2 A 1 2 2 4 3 6

A "real-world" example of Reactive Programming Excel spreadsheet: A 1 2 2 4 3 =A1 + A2 A 1 2 2 4 3 6 A 1 2 2 5 3 7

RxJava Contract void onNext(T t); void onError(Throwable throwable); void onCompleted();

RxJava Contract void onNext(T t); void onError(Throwable throwable); void onCompleted(); T get(); Iterable get(); vs. Imperative Programming

RxJava Contract void onNext(T t); void onError(Throwable throwable); void onCompleted(); T get() throws Throwable; Iterable get() throws Throwable; vs. Imperative Programming

In Action Observable.just(1, 2, 3, 4) .subscribe(new Subscriber() { @Override public void onNext(Integer integer) { System.out.println(integer); } @Override public void onCompleted() { System.out.println("Done!"); } ! @Override public void onError(Throwable e) { System.err.println("An error occurred: " + e.getMessage()); } }); ! > 1 > 2 > 3 > 4 > Done!

Java 8 to the Rescue!

Java 8 to the Rescue! Java 8 to the Rescue!

Java 8 to the Rescue! Java 8 to the Rescue! Yes, on Android!

Retrolambda ! https://github.com/evant/gradle-retrolambda Observable.just(1, 2, 3, 4).subscribe( integer -> System.out.println(integer), error -> System.err.println("An error occurred: " + error.getMessage()), () -> System.out.println(“Done!"));

Observable.just(1, 2, 3, 4).subscribe( + integer -> System.out.println(integer), - System.out::println, error -> System.err.println("An error occurred: " + error.getMessage()), () -> System.out.println("Done!")); Retrolambda ! https://github.com/evant/gradle-retrolambda

Observable.just(1, 2, 3, 4).subscribe( + integer -> System.out.println(integer), - System.out::println, error -> System.err.println("An error occurred: " + error.getMessage()), () -> System.out.println("Done!")); Retrolambda ! https://github.com/evant/gradle-retrolambda Not only is this more concise, Java 8 lambdas don't have an implicit reference to their enclosing block's class like anonymous classes do (unless you explicitly refer to MyClass.this)

So why would I actually use this? Ok.

Long running tasks

(Potentially) long running tasks

(Potentially) long running tasks db.loadSomeData() .subscribe(data -> textView.setText(data.toString));

(Potentially) long running tasks db.loadSomeData() + .subscribeOn(Schedulers.io()) + .observeOn(AndroidSchedulers.mainThread()) .subscribe(data -> textView.setText(data.toString));

(Potentially) long running tasks Observable loadSomeData() { // loading is un-opinionated about // what thread it is running on } db.loadSomeData() + .subscribeOn(Schedulers.io()) + .observeOn(AndroidSchedulers.mainThread()) .subscribe(data -> textView.setText(data.toString));

(Potentially) long running tasks Observable loadSomeData() { // loading is un-opinionated about // what thread it is running on } db.loadSomeData() .subscribeOn(Schedulers.newThread()) .observeOn(AndroidSchedulers.mainThread()) .subscribe(data -> textView.setText(data.toString));

Observables + SQL

Observables + SQL shameless plug for github.com/venmo/cursor-utils

Observables + SQL shameless plug for github.com/venmo/cursor-utils IterableCursor: turn a Cursor row into a T IterableCursorAdapter adapter = new IterableCursorAdapter(context); db.loadSomeData() .subscribeOn(Schedulers.io()) .observeOn(AndroidSchedulers.mainThread()) .subscribe(adapter::changeCursor);

Observables + SQL Because IterableCursor extends Iterable, you can easily transform your queries into Observables:

Observables + SQL db.loadSomeData() .subscribeOn(Schedulers.io()) .observeOn(AndroidSchedulers.mainThread()) .flatMap(cursor -> Observable.from(cursor)) .onNext(pojo -> /* ... */); Because IterableCursor extends Iterable, you can easily transform your queries into Observables:

Observables + SQL Because IterableCursor extends Iterable, you can easily transform your queries into Observables: db.loadSomeData() .subscribeOn(Schedulers.io()) .observeOn(AndroidSchedulers.mainThread()) .flatMap(cursor -> Observable.from(cursor)) - .onNext(pojo -> /* ... */); + .toList() + .onNext(pojoList -> /* ... */);

Observables have a semi-declaritive syntax

Observables have a semi-declaritive syntax // detect when the user pauses typing, // then search the api ViewObservable.text(myTextView) .map(TextView::getText) .delay(400, TimeUnit.MILLISECONDS) .filter(delayedText -> TextUtils.equals( delayedText, myTextView.getText())) .subscribe(api::search);

Observables have a semi-declaritive syntax // detect when the user pauses typing, // then search the api ViewObservable.text(myTextView) .map(TextView::getText) + .map(CharSequence::toString()) .delay(400, TimeUnit.MILLISECONDS) .filter(delayedText -> TextUtils.equals( delayedText, myTextView.getText())) .subscribe(api::search); // make sure you're always dealing with immutable objects!

Form Validation

Form Validation Observable validName = ViewObservable .text(nameTextView) .map(view -> TextUtils.isEmpty(view.getText())); Observable validTermsOfService = ViewObservable.input(tosCheckBox, true); Observable validBirthdate = // ...

Form Validation

Form Validation Func3 combinator = (name, tos, date) -> name && tos && date); Observable.combineLatest(validName, validTermsOfService, validBirthdate, combinator) .subscribe(allValid -> { if (allValid) { submitButton.setVisibility(View.VISIBLE); } else { submitButton.setVisibility(View.GONE); } });

Form Validation Func3 combinator = (name, tos, date) -> name && tos && date); Observable.combineLatest(validName, validTermsOfService, validBirthdate, combinator) .subscribe(allValid -> { if (allValid) { submitButton.setVisibility(View.VISIBLE); } else { submitButton.setVisibility(View.GONE); } }); combineLatest and FuncX are type-safe! Adding a new input source in one area but not the other will break at compile-time instead of a silent runtime mistake

Composability

sp? Composability

API Client

API Client Observable getMetaData() {/* ... */} Observable postPicture(String url, Bitmap b) {/* ... */} Observable linkPostToPermalink(Strind id, Permalink p) {/* ... */}

API Client Observable getMetaData() {/* ... */} Observable postPicture(String url, Bitmap b) {/* ... */} Observable linkPostToPermalink(Strind id, Permalink p) {/* ... */} ! // if any of these fail, i.e. call `onError` instead of `onCompleted`, // the subsequent calls are not made getMetaData() .flatMap(metadata -> postPicture(metadata.getUrl(), bitmap) .flatMap(permalink -> linkPostToPermalink(postId, permalink) .subscribe(/* ... */);

API Client Observable getMetaData() {/* ... */} Observable postPicture(String url, Bitmap b) {/* ... */} Observable linkPostToPermalink(Strind id, Permalink p) {/* ... */} ! // if any of these fail, i.e. call `onError` instead of `onCompleted`, // the subsequent calls are not made getMetaData() .flatMap(metadata -> postPicture(metadata.getUrl(), bitmap) .flatMap(permalink -> linkPostToPermalink(postId, permalink) + .subscribeOn(Schedulers.io()) + .observeOn(AndroidSchedulers.mainThread()) .subscribe(/* ... */);

API Client

API Client Observable first = // ... Observable second = // ... Observable third = // ... Func3 combineFunc = // ...

API Client // wait for all 3 to complete Observable.combineLatest(first, second, third, combineFunc) .subscribe((R r) -> doSomethingAwesome()); Observable first = // ... Observable second = // ... Observable third = // ... Func3 combineFunc = // ...

API Client // wait for all 3 to complete Observable.combineLatest(first, second, third, combineFunc) .subscribe((R r) -> doSomethingAwesome()); Observable first = // ... Observable second = // ... Observable third = // ... Func3 combineFunc = // ... Make sure each of first, second, and third handles off-loading/concurrency concerns. !

RxEventBus

RxEventBus Subject // observe a T, emit an R (can be the same) AsyncSubject: last emitted item BehaviorSubject: most recent + all future items PublishSubject: all future items ReplaySubject: caches all* items, emits each on subscribing

RxEventBus class RxEventBus { private Subject subject = PublishSubject.create(); Subscription subscribe(Action1 super T> action, Class clazz) { return subject.ofType(clazz).subscribe(action); } void post(T t) { subject.onNext(t); } void post(Observable observable) { observable.subscribe(subject); } }

RxEventBus class RxEventBus { // ... /** Forwarding terminal events will terminate your Subject! */ void post(Observable observable) { observable.materialize().map((Notification notification) -> { if (notification.isOnNext()) { return notification.getValue(); } else if (notification.isOnError()) { return new OnErrorEvent(notification.getThrowable()); } return new OnCompletedEvent(); }).subscribe(subject); } }

RxEventBus class RxEventBus { // ... /** Forwarding terminal events will terminate your Subject! */ void post(Observable observable) { observable.materialize().map((Notification notification) -> { if (notification.isOnNext()) { return notification.getValue(); } else if (notification.isOnError()) { return new OnErrorEvent(notification.getThrowable()); } return new OnCompletedEvent(); }).subscribe(subject); } }

Simplified Testing

Simplified Testing Observable sleepObservable(long timeout, TimeUnit unit) { return Observable.create((Subscriber super T> subscriber) -> { SystemClock.sleep(unit.toMillis(timeout)); subscriber.onCompleted(); }); }

Simplified Testing Observable sleepObservable(long timeout, TimeUnit unit) { return Observable.create((Subscriber super T> subscriber) -> { SystemClock.sleep(unit.toMillis(timeout)); subscriber.onCompleted(); }); } Database db = Mockito.spy(db); doAnswer(mockedMethod -> { return sleepObservable(2, SECONDS) .concatWith(mockedMethod.callRealMethod()); }).when(db).loadSomeData()

Simplified Testing AtomicBoolean wasClicked = new AtomicBoolean(false); new Thread(() -> { SystemClock.sleep(1000); assertTrue(wasClicked.get()); }).start(); onView(withId(R.id.my_view) .perform(setOnClickListener((v) -> wasClicked.set(true)) .perform(click(); // if your data is being loaded on the main-thread, // the view will not be clicked

Look familiar? UNEXPECTED TOP-LEVEL EXCEPTION: java.lang.IllegalArgumentException: method ID not in [0, 0xffff]: 65536 at com.android.dx.merge.DexMerger$6.updateIndex(DexMerger.java:501) at com.android.dx.merge.DexMerger$IdMerger.mergeSorted(DexMerger.java:276) at com.android.dx.merge.DexMerger.mergeMethodIds(DexMerger.java:490) at com.android.dx.merge.DexMerger.mergeDexes(DexMerger.java:167) at com.android.dx.merge.DexMerger.merge(DexMerger.java:188) at com.android.dx.command.dexer.Main.mergeLibraryDexBuffers(Main.java:439) at com.android.dx.command.dexer.Main.runMonoDex(Main.java:287) at com.android.dx.command.dexer.Main.run(Main.java:230) at com.android.dx.command.dexer.Main.main(Main.java:199) at com.android.dx.command.Main.main(Main.java:103)

Look familiar? UNEXPECTED TOP-LEVEL EXCEPTION: java.lang.IllegalArgumentException: method ID not in [0, 0xffff]: 65536 at com.android.dx.merge.DexMerger$6.updateIndex(DexMerger.java:501) at com.android.dx.merge.DexMerger$IdMerger.mergeSorted(DexMerger.java:276) at com.android.dx.merge.DexMerger.mergeMethodIds(DexMerger.java:490) at com.android.dx.merge.DexMerger.mergeDexes(DexMerger.java:167) at com.android.dx.merge.DexMerger.merge(DexMerger.java:188) at com.android.dx.command.dexer.Main.mergeLibraryDexBuffers(Main.java:439) at com.android.dx.command.dexer.Main.runMonoDex(Main.java:287) at com.android.dx.command.dexer.Main.run(Main.java:230) at com.android.dx.command.dexer.Main.main(Main.java:199) at com.android.dx.command.Main.main(Main.java:103) RxJava is pretty big - you may/may not need ProGuard

Ron Shapiro shapiro.rd@gmail.com @rdshapiro ! venmo.com/ronshapiro Questions?