RxJava Easy Wins - Droidcon NYC 2014

Everyone keeps talking about Reactive Programming and RxJava, but how is it actually used in Android programming? This talk will discuss the building blocks of reactive programming, Observables, and how to understand the core parts of push-not-pull programming. Learn how to easily do work on a background thread in one line while still updating your UI on the main thread, easily implement a Publish-Subscribe pattern with Observables and Subjects, and take advantage of RxJava to make form-based UIs simple. Finally, we'll look at how using Observables can improve testing and ensure that your code will not block the UI thread if operations are blocked.

Transcript

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

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2. 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:+'
   

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

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

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5. A "real-world" example of Reactive Programming
   

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

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

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

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9. 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
   

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

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

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

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13. 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!
    

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14. Java 8 to
    the Rescue!
    

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15. Java 8 to
    the Rescue!
    Java 8 to
    the Rescue!
    

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

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17. 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!"));
    

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18. 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
    

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19. 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)
    

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20. So why would I actually use this?
    Ok.
    

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21. Long running tasks
    

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22. (Potentially) long running tasks
    

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

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

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25. (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));
    

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26. (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));
    

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27. Observables + SQL
    

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

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29. 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);
    

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

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31. 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:
    

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32. 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 -> /* ... */);
    

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33. Observables have a semi-declaritive syntax
    

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34. 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);
    

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35. 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!
    

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36. Form Validation
    

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

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38. Form Validation
    

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39. 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);
    }
    });
    

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40. 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
    

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41. Composability
    

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42. sp?
    Composability
    

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43. API Client
    

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

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45. 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(/* ... */);
    

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46. 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(/* ... */);
    

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47. API Client
    

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

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49. 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 = // ...
    

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50. 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.
    !
    

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51. RxEventBus
    

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52. 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
    

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53. 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);
    }
    }
    

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54. 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);
    }
    }
    

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55. 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);
    }
    }
    

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56. Simplified Testing
    

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

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58. 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()
    

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59. 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
    

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60. 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)
    

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61. 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
    

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

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