RxJava for Android Developers (Austin Droids Meetup)

The principles of Reactive Programming will be introduced, using RxJava and other related libraries as a vehicle.

Specific examples targeting the Android platform will be demonstrated.

No prior knowledge of Reactive Programming or RxJava is needed.

Presented at http://www.meetup.com/Austin-Android/events/226870556/

Example code: https://github.com/jpetitto/rxjava-android-example

Written narrative: http://johnpetitto.com/rxjava-android-devs/

Transcript

1. RxJava for Android Developers John Petitto IBM Mobile Innovation Lab
2. Android Development Deals with lots of asynchronous programming • Can’t block the main thread • I/O (network, storage access) • Other long running operations
3. Async Programming is Hard Composition
4. Async Programming is Hard Composition + Concurrency
5. Async Programming is Hard Composition + Concurrency + Error Handling
6. Async Programming is Hard Composition + Concurrency + Error Handling Each of these are hard enough on their own
7. Traditional Android Developer’s Toolbox • Lots of callbacks • AsyncTask of course • Handlers + Loopers • Fall back to java.util.concurrent from time to time • Threads? None of these help address all three issues
8. RxJava to the rescue!
9. So what is RxJava, anyways? RxJava is a library for producing, composing and consuming asynchronous streams. It provides powerful abstractions for concurrency and makes error handling a breeze.
10. Observables (Producers) Observable.create(new Observable.OnSubscribe<String>() { @Override public void call(Subscriber<? super String> subscriber) { subscriber.onNext("Hello"); subscriber.onNext("Austin Droids!"); subscriber.onCompleted(); } });
11. Observables (Producers) Observable.create(subscriber -> { subscriber.onNext("Hello"); subscriber.onNext("Austin Droids!"); subscriber.onCompleted(); }); That’s better!
12. Observables (Producers) Observable.create(subscriber -> { subscriber.onNext("Hello"); subscriber.onNext("Austin Droids!"); subscriber.onCompleted(); });
13. Observables (Producers) Observable.create(subscriber -> { subscriber.onNext("Hello"); subscriber.onNext("Austin Droids!"); subscriber.onCompleted(); });
14. Observables (Producers) Observable.create(subscriber -> { subscriber.onNext("Hello"); subscriber.onNext("Austin Droids!"); subscriber.onCompleted(); });
15. Observables (Producers) Observable.create(subscriber -> { subscriber.onNext("Hello"); subscriber.onNext("Austin Droids!"); subscriber.onCompleted(); });
16. Observables (Producers) Observable.create(subscriber -> { subscriber.onNext("Hello"); subscriber.onNext("Austin Droids!"); subscriber.onError(new Exception("iOS users not allowed!")); });
17. Observables (Producers) Observable.just("Hello", "again...");
18. Observables (Producers) Observable.just("Hello", "again..."); Observable.from(Arrays.asList("Hello", "again...")); Observable.from(new String[]{"Hello", "again..."}); Observable.concat(Observable.just("Hello"), Observable.just("again...")); Observable.merge(Observable.just("Hello", "again..."), Observable.never()); ... Factory Overload!
19. Observables (Producers) Observable.just("Hello", "again..."); Observable.from(Arrays.asList("Hello", "again...")); Observable.from(new String[]{"Hello", "again..."}); Observable.concat(Observable.just("Hello"), Observable.just("again...")); Observable.merge(Observable.just("Hello", "again..."), Observable.never()); ... Factory Overload!
20. Observables (Producers) Observable.just("Hello", "again..."); Observable.from(Arrays.asList("Hello", "again...")); Observable.from(new String[]{"Hello", "again..."}); Observable.concat(Observable.just("Hello"), Observable.just("again...")); Observable.merge(Observable.just("Hello", "again..."), Observable.never()); ... Factory Overload!
21. Observables (Producers) Observable.just("Hello", "again..."); Observable.from(Arrays.asList("Hello", "again...")); Observable.from(new String[]{"Hello", "again..."}); Observable.concat(Observable.just("Hello"), Observable.just("again...")); Observable.merge(Observable.just("Hello", "again..."), Observable.never()); ... Factory Overload!
22. Observables (Producers) Observable.just("Hello", "again..."); Observable.from(Arrays.asList("Hello", "again...")); Observable.from(new String[]{"Hello", "again..."}); Observable.concat(Observable.just("Hello"), Observable.just("again...")); Observable.merge(Observable.just("Hello", "again..."), Observable.never()); ... Factory Overload!
23. enum Version { JELLY_BEAN, KIT_KAT, LOLLIPOP, MARSHMALLOW } public List<Version> getSupportedVersions() { return Arrays.asList(KIT_KAT, LOLLIPOP, MARSHMALLOW); }
24. Subscribers (Consumers) Observable.defer(() -> Observable.from(getSupportedVersions()));
25. Subscribers (Consumers) Observable.defer(() -> Observable.from(getSupportedVersions())) .subscribe();
26. Subscribers (Consumers) Observable.defer(() -> Observable.from(getSupportedVersions())) .subscribe(version -> print(version.toString()));
27. Subscribers (Consumers) Observable.defer(() -> Observable.from(getSupportedVersions())) .subscribe(version -> print(version.toString()), throwable -> print("Error: " + throwable.getMessage()));
28. Subscribers (Consumers) Observable.defer(() -> Observable.from(getSupportedVersions())) .subscribe(version -> print(version.toString()), throwable -> print("Error: " + throwable.getMessage()), () -> print("Don't see your phone's version? Time to upgrade!"));
29. Subscribers (Consumers) Observable.defer(() -> Observable.from(getSupportedVersions())) .subscribe(new Subscriber<Version>() { @Override public void onCompleted() { print("Don't see your phone's version? Time to upgrade!"); } @Override public void onError(Throwable e) { print("Error: " + e.getMessage()); } @Override public void onNext(Version version) { print(version.toString()); } });
30. Operators Observable.from(getSupportedVersions());
31. Operators Observable.from(getSupportedVersions()) .map(version -> version.toString());
32. Operators Observable.from(getSupportedVersions()) .map(version -> version.toString()) .filter(name -> !name.startsWith("K"));
33. public List<String> getDevices(Version version) { switch (version) { case KIT_KAT: return Arrays.asList("Samsung Galaxy S3", "LG Optimus G"); case LOLLIPOP: return Arrays.asList("Nexus 4", "Samsung Galaxy S4"); case MARSHMALLOW: return Arrays.asList("Nexus 5", "Nexus 6"); default: return Collections.emptyList(); } }
34. Operators Observable.from(getSupportedVersions()) .flatMap(version -> Observable.from(getDevices(version)));
35. Operators Observable.from(getSupportedVersions()) .concatMap(version -> Observable.from(getDevices(version))); When order is important
36. Operators Operators for... • Transforming ◦ map, flatMap, concatMap, scan, buffer, cast • Filtering ◦ filter, take, skip, first, last, distinct • Combining ◦ zip, merge, mergeDelayError, startWith • Creating ◦ create, just, from, defer, interval, range • And more! ◦ cache, materialize, delay, doOnNext, ...
37. Operators And if you’re still not happy…
38. Operators And if you’re still not happy… Implement your own and apply it with the lift() operator or…
39. Operators And if you’re still not happy… Implement your own and apply it with the lift() operator or… apply multiple operators with compose()
40. I’ve got a confession to make
41. So far all of our Observables have been synchronous
42. Enter Schedulers
43. Schedulers (Multithreading) • Abstraction for concurrency • Determines the how... ◦ Thread, Thread Pool, Executor, etc… • … and the when ◦ Either now or in the future
44. Schedulers (Multithreading) Observable.from(getSupportedVersions()) .subscribe(); Synchronous...
45. Schedulers (Multithreading) Observable.from(getSupportedVersions()) .subscribeOn(Schedulers.newThread()) .subscribe(); Asynchronous!
46. Schedulers (Multithreading) Observable.from(getSupportedVersions()) .subscribeOn(Schedulers.newThread()) .subscribe(); Pre-defined Schedulers
47. Schedulers (Multithreading) Observable.from(getSupportedVersions()) .subscribeOn(Schedulers.io()) .subscribe(); Pre-defined Schedulers
48. Schedulers (Multithreading) Observable.from(getSupportedVersions()) .subscribeOn(Schedulers.immediate()) .subscribe(); Pre-defined Schedulers
49. Schedulers (Multithreading) Observable.from(getSupportedVersions()) .subscribeOn(Schedulers.from(executor)) .subscribe(); Pre-defined Schedulers
50. Schedulers (Multithreading) Observable.from(getSupportedVersions()) .subscribeOn(Schedulers.from(executor)) .observeOn(AndroidSchedulers.mainThread()) .subscribe();
51. Schedulers (Multithreading) Observable.from(getSupportedVersions()) .subscribeOn(Schedulers.from(executor)) .observeOn(AndroidSchedulers.mainThread()) .subscribe();
52. Schedulers (Multithreading) Observable.from(getSupportedVersions()) .subscribeOn(Schedulers.from(executor)) .filter(version -> version != KIT_KAT) .observeOn(Schedulers.computation()) .map(version -> expensiveComputation(version)) .observeOn(AndroidSchedulers.mainThread()) .subscribe();
53. Schedulers (Multithreading) Observable.from(getSupportedVersions()) .filter(version -> version != KIT_KAT) .observeOn(Schedulers.computation()) .map(version -> expensiveComputation(version)) .observeOn(AndroidSchedulers.mainThread()) .subscribeOn(Schedulers.from(executor)) .subscribe();
54. Schedulers (Multithreading) Observable.from(getSupportedVersions()) .filter(version -> version != KIT_KAT) .observeOn(Schedulers.computation()) .map(version -> expensiveComputation(version)) .observeOn(AndroidSchedulers.mainThread()) .subscribeOn(Schedulers.from(executor)) .subscribe(); compile 'io.reactivex:rxandroid:1.1.0'
55. Schedulers (Multithreading) Observable.interval(2, TimeUnit.SECONDS); Implicitly Async Observables
56. Schedulers (Multithreading) Observable.interval(2, TimeUnit.SECONDS, Schedulers.computation()); Implicitly Async Observables
57. Since we all diligently test our code, you probably want to know how to test an async stream
58. Testing public Observable<Version> getSupportedVersions() { return Observable.just(KIT_KAT, LOLLIPOP, MARSHMALLOW) .subscribeOn(Schedulers.newThread()); } How might we test this?
59. Testing List<Version> results = new ArrayList<>(); getSupportedVersions() .toBlocking() .subscribe(version -> results.add(version)); assertEquals(Arrays.asList(KIT_KAT, LOLLIPOP, MARSHMALLOW), results); This works, but there’s a better way
60. Testing TestSubscriber<Version> testSubscriber = new TestSubscriber<>(); getSupportedVersions() .subscribe(testSubscriber); testSubscriber.assertValues(KIT_KAT, LOLLIPOP, MARSHMALLOW); testSubscriber.assertNoErrors(); testSubscriber.assertCompleted();
61. Testing TestScheduler testScheduler = Schedulers.test(); TestSubscriber<Long> testSubscriber = new TestSubscriber<>(); Observable.interval(2, TimeUnit.SECONDS, testScheduler) .subscribe(testSubscriber); testSubscriber.assertNoValues(); testScheduler.advanceTimeBy(2, TimeUnit.SECONDS); testSubscriber.assertValue(0L); Time Warp
62. Testing TestScheduler testScheduler = Schedulers.test(); TestSubscriber<Long> testSubscriber = new TestSubscriber<>(); Observable.interval(2, TimeUnit.SECONDS, testScheduler) .subscribe(testSubscriber); testSubscriber.assertNoValues(); testScheduler.advanceTimeBy(2, TimeUnit.SECONDS); testSubscriber.assertValue(0L);
63. Testing TestScheduler testScheduler = Schedulers.test(); TestSubscriber<Long> testSubscriber = new TestSubscriber<>(); Observable.interval(2, TimeUnit.SECONDS, testScheduler) .subscribe(testSubscriber); testSubscriber.assertNoValues(); testScheduler.advanceTimeBy(2, TimeUnit.SECONDS); testSubscriber.assertValue(0L);
64. Testing TestScheduler testScheduler = Schedulers.test(); TestSubscriber<Long> testSubscriber = new TestSubscriber<>(); Observable.interval(2, TimeUnit.SECONDS) .observeOn(testScheduler) .subscribe(testSubscriber); testSubscriber.assertNoValues(); testScheduler.advanceTimeBy(2, TimeUnit.SECONDS); testSubscriber.assertValue(0L);
65. Testing TestScheduler testScheduler = Schedulers.test(); TestSubscriber<Long> testSubscriber = new TestSubscriber<>(); Observable.interval(2, TimeUnit.SECONDS) .observeOn(testScheduler) .subscribe(testSubscriber); testSubscriber.assertNoValues(); testScheduler.advanceTimeBy(2, TimeUnit.SECONDS); testSubscriber.assertValue(0L);
66. RxJava in a “real” app
67. git clone https://github.com/jpetitto/rxjava-android-example.git
69. Searching subscription = RxTextView.textChanges(searchBar) .observeOn(Schedulers.io()) .debounce(1, TimeUnit.SECONDS) .filter(charSequence -> charSequence.length() > 0) .switchMap(charSequence -> interactor.searchUsers(charSequence.toString())) .flatMap(searchResult -> Observable.from(searchResult.getItems()).limit(20).toList()) .observeOn(AndroidSchedulers.mainThread()) .subscribe(searchItems -> adapter.refreshItems(searchItems), throwable -> Log.e(TAG, throwable.getMessage()));
70. Searching compile 'com.jakewharton.rxbinding:rxbinding:0.3.0' subscription = RxTextView.textChanges(searchBar) .observeOn(Schedulers.io()) .debounce(1, TimeUnit.SECONDS) .filter(charSequence -> charSequence.length() > 0) .switchMap(charSequence -> interactor.searchUsers(charSequence.toString())) .flatMap(searchResult -> Observable.from(searchResult.getItems()).limit(20).toList()) .observeOn(AndroidSchedulers.mainThread()) .subscribe(searchItems -> adapter.refreshItems(searchItems), throwable -> Log.e(TAG, throwable.getMessage()));
71. Searching subscription = RxTextView.textChanges(searchBar) .observeOn(Schedulers.io()) .debounce(1, TimeUnit.SECONDS) .filter(charSequence -> charSequence.length() > 0) .switchMap(charSequence -> interactor.searchUsers(charSequence.toString())) .flatMap(searchResult -> Observable.from(searchResult.getItems()).limit(20).toList()) .observeOn(AndroidSchedulers.mainThread()) .subscribe(searchItems -> adapter.refreshItems(searchItems), throwable -> Log.e(TAG, throwable.getMessage())); Move off of main thread for search
72. Searching subscription = RxTextView.textChanges(searchBar) .observeOn(Schedulers.io()) .debounce(1, TimeUnit.SECONDS) .filter(charSequence -> charSequence.length() > 0) .switchMap(charSequence -> interactor.searchUsers(charSequence.toString())) .flatMap(searchResult -> Observable.from(searchResult.getItems()).limit(20).toList()) .observeOn(AndroidSchedulers.mainThread()) .subscribe(searchItems -> adapter.refreshItems(searchItems), throwable -> Log.e(TAG, throwable.getMessage())); Only perform search after 1 second of no change
73. Searching subscription = RxTextView.textChanges(searchBar) .observeOn(Schedulers.io()) .debounce(1, TimeUnit.SECONDS) .filter(charSequence -> charSequence.length() > 0) .switchMap(charSequence -> interactor.searchUsers(charSequence.toString())) .flatMap(searchResult -> Observable.from(searchResult.getItems()).limit(20).toList()) .observeOn(AndroidSchedulers.mainThread()) .subscribe(searchItems -> adapter.refreshItems(searchItems), throwable -> Log.e(TAG, throwable.getMessage())); Ignore empty search bar
74. Searching subscription = RxTextView.textChanges(searchBar) .observeOn(Schedulers.io()) .debounce(1, TimeUnit.SECONDS) .filter(charSequence -> charSequence.length() > 0) .switchMap(charSequence -> interactor.searchUsers(charSequence.toString())) .flatMap(searchResult -> Observable.from(searchResult.getItems()).limit(20).toList()) .observeOn(AndroidSchedulers.mainThread()) .subscribe(searchItems -> adapter.refreshItems(searchItems), throwable -> Log.e(TAG, throwable.getMessage())); Make network call with Retrofit
75. Network Call public interface GitHubService { @GET("/search/users?") Observable<SearchResult> searchUsers(@Query("q") String query); } compile 'com.squareup.retrofit2:retrofit:2.0.0-beta3'
76. Network Call public interface GitHubService { @GET("/search/users?") Observable<SearchResult> searchUsers(@Query("q") String query); }
77. Network Call public Observable<SearchResult> searchUsers(String query) { return Observable.concat(cachedObservable(query), networkObservable(query)).first(); }
78. Network Call public Observable<SearchResult> searchUsers(String query) { return Observable.concat(cachedObservable(query), networkObservable(query)).first(); } Consume cached results first
79. Network Call public Observable<SearchResult> searchUsers(String query) { return Observable.concat(cachedObservable(query), networkObservable(query)).first(); } Otherwise make network call
80. Network Call public Observable<SearchResult> searchUsers(String query) { return Observable.concat(cachedObservable(query), networkObservable(query)).first(); } Prevents unnecessary network call
81. Network Call private Observable<SearchResult> cachedObservable(String query) { return Observable.just(cache.get(query)).filter(result -> result != null); }
82. Network Call private Observable<SearchResult> networkObservable(String query) { return service.searchUsers(query).doOnNext(result -> cache.put(query, result)); }
83. Network Call private Observable<SearchResult> networkObservable(String query) { return service.searchUsers(query).doOnNext(result -> cache.put(query, result)); } Store result in cache
84. Network Call Retrofit retrofit = new Retrofit.Builder() .baseUrl("https://api.github.com") .addCallAdapterFactory(RxJavaCallAdapterFactory.create()) .addConverterFactory(GsonConverterFactory.create()) .build(); LruCache<String, SearchResult> cache = new LruCache<>(5 * 1024 * 1024); // 5MiB GitHubInteractor interactor = new GitHubInteractor(retrofit, cache);
85. Network Call Retrofit retrofit = new Retrofit.Builder() .baseUrl("https://api.github.com") .addCallAdapterFactory(RxJavaCallAdapterFactory.create()) .addConverterFactory(GsonConverterFactory.create()) .build(); LruCache<String, SearchResult> cache = new LruCache<>(5 * 1024 * 1024); // 5MiB GitHubInteractor interactor = new GitHubInteractor(retrofit, cache); compile 'com.squareup.retrofit:adapter-rxjava:2.0.0-beta3'
86. Searching subscription = RxTextView.textChanges(searchBar) .observeOn(Schedulers.io()) .debounce(1, TimeUnit.SECONDS) .filter(charSequence -> charSequence.length() > 0) .switchMap(charSequence -> interactor.searchUsers(charSequence.toString())) .flatMap(searchResult -> Observable.from(searchResult.getItems()).limit(20).toList()) .observeOn(AndroidSchedulers.mainThread()) .subscribe(searchItems -> adapter.refreshItems(searchItems), throwable -> Log.e(TAG, throwable.getMessage())); Cancel previous network call (if on-going)
87. Searching subscription = RxTextView.textChanges(searchBar) .observeOn(Schedulers.io()) .debounce(1, TimeUnit.SECONDS) .filter(charSequence -> charSequence.length() > 0) .switchMap(charSequence -> interactor.searchUsers(charSequence.toString())) .flatMap(searchResult -> Observable.from(searchResult.getItems()).limit(20).toList()) .observeOn(AndroidSchedulers.mainThread()) .subscribe(searchItems -> adapter.refreshItems(searchItems), throwable -> Log.e(TAG, throwable.getMessage())); Sanitize results for consumption
88. Searching subscription = RxTextView.textChanges(searchBar) .observeOn(Schedulers.io()) .debounce(1, TimeUnit.SECONDS) .filter(charSequence -> charSequence.length() > 0) .switchMap(charSequence -> interactor.searchUsers(charSequence.toString())) .flatMap(searchResult -> Observable.from(searchResult.getItems()).limit(20).toList()) .observeOn(AndroidSchedulers.mainThread()) .subscribe(searchItems -> adapter.refreshItems(searchItems), throwable -> Log.e(TAG, throwable.getMessage())); Post results back on main thread
89. Searching subscription = RxTextView.textChanges(searchBar) .observeOn(Schedulers.io()) .debounce(1, TimeUnit.SECONDS) .filter(charSequence -> charSequence.length() > 0) .switchMap(charSequence -> interactor.searchUsers(charSequence.toString())) .flatMap(searchResult -> Observable.from(searchResult.getItems()).limit(20).toList()) .observeOn(AndroidSchedulers.mainThread()) .subscribe(searchItems -> adapter.refreshItems(searchItems), throwable -> Log.e(TAG, throwable.getMessage())); Kick off work Starts on main thread since we’re in MainActivity
90. Searching subscription = RxTextView.textChanges(searchBar) .observeOn(Schedulers.io()) .debounce(1, TimeUnit.SECONDS) .filter(charSequence -> charSequence.length() > 0) .switchMap(charSequence -> interactor.searchUsers(charSequence.toString())) .flatMap(searchResult -> Observable.from(searchResult.getItems()).limit(20).toList()) .observeOn(AndroidSchedulers.mainThread()) .subscribe(searchItems -> adapter.refreshItems(searchItems), throwable -> Log.e(TAG, throwable.getMessage())); Update RecylerView adapter
91. Searching subscription = RxTextView.textChanges(searchBar) .observeOn(Schedulers.io()) .debounce(1, TimeUnit.SECONDS) .filter(charSequence -> charSequence.length() > 0) .switchMap(charSequence -> interactor.searchUsers(charSequence.toString())) .flatMap(searchResult -> Observable.from(searchResult.getItems()).limit(20).toList()) .observeOn(AndroidSchedulers.mainThread()) .subscribe(searchItems -> adapter.refreshItems(searchItems), throwable -> Log.e(TAG, throwable.getMessage())); Handle error events
92. Searching subscription = RxTextView.textChanges(searchBar) .observeOn(Schedulers.io()) .debounce(1, TimeUnit.SECONDS) .filter(charSequence -> charSequence.length() > 0) .switchMap(charSequence -> interactor.searchUsers(charSequence.toString())) .flatMap(searchResult -> Observable.from(searchResult.getItems()).limit(20).toList()) .observeOn(AndroidSchedulers.mainThread()) .subscribe(searchItems -> adapter.refreshItems(searchItems), throwable -> Log.e(TAG, throwable.getMessage())); Manage resources
93. Searching subscription = RxTextView.textChanges(searchBar) .observeOn(Schedulers.io()) .debounce(1, TimeUnit.SECONDS) .filter(charSequence -> charSequence.length() > 0) .switchMap(charSequence -> interactor.searchUsers(charSequence.toString())) .flatMap(searchResult -> Observable.from(searchResult.getItems()).limit(20).toList()) .observeOn(AndroidSchedulers.mainThread()) .subscribe(searchItems -> adapter.refreshItems(searchItems), throwable -> Log.e(TAG, throwable.getMessage())); Manage resources
94. Subscriptions Subscription subscription; @Override protected void onDestroy() { super.onDestroy(); subscription.unsubscribe(); }
95. What’s Coming • The Single API ◦ Currently marked @Beta ◦ Simpler semantics due to single value ◦ Suitable for common request/response scenarios ◦ Composable with Observables • RxJava 2.0 ◦ Also in beta ◦ Takes advantage of Java 8 features ◦ Follows the Reactive Streams spec (http://www.reactive-streams.org/) ◦ 1.x will still be supported for us Android devs
96. Additional Resources • ReactiveX http://reactivex.io/ • RxJava https://github.com/ReactiveX/RxJava • RxAndroid https://github.com/ReactiveX/RxAndroid • RxBinding https://github.com/JakeWharton/RxBinding • RxMarbles http://rxmarbles.com/
97. RxJava for Android Developers twitter.com/johnpetitto github.com/jpetitto johnpetitto.com