Functional on Android

On Android, more and more developers are attracted to the functional programming-style concept of declarative data manipulation using lambdas. Java 8 has a new steams API, but it's limited to Android N. Backports exist, but it's RxJava that's all the rage, with its elegant threading solution. How do we use lambdas, streams and Rx effectively on Android? Orientation changes and background tasks? I propose to stop worrying about the lifecycle and cache your way into a blissful user experience.

Transcript

1. Functional on Android Lambdas, Rx and streams in your app Eric Kok http://2312.nl
3. Functional in Java
4. Functional in Java Lambdas? Java 8? Rx? Streams?
5. List<String> numbers = Arrays.asList("5", "3", "4", "1", "2");
 Collections.sort(numbers, new Comparator<String>() {
 @Override
 public int compare(String s1, String s2) {
 Integer i1 = Integer.valueOf(s1);
 Integer i2 = Integer.valueOf(s2);
 return i1.compareTo(i2);
 }
 }); Lambdas
6. List<String> numbers = Arrays.asList("5", "3", "4", "1", "2");
 Collections.sort(numbers, new Comparator<String>() {
 @Override
 public int compare(String s1, String s2) {
 Integer i1 = Integer.valueOf(s1);
 Integer i2 = Integer.valueOf(s2);
 return i1.compareTo(i2);
 }
 }); Lambdas
7. List<String> numbers = Arrays.asList("5", "3", "4", "1", "2");
 Collections.sort(numbers,
 @Override
 public int compare(String s1, String s2) {
 Integer i1 = Integer.valueOf(s1);
 Integer i2 = Integer.valueOf(s2);
 return i1.compareTo(i2);
 }
 ); Lambdas
8. List<String> numbers = Arrays.asList("5", "3", "4", "1", "2");
 Collections.sort(numbers,
 
 ( s1, s2) {
 Integer i1 = Integer.valueOf(s1);
 Integer i2 = Integer.valueOf(s2);
 return i1.compareTo(i2);
 }
 ); Lambdas
9. List<String> numbers = Arrays.asList("5", "3", "4", "1", "2");
 Collections.sort(numbers,
 
 ( s1, s2) -> {
 Integer i1 = Integer.valueOf(s1);
 Integer i2 = Integer.valueOf(s2);
 return i1.compareTo(i2);
 }
 ); Lambdas
10. List<String> numbers = Arrays.asList("5", "3", "4", "1", "2");
 Collections.sort(numbers, (s1, s2) -> 
 Integer.valueOf(s1).compareTo(Integer.valueOf(s2)));
11. List<String> numbers = Arrays.asList("5", "3", "4", "1", "2");
 Collections.sort(numbers, (s1, s2) -> 
 Integer.valueOf(s1).compareTo(Integer.valueOf(s2))); private int compareAsInt(String s1, String s2) {
 return Integer.valueOf(s1).compareTo(Integer.valueOf(s2));
 }
12. List<String> numbers = Arrays.asList("5", "3", "4", "1", "2");
 Collections.sort(numbers, (s1, s2) -> compareAsInt(s1, s2)); private int compareAsInt(String s1, String s2) {
 return Integer.valueOf(s1).compareTo(Integer.valueOf(s2));
 }
13. List<String> numbers = Arrays.asList("5", "3", "4", "1", "2");
 Collections.sort(numbers, this::compareAsInt); private int compareAsInt(String s1, String s2) {
 return Integer.valueOf(s1).compareTo(Integer.valueOf(s2));
 }
14. Streaming List<String> numbers = Arrays.asList("5", "3", "4", "1", "2");
 
 for (String number : numbers) {
 Integer i = Integer.valueOf(number); // convert to int
 if (i % 2 == 0) { // filter even
 log(i);
 }
 }
15. Streaming List<String> numbers = Arrays.asList("5", "3", "4", "1", "2");
 
 numbers.stream() // start streaming
 .map(Integer::valueOf) // convert to int
 .filter(i -> i % 2 == 0) // filter even
 .forEach(this::log); 
 

16. Streaming Java 8 and Android N (minSdk 24) List<String> numbers = Arrays.asList("5", "3", "4", "1", "2");
 
 numbers.stream() // start streaming
 .map(Integer::valueOf) // convert to int
 .filter(i -> i % 2 == 0) // filter even
 .forEach(this::log); 
 

17. Streaming StreamSupport List<String> numbers = Arrays.asList("5", "3", "4", "1", "2");
 
 StreamSupport.stream(numbers) // start streaming
 .map(Integer::valueOf)
 .filter(i -> i % 2 == 0)
 .forEach(this::log); 
 

18. Streaming List<String> numbers = Arrays.asList("5", "3", "4", "1", "2");
 
 Observable.from(numbers) // create observable
 .map(Integer::valueOf)
 .filter(i -> i % 2 == 0)
 .subscribe(this::log); // start emitting 
 
 RxJava?
19. Reactive programming • Sources of data that stream • Combining and transforming streams • Think: spreadsheet
20. • Collection streaming (push) • Java 8 / Android N / Streamsupport • Reactive interfaces (pull) • RxJava 1 and 2 • Akka streams • Reactor Core • Java 9 Flow
21. RxJava • Streaming data with • transformations • lifecycle callbacks on emit, completion, error • threading • reactive pull • backpressure
22. List<Integer> ids = Arrays.asList(5, 3, 4, 1, 2);
 
 Observable.from(ids)
 
 .map(i -> Network.blockingCall(i))
 .filter(item -> item.shouldShow())
 
 .subscribe(this::log);
23. List<Integer> ids = Arrays.asList(5, 3, 4, 1, 2);
 
 Observable.from(ids)
 
 .map(i -> Network.getBeer(i))
 .filter(beer -> beer.rating > 0.8F)
 
 .subscribe(this::showBeer);
24. List<Integer> ids = Arrays.asList(5, 3, 4, 1, 2);
 
 Observable.from(ids)
 .subscribeOn(io()) // emit on io
 .map(i -> Network.getBeer(i))
 .filter(beer -> beer.rating > 0.8F) 
 .observeOn(mainThread()) // switch to main thread
 .subscribe(this::showBeer);
25. List<Integer> ids = Arrays.asList(5, 3, 4, 1, 2);
 
 Observable.from(ids)
 .subscribeOn(io())
 .map(i -> Network.getBeer(i))
 .filter(beer -> beer.rating > 0.8F) .take(1) // only request 1
 .observeOn(mainThread())
 .subscribe(this::showBeer);
26. Rx on Android
27. RxView.clicks(button1)
 
 
 .scan(0, (integer, click) -> integer + 1) // running count
 .map(i -> Integer.toString(i)) // int to string
 
 .subscribe(RxTextView.text(textview1)); RxBinding and RxAndroid
28. RxView.clicks(button1)
 
 .subscribeOn(Schedulers.computation()) // switch to computation
 .scan(0, (integer, click) -> integer + 1) // running count
 .map(i -> Integer.toString(i)) // int to string
 
 .subscribe(RxTextView.text(textview1)); RxBinding and RxAndroid
29. RxView.clicks(button1)
 
 .subscribeOn(Schedulers.computation()) // switch to computation
 .scan(0, (integer, click) -> integer + 1) // running count
 .map(i -> Integer.toString(i)) // int to string
 
 .subscribe(RxTextView.text(textview1)); RxBinding and RxAndroid CalledFromWrongThreadException
30. RxBinding and RxAndroid RxView.clicks(button1)
 
 .subscribeOn(Schedulers.computation()) // switch to computation
 .scan(0, (integer, click) -> integer + 1) // running count
 .map(i -> Integer.toString(i)) // int to string
 .observeOn(AndroidSchedulers.mainThread()) // switch to main thread
 .subscribe(RxTextView.text(textview1)); CalledFromWrongThreadException
31. RxBinding and RxAndroid RxView.clicks(button1)
 
 .subscribeOn(Schedulers.computation()) // switch to computation
 .scan(0, (integer, click) -> integer + 1) // running count
 .map(i -> Integer.toString(i)) // int to string
 .observeOn(AndroidSchedulers.mainThread()) // switch to main thread
 .subscribe(RxTextView.text(textview1)); IllegalStateException: Must be called from the main thread
32. RxBinding and RxAndroid RxView.clicks(button1)
 .subscribeOn(AndroidSchedulers.mainThread()) // emit on computation
 .observeOn(Schedulers.computation()) // switch to computation
 .scan(0, (integer, click) -> integer + 1) // running count
 .map(i -> Integer.toString(i)) // int to string
 .observeOn(AndroidSchedulers.mainThread()) // switch to main thread
 .subscribe(RxTextView.text(textview1)); IllegalStateException: Must be called from the main thread
33. Observable.combineLatest(
 RxCompoundButton.checkedChanges(agreeTermsCheck),
 RxCompoundButton.checkedChanges(noCommentCheck),
 RxTextView.textChanges(reviewEdit).map(s -> s.length() > 80), 
 (agreeTerms, noComment, hasReview) -> agreeTerms && (noComment || hasReview))
 .subscribe(RxView.enabled(submitButton));
34. RxTextView.textChanges(searchEdit) 
 .debounce(1, TimeUnit.SECONDS, mainThread()) // delay emission 
 .subscribe(this::startQuery));
35. RxTextView.textChanges(searchEdit)
 
 .debounce(1, TimeUnit.SECONDS, io()) // on background
 .flatMap(query -> Network.searchBeer(query.toString()))
 
 .subscribe(this::showBeers);
36. RxTextView.textChanges(searchEdit)
 .subscribeOn(mainThread()) // on ui
 .debounce(1, TimeUnit.SECONDS, io())
 .flatMap(query -> Network.searchBeer(query.toString()))
 .observeOn(mainThread()) // to ui
 .subscribe(this::showBeers);
37. BehaviorSubject<Float> syncProgress = BehaviorSubject.create();
 syncProgress.onNext(0.15F); // post event
 syncProgress .subscribe(RxView.progress(progressBar));

38. BehaviorSubject<Float> syncProgress = BehaviorSubject.create();
 syncProgress.onNext(0.15F);
 syncProgress .subscribe(RxView.progress(progressBar)) 
 syncProgress
 .map(f -> f >= 1F)
 .subscribe(RxView.visibility(syncedItemList));
39. Caching
40. Caching in memory: subjects • ReplaySubject • re-emits all • BehaviorSubject • re-emits last item • PublishSubject • re-emits none
41. Caching in memory: cache() Observable<Beer> cached = Network.getBeer(8)
 .cache();
 cached.subscribe(this::showBeerHere) cached.subscribe(this::showBeerThere)
42. Caching in memory: cache() Observable<Beer> cached = Network.getBeer(8)
 .replay(30, TimeUnit.MINUTES).autoConnect();
 cached.subscribe(this::showBeerHere) cached.subscribe(this::showBeerThere
43. Lifecycle management • Keep Subject or cache()’s stream in • singleton (Repository) • retained fragment • nonConfigurationInstance
44. protected void onCreate(Bundle savedInstanceState) {
 ...
 
 Network.findByBrewer("Piwoteka")
 .subscribeOn(io())
 .observeOn(mainThread())
 .subscribe(this::showBeers); }
 

45. private Observable<Beer> beerCall; protected void onCreate(Bundle savedInstanceState) {
 ...
 
 if (beerCall == null) {
 beerCall = Network.findByBrewer("Piwoteka").cache();
 }
 beerCall
 .subscribeOn(io())
 .observeOn(mainThread())
 .subscribe(this::showBeers);
 }
 

46. private Observable<Beer> beerCall; protected void onCreate(Bundle savedInstanceState) {
 ...
 
 beerCall = (Observable<Beer>) getLastNonConfigurationInstance();
 if (beerCall == null) {
 beerCall = Network.findByBrewer("Piwoteka").cache();
 }
 beerCall
 .subscribeOn(io())
 .observeOn(mainThread())
 .subscribe(this::showBeers);
 }
 
 @Override
 public Object onRetainNonConfigurationInstance() {
 return beerCall;
 }
47. RxCupboard • Rx wrapper around Hugo’s Cupboard • Minimal boilerplate • Relies heavy on defaults public class Beer {
 public Long _id;
 public String brewer;
 public String name;
 } cupboard().register(Beer.class); // one-time registration RxDatabase rxdb = RxCupboard.withDefault( new SQLiteOpenHelper(context).getWritableDatabase());
48. Observable<Beer> allBeers = rxdb.query(Beer.class); 
 Observable<Beer> piwotekaBeers = rxdb.query(Beer.class, "brewer = ?", “Piwoteka"); 
 Observable<Beer> beerWithId8 = rxdb.get(Beer.class, 8);
49. source.subscribe(beer -> rxdb.put()); rxdb.putRx(beer).map(...).subscribe(...);
50. source.subscribe(beer -> rxdb.put()); rxdb.putRx(beer).map(...).subscribe(...); Network.findByBrewer("Piwoteka")
 .map(rxdb::putRx) // insert/update every beer
 .toList()
 .subscribe(this::showBeerList);
51. Observable<DatabaseChange> changes = rxdb.changes(); 
 Observable<DatabaseChange<Beer>> changes = rxdb.changes(Beer.class);
52. Observable<DatabaseChange> changes = rxdb.changes(); 
 Observable<DatabaseChange<Beer>> changes = rxdb.changes(Beer.class); Observable<Beer> changedBeers = rxdb.changes(Beer.class)
 .map(change -> change.entity());
53. Observable<DatabaseChange> changes = rxdb.changes(); 
 Observable<DatabaseChange<Beer>> changes = rxdb.changes(Beer.class); Observable<Beer> addedBeers = rxdb.changes(Beer.class)
 .ofType(DatabaseChange.DatabaseInsert.class)
 .map(insertion -> (Beer) insertion.entity()); // cast :( Observable<Beer> changedBeers = rxdb.changes(Beer.class)
 .map(change -> change.entity());
54. Caching in db protected void onCreate(Bundle savedInstanceState) {
 ...
 Observable<Beer> db = rxdb.get(Beer.class, 8) .filter(beer -> beer != null); Observable<Beer> fresh = Network.getBeer(8) .doOnNext(beer -> rxdb.put()); } Only cached or fresh entity
55. Caching in db Only cached or fresh entity protected void onCreate(Bundle savedInstanceState) {
 ...
 Observable<Beer> db = rxdb.get(Beer.class, 8) .filter(beer -> beer != null); Observable<Beer> fresh = Network.getBeer(8) .doOnNext(beer -> rxdb.put()); Observable.concat(db, fresh) .subscribeOn(io())
 .observeOn(mainThread())
 .subscribe(this::showBeer); }
56. Caching in db protected void onCreate(Bundle savedInstanceState) {
 ...
 Observable<Beer> db = rxdb.get(Beer.class, 8) .filter(beer -> beer != null); Observable<Beer> fresh = Network.getBeer(8) .doOnNext(beer -> rxdb.put()); Observable.concat(db, fresh).first() .subscribeOn(io())
 .observeOn(mainThread())
 .subscribe(this::showBeer); } Only cached or fresh entity
57. Caching in db protected void onCreate(Bundle savedInstanceState) {
 ... Observable<Beer> db = rxdb.query(Beer.class, "brewer = ?", “Piwoteka”); Observable<Beer> fresh = Network.findByBrewer("Piwoteka") .map(rxdb::putRx); Observable.merge(db, fresh) .subscribeOn(io())
 .observeOn(mainThread())
 .subscribe(this::showBeers); } Combine cached and fresh entities
58. Caching in db protected void onCreate(Bundle savedInstanceState) {
 ... Observable<Beer> db = rxdb.query(Beer.class, "brewer = ?", “Piwoteka”); Observable<Beer> fresh = Network.findByBrewer("Piwoteka") .map(rxdb::putRx); Observable.merge(db, fresh).distinct() .subscribeOn(io())
 .observeOn(mainThread())
 .subscribe(this::showBeers); } Combine cached and fresh entities
59. Here be dragons • Beware of leaking subscriptions • Hot observables (no onComplete) • Long running operations (temporary leak) private Subscription sub; protected void onStart() { ... sub = hotSource.subscribe(this::showBeers) } protected void onStop() { sub.unsubscribe(); }
60. Here be dragons • Beware of leaking subscriptions • Hot observables (no onComplete) • Long running operations (temporary leak) private Subscription sub; protected void onResume() { ... sub = hotSource.subscribe(this::showBeers) } protected void onPause() { sub.unsubscribe(); }
61. Here be dragons Observable.create(subscriber -> {
 for (String number : numbers) {
 subscriber.onNext(number);
 }
 }); Observable.from(numbers); don’t use .create() use .from()
62. Here be dragons Observable.create(subscriber -> {
 someButton.setOnClickListener(view -> {
 subscriber.onNext(null);
 });
 }); Observable.fromEmitter(subscriber -> {
 subscriber.onNext(null);
 }, Emitter.BackpressureMode.DROP); don’t use .create() use .fromEmitter()
63. Here be dragons Observable.just(Network.blockingCall(“query")); Observable.fromCallable(() -> Network.blockingCall("query")); beware of blocking code use .fromCallable()
64. RxJava 2 • the future! • build on Java 8 reactive-streams interfaces • explicit backpressure • Flowable vs Observable (and Subject vs Processor) • .subscribe() doesn't return Subscriber • creation via .create()
65. Agera? • Reactive framework from Google Play Movies team • + MVP tied Android lifecycle • value-less signals and manual fetching • race conditions • no stream completion/errors
66. Recommended • RxBinding (view binding) • Retrofit (network calls) • RxLifecycle (automatic unsubscriptions) • android-reactive-location (location updates) • RxTuples (pairing model objects) • RxCupboard :)
67. Jack compiler compileSdkVersion 24 
 jackOptions {
 enabled true
 }
68. Jack compiler • Take a real-world app RateBeer • Set compileSdk and remove retrolambda compileSdkVersion 24 
 jackOptions {
 enabled true
 } 0 errors