Recipes in RxJava for Android Sasa Sekulic co-author of the Manning book “Grokking Rx”(with Fabrizio Chignoli and Ivan Morgillo); developer of the UN-WFP ShareTheMeal app; cofounder of Alter Ego Solutions @sasa_sekulic | www.alterego.solutions 

UN World Food Programme - ShareTheMeal We provide food to schoolchildren in need – over 5.000.000 meals shared! It costs only €0.40 to feed one child for a day. www.sharethemeal.org Google Play Best of 2015, SXSW 2015 Innovation Award for New Economy Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Basics of RxJava  Observer  Observable  Subscription  Subscriber (Observer & Subscription)  Subject (Observer & Observable) Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Observable creation from any object  just(), from() – simple, executed immediately upon creation  create() – executed upon subscription but need to take care of contract calls  defer()/fromCallable() – simple, but executed upon subscription Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Observable creation –example 1: too simple  just(), from() – simple, executed immediately upon creation public Observable exampleBlocking() { SharedPreferences prefs = context.getSharedPreferences("prefs", Context.MODE_PRIVATE); return Observable.just(prefs.getBoolean("boolean", false)); } Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Observable creation –example 2: too complicated  create() – executed upon subscription but need to take care of contract calls public Observable exampleTooComplicated() { SharedPreferences prefs = context.getSharedPreferences("prefs", Context.MODE_PRIVATE); return Observable.create(new Observable.OnSubscribe() { @Override public void call(Subscriber subscriber) { if (subscriber.isUnsubscribed()) { return; } subscriber.onNext(prefs.getBoolean("boolean", false)); if (!subscriber.isUnsubscribed()) { subscriber.onCompleted(); } } }); } Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Observable creation –create() isn't as easy as it looks  think about the unsubscription/backpressure chain Observable.create(s -> { int i = 0; while (true) { s.onNext(i++); } }).subscribe(System.out::println);  for list of many more other problems: http://akarnokd.blogspot.hu/2015/05/pitfalls-of-operator-implementations.html Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Observable creation –example 3: just right  defer() – simple, but executed upon subscription  fromCallable() – the same, for methods that return values public Observable exampleJustRight() { SharedPreferences prefs = context.getSharedPreferences("prefs", Context.MODE_PRIVATE); return Observable.defer(new Func0>() { @Override public Observable call() { return Observable.just(prefs.getBoolean("boolean", false)); } }); } Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Observable creation –special case of from()  from(Future) – blocking, cannot unsubscribe (but you can specify timeout or Scheduler)  from(Iterable), from(T[]) – convert Iterable/array event into events from Iterable/array List list = new ArrayList<>(); Observable.just(list).subscribe(new Observer>(){ ... } //takes List, emits List Observable.from(list).subscribe(new Observer(){ ... } //takes List, emits Boolean items from the list Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Observable creation –forEach() equivalent ops List list = new ArrayList<>(); Observable.from(list) .map(aBoolean -> !aBoolean) //returns value .flatMap(aBoolean -> Observable.just(aBoolean.hashCode()) //returns observable .toList() //or .toSortedList() .subscribe(new Observer>() { ... Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Subjects  they're Hot observables – the values are generated independently of existence of subscribers  use them to send values to Observers outside creation block  enable you to have communication between the Observer and Observable-value generator  access onNext(), onError() and onCompleted() whenever you want  single-threaded by default, don't mix threads when calling onXXX()!  for thread safety, wrap them in a SerializedSubject() or call Subject.toSerialized().onXXX() Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Subjects - types  PublishSubject – doesn’t store any states, just sends what it receives while subscribed (you miss anything in between) – epitome of Hot observable  BehaviourSubject – remembers the last value – emptied after onCompleted()/onError() - can be initialized with a value Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Subjects – types, cont'd  AsyncSubject – remembers the last value – after onCompleted() subscribers receive the last value and the onCompleted() event – after onError() subscribers receive just the onError() event  ReplaySubject – remembers everything but can be limited (create(int bufferCapacity), createWithSize(int size), createWithTime(long time, TimeUnit unit, final Scheduler scheduler), createWithTimeAndSize(long time, TimeUnit unit, int size, final Scheduler scheduler)) Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Subjects – music player example UX Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Subjects – music player example states Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Subjects – music player example Observer public class MusicPlayer { public enum PLAYER_STATE {STOPPED, PLAYING} Observer playerObserver = new Observer() { @Override public void onNext(PLAYER_STATE state) { currentPlayerState = state; if (state == PLAYER_STATE.PLAYING) { startSong(); showPauseButton(); } else if (state == PLAYER_STATE.STOPPED) { stopSong(); showPlayButton(); } } ... }; } Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Subjects – music player example Subject public class MusicPlayer { PLAYER_STATE currentPlayerState = PLAYER_STATE.STOPPED; BehaviorSubject playerStateSubject = BehaviorSubject.create(currentPlayerState); public MusicPlayer() { playerStateSubject.subscribe(playerObserver); } private void pressButton() { if (currentPlayerState == PLAYER_STATE.PLAYING) { playerStateSubject.onNext(PLAYER_STATE.STOPPED); } else if (currentPlayerState == PLAYER_STATE.STOPPED) { playerStateSubject.onNext(PLAYER_STATE.PLAYING); } } } Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Subjects – music player example optimization  We don't need currentPlayerState, just use playerStateSubject.getValue()  Don't expose Subject to others: when offering them for subscribing, use getObservable() public Observable getPlayerStateObservable() { return playerStateSubject.asObservable(); }  Use Subscription to control Observer lifecycle! Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Subscription  Subscription is a relationship between the Observer and the Observable public MusicPlayer() { Subscription stateSub = playerStateSubject.subscribe(playerObserver); }  Subscription is returned by the subscribe() public MusicPlayer() { Subscription stateSub = playerStateSubject //Observable .filter(state -> state == PLAYING) //Observable .map(state -> someMethod()) //Observable .subscribe(playerObserver); //Subscription }  Very simple: isUnsubscribed(), unsubscribe() Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Subscription – usage  To control the lifecycle and/or release the resources, call it in onPause()/onStop()/onDestroy(): @Override protected void onDestroy() { super.onDestroy(); if (mPlayerSubscription != null && !mPlayerSubscription.isUnsubscribed()) { mPlayerSubscription.unsubscribe(); } } If you don't want to do null checks, initialize the subscription: Subscription mPlayerSubscription = Subscriptions.unsubscribed(); //or Subscriptions.empty(); Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Subscription – usage, cont'd To prevent multiple executions/subscriptions: public MusicPlayer() { if (mPlayerSubscription == null || mPlayerSubscription.isUnsubscribed()) { mPlayerSubscription = playerStateSubject.subscribe(playerObserver); } }  Important: unsubscribing has no impact on the observable or other subscriptions! Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

CompositeSubscription – grouping Subscriptions When you need to control multiple subscriptions at the same time: CompositeSubscription activitySubscriptions = new CompositeSubscription(); @Override protected void onCreate() { mPlayerSub = playerStateSubject.subscribe(playerObserver); mPlayerSub2 = playerStateSubject.subscribe(playerObserver2); activitySubscriptions = new CompositeSubscriptions(mPlayerSub, mPlayerSub2); } @Override protected void onResume() { mPlayerSub3 = playerStateSubject.subscribe(playerObserver3); activitySubscriptions.add(mPlayerSub3); } Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

CompositeSubscription – grouping Subscriptions (cont'd) When you need to control multiple subscriptions at the same time: removing with remove(), unsubscribing all with unsubscribe(), and clear() unsubscribes all and removes them from the CompositeSubscription: @Override protected void onPause() { mPlayerSub3 = playerStateSubject.subscribe(playerObserver3); activitySubscriptions.remove(mPlayerSub3); } @Override protected void onDestroy() { super.onDestroy(); if (activitySubscriptions.hasSubscriptions() && !activitySubscriptions.isUnsubscribed()) { activitySubscriptions.unsubscribe(); } } Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Subscription –control the lifecycle  Manually, on the Activity or Fragment level – create when you want, unsubscribe in onPause()/onStop()/onDestroy()  Automatically, on the Activity or Fragment level – use https://github.com/trello/RxLifecycle public class MainActivity extends RxAppCompatActivity { @Override protected void onCreate(Bundle savedInstanceState) { Observable.interval(1, TimeUnit.SECONDS) .compose(this.bindUntilEvent(ActivityEvent.PAUSE)) //binds to onPause() .subscribe(onCreateObserver); } } .compose(this.bindUntilEvent(ActivityEvent.DESTROY)) //binds to onDestroy() .compose(this.bindToLifecycle()) //binds automatically to opposite method (if called from onResume() it will be bound to onPause() One very important note: RxLifecycle doesn't call unsubscribe() but calls onCompleted() instead! Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Subscription – control the lifecycle (global)  Manually on the global, Application level – create when you want, and unsubscribe? Never, so do the transactions atomically, close the observables and unsubscribe from activities/fragments: public class SplashActivity extends AppCompatActivity { Subscription mUserSubscription = Subscriptions.empty(); @Override protected void onCreate(Bundle savedInstanceState) { mUserManager.getUserObservable() .subscribe(user -> continue(), throwable -> showError(throwable)); } @Override protected void onDestroy() { if (!mUserSubscription.isUnsubscribed()) mUserSubscription.unsubscribe(); } } Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Subscription – control the lifecycle (global, cont'd) public UserManager () { //from() will automatically call onCompleted() so no pending references Observable.from(loadUserFromSavedPreferences()) .subscribe(user -> mUserSubject.onNext(user), throwable -> mUserSubject.onError(throwable)); } Observable getUserObservable() { return mUserSubject.asObservable(); } }  There's no automated way to manage lifecycle on the global level! Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Architecture example (bound to App context) Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Manager example (bound to App context) loads a configuration, then loads a saved user, and propagates it: public UserManager () { public UserManager() { mConfigurationManager.getCurrentConfigObservable() .flatMap(config -> loadUserFromSavedPreferences()) .subscribe(user -> mUserSubject.onNext(user), throwable -> mUserSubject.onError(throwable)); } Observable getUserObservable() { return mUserSubject.asObservable(); } } Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Manager example - error handling  sending onError() closes the subscription and no more events are sent!  onError() handles ALL errors in the chain!  to prevent this, use error handling methods: – onErrorReturn() - use other value – onErrorResumeNext() - use other observable – onExceptionResumeNext() - handles Exceptions but not Throwables mConfigurationManager.getCurrentConfigObservable() .onErrorReturn(config -> Config.Empty) .flatMap(config -> loadUserFromSavedPreferences()) .onErrorReturn(user -> User.Empty) .subscribe(user -> mUserSubject.onNext(user); Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Manager example - splitting subscriptions  the whole of the chain is an Observable until the subscribe()  if you don’t use error management (onErrorReturn()/onErrorResumeNext()…), all subscriptions have to have onError()– but that is always a good idea!!! Observable Config.Empty); Subscription configValidSub = configObservable .filter(config -> config != null && config != Config.Empty) .flatMap(config -> loadUserFromSavedPreferences()) .onErrorReturn(user -> User.Empty) .subscribe(user -> mUserSubject.onNext(user)); Subscription configInvalidSub = configObservable .filter(config -> config == null || config == Config.Empty) .subscribe(config -> Log.d("App", "config empty!"); Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Manager example - reusing observable chains  if you have a part of the chain that repeats itself – extract it in a Transformer () Subscription configValidUserValidSub = configObservable .filter(config -> config != null && config != Config.Empty) .flatMap(config -> loadUserFromSavedPreferences()) .onErrorReturn(user -> User.Empty) .filter(user -> user != User.Empty) .subscribe(user -> mUserSubject.onNext(user)); Subscription configValidUserInalidSub = configObservable .filter(config -> config != null && config != Config.Empty) .flatMap(config -> loadUserFromSavedPreferences()) .onErrorReturn(user -> User.Empty) .filter(user -> user == User.Empty) .subscribe(user -> Log.d("APP", "user empty!"); Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Manager example - reusing observable chains (cont'd) Observable.Transformer convertConfigToUser() { return new Observable.Transformer() { @Override public Observable call(Observable configObservable) { return configObservable .filter(config -> config != null && config != Config.Empty) .flatMap(config -> loadUserFromSavedPreferences()) .onErrorReturn(user -> User.Empty); } }; } Subscription configValidUserValidSub = configObservable .compose(convertConfigToUser()) .filter(user -> user != User.Empty) .subscribe(user -> mUserSubject.onNext(user)); Subscription configValidUserInalidSub = configObservable .compose(convertConfigToUser()) .filter(user -> user == User.Empty) .subscribe(user -> Log.d("APP", "user empty!"); Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Manager example - multithreading  RxJava is asynchronous by default, but not multithreaded by default!  use explicit methods to change the thread:  subscribeOn() – it changes the upstream thread (until that point)  observeOn() – it changes the downstream thread (after that point) Observable.Transformer convertConfigToUser() { return new Observable.Transformer() { @Override public Observable call(Observable configObservable) { return configObservable .filter(config -> config != null && config != Config.Empty) .flatMap(config -> loadUserFromSavedPreferences()) .onErrorReturn(user -> User.Empty) .subscribeOn(Schedulers.io()) //everything up to this uses io() thread .observeOn(AndroidSchedulers.mainThread()); // everything after this // uses UI thread } }; } Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Multithreading – subscribeOn vs observeOn  subscribeOn() – it changes the upstream thread (until that point)  observeOn() – it changes the downstream thread (after that point) Observable.fromCallable(()-> doSomething1(); //executed on IO thread //if there's no subscribeOn(), execs on calling thread ) .subscribeOn(Schedulers.io()) .filter(something -> doSomething2()) //subscribe call AND everything up to this uses io() thread .observeOn(Schedulers.computation()); //changes thread to computation! .flatMap(something -> doSomething3()) .onErrorReturn(user -> User.Empty) .subscribeOn(Schedulers.computation()) //doesn't matter!!! wasteful! .observeOn(AndroidSchedulers.mainThread()) //switches to UI thread .subscribe(result -> doSomething4()); Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Manager example - multithreading (cont'd)  RxJava Scheduler:  computation() – bound (limited by CPU cores)  io() - unbound  immediate() – executes immediately on current thread  trampoline() – executes after finishing, on current thread  newThread() – creates new thread  from(Executor) – use custom executor  test() – uses TestScheduler with manual scheduling controls  rxandroid library: AndroidScheduler.mainThread()  Multithreading is hard, RxJava makes it easier – but it's still not easy!  do not specify the thread unless you really need to Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic 

Further reading/watching  https://github.com/ReactiveX/RxJava  RxJava Single discussion: https://github.com/ReactiveX/RxJava/issues/1594  RxJava Subscriber discussion: https://github.com/ReactiveX/RxJava/issues/792  New viewbinding library using RxJava: https://github.com/alter-ego/androidbound  ReactiveX homepage: http://reactivex.io/  Interactive marble diagrams for operators: http://www.rxmarbles.com/  Ben Christensen's talks: https://speakerdeck.com/benjchristensen/  David Karnok's Advanced RxJava blog: http://akarnokd.blogspot.hu/  Grokking Rx MEAP: http://bit.ly/grokking-rx  50% discount coupon until May 2: mlmorgillo Recipes in RxJava for Android | #GrokkingRx | @sasa_sekulic