RxJava in a nutshell - Speaker Deck

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RxJava in a nutshell Alexey Bykov

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Alexey Bykov * Google Developer Expert for Android * Android Engineer @Revolut * EX: Android Academy MSK community lead * EX: Podlodka Crew & AppsConf

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RxJava — complicated

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Goal To make RxJava easier for you

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Lifecycle

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//Ordering? Observable.just("Hey") .subscribeOn(Schedulers.io()) .map { printThread() it.length } .observeOn(AndroidSchedulers.mainThread()) .doOnSubscribe { printThread() } .subscribeOn(Schedulers.computation()) .flatMap { printThread() Observable.timer(1, TimeUnit.SECONDS) .subscribeOn(Schedulers.single()) .doOnSubscribe { printThread() } }.subscribe { printThread() } //Threading?

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//1. Creation

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public static Observable just(T item) { ObjectHelper.requireNonNull(item, "item is null"); return RxJavaPlugins.onAssembly( new ObservableJust(item) ); } Observable.just("Hey")

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Observable.just("Hey") public static Observable just(T item) { ObjectHelper.requireNonNull(item, "item is null"); return RxJavaPlugins.onAssembly( new ObservableJust(item) ); }

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Observable.just("Hey") public static Observable just(T item) { ObjectHelper.requireNonNull(item, "item is null"); return RxJavaPlugins.onAssembly( new ObservableJust(item) ); }

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subscribeOn(Schedulers.io()) public final Observable subscribeOn(Scheduler scheduler) { ObjectHelper.requireNonNull(scheduler, "scheduler is null»); return RxJavaPlugins.onAssembly( new ObservableSubscribeOn(this, scheduler) ); }

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Observable.just("Hey")  .subscribeOn(Schedulers.io()) .map { printThread() it.length } .observeOn(AndroidSchedulers.mainThread()) .doOnSubscribe { printThread() } .subscribeOn(Schedulers.computation()) .flatMap { printThread() Observable.timer(1, TimeUnit.SECONDS) .subscribeOn(Schedulers.single()) .doOnSubscribe { printThread() } }.subscribe { printThread() } Store reference

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//2. Subscription

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Observable.just("Hey") .subscribeOn(Schedulers.io()) .map { printThread() it.length } .observeOn(AndroidSchedulers.mainThread()) .doOnSubscribe { printThread() } .subscribeOn(Schedulers.computation()) .flatMap { printThread() Observable.timer(1, TimeUnit.SECONDS) .subscribeOn(Schedulers.single()) .doOnSubscribe { printThread() } }.subscribe { printThread() } Subscribe Subscribe

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Observable.just("Hey") .subscribeOn(Schedulers.io()) .map { printThread() it.length } .observeOn(AndroidSchedulers.mainThread()) .doOnSubscribe { printThread() } .subscribeOn(Schedulers.computation()) .flatMap { printThread() Observable.timer(1, TimeUnit.SECONDS) .subscribeOn(Schedulers.single()) .doOnSubscribe { printThread() } }.subscribe { printThread() } Subscribe Subscribe //1. Change thread: Computation

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Observable.just("Hey") .subscribeOn(Schedulers.io()) .map { printThread() it.length } .observeOn(AndroidSchedulers.mainThread()) .doOnSubscribe { printThread() } .subscribeOn(Schedulers.computation()) .flatMap { printThread() Observable.timer(1, TimeUnit.SECONDS) .subscribeOn(Schedulers.single()) .doOnSubscribe { printThread() } }.subscribe { printThread() } Subscribe Subscribe //1. Change thread: Computation Subscribe

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Observable.just("Hey") .subscribeOn(Schedulers.io()) .map { printThread() it.length } .observeOn(AndroidSchedulers.mainThread()) .doOnSubscribe { printThread() } .subscribeOn(Schedulers.computation()) .flatMap { printThread() Observable.timer(1, TimeUnit.SECONDS) .subscribeOn(Schedulers.single()) .doOnSubscribe { printThread() } }.subscribe { printThread() } Subscribe Subscribe //1. Change thread: Computation //2. Execute callback: Computation Subscribe

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Observable.just("Hey") .subscribeOn(Schedulers.io()) .map { printThread() it.length } .observeOn(AndroidSchedulers.mainThread()) .doOnSubscribe { printThread() } .subscribeOn(Schedulers.computation()) .flatMap { printThread() Observable.timer(1, TimeUnit.SECONDS) .subscribeOn(Schedulers.single()) .doOnSubscribe { printThread() } }.subscribe { printThread() } Subscribe Subscribe //1. Change thread: Computation //2. Execute callback: Computation Subscribe Subscribe

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Observable.just("Hey") .subscribeOn(Schedulers.io()) .map { printThread() it.length }  .observeOn(AndroidSchedulers.mainThread()) .doOnSubscribe { printThread() } .subscribeOn(Schedulers.computation()) .flatMap { printThread() Observable.timer(1, TimeUnit.SECONDS) .subscribeOn(Schedulers.single()) .doOnSubscribe { printThread() } }.subscribe { printThread() } Subscribe Subscribe //1. Change thread: Computation //2. Execute callback: Computation Subscribe Subscribe Subscribe

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Observable.just("Hey") .subscribeOn(Schedulers.io()) .map { printThread() it.length }  .observeOn(AndroidSchedulers.mainThread()) .doOnSubscribe { printThread() } .subscribeOn(Schedulers.computation()) .flatMap { printThread() Observable.timer(1, TimeUnit.SECONDS) .subscribeOn(Schedulers.single()) .doOnSubscribe { printThread() } }.subscribe { printThread() } Subscribe Subscribe //1. Change thread: Computation //2. Execute callback: Computation Subscribe Subscribe Subscribe Subscribe

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Observable.just("Hey") .subscribeOn(Schedulers.io()) .map { printThread() it.length }  .observeOn(AndroidSchedulers.mainThread()) .doOnSubscribe { printThread() } .subscribeOn(Schedulers.computation()) .flatMap { printThread() Observable.timer(1, TimeUnit.SECONDS) .subscribeOn(Schedulers.single()) .doOnSubscribe { printThread() } }.subscribe { printThread() } Subscribe Subscribe //1. Change thread: Computation //2. Execute callback: Computation Subscribe Subscribe Subscribe Subscribe //3. Change thread: IO

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Observable.just("Hey") .subscribeOn(Schedulers.io()) .map { printThread() it.length }  .observeOn(AndroidSchedulers.mainThread()) .doOnSubscribe { printThread() } .subscribeOn(Schedulers.computation()) .flatMap { printThread() Observable.timer(1, TimeUnit.SECONDS) .subscribeOn(Schedulers.single()) .doOnSubscribe { printThread() } }.subscribe { printThread() } Subscribe Subscribe //1. Change thread: Computation //2. Execute callback: Computation Subscribe Subscribe Subscribe Subscribe //3. Change thread: IO Subscribe

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//3. Emission

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Observable.just("Hey") .subscribeOn(Schedulers.io()) .map { printThread() it.length } .observeOn(AndroidSchedulers.mainThread()) .doOnSubscribe { printThread() } .subscribeOn(Schedulers.computation()) .flatMap { printThread() Observable.timer(1, TimeUnit.SECONDS) .subscribeOn(Schedulers.single()) .doOnSubscribe { printThread() } }.subscribe { printThread() } Emit «Hey», IO Emit «Hey», IO

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Observable.just("Hey") .subscribeOn(Schedulers.io()) .map { printThread() it.length } .observeOn(AndroidSchedulers.mainThread()) .doOnSubscribe { printThread() } .subscribeOn(Schedulers.computation()) .flatMap { printThread() Observable.timer(1, TimeUnit.SECONDS) .subscribeOn(Schedulers.single()) .doOnSubscribe { printThread() } }.subscribe { printThread() } //1. Print thread, IO Emit «Hey», IO Emit «Hey», IO

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Observable.just("Hey") .subscribeOn(Schedulers.io()) .map { printThread() it.length } .observeOn(AndroidSchedulers.mainThread()) .doOnSubscribe { printThread() } .subscribeOn(Schedulers.computation()) .flatMap { printThread() Observable.timer(1, TimeUnit.SECONDS) .subscribeOn(Schedulers.single()) .doOnSubscribe { printThread() } }.subscribe { printThread() } //1. Print thread, IO Emit «Hey», IO Emit «Hey», IO Emit «Hey», IO

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Observable.just("Hey") .subscribeOn(Schedulers.io()) .map { printThread() it.length } .observeOn(AndroidSchedulers.mainThread()) .doOnSubscribe { printThread() } .subscribeOn(Schedulers.computation()) .flatMap { printThread() Observable.timer(1, TimeUnit.SECONDS) .subscribeOn(Schedulers.single()) .doOnSubscribe { printThread() } }.subscribe { printThread() } //1. Print thread, IO //2. Change thread, Main Emit «Hey», IO Emit «Hey», IO Emit «Hey», IO

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Observable.just("Hey") .subscribeOn(Schedulers.io()) .map { printThread() it.length } .observeOn(AndroidSchedulers.mainThread()) .doOnSubscribe { printThread() } .subscribeOn(Schedulers.computation()) .flatMap { printThread() Observable.timer(1, TimeUnit.SECONDS) .subscribeOn(Schedulers.single()) .doOnSubscribe { printThread() } }.subscribe { printThread() } //1. Print thread, IO //2. Change thread, Main Emit «Hey», IO Emit «Hey», IO Emit «Hey», IO Emit «Hey», Main

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Observable.just("Hey") .subscribeOn(Schedulers.io()) .map { printThread() it.length } .observeOn(AndroidSchedulers.mainThread()) .doOnSubscribe { printThread() } .subscribeOn(Schedulers.computation()) .flatMap { printThread() Observable.timer(1, TimeUnit.SECONDS) .subscribeOn(Schedulers.single()) .doOnSubscribe { printThread() } }.subscribe { printThread() } //1. Print thread, IO //2. Change thread, Main Emit «Hey», IO Emit «Hey», IO Emit «Hey», IO Emit «Hey», Main Emit «Hey», Main

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Observable.just("Hey") .subscribeOn(Schedulers.io()) .map { printThread() it.length } .observeOn(AndroidSchedulers.mainThread()) .doOnSubscribe { printThread() } .subscribeOn(Schedulers.computation()) .flatMap { printThread() Observable.timer(1, TimeUnit.SECONDS) .subscribeOn(Schedulers.single()) .doOnSubscribe { printThread() } }.subscribe { printThread() } //1. Print thread, IO //2. Change thread, Main Emit «Hey», IO Emit «Hey», IO Emit «Hey», IO Emit «Hey», Main Emit «Hey», Main Emit «Hey», Main

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Observable.just("Hey") .subscribeOn(Schedulers.io()) .map { printThread() it.length } .observeOn(AndroidSchedulers.mainThread()) .doOnSubscribe { printThread() } .subscribeOn(Schedulers.computation()) .flatMap { printThread() Observable.timer(1, TimeUnit.SECONDS) .subscribeOn(Schedulers.single()) .doOnSubscribe { printThread() } }.subscribe { printThread() } //1. Print thread, IO //2. Change thread, Main //3. Print thread, Main Emit «Hey», IO Emit «Hey», IO Emit «Hey», IO Emit «Hey», Main Emit «Hey», Main Emit «Hey», Main

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Observable.just("Hey") … .observeOn(AndroidSchedulers.mainThread()) … .flatMap { printThread() //Own lifecycle: Creation, Subscription, Emission Observable.timer(1, TimeUnit.SECONDS) .subscribeOn(Schedulers.single()) .doOnSubscribe { printThread() } }.subscribe { printThread() }

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Observable.just("Hey") … .observeOn(AndroidSchedulers.mainThread()) … .flatMap { printThread() 1. Creation Observable.timer(1, TimeUnit.SECONDS) .subscribeOn(Schedulers.single()) .doOnSubscribe { printThread() } }.subscribe { printThread() }

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Observable.just("Hey") … .observeOn(AndroidSchedulers.mainThread()) … .flatMap { printThread() Observable.timer(1, TimeUnit.SECONDS) .subscribeOn(Schedulers.single()) .doOnSubscribe { printThread() } }.subscribe { printThread() }

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Observable.just("Hey") … .observeOn(AndroidSchedulers.mainThread()) … .flatMap { printThread() 2. Subscription Observable.timer(1, TimeUnit.SECONDS) .subscribeOn(Schedulers.single()) .doOnSubscribe { printThread() } }.subscribe { printThread() }

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public static Observable timer(long delay, TimeUnit unit) { return timer(delay, unit, Schedulers.computation() ); } Observable.timer

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public static Observable timer(long delay, TimeUnit unit) { return timer(delay, unit, Schedulers.computation() ); } Observable.timer

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public static Observable timer(long delay, TimeUnit unit) { return timer(delay, unit, Schedulers.computation() ); } public static Observable timer(long delay, TimeUnit unit, Scheduler scheduler) { ObjectHelper.requireNonNull(unit, "unit is null"); ObjectHelper.requireNonNull(scheduler, "scheduler is null"); return RxJavaPlugins.onAssembly( new ObservableTimer(Math.max(delay, 0L), unit, scheduler)); }

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Observable.just("Hey") … .observeOn(AndroidSchedulers.mainThread()) … .flatMap { printThread() 3. Emission Observable.timer(1, TimeUnit.SECONDS) .subscribeOn(Schedulers.single()) .doOnSubscribe { printThread() } }.subscribe { printThread() }

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Lifecycle: Conclusions

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Lifecycle: Conclusions //1. Creation (From builder until terminal operator reached)

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Builder operators example //1. just //2. fromCallable //3. defer //4. interval //5. delay https://reactivex.io/documentation/operators.html etc…

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Terminal operators example //1. subscribe() //2. blockingGet() … https://reactivex.io/documentation/operators.html etc…

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Lifecycle: Conclusions //1. Creation (until terminal operator reached) //2. Subscription (until top(builder) operator reached)

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Executable on subscription step //1. subscribeOn //2. startWith //3. doOnSubscribe https://reactivex.io/documentation/operators.html etc…

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Lifecycle: Conclusions //1. Creation (until terminal operator reached) //2. Subscription (until top(builder) operator reached) //3. Emission

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Executable on emission step //1. observeOn //2. doOnNext, doOnError, catch //3. *map family https://reactivex.io/documentation/operators.html //4. disctinctUntilChange etc…

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Opinion: only latest to the top subscribeOn will be executed and applied

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Observable.just("Hey") .subscribeOn(Schedulers.io()) .subscribeOn(Schedulers.io()) .subscribeOn(Schedulers.io()) .subscribe{ }

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Observable.just("Hey") .doOnSubscribe { } .subscribeOn(Schedulers.io()) .doOnSubscribe { } .subscribeOn(Schedulers.io()) .doOnSubscribe { } .subscribeOn(Schedulers.io()) .subscribe { }

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Observable.just("Hey") .doOnSubscribe { } .subscribeOn(Schedulers.io()) .doOnSubscribe { } .subscribeOn(Schedulers.io()) .doOnSubscribe { } .subscribeOn(Schedulers.io()) .subscribe { }

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Observable.just("Hey") .doOnSubscribe { } .subscribeOn(Schedulers.io()) .doOnSubscribe { } .subscribeOn(Schedulers.io()) .doOnSubscribe { } .subscribeOn(Schedulers.io()) .subscribe { } //Thread 1 //Thread 2 //Thread1

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Opinion: only latest to the top subscribeOn will be executed and applied All subscribeOn will be executed, but latest applied

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Case: Efficient Search

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No content

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private val subject = PublishSubject.create() fun onTextInserted(text:String) { subject.onNext(text) }

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init { } subject.debounce(300, TimeUnit.MILLISECONDS)

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init { } subject.debounce(300, TimeUnit.MILLISECONDS) .switchMap { getData() }

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init { } subject.debounce(300, TimeUnit.MILLISECONDS) .switchMap { getData() } .distinctUntilChanged()

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init { } subject.debounce(300, TimeUnit.MILLISECONDS) .switchMap { getData() } .distinctUntilChanged() .observeOn(AndroidSchedullers.mainThread())

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init { subject.debounce(300, TimeUnit.MILLISECONDS) } .switchMap { getData() } .distinctUntilChanged() .observeOn(AndroidSchedullers.mainThread()) .subscribeTillCleared(::setState,::processError)

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Case: Shared http request

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The same content

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The same content

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The same content PesRepository

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The same content PesRepository Network Network Network

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Idea: PesRepository Network Shared request

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Connectable Observable

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Connectable Observable //1. Doesn’t start when you executed subscribe() //2. Starts when you executed connect()

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val pes: Single = service.getPes()

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val pes: Observable = service.getPes()

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val pes: ConnectableObservable = getFromNetwork().publish()

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val pes: ConnectableObservable = getFromNetwork().publish() pes.subscribe() pes.subscribe() pes.subscribe() // nothing will happen

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val pes: ConnectableObservable = getFromNetwork().publish() pes.subscribe() pes.subscribe() pes.subscribe() pes.connect()

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val pes: ConnectableObservable = getFromNetwork().publish() pes.subscribe() pes.subscribe() pes.subscribe() pes.connect() Shared request

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val pes: Observable = getFromNetwork().publish().refCount() pes.subscribe() pes.subscribe() pes.subscribe() Shared request

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val pes: Observable = getFromNetwork().share() pes.subscribe() pes.subscribe() pes.subscribe() Shared request

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Case: Mediator pattern

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class LogoutMediator { } private val subject = PublishSubject.create()

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class LogoutMediator { } fun logoutRequired() { } subject.onNext() private val subject = PublishSubject.create()

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class LogoutMediator { } private val subject = PublishSubject.create() fun logoutRequired() {…}

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class LogoutMediator { } private val subject = PublishSubject.create() fun logoutRequired() {…} fun observeLogout(): Observable = subject

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class LogoutMediator { } private val subject = PublishSubject.create() fun logoutRequired() {…} fun observeLogout(): Observable = subject .toSeriallised()

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Case: Polling

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No content

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Observable.interval(0, 1, TimeUnit.SECONDS)

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Observable.interval(0, 1, TimeUnit.SECONDS) .switchMap { getData() }

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Observable.interval(0, 1, TimeUnit.SECONDS) .switchMap { getData() } .observaOn(AndroidSchedullers.mainThread()) .subscribe(::processData,::processError)

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Case: Testing

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class Repository….

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class Repository…. fun getDog() :Single = service.getDog() .subscribeOn(Schedullers.io() .map(::toDomain())

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fun test() @Test

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fun test() = when(service.getDog()).thenReturn(Single.just(DogDto(…))) @Test

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fun test() = when(service.getDog()).thenReturn(Single.just(DogDto(…))) repository.getDog() .test() .assertNoErrors() .assertValue(Dog(…)) @Test

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fun test() = when(service.getDog()).thenReturn(Single.just(DogDto(…))) repository.getDog() .test() .assertNoErrors() .assertValue(Dog(…)) @Test It doesn’t work ¯\_(ツ)_/¯

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class Repository…. fun getDog() :Single = service.getDog() .subscribeOn(Schedullers.io() .map(::toDomain())

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class Repository…. fun getDog() :Single = service.getDog() .subscribeOn(Schedullers.io() .map(::toDomain())

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• class RxTrampolineSchedulerRule : TestRule { • • override fun apply(base: Statement, description: Description): Statement { • return object : Statement() { • @Throws(Throwable::class) • override fun evaluate() { • RxJavaPlugins.setIoSchedulerHandler { Schedulers.trampoline() } • RxJavaPlugins.setComputationSchedulerHandler { Schedulers.trampoline() } • RxJavaPlugins.setNewThreadSchedulerHandler { Schedulers.trampoline() } • RxAndroidPlugins.setInitMainThreadSchedulerHandler { Schedulers.trampoline() } • • try { • base.evaluate() • } finally { • RxJavaPlugins.reset() • RxAndroidPlugins.reset() • } • } • } • }

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fun test() = when(service.getDog()).thenReturn(Single.just(DogDto(…))) repository.getDog() .test() .assertNoErrors() .assertValue(Dog(…)) @Test

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fun test() = when(service.getDog()).thenReturn(Single.just(DogDto(…))) repository.getDog() .test() .assertNoErrors() .assertValue(Dog(…)) @Test val rule = RxTrampolineSchedullerRule() @Rule @JvmField

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Case: Disposing

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interface Disposable { } fun dispose() fun isDisposed()

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class ViewModel…. init { interact.getDog() .observeOn(MainThread()) .subscribe(::processData,::processError) }

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class ViewModel…. init { disposable = interact.getDog() .observeOn(MainThread()) .subscribe(::processData,::processError) } private lateinit var disposable:Disposable

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class ViewModel…. init {…} private lateinit var disposable:Disposable fun onCleared() { } disposable.dispose()

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class ViewModel…. init {…} private lateinit var disposable:Disposable fun onCleared() { } disposable.dispose() Bad practise

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class ViewModel…. private val compositeDisposable = CompositeDisposable() }

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class ViewModel…. init { val disposable = interact.getDog() .observeOn(MainThread()) .subscribe(::processData,::processError) } private val compositeDisposable = CompositeDisposable() compositeDisposable.add(disposable)

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class ViewModel…. init {…} fun onCleared() { } compositeDisposable.clear() private val compositeDisposable = CompositeDisposable

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We can do better :)

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shorturl.at/zEJPT ViewModel extensions

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class ViewModel…. init { val disposable = interact.getDog() .observeOn(MainThread()) .subscribeTillCleared(::processData,::processError) }

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Conclusion * RxJava complicated, but you can handle it * RxJava won’t disappear soon * New project — Kotlin Coroutines Flow * You need to know it :)