Reactive Programming with RxJava

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Reactive Programming with RxJava Duarte Nunes duarte@midokura.com

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Agenda Functional reactive rx fundamental types operators Taming the Sequence concurrency

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Reactive

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Reactive re·ac·tive adjective \r ē - ˈ ak-tiv\ ! 1 of, relating to, or marked by reaction or reactance 2 readily responsive to a stimulus

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Reactive re·ac·tive adjective \r ē - ˈ ak-tiv\ ! 1 of, relating to, or marked by reaction or reactance 2 readily responsive to a stimulus P

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Reactive re·ac·tive adjective \r ē - ˈ ak-tiv\ ! 1 of, relating to, or marked by reaction or reactance 2 readily responsive to a stimulus P E

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Reactive re·ac·tive adjective \r ē - ˈ ak-tiv\ ! 1 of, relating to, or marked by reaction or reactance 2 readily responsive to a stimulus P E

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Reactive re·ac·tive adjective \r ē - ˈ ak-tiv\ ! 1 of, relating to, or marked by reaction or reactance 2 readily responsive to a stimulus P E REACTIVE

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Reactive re·ac·tive adjective \r ē - ˈ ak-tiv\ ! 1 of, relating to, or marked by reaction or reactance 2 readily responsive to a stimulus Asynchronous Scalable Push VAlueS P E REACTIVE

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Functional

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Functional func·tion·al adjective \ ˈ f əŋ (k)-shn ə l, -sh ə -n ə l\ ! 1 of, relating to, or indicating mathematical functions 2 capable of functioning; working

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Functional func·tion·al adjective \ ˈ f əŋ (k)-shn ə l, -sh ə -n ə l\ ! 1 of, relating to, or indicating mathematical functions 2 capable of functioning; working

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Functional func·tion·al adjective \ ˈ f əŋ (k)-shn ə l, -sh ə -n ə l\ ! 1 of, relating to, or indicating mathematical functions 2 capable of functioning; working Functions HIGHER-ORDER (MOSTLY) PURe Composable

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RxJava “a library for composing asynchronous and event-based programs using observable sequences for the Java VM" https://github.com/Netflix/RxJava A Java port of Rx https://github.com/Reactive-Extensions Reactive Extensions

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Callbacks GWT AsyncCallback 1 service.getShapes(dbName, new AsyncCallback() { 2 def onSuccess(result: Array[Shape]): Unit = { /* ... */ } 3 def onFailure(caught: Throwable): Unit { /* ... */ } 4 });

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Callbacks GWT AsyncCallback 1 service.getShapes(dbName, new AsyncCallback() { 2 def onSuccess(result: Array[Shape]): Unit = { /* ... */ } 3 def onFailure(caught: Throwable): Unit { /* ... */ } 4 }); one-shot No cancellation asynchronous computations are not values

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Observers 1 trait Observer[-A] { 2 def onNext(value: A): Unit = {} 3 def onError(error: Throwable): Unit= {} 4 def onCompleted(): Unit = {} 5 }

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Futures? 1 trait Future[+A] { 2 def onComplete[B](f: Try[A] => B): Unit 3 def isCompleted: Boolean 4 def value: Option[Try[A]] 5 }

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Futures? 1 trait Future[+A] { 2 def onComplete[B](f: Try[A] => B): Unit 3 def isCompleted: Boolean 4 def value: Option[Try[A]] 5 } asynchronously stream back results Goal

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Futures? 1 trait Future[+A] { 2 def onComplete[B](f: Try[A] => B): Unit 3 def isCompleted: Boolean 4 def value: Option[Try[A]] 5 } asynchronously stream back results Goal 1 trait Future[+A] { 2 def subscribe(observer: Observer[A]): Unit 3 def tryCancel(): Boolean 4 }

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Observables 1 trait Observable[+T] { 2 def subscribe(observer: Observer[T]): Subscription 3 }

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Duality ITERATOR OBSERVER def next(): A throws Throwable def hasNext(): Boolean def onNext(a: A): Unit def onError(t: Throwable): Unit def onCompleted(): Unit ITERATABLE OBSERVABLE def iterator: Iterator[A] def subscribe(o: Observer[A]): Subscription getDataFromMemory() drop(10) take(5) map { _ + "_transformed" } foreach println getDataFromNetwork() drop(10) take(5) map { _ + "_transformed" } subscribe println

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Interacting with data Async SYNC Single Multiple A Iterable[A] Observable[A] Future[A]

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Observables are… produced & consumed with side-effects sequences of data in motion! composed functionally synchronous or asynchronous

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Concurrency is abstracted def subcribe(observer: Observer[A]): Subscription Calling thread callback thread Calling thread

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Concurrency is abstracted Calling thread callback threads thread pool def subcribe(observer: Observer[A]): Subscription

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Concurrency is abstracted Calling thread callback thread Actor def subcribe(observer: Observer[A]): Subscription

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Concurrency is abstracted Calling thread callback thread nio Event loop def subcribe(observer: Observer[A]): Subscription

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Contract OnNext(t)* (OnCompleted() | OnError(e))? after zero or more OnNext calls, either one of OnCompleted or OnError will optionally be called observers can assume to be synchronised conceptually they run under a lock, similar to the reactor pattern Observable.synchronize UNsubscribing May not cancel Outsanding work work already in progress might still complete as it is not always safe to abort work in progress, but should not be signaled to unsubscribed observers

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Observable.create 1 def create(subscribe: Observer[A] => Subscription): Observable[A] 2 3 Observable.create(observer => { 4 try { 5 observer.onNext(...) 6 observer.onCompleted(); 7 } catch(Exception e) { 8 observer.onError(e); 9 } 10 }) x

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Interval interval( ) …

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Interval interval( ) … Cold Observable

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1 def myInterval(d: Duration): Observable[Long] = { 2 val t = new Timer(true) 3 Observable.create(observer => { 4 @volatile var done = false 5 var c = 0l 6 def createTimerTask() = new TimerTask() { 7 def run() { 8 if (done) return 9 observer.onNext(c) 10 c += 1 11 t.schedule(createTimerTask(), d.toMillis) 12 }} 13 t.schedule(createTimerTask(), d.toMillis) 14 Subscription { done = true; } 15 }) 16 }

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Subscriptions life-cycle management unsubscribe unsubscribe unsubscribe subscription boolean Composite refcount multiassignment serial

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Subjects onXXX onXXX replaysubject behavioursubject caches values remembers last publication requires default value asyncsubject caches the last value waits until sequence is over

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From future 1 def from[T](f: Future[T])(implicit ec: ExecutionContext): 2 Observable[T] = { 3 val s = AsyncSubject[T]() 4 f.onComplete { 5 case Success(c) => 6 s.onNext(c) 7 s.onCompleted() 8 case Failure(e) => 9 s.onError(e) 10 } 11 s 12 } x

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assume 1 implicit class ObservableOps[A](val o: Observable[A]) extends AnyVal { 2 // operator definitions go here 3 } 4 5 Observable.interval(200 millis) take(3) filter (isEven _)

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Take take(3)

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1 def myTake(n: Int): Observable[A] = 2 Observable.create(observer => { 3 var c = n 4 var s = CompositeSubscription() 5 s += o.subscribe( 6 ((a: A) => { 7 if (c > 0) { 8 observer.onNext(a) 9 c -= 1 10 if (c == 0) { 11 s.unsubscribe() 12 observer.onCompleted() 13 } 14 } 15 }), 16 observer.onError _, 17 observer.onCompleted _) 18 s 19 })

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Filter x x 1 def myFilter(p: A => Boolean): Observable[A] = 2 Observable.create(observer => { 3 o.subscribe( 4 (a: A) => { 5 if (p(a)) 6 observer.onNext(a) 7 }, 8 observer.onError _, 9 observer.onCompleted _) 10 }) ﬁlter(isEven)

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Any = true = false any() any()

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1 def myAny(): Observable[Boolean] = 2 Observable.create(observer => { 3 var s = CompositeSubscription() 4 var hasReturned = false 5 s += o.subscribe( 6 (_: A) => { 7 if (!hasReturned) { 8 hasReturned = true 9 observer.onNext(true) 10 observer.onCompleted() 11 s.unsubscribe 12 } 13 }, 14 observer.onError _, 15 () => { 16 if (!hasReturned) { 17 observer.onNext(false) 18 observer.onCompleted() 19 } 20 }) 21 s 22 })

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FoldLeft 1 def myFoldLeft[B](z: B)(f: (B, A) => B): Observable[B] = 2 Observable.create(observer => { 3 var acc = z 4 o.subscribe( 5 (a: A) => acc = f(acc, a), 6 observer.onError _, 7 () => { 8 observer.onNext(acc) 9 observer.onCompleted() 10 }) 11 }) foldLeft(0)(_ + _)

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FlatMap flatMap { }

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1 def flatMap[R](f: T => Observable[R]): Observable[R] 2 3 4 def filter(predicate: A => Boolean): Observable[A] = 5 o flatMap { x => 6 if (predicate(x)) 7 Observable.items(x); 8 else 9 Observable.empty[A](); 10 } 11 12 def map[R](func: T => R): Observable[R] = { 13 o flatMap (func andThen Obsersable.items) 14 }

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Anamorphisms a => observable[a] Catamorphisms Bind / flatMap observable[a] => B observable[a] => Observable[b]

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Side-effects

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Side-effects compose data in the pipeline

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Side-effects compose data in the pipeline use map to introduce state

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Side-effects compose data in the pipeline sometimes side-effects are unavoidable use map to introduce state

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Side-effects compose data in the pipeline sometimes side-effects are unavoidable use map to introduce state def doOnNext(onNext: A => Unit): Observable[A]

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Side-effects compose data in the pipeline sometimes side-effects are unavoidable beware of mutable elements use map to introduce state def doOnNext(onNext: A => Unit): Observable[A]

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Observable .toBlockingObservable Leaving the monad 1 def foreach(f: A => Unit): Unit 2 3 def withFilter(p: A => Boolean): WithFilter[A] 4 5 def toIterable: Iterable[A] 6 7 def single: A 8 9 def singleOption: Option[A] 10 11 def toFuture: Future[A]

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Error Handling

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Error Handling 1 def retry(retryCount: Int): Observable[T] retry

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Error Handling 1 def retry(retryCount: Int): Observable[T] retry onErrorResumeNext { } x

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Combining Sequences concat

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Combining Sequences concat sequential concat repeat startwith

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Combining Sequences merge x x

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Combining Sequences merge concurrent amb switch zip merge x x

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1 def myMerge(other: Observable[A]): Observable[A] = 2 Observable.create(observer => { 3 val s = CompositeSubscription() 4 val obs = createObserver(s) 5 s += o.subscribe(obs) 6 s += other.subscribe(obs) 7 s 8 }) 1 def createObserver(s: Subscription): Observer[A] = { 2 var done = false 3 Observer( 4 (a: A) => { 5 synchronized 6 if (!done) 7 observer.onNext(a) 8 }, t => { 9 synchronized 10 if (!done) { 11 done = true; s.unsubscribe; observer.onError(t) 12 } 13 }, () => { 14 synchronized 15 if (!done) { 16 done = true; s.unsubscribe; observer.onCompleted() 17 } 18 }) 19 }

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switch 1 def switch[U](implicit evidence: Observable[T] <:< Observable[Observable[U]]): Observable[U] switch

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An example switch observeChanges(output) map(word => completions(word)) throttle(10 millis) textChanges(input)

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time axis is an unknown

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time axis is an unknown Time shifted sequences maps data on the time plane Buffer Delay Sample Throttle Timeout

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time axis is an unknown Time shifted sequences maps data on the time plane Buffer Delay Sample sequences of coincidence Throttle Timeout Buffer Join Window sliding windows

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Sharing Observable

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Sharing publish Observable ConnectableObservable

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Sharing Subject publish Observable ConnectableObservable

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Sharing Subject publish Observable ConnectableObservable

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Sharing Subject publish connect / refCount Observable ConnectableObservable

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Sharing Subject publish connect / refCount Observable ConnectableObservable

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Sharing Subject publish connect / refCount Observable ConnectableObservable

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Sharing Subject publish connect / refCount 1 val o = Observable.interval(200 millis) 2 val c: ConnectableObservable[Long] = o.publish 3 c.subscribe(x => println(x)) 4 val s = c.connect 5 c.subscribe(x => println(x)) 6 s.unsubscribe Observable ConnectableObservable

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from iterable infinity and beyond? 1 def myFrom[A](xs: Iterable[A]): Observable[A] = 2 Observable.create(observer => { 3 xs foreach (observer.onNext _) 4 observer.onCompleted() 5 Subscription { } 6 })

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Concurrency 1 object Future { 2 def apply[A](body: =>T)(implicit executor: ExecutionContext): Future[A] 3 }

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Concurrency 1 object Future { 2 def apply[A](body: =>T)(implicit executor: ExecutionContext): Future[A] 3 } 1 trait Observable[A] { 2 def observeOn(scheduler: Scheduler): Observable[A] 3 }

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Concurrency 1 object Future { 2 def apply[A](body: =>T)(implicit executor: ExecutionContext): Future[A] 3 } 1 trait Observable[A] { 2 def observeOn(scheduler: Scheduler): Observable[A] 3 }

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Schedulers 1 trait Scheduler { 2 def schedule(action: => Unit): Subscription 3 def schedule(action: Scheduler => Subscription): Subscription 4 def scheduleRec(work: (=>Unit) => Unit): Subscription 5 // ... 6 }

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from iterable 1 def myFrom2[A](xs: Iterable[A], s: Scheduler): Observable[A] = 2 Observable.create(observer => { 3 s schedule { 4 xs foreach (observer.onNext _) 5 observer.onCompleted() 6 } 7 }) again

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from iterable again 1 def myFrom2[A](xs: Iterable[A], s: Scheduler): Observable[A] = 2 Observable.create(observer => { 3 s schedule { 4 xs foreach (observer.onNext _) 5 observer.onCompleted() 6 } 7 }) still Wrong 1 def myFrom3[A](xs: Iterable[A], s: Scheduler): Observable[A] = 2 Observable.create(observer => { 3 val it = xs.iterator 4 s.scheduleRec(self => { 5 if (it.hasNext) { 6 observer.onNext(it.next()) 7 self 8 } else observer.onCompleted() 9 })}) again and

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R 1 implicit class SchedulerOps(val s: Scheduler) extends AnyVal { 2 3 def mySchedule(work: (=>Unit) => Unit): Subscription = 4 s.schedule(scheduler => { 5 6 val subscription = MultipleAssignmentSubscription() 7 8 def loop(): Unit = { 9 subscription.subscription = scheduler.schedule { 10 work { loop() } 11 } 12 } 13 14 loop() 15 16 subscription 17 }) 18 } ecursive scheduling

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Thank you Questions?