RxJava. and WHY you should care. Jeroen Tietema

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1 RxJava and WHY you should care Jeroen Tietema

2 Overview intro of RxJava why? some tips

3 What is RxJava? Java implementation of Reactive Extensions Reactive Extensions is.net implementation of Reactive Programming

4 What is Reactive programming? Reactive programming is a programming paradigm oriented around data ᴀ밄ows and the propagation of change. This means that it should be possible to express static or dynamic data ᴀ밄ows with ease in the programming languages used, and that the underlying execution model will automatically propagate changes through the data ᴀ밄ow. Wikipedia (Reactive Programming) TLDR; something with data ᴀ밄ows?

5 What is Reactive Extensions? Using Rx, developers represent asynchronous data streams with Observables, query asynchronous data streams using LINQ operators, and parameterize the concurrency in the asynchronous data streams using Schedulers. Simply put, Rx = Observables + LINQ + Schedulers. Microsoft Reactice Extensions Homepage

6 RxJava concepts Observables and the Observable Contract Subscribing and subscriptions operators

7 Observables Observables emit data when you subscribe to them. onnext (0.. ) onerror (0..1) oncompleted (0..1) Cannot call both onerror and oncompleted. Should never do any calls after the Subscriber unsubscribes. When multiple observers subscribe to the same Observable, it is up to the Observable to decide what to do with even emission.

8 Subscribers and Subscriptions Observable<Integer> myints = Observable.just(1, 2, 3, 4, 5);

9 Subscribers and Subscriptions Observable<Integer> myints = Observable.just(1, 2, 3, 4, 5); Subscription s = myints.subscribe(new Subscriber<Integer>() { void onnext(integer i) { // your Integer has arrived void oncompleted() { // Happy Friday!!! void onerror(throwable e) { // :' ( );

10 Subscribers and Subscriptions Observable<Integer> myints = Observable.just(1, 2, 3, 4, 5); Subscription s = myints.subscribe(new Subscriber<Integer>() { void onnext(integer i) { // your Integer has arrived void oncompleted() { // Happy Friday!!! void onerror(throwable e) { // :' ( ); // boring, boring! s.unsubscribe();

11 Operators Observable<Integer> myints = Observable.just(1, 2, 3, 4, 5);

12 Operators Observable<Integer> myints = Observable.just(1, 2, 3, 4, 5); Observable<Integer> evennumbers = myints.filter(i > i % 2 == 0)

13 Operators Observable<Integer> myints = Observable.just(1, 2, 3, 4, 5); Observable<Integer> evennumbers = myints.filter(i > i % 2 == 0); Observable<String> myintsasstrings = evennumbers.map(i > "My lovely int " + i);

14 Operators Observable<Integer> myints = Observable.just(1, 2, 3, 4, 5); Observable<Integer> evennumbers = myints.filter(i > i % 2 == 0); Observable<String> myintsasstrings = evennumbers.map(i > "My lovely int " + i); // Subscribing results in: onnext("my lovely int 2"); onnext("my lovely int 4"); oncompleted();

15 Operators (continued) Observable<Integer> helloasyncints = Observable.just(1, 2, 3, 4); // start emitting on Schedulers.io().subscribeOn(Schedulers.io()) // emit the results on main UI thread.observeon(androidschedulers.mainthread());

16 Operators (continued) Observable<Integer> helloasyncints = Observable.just(1, 2, 3, 4); // start emitting on Schedulers.io().subscribeOn(Schedulers.io()) // emit the results on main UI thread.observeon(androidschedulers.mainthread()); You can call observeon multiple times! AndroidSchedulers comes from RxAndroid

17 Operators (continued) Observable<Integer> helloasyncints = Observable.just(1, 2, 3, 4); // add a delay to emitted items on // Schedulers.computation().delay(500, TimeUnit.MILLISECONDS)

18 But WHY? Leonid Mamchenkov

19 Retrofit vs RxJava (async) myretrofitapi.somecall( new Callback<SomePojo>() { void onsuccess(somepojo mypojo) { // your awesomeness void onerror(retrofiterror error) { // deal with it ); Subscription s = myobservableapi.somecall().subscribeon(schedulers.io()).observeon(androidschedulers.mainthread()).subscribe(new Subscriber<SomePojo>() { void onnext(somepojo mypojo) { // your awesomeness void onerror(throwable error) { // deal with it void oncompleted() { // but... why? );

20 Retrofit vs RxJava (sync) SomePojo mypojo = myretrofitapi.somecall(); Subscription s = myobservableapi.somecall().subscribe(new Subscriber<SomePojo>() { void onnext(somepojo mypojo) { // your awesomeness void onerror(throwable error) { // deal with it void oncompleted() { // but... why? );

21 Freddie Alequin

Threading dilemma (or not?) // sync SomePojo mypojo = myretrofitapi.somecall(); Observable<SomePojo> myobservableapi.somecall(); // async myretrofitapi.

22 Threading dilemma (or not?) // sync SomePojo mypojo = myretrofitapi.somecall(); Observable<SomePojo> myobservableapi.somecall(); // async myretrofitapi.somecall(mycallback); Observable<SomePojo> myobservableapi.somecall(); Decisions around threading are big decisions, but we end up making them in a ad-hoc fashion RxJava: You can worry about threading later!

23 For example

24 Observable<Account> observable = mydatabase.findaccount();

25 Observable<HomePageFeed> observable = mydatabase.findaccount().flatmap(myaccount > myapi.fetchhomepagefeed(myaccount))

26 Observable<HomePageFeed> observable = mydatabase.findaccount().flatmap(myaccount > myapi.fetchhomepagefeed(myaccount)).doonnext(myhomepagefeed > mycache.store(myhomepagefeed))

27 Observable<HomePageFeed> observable = mydatabase.findaccount().flatmap(myaccount > myapi.fetchhomepagefeed(myaccount)).doonnext(myhomepagefeed > mycache.store(myhomepagefeed)).subscribeon(schedulers.io()).observeon(androidschedulers.mainthread());

28 Subscription s = mydatabase.findaccount().flatmap(myaccount > myapi.fetchhomepagefeed(myaccount)).doonnext(myhomepagefeed > mycache.store(myhomepagefeed)).subscribeon(schedulers.io()).observeon(androidschedulers.mainthread()).subscribe(new Subscriber<HomePageFeed>() { void onnext(homepagefeed myhomepagefeed) { // render homepage feed void onerror(throwable error) { // show error dialog void oncompleted(){ );

29 Compared to:

30 mydatabase.findaccount(new DatabaseCallback<Account>() { ); void onquerycomplete(account account){

31 mydatabase.findaccount(new DatabaseCallback<Account>() { void onquerycomplete(account account){ myapi.gethomepage(account, new ApiCallback<HomePageFeed>() { void onresponse(homepagefeed homepage) { ); );

32 mydatabase.findaccount(new DatabaseCallback<Account>() { void onquerycomplete(account account){ myapi.gethomepage(account, new ApiCallback<HomePageFeed>() { ); void onresponse(homepagefeed homepage) { mycache.store(homepage); );

33 mydatabase.findaccount(new DatabaseCallback<Account>() { void onquerycomplete(account account){ myapi.gethomepage(account, new ApiCallback<HomePageFeed>() { ); void onresponse(homepagefeed homepage) { mycache.store(homepage); // < Disk IO on UI thread! );

34 mydatabase.findaccount(new DatabaseCallback<Account>() { void onquerycomplete(account account){ myapi.gethomepage(account, new ApiCallback<HomePageFeed>() { ); void onresponse(homepagefeed homepage) { new AsyncTask<Void, Void, Void>() { Void doinbackground(void... voids) { mycache.store(homepage); return null;.execute(); );

35 Benefits writing complicated threaded code becomes (relatively) easy. clear pattern to cancel background tasks through Subscriptions

40 Operators, operators, operators!! manipulating data can do threading but also replace conditional logic rjp

41 For example Subscription s = mycache.fetchhomepagefeed().concat(myapi.fetchhomepagefeed()).first().subscribe(new Subscriber // etc.

42 For example Subscription s = mycache.fetchhomepagefeed().concat(myapi.fetchhomepagefeed()).first().subscribe(new Subscriber // etc. vs HomePageFeed myfeed = mycache.fetchhomepagefeed(); if (myfeed == null) { myfeed = myapi.fetchhomepagefeed(); return myfeed;

43 Search example class MySearchView extends TextView { PublishSubject<String> subject = PublishSubject.create(); void init() { addtextchangedlistener(new TextWatcher() { //.. void aftertextchanged(editable s) { subject.onnext(s.tostring()); ); Observable<String> gettextobservable() { return subject;

44 Search example mysearchview.gettextobservable().filter(text > text.length > 2)

45 Search example mysearchview.gettextobservable().filter(text > text.length > 2).debounce(400, TimeUnit.MILLISECONDS)

46 Search example mysearchview.gettextobservable().filter(text > text.length > 2).debounce(400, TimeUnit.MILLISECONDS).observeOn(Schedulers.io()).flatMap(text > myapi.search(text))

47 Search example mysearchview.gettextobservable().filter(text > text.length > 2).debounce(400, TimeUnit.MILLISECONDS).observeOn(Schedulers.io()).flatMap(text > myapi.search(text)).observeon(androidschedulers.mainthread())

48 Search example Subscription s = mysearchview.gettextobservable().filter(text > text.length > 2).debounce(400, TimeUnit.MILLISECONDS).observeOn(Schedulers.io()).flatMap(text > myapi.search(text)).observeon(androidschedulers.mainthread()).subscribe(new Subscriber() { void onnext(searchresults results) { // render results void onerror(throwable someerror) { void oncompleted() { );

49 Search vanilla example mytextview.addtextchangedlistener(new TextWatcher() { void aftertextchanged(final Editable s) { if (s.tostring().length > 2) { );

50 Search vanilla example mytextview.addtextchangedlistener(new TextWatcher() { void aftertextchanged(final Editable s) { if (s.tostring().length > 2) { myhandler.postdelayed(new Runnable() void run() { );, 400L);

51 Search vanilla example mytextview.addtextchangedlistener(new TextWatcher() { void aftertextchanged(final Editable s) { if (s.tostring().length > 2) { myhandler.postdelayed(new Runnable() void run() { myasynctask = new FetchSuggestionsTask(); myasynctask.execute(s.tostring());, 400L); );

52 Search vanilla example mytextview.addtextchangedlistener(new TextWatcher() { void aftertextchanged(final Editable s) { if (s.tostring().length > 2) { if (mycallback!= null) { myhandler.removecallbacks(mycallback); mycallback = new Runnable() void run() { myasynctask = new FetchSuggestionsTask(); myasynctask.execute(s.tostring()); myhandler.postdelayed(mycallback, 400L); );

53 Search example Subscription s = mysearchview.gettextobservable().filter(text > text.length > 2).debounce(400, TimeUnit.MILLISECONDS).observeOn(Schedulers.io()).flatMap(text > myapi.search(text)).observeon(androidschedulers.mainthread()).subscribe(new Subscriber() { void onnext(searchresults results) { // render results void onerror(throwable someerror) { void oncompleted() { );

54 Craig Sunter

55 Creating Observables class MyObservableApi { public Observable<HomePageFeed> gethomepage(account account) { // wrong return Observable.just( mysynclegacyapi.gethomepage(account));

56 Defer + just = your friend! class MyObservableApi { public Observable<HomePageFeed> gethomepage(account account) { // right! return Observable.defer(() > Observable.just( mysynclegacyapi.gethomepage(account));

57 Testing model public void gethomepage_givenvalidaccount_returnshomepage() { TestSubscriber<HomePageFeed> s = new TestSubscriber<>(); myobservableapi.subscribe(s); s.assertcompleted(); s.assertvaluecount(1); HomePageFeed h = s.getonnextvalues().get(0);

58 Testing Presenter layer (Rx HQ) Inject your Schedulers Schedulers.immediate() will su 鍉䨈 ce most of the time. Use TestScheduler for more advanced use cases.

59 Testing time based public void test400milliesdelay() { TestScheduler testscheduler = new TestScheduler(); TestSubscriber<Integer> testsubscriber = new TestSubscriber<>(); Observable.just(1).delay(400, TimeUnit.MILLISECONDS, testscheduler).subscribe(testsubscriber); testscheduler.advancetimeby(200, TimeUnit.MILLISECONDS); testsubscriber.assertnovalues(); testscheduler.advancetimeby(200, TimeUnit.MILLISECONDS); testsubscriber.assertvaluecount(1); testsubscriber.assertcompleted();

60 Useful links Functional Reactive Programming with RxJava, Ben Christensen, Netᴀ밄ix Going Reactive, An Android Architectural Journey Matthias Käppler, Soundcloud Grokking RxJava, Dan Lew, Trello

61 Questions? Beatnik Photos

Thank you Slides are at: https://jeroen.tietema.

62 Thank you Slides are at: Contact me / jeroen@tietema.net

Reactive Programming in Java. Copyright - Syncogni Consulting Pvt Ltd. All rights reserved.

Reactive Programming in Java. Copyright - Syncogni Consulting Pvt Ltd. All rights reserved. Reactive Programming in Java Copyright - Syncogni Consulting Pvt Ltd. All rights reserved. Prerequisites: Functional Programming as in Java 8 Streams of Java 8 Lambda expressions Method references Expectations