Reactive and Functional Programming David Tiago Conceição Involves Android Dev BR @davidtiagocon Tales of failure and learning 

Once upon a time... 

Asynchronous code ● Callbacks 

Asynchronous code ● Callbacks someObject.longNetworkOperation( new NetworkOperationCallback() { void onOperationResult(Result result) { updateScreen(result); } }); 

Asynchronous code ● Callbacks someObject.longNetworkOperation( new NetworkOperationCallback() { void onOperationResult(Result result) { updateScreen(result); } void onSomethingWentWrong(WhatWentWrong what) { showError(what); } }); 

Asynchronous code ● Callbacks someObject.longNetworkOperation( new NetworkOperationCallback(){ void onOperationResult(Result result){ anotherObject.doAnotherLongOperation( result, new NetworkOperationCallback(){ void onOperationResult(Result result){ finallyShowTheResult(result); ... 

Asynchronous code ● Callbacks someObject.longNetworkOperation( new NetworkOperationCallback(){ void onOperationResult(Result result){ anotherObject.doAnotherLongOperation( result, new NetworkOperationCallback(){ void onOperationResult(Result result){ finallyShowTheResult(result); } void onSomethingWentWrong(WhatWentWrong what){ showError(what); } } } void onSomethingWentWrong(WhatWentWrong what){ showError(what); } }); 

Asynchronous code ● Callbacks someObject.longNetworkOperation( new NetworkOperationCallback(){ void onOperationResult(Result result){ anotherObject.doAnotherLongOperation( result, new NetworkOperationCallback(){ void onOperationResult(Result result){ runOnUiThread(new Runnable(){ void run(){ finallyShowTheResult(result); } }); } void onSomethingWentWrong(WhatWentWrong what){ runOnUiThread(new Runnable(){ showError(what); }); } } } void onSomethingWentWrong(WhatWentWrong what){ showError(what); } }); 

Asynchronous code ● Callbacks someObject.longNetworOperation( new NetworkOperationCallback(){ void onOperationResult(Result result){ anotherObject.doAnotherLongOperation( result, new NetworkOperationCallback(){ void onOperationResult(Result result){ runOnUiThread(new Runnable(){ void run(){ finallyShowTheResult(result); } }); } void onSomethingWentWrong(WhatWentWrong what){ runOnUiThread(new Runnable(){ showError(what); }); } } } void onSomethingWentWrong(WhatWentWrong what){ showError(what); } }); https://storage.googleapis.com/material-design/publish/material_v_11/assets/0Bzhp5Z4wHba3NnpEM2lLc0NKd1U/notifications_templates_04_progress.png 

Asynchronous code 

Asynchronous code ● AsyncTasks private class NetworkOperation extends AsyncTask { protected Long doInBackground(Input... inputs) { return someLongNetworkOperation(inputs); } protected void onProgressUpdate(Integer... progress) { setProgressPercent(progress[0]); } protected void onPostExecute(Long result) { showDialog("Downloaded " + result + " bytes"); } } 

Asynchronous code ● IntentService public class NetworkService extends IntentService { @Override protected void onHandleIntent(Intent workIntent) { longNetworkOperation(workIntent); } } 

Asynchronous code 

https://thumb7.shutterstock.com/display_pic_with_logo/164459732/518981521/stock-photo-react-stamp-sign-text-word-logo-black-518981521.jpg 

Reactive Extensions “An API for asynchronous programming with observable streams.” http://reactivex.io/ 

Reactive Extensions “An API for asynchronous programming with observable streams.” “The Observer pattern done right.” http://reactivex.io/ 

RxJava ● Creating an Observable Single.fromCallable(() -> { return someNetworkOperation();}) .subscribeWith(new DisposableSingleObserver() { public void onSuccess(NetworkResult result) { updateScreen(result); } public void onError(Throwable e) { showError(e); } }); 

RxJava ● Creating an Observable Single.fromCallable(() -> { return someNetworkOperation(); }) .subscribeOn(Schedulers.io()) .observeOn(AndroidSchedulers.mainThread()) .subscribeWith(new DisposableSingleObserver() { public void onSuccess(NetworkResult result) { updateScreen(result); } public void onError(Throwable e) { showError(e); } }); 

RxJava ● Creating an Observable Single.fromCallable(() -> { return someNetworkOperation(); }) .flatMap(networkResult -> { return anotherNetworkOperation(networkResult); }) .subscribeOn(Schedulers.io()) .observeOn(AndroidSchedulers.mainThread()) .subscribeWith(new DisposableSingleObserver() { public void onSuccess(NetworkResult result) { updateScreen(result); } public void onError(Throwable e) { showError(e); } }); 

RxJava ● Creating an Observable compositeDisposable = new CompositeDisposable(); // ... compositeDisposable.add(createStream()); // ... compositeDisposable.clear(); https://storage.googleapis.com/material-design/publish/material_v_11/assets/0Bzhp5Z4wHba3NnpEM2lLc0NKd1U/notifications_templates_04_progress.png 

RxJava 

RxJava 

Reactive Extensions “CREATE, COMBINE, LISTEN” http://reactivex.io/ 

RxJava ● Create Single.fromCallable(() -> { return someNetworkOperation();}) .subscribeWith(new DisposableSingleObserver() { public void onSuccess(NetworkResult result) { updateScreen(result); } public void onError(Throwable e) { showError(e); } }); 

RxJava ● Combine Single.fromCallable(() -> { return someNetworkOperation(); }) .flatMap(networkResult -> { return anotherNetworkOperation(networkResult); }) .subscribeOn(Schedulers.io()) .observeOn(AndroidSchedulers.mainThread()) .subscribeWith(new DisposableSingleObserver() { public void onSuccess(NetworkResult result) { updateScreen(result); } public void onError(Throwable e) { showError(e); } }); 

RxJava ● Listen Single.fromCallable(() -> { return someNetworkOperation(); }) .flatMap(networkResult -> { return anotherNetworkOperation(networkResult); }) .subscribeOn(Schedulers.io()) .observeOn(AndroidSchedulers.mainThread()) .subscribeWith(new DisposableSingleObserver() { public void onSuccess(NetworkResult result) { updateScreen(result); } public void onError(Throwable e) { showError(e); } }); 

RxJava ● Stream Single.fromCallable(() -> { return someNetworkOperation(); }) .flatMap(networkResult -> { return anotherNetworkOperation(networkResult); }) .subscribeOn(Schedulers.io()) .observeOn(AndroidSchedulers.mainThread()) .subscribeWith(new DisposableSingleObserver() { public void onSuccess(NetworkResult result) { updateScreen(result); } public void onError(Throwable e) { showError(e); } }); 

Growing Strong ● Start composing ○ Large streams composed by smaller ● Less logic ● More data flow 

Growing Strong class SomeNetworkClass { Observable longNetworkOperation(Parameters parameters){ // Compose networkParameters return NetworkLibrary.createNetworkRequest(networkParameters); } } // ... someNetworkInstance.longNetworkOperation(parameters) .doOnNext(Cache::saveToCache) .subscribeWith( // subscriber ... 

Growing Strong class SomeNetworkClass { Observable longNetworkOperation(Parameters parameters){ // Compose networkParameters return NetworkLibrary.createNetworkRequest(networkParameters); } } // ... someNetworkInstance.longNetworkOperation(parameters) .doOnNext(Cache::saveToCache) .subscribeWith( // subscriber ... 

Race Conditions moviesRemoteRepository.getUpcoming(currentPageCount) .map(new Function() { public Movie apply(Movie movie) throws Exception { return MovieUtil.mapMovieFields( movie, imageConfiguration, genres); } }) .toList() //... 

Race conditions moviesRemoteRepository.getUpcoming(currentPageCount) .map(new Function() { public Movie apply(Movie movie) throws Exception { return MovieUtil.mapMovieFields( movie, imageConfiguration, genres); } }) .toList() //... 

Reactive Extensions “ReactiveX is a combination of the best ideas from the Observer pattern, the Iterator pattern, and functional programming.” http://reactivex.io/ 

Functional Programming ● Mathematical concepts ● High order functions ● Immutability ● Lambdas ● Monads ● Pure functions 

Pure Functions ● Output determined by the input ● No side effects ● An input generates always the same output 

Pure Functions int sum(int firstNumber, int secondNumber){ return firstNumber + secondNumber; } 

Pure Functions moviesRemoteRepository.getUpcoming(currentPageCount) .merge(//... Stream of more data .combineLatest( //... Stream of more data .toList() //... 

Pure Functions ● Easily processed in parallel ○ Highly scalable ● Independent Streams ● Easy to read, maintain and test 

Reactive Architectures ● Communication through streams 

Reactive Architectures ● Communication through streams ● Components reacting to each other 

Everything as stream ● User inputs ● I/O ● Internal events ● System events 

What could go wrong? ● State, error handling, and much more 

Propagating state ● Single source of truth ● Hot Observables ● Mapping, flat mapping and others 

Handling errors ● onError invalidates the stream ● Error as state ● Propagate an error state ● Avoids invalidation 

Much more ● Complex streams ● Weird use cases 

And now? ● Key architectural component ● Can’t work without it 

How to know more? ● ReactiveX reactivex.io ● RxJava for Android App Development oreilly.com/programming/free/rxjava-for-android-app-development.csp ● Reactive Programming on Android with RxJava leanpub.com/reactiveandroid ● Managing State with RxJava by Jake Wharton www.youtube.com/watch?v=0IKHxjkgop4 ● Object-Oriented vs. Functional Programming oreilly.com/programming/free/object-oriented-vs-functional-programming.csp 

slack.AndroidDevBR.org Android Dev BR 

Thank you! David Tiago Conceição Android Developer @ Involves Android Dev BR @davidtiagoconceicao twitter.com/@davidtiagocon github.com/davidtcdeveloper Slides