Let it Flow !

Reactive Functional Programming becomes more and more recognised as a great way to build reliable and maintainable software.
Functional programming paradigms such as Immutability and avoiding statefull objects are considered good practices and something to strive for.
That’s great! However it is not always straightforward to apply those principles, let alone to create a whole architecture around them.
Refactoring from an existing code base we will see how we can craft a great architecture and simplify our presentation layer by pushing more concerns as part of our domain.

The code presented in this talk is available here: https://github.com/Dorvaryn/unidirectionalDataFlow

Transcript

1. Let it Flow! Unidirectional data flow architecture on Android
2. @Dorvaryn +BenjaminAugustin-Dorvaryn Dorvaryn Benjamin Augustin Android Software Craftsman
3. What is Unidirectional Data Flow ?
4. • Remove state in UI • Push based • UI reactive Unidirectional Data Flow ?
6. • Apps get more complex • Predictability • Forces us to do things right What’s the point ?
7. Let’s build an app
16. Let’s start with getting data
18. You can use any language
19. Kotlin Syntax fun aFunction(parameter: String): Int { return parameter.length } var thisCanBeNull: Int? = null val lambda = { x: Int, y: Int -> x + y }
21. Example of Views interface CatsView { fun attach(listener: CatsPresenter.CatClickedListener) fun display(cats: Cats) fun display(favouriteCats: FavouriteCats) }
22. Example of Views interface LoadingView { fun attach(retryListener: RetryClickedListener ) fun showLoadingIndicator() fun showLoadingScreen() fun showData() fun showEmptyScreen() fun showErrorIndicator() fun showErrorScreen() }
23. Service interface CatService { fun getCat(id: Int): Observable<Cat> }
24. fun startPresenting() { catsView.attach(catClickedListener) loadingView.attach(retryListener) loadingView.showLoadingScreen() subscriptions = catsService.getCats() .subscribe(catsObserver) } What does not belong here ?
25. fun startPresenting() { catsView.attach(catClickedListener) loadingView.attach(retryListener) loadingView.showLoadingScreen() subscriptions = catsService.getCats() .subscribe(catsObserver) } Loading when we request data
26. private val catsObserver = object : Observer<Cats> { var displayedCats: Cats? = null override fun onNext(cats: Cats) { catsView.display(cats); displayedCats = cats loadingView.showLoadingIndicator () } override fun onError(e: Throwable?) { if (displayedCats == null) { loadingView.showErrorScreen () } else { loadingView.showErrorIndicator () } } override fun onCompleted() { if (displayedCats == null) { loadingView.showEmptyScreen () } else { loadingView.showData() } } What is the problem ?
27. private val catsObserver = object : Observer<Cats> { var displayedCats: Cats? = null override fun onNext(cats: Cats) { catsView.display(cats); displayedCats = cats loadingView.showLoadingIndicator () } override fun onError(e: Throwable?) { if (displayedCats == null) { loadingView.showErrorScreen () } else { loadingView.showErrorIndicator () } } override fun onCompleted() { if (displayedCats == null) { loadingView.showEmptyScreen () } else { loadingView.showData() } } Presenter now has state
28. val retryListener = object : RetryClickedListener { override fun onRetry() { stopPresenting() startPresenting() } } What is wrong here ?
29. val retryListener = object : RetryClickedListener { override fun onRetry() { stopPresenting() startPresenting() } } Retry restarting state
30. class PersistedCatsService(...) : CatsService { override fun getCats() = repository.readCats() .flatMap { updateFromRemoteIfOutdated(it) } .switchIfEmpty(fetchRemoteCats()) } Service is request response based
32. data class Event<T> ( val status: Status, val data: T?, val error: Throwable? ) enum class Status { LOADING, IDLE, ERROR } Modeling context
33. abstract class EventObserver<T>: DataObserver<Event<T>> { override fun onNext(p0: Event<T>) { when (p0.status) { Status.LOADING -> onLoading(p0) Status.IDLE -> onIdle(p0) Status.ERROR -> onError(p0) } } abstract fun onLoading(event: Event<T>); abstract fun onIdle(event: Event<T>); abstract fun onError(event: Event<T>); Bit of tooling
34. fun startPresenting() { catsView.attach(catClickedListener) loadingView.attach(retryListener) subscriptions = catsService.getCats() .subscribe(catsObserver) } No more state based actions in Presenter
35. private val catsObserver = object : DataObserver<Cats> { override fun onNext(p0: Cats) { catsView.display(p0); } } Displaying data is isolated
36. private val catsEventsObserver = object : EventObserver <Cats>() { override fun onLoading(event: Event<Cats>) { if (event.data != null) { loadingView.showLoadingIndicator () } else { loadingView.showLoadingScreen () } } override fun onIdle(event: Event<Cats>) { if (event.data != null) { loadingView.showData() } else { loadingView.showEmptyScreen () } } override fun onError(event: Event<Cats>) { if (event.data != null) { loadingView.showErrorIndicator () } else { loadingView.showErrorScreen () } } Displaying data is isolated
37. private val catsEventsObserver = object : EventObserver <Cats>() { override fun onLoading(event: Event<Cats>) { if (event.data != null) { loadingView.showLoadingIndicator () } else { loadingView.showLoadingScreen () } } override fun onIdle(event: Event<Cats>) { if (event.data != null) { loadingView.showData() } else { loadingView.showEmptyScreen () } } override fun onError(event: Event<Cats>) { if (event.data != null) { loadingView.showErrorIndicator () } else { loadingView.showErrorScreen () } } No state in Presenter
38. val retryListener = object : RetryClickedListener { override fun onRetry() { catsService.refreshCats() } } Retry is now an Action
39. class PersistedCatsService(...) : CatsService { val catsSubject = BehaviorSubject.create( Event<Cats>(Status.IDLE, null, null) ) override fun getCatsEvents(): Observable<Event<Cats>> { return catsSubject.asObservable() .startWith(initialiseSubject()) .distinctUntilChanged() } } Service with events
40. class PersistedCatsService(...) : CatsService { private fun initialiseSubject(): Observable<Event<Cats>> { if (isInitialised(catsSubject)) { return Observable.empty() } return repository.readCats() .flatMap { updateFromRemoteIfOutdated (it) } .switchIfEmpty(fetchRemoteCats()) .compose(asEvent<Cats>()) .doOnNext { catsSubject.onNext(it) } } } Service with events
41. class PersistedCatsService(...) : CatsService { private fun getCats(): Observable<Cat>> { return getCatsEvents().compose(asData()) } } Data flow comes from events
42. class PersistedCatsService(...) : CatsService { override fun refreshCats() { fetchRemoteCats() .compose(asEvent<Cats>()) .subscribe { catsSubject.onNext(it) } } } Refresh is now an Action
44. • Stateless UI • Domain is empowered • Background push compatible What did we gain ?
51. data class FavouriteCats( val favourites: Map<Cat, FavouriteState> ) { … } Modeling context, again
52. enum class FavouriteState { FAVOURITE, PENDING_FAVOURITE, UN_FAVOURITE, PENDING_UN_FAVOURITE } Modeling context, again
53. fun startPresenting() { catsView.attach(catClickedListener) loadingView.attach(retryListener) subscriptions.add( catsService.getCatsEvents() .subscribe(catsEventsObserver) ) subscriptions.add( catsService.getCats() .subscribe(catsObserver) ) subscriptions.add( favouriteCatsService.getFavouriteCats () .subscribe(favouriteCatsObserver) ) } UI just registers itself
54. fun startPresenting() { catsView.attach(catClickedListener) loadingView.attach(retryListener) subscriptions.add( catsService.getCatsEvents() .subscribe(catsEventsObserver) ) subscriptions.add( catsService.getCats() .subscribe(catsObserver) ) subscriptions.add( favouriteCatsService.getFavouriteCats() .subscribe( favouriteCatsObserver) ) } UI just registers itself
55. private val favCatsObserver = object : DataObserver<FavouriteCats> { override fun onNext(favouriteCats: FavouriteCats) { catsView.display(favouriteCats) } } Displaying data is isolated
56. fun display(cat: Cat, favouriteState : FavouriteState , ...) { ... favouriteIndicator.setImageDrawable (favouriteDrawable (favouriteState )) favouriteIndicator.isEnabled = favouriteState == FavouriteState .FAVOURITE || favouriteState == FavouriteState .UN_FAVOURITE ... } private fun favouriteDrawable(favouriteState : FavouriteState ) = when (favouriteState ) { FavouriteState .FAVOURITE -> android.R.drawable.star_big_on FavouriteState .PENDING_FAVOURITE -> android.R.drawable.star_big_on FavouriteState .PENDING_UN_FAVOURITE -> android.R.drawable.star_big_off FavouriteState .UN_FAVOURITE -> android.R.drawable.star_big_off } View Reacts
57. override fun onFavouriteClicked(cat: Cat, currentState: FavouriteState) { if (currentState == FavouriteState.FAVOURITE) { favouriteCatsService.removeFromFavourite(cat) } else if (currentState == FavouriteState.UN_FAVOURITE) { favouriteCatsService.addToFavourite(cat) } } Modification are actions
58. class PersistedFavouriteCatsService (...): FavouriteCatsService { val favouriteCatsSubject: = BehaviorSubject .create( Event<FavouriteCats>(Status.IDLE, null, null) ) override fun getFavouriteCatsEvents(): Observable<Event<FavouriteCats>> { return favouriteCatsSubject.asObservable() .startWith(initialiseSubject ()) .distinctUntilChanged () } } Similar structure in the Service
59. override fun addToFavourite(cat: Cat) { api.addToFavourite (cat) .map { Pair(cat, FavouriteState .FAVOURITE) } .onErrorReturn { Pair(cat, FavouriteState .UN_FAVOURITE) } .startWith(Pair(cat, FavouriteState .PENDING_FAVOURITE)) .doOnNext { repository.saveCatFavoriteStatus (it) } .subscribe { val value = favouriteCatsSubject.value val favouriteCats = value.data ?: FavouriteCats(mapOf()) favouriteCatsSubject.onNext(Event.of(favouriteCats.put(it))) } } Modification are actions
60. override fun addToFavourite(cat: Cat) { api.addToFavourite (cat) .map { Pair(cat, FavouriteState. FAVOURITE) } .onErrorReturn { Pair(cat, FavouriteState .UN_FAVOURITE) } .startWith(Pair(cat, FavouriteState .PENDING_FAVOURITE)) .doOnNext { repository.saveCatFavoriteStatus (it) } .subscribe { val value = favouriteCatsSubject.value val favouriteCats = value.data ?: FavouriteCats(mapOf()) favouriteCatsSubject.onNext(Event.of(favouriteCats.put(it))) } } Modification are actions
61. override fun addToFavourite(cat: Cat) { api.addToFavourite (cat) .map { Pair(cat, FavouriteState .FAVOURITE) } .onErrorReturn { Pair(cat, FavouriteState. UN_FAVOURITE) } .startWith(Pair(cat, FavouriteState .PENDING_FAVOURITE)) .doOnNext { repository.saveCatFavoriteStatus (it) } .subscribe { val value = favouriteCatsSubject.value val favouriteCats = value.data ?: FavouriteCats(mapOf()) favouriteCatsSubject.onNext(Event.of(favouriteCats.put(it))) } } Modification are actions
62. override fun addToFavourite(cat: Cat) { api.addToFavourite (cat) .map { Pair(cat, FavouriteState .FAVOURITE) } .onErrorReturn { Pair(cat, FavouriteState .UN_FAVOURITE) } .startWith(Pair(cat, FavouriteState. PENDING_FAVOURITE)) .doOnNext { repository.saveCatFavoriteStatus (it) } .subscribe { val value = favouriteCatsSubject.value val favouriteCats = value.data ?: FavouriteCats(mapOf()) favouriteCatsSubject.onNext(Event.of(favouriteCats.put(it))) } } Modification are actions
63. override fun addToFavourite(cat: Cat) { api.addToFavourite (cat) .map { Pair(cat, FavouriteState .FAVOURITE) } .onErrorReturn { Pair(cat, FavouriteState .UN_FAVOURITE) } .startWith(Pair(cat, FavouriteState .PENDING_FAVOURITE)) .doOnNext { repository.saveCatFavoriteStatus( it) } .subscribe { val value = favouriteCatsSubject.value val favouriteCats = value.data ?: FavouriteCats(mapOf()) favouriteCatsSubject.onNext(Event.of(favouriteCats.put(it))) } } Modification are actions
64. override fun addToFavourite(cat: Cat) { api.addToFavourite (cat) .map { Pair(cat, FavouriteState .FAVOURITE) } .onErrorReturn { Pair(cat, FavouriteState .UN_FAVOURITE) } .startWith(Pair(cat, FavouriteState .PENDING_FAVOURITE)) .doOnNext { repository.saveCatFavoriteStatus (it) } .subscribe { val value = favouriteCatsSubject.value val favouriteCats = value.data ?: FavouriteCats(mapOf()) favouriteCatsSubject.onNext(Event.of(favouriteCats.put(it))) } } Modification are actions
65. • Stateless UI • Domain is empowered • Eventual consistency is now easy What did we gain ?
67. Demo Time !
69. • Aim for stateless UI • Model your context • Make your domain responsible • UI displays data • UI sends actions In short
71. @Dorvaryn +BenjaminAugustin-Dorvaryn Dorvaryn https://github.com/Dorvaryn/unidirectionalDataFlow/