Let it Flow ! - Speaker Deck

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Let it Flow! Unidirectional data flow architecture on Android

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@Dorvaryn +BenjaminAugustin-Dorvaryn Dorvaryn Benjamin Augustin Android Software Craftsman

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What is Unidirectional Data Flow ?

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● Remove state in UI ● Push based ● UI reactive Unidirectional Data Flow ?

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● Apps get more complex ● Predictability ● Forces us to do things right What’s the point ?

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Let’s build an app

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Let’s start with getting data

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You can use any language

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Kotlin Syntax fun aFunction(parameter: String): Int { return parameter.length } var thisCanBeNull: Int? = null val lambda = { x: Int, y: Int -> x + y }

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Example of Views interface CatsView { fun attach(listener: CatsPresenter.CatClickedListener) fun display(cats: Cats) fun display(favouriteCats: FavouriteCats) }

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Example of Views interface LoadingView { fun attach(retryListener: RetryClickedListener ) fun showLoadingIndicator() fun showLoadingScreen() fun showData() fun showEmptyScreen() fun showErrorIndicator() fun showErrorScreen() }

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Service interface CatService { fun getCat(id: Int): Observable }

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fun startPresenting() { catsView.attach(catClickedListener) loadingView.attach(retryListener) loadingView.showLoadingScreen() subscriptions = catsService.getCats() .subscribe(catsObserver) } What does not belong here ?

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private val catsObserver = object : Observer { 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 ?

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val retryListener = object : RetryClickedListener { override fun onRetry() { stopPresenting() startPresenting() } } What is wrong here ?

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val retryListener = object : RetryClickedListener { override fun onRetry() { stopPresenting() startPresenting() } } Retry restarting state

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class PersistedCatsService(...) : CatsService { override fun getCats() = repository.readCats() .flatMap { updateFromRemoteIfOutdated(it) } .switchIfEmpty(fetchRemoteCats()) } Service is request response based

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data class Event ( val status: Status, val data: T?, val error: Throwable? ) enum class Status { LOADING, IDLE, ERROR } Modeling context

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abstract class EventObserver: DataObserver> { override fun onNext(p0: Event) { when (p0.status) { Status.LOADING -> onLoading(p0) Status.IDLE -> onIdle(p0) Status.ERROR -> onError(p0) } } abstract fun onLoading(event: Event); abstract fun onIdle(event: Event); abstract fun onError(event: Event); Bit of tooling

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fun startPresenting() { catsView.attach(catClickedListener) loadingView.attach(retryListener) subscriptions = catsService.getCats() .subscribe(catsObserver) } No more state based actions in Presenter

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private val catsObserver = object : DataObserver { override fun onNext(p0: Cats) { catsView.display(p0); } } Displaying data is isolated

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private val catsEventsObserver = object : EventObserver () { override fun onLoading(event: Event) { if (event.data != null) { loadingView.showLoadingIndicator () } else { loadingView.showLoadingScreen () } } override fun onIdle(event: Event) { if (event.data != null) { loadingView.showData() } else { loadingView.showEmptyScreen () } } override fun onError(event: Event) { if (event.data != null) { loadingView.showErrorIndicator () } else { loadingView.showErrorScreen () } } Displaying data is isolated

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val retryListener = object : RetryClickedListener { override fun onRetry() { catsService.refreshCats() } } Retry is now an Action

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class PersistedCatsService(...) : CatsService { val catsSubject = BehaviorSubject.create( Event(Status.IDLE, null, null) ) override fun getCatsEvents(): Observable> { return catsSubject.asObservable() .startWith(initialiseSubject()) .distinctUntilChanged() } } Service with events

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class PersistedCatsService(...) : CatsService { private fun initialiseSubject(): Observable> { if (isInitialised(catsSubject)) { return Observable.empty() } return repository.readCats() .flatMap { updateFromRemoteIfOutdated (it) } .switchIfEmpty(fetchRemoteCats()) .compose(asEvent()) .doOnNext { catsSubject.onNext(it) } } } Service with events

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class PersistedCatsService(...) : CatsService { private fun getCats(): Observable> { return getCatsEvents().compose(asData()) } } Data flow comes from events

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class PersistedCatsService(...) : CatsService { override fun refreshCats() { fetchRemoteCats() .compose(asEvent()) .subscribe { catsSubject.onNext(it) } } } Refresh is now an Action

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● Stateless UI ● Domain is empowered ● Background push compatible What did we gain ?

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data class FavouriteCats( val favourites: Map ) { … } Modeling context, again

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enum class FavouriteState { FAVOURITE, PENDING_FAVOURITE, UN_FAVOURITE, PENDING_UN_FAVOURITE } Modeling context, again

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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

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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

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private val favCatsObserver = object : DataObserver { override fun onNext(favouriteCats: FavouriteCats) { catsView.display(favouriteCats) } } Displaying data is isolated

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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

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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

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class PersistedFavouriteCatsService (...): FavouriteCatsService { val favouriteCatsSubject: = BehaviorSubject .create( Event(Status.IDLE, null, null) ) override fun getFavouriteCatsEvents(): Observable> { return favouriteCatsSubject.asObservable() .startWith(initialiseSubject ()) .distinctUntilChanged () } } Similar structure in the Service

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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

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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

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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

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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

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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

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● Stateless UI ● Domain is empowered ● Eventual consistency is now easy What did we gain ?

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Demo Time !

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● Aim for stateless UI ● Model your context ● Make your domain responsible ● UI displays data ● UI sends actions In short

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@Dorvaryn +BenjaminAugustin-Dorvaryn Dorvaryn https://github.com/Dorvaryn/unidirectionalDataFlow/