Flowing Things, not so strange in the MVI world

Migrating an Rx MVI pattern completely to coroutines using Flow. In the MVI world, there was a missing piece from the coroutines framework and due to this, it always felt strange to completely adopt coroutines. Recently, with the introduction of Co-routines Flow library, things are not so strange anymore. In this talk we will have a look at Coroutines Flow library, its need and how it compares with the reactive world. We will then learn to migrate an Rx MVI pattern to use coroutines Flow.

Transcript

1. FLOWING THINGS Not so strange in the MVI world
2. Garima Jain @ragdroid
3. @ragdroid #droidconNYC Who?
4. Who? • Rx background • Coroutines • MVI • Stranger Things?
5. @ragdroid #droidconNYC What?
6. What? • Chapter One : Coroutines and Flow • Chapter Two : Channels and Flows • Chapter Three : MVI • Chapter Four : Rx to Flow • Summary
7. CHAPTER ONE THE FLOW
8. Coroutines • Lightweight thread • Run computations without blocking • Can be suspended
9. Coroutines • Coroutine Scope ★ ViewModel Scope ★ Lifecycle Scope • Coroutine Builders ★ launch { } - fire and forget ★ async { } - await() result ★ within a scope, Structured Concurrency
10. Channels • Communication between different coroutines • Similar to BlockingQueue
11. Flow • Cold asynchronous stream that sequentially emits values • Utilizes coroutines and channels • Experimental Recently Stable APIs in 1.3.0
12. Flow Builders ★ flowOf( ) ★ asFlow( ) ★ flow { } ★ channelFlow { }
13. Flow Operators • Intermediate : ★ map, filter, take, zip, etc. • Terminal : ★ collect, single, reduce, etc.
14. Flow Operators fun main() = runBlocking { val flow = flow { for (i in 1..10) { delay(500L) emit(i) } }.filter { it % 2 == 0 } flow.collect { println(it) } } 2 4 6 8 10
15. Flow Constraints ★ Context Preservation ★ Exception Transparency
16. Context Preservation val flowA = flowOf(1, 2, 3) .map { it + 1 } .flowOn(ctxA) val filtered = flowA .filter { it == 3 } withContext(Dispatchers.Main) { val result = filtered.single() myUi.text = result } https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/-flow/ // Will be executed in ctxA // Changes the upstream context: flowOf and map // Pure operator without a context yet // All non-encapsulated operators will be executed in Main: filter and single
17. CHAPTER TWO CONSUMER, DO YOU COPY? Hot Channels and Cold Flows
18. fun main() = runBlocking { val job = launch { produce { println(“We are hot") send(1) } } job.invokeOnCompletion { println("Completed") } delay(1000L) println("Done waiting") } Channels Done waiting Delay We are hot
19. Channels fun main() = runBlocking { val job = launch { val hotChannel = produce { println(“We are hot") send(1) } channel.consumeEach { println(it) } } job.invokeOnCompletion { println("Completed") } delay(1000L) println("Done waiting") } 1 Completed Done waiting We are hot
20. Flows fun main() = runBlocking { val job = launch { val coldFlow = flow { println(“We are cold”) emit(1) } } job.invokeOnCompletion { println("Completed") } delay(1000L) println("Done waiting") } Completed Done waiting
21. Flows fun main() = runBlocking { val job = launch { val coldFlow = flow { println("We are cold") emit(1) } coldFlow.collect { println(it) } } job.invokeOnCompletion { println("Completed") } delay(1000L) println("Done waiting") } We are cold 1 Completed Done waiting
22. Channel Gotchas • Channels are a pathway to a different dimension (Upside Down) • If we they are open, another dimension exists • Existence of another dimension could be dangerous • It could use up some heavy resources
23. Channel Gotchas Easy to make mistakes
24. Channel Gotchas Easy to make mistakes
25. CHAPTER THREE THE BATTLE OF ACTIONS MVI
26. Model View Intent
27. Model View Intent State View Intention
28. State View Intention User Intent State View Changes Updates Seen Interacts
29. View ViewModel Data Intent newState Action Result previousState Reducer State View Intention State MVI on top of MVVM
30. View ViewModel Data Intent newState Action Result previousState Reducer State View Intention State 4 3 5 1 2 MVI on top of MVVM
31. State View Intention 1 3 2 4 5 Action Events Actions to Results Reduce Results to new State Plugging-in the pieces State stream
32. CHAPTER FOUR VANISHING OF Rx FLOWABLES Migrating an Rx MVI pattern to use Flow
33. Demo
34. Actions sealed class MainAction { object PullToRefresh object LoadData data class LoadDescription(val characterId: Long) }
35. Actions - ViewModel interface ViewModel { fun onAction(action: MainAction) ... } Rx
36. Actions - ViewModel interface ViewModel { fun onAction(action: MainAction) ... } Flow
37. Action Events viewModel.onAction(MainAction.LoadData) viewModel.onAction(..)
38. viewModel.onAction(MainAction.LoadData) viewModel.onAction(..) refreshLayout.setOnRefreshListener { viewModel.onAction(MainAction.PullToRefresh) } Action Events
39. viewModel.onAction(MainAction.LoadData) viewModel.onAction(..) override fun onCharacterDescriptionClicked(itemId: Long) { viewModel.onAction(MainAction.LoadDescription(itemId)) } refreshLayout.setOnRefreshListener { viewModel.onAction(MainAction.PullToRefresh) } Action Events
40. State View Intention 1 Action Events 3 2 4 5 Actions to Results Reduce Results to new State State stream Plugging-in the pieces
41. State View Intention 1 Action Events 3 2 4 5 Actions to Results Reduce Results to new State State stream Plugging-in the pieces
42. State View Intention 1 Action Events 3 2 4 5 Actions to Results Reduce Results to new State State stream Plugging-in the pieces same
43. Actions to Results sealed class MainResult { object Loading object LoadingError data class LoadingComplete(val characters: List<CharacterMarvel>) object PullToRefreshing: MainResult() object PullToRefreshError data class PullToRefreshComplete(val characters: List<CharacterMarvel>) sealed class DescriptionResult(val characterId: Long) { data class DescriptionLoading(private val id: Long) data class DescriptionError(private val id: Long) data class DescriptionLoadComplete(private val id: Long, val description: String) } }
44. View ViewModel Data Intent newState Action Result previousState Reducer State View Intention State
45. View ViewModel Data Intent newState Action Result previousState Reducer State View Intention State Repository Network
46. Data Layer @GET("characters/{characterId}") fun getCharactersSingle(...): Single<List<CharacterMarvel>> fun fetchCharactersSingle(): Single<List<CharacterMarvel>> = marvelApi.getCharactersSingle( ... ) MarvelApi MainRepository
47. Actions to Results - LoadData fun loadingResult(loadDataActionStream: Flowable<MainAction.LoadData>) : Flowable<MainResult> { return loadDataActionStream .observeOn(Schedulers.io()) .flatMap { mainRepository.fetchCharactersSingle().toFlowable() .map { list -> MainResult.LoadingComplete(list) } .startWith(MainResult.Loading) .onErrorReturn { error -> navigate(MainNavigation.Snackbar(error.message)) MainResult.LoadingError } } } ViewModel
48. Data Layer @GET("characters/{characterId}") fun getCharactersSingle(...): Single<List<CharacterMarvel>> MarvelApi - Rx
49. Data Layer @GET("characters/{characterId}") suspend fun getCharacters(…): List<CharacterMarvel> MarvelApi - Coroutines 2.6.0
50. Data Layer fun fetchCharactersSingle(): Single<List<CharacterMarvel>> = marvelApi.getCharactersSingle( ... ) MainRepository - Rx
51. Data Layer suspend fun fetchCharacters(): List<CharacterMarvel> = marvelApi.getCharacters( ... ) MainRepository - Coroutines
52. fun loadingResult(actions: Flowable<...>) : Flowable<MainResult> = actions .observeOn(Schedulers.io()) .flatMap { mainRepository.fetchCharactersFlowable() .map { MainResult.LoadingComplete } .startWith(MainResult.Loading) .onErrorReturn { error -> navigate(MainNavigation.Snackbar) MainResult.LoadingError } } fun loadingResult(actions: Flowable<MainAction.LoadData>) : Flowable<MainResult> = actions .observeOn(Schedulers.io()) .flatMap { mainRepository.fetchCharactersSingle().toFlowable() .map { states -> MainResult.LoadingComplete(states) } .startWith(MainResult.Loading) .onErrorReturn { error -> navigate(MainNavigation.Snackbar(error.message) MainResult.LoadingError } } Actions to Results - Load Data RxViewModel
53. fun loadingResult(actions: Flowable<...>) : Flowable<MainResult> = actions .observeOn(Schedulers.io()) .flatMap { mainRepository.fetchCharactersFlowable() .map { MainResult.LoadingComplete } .startWith(MainResult.Loading) .onErrorReturn { error -> navigate(MainNavigation.Snackbar) MainResult.LoadingError } } RxViewModel fun loadingResult(actionsFlow: Flow<MainAction.LoadData>) : Flow<MainResult> = actionsFlow.flatMapMerge { flow { emit(MainResult.Loading) val characters = mainRepository.fetchCharacters() emit(MainResult.LoadingComplete(characters)) }.catch { navigate(MainNavigation.Snackbar(it.message) emit(MainResult.LoadingError) } } FlowViewModel Actions to Results - Load Data
54. fun loadingResult(actions: Flowable<...>) : Flowable<MainResult> = actions .observeOn(Schedulers.io()) .flatMap { mainRepository.fetchCharactersFlowable() .map { MainResult.LoadingComplete } .startWith(MainResult.Loading) .onErrorReturn { error -> navigate(MainNavigation.Snackbar) MainResult.LoadingError } } RxViewModel FlowViewModel fun loadingResult(actionsFlow: Flow<MainAction.LoadData>) : Flow<MainResult> = actionsFlow.flatMapMerge { flow { emit(mainRepository.fetchCharacters()) } .map { MainResult.LoadingComplete(characters) } .onStart { emit(MainResult.Loading) } .catch { navigate(MainNavigation.Snackbar(it.message) emit(MainResult.LoadingError) } } Actions to Results - Load Data
55. fun pullToRefreshResult(actions: Flowable<MainAction.LoadData>) : Flowable<MainResult> = ... Actions to Results - Pull To Refresh RxViewModel
56. fun pullToRefreshResult(actions: Flowable<MainAction.LoadData>) : Flow<MainResult> = ... Actions to Results - Pull To Refresh FlowViewModel
57. fun loadDesciptionResult(actions: Flowable<MainAction.LoadData>) : Flowable<MainResult> = ... Actions to Results - Load Desciption RxViewModel
58. fun loadDesciptionResult(actions: Flowable<MainAction.LoadData>) : Flow<MainResult> = ... Actions to Results - Load Desciption FlowViewModel
59. Actions to Results fun actionsToResultTransformer(actions: Flowable<MainAction>): Flowable<MainResult> = actions.publish { shared -> Flowable.merge(loadingResult(shared.ofType(MainAction.LoadData::class.java)), loadDescriptionResult(shared.ofType(MainAction.LoadDescription::class.java)), pullToRefreshResult(shared.ofType(MainAction.PullToRefresh::class.java))) } RxViewModel
60. Actions to Results fun Flow<MainAction>.actionsToResultTransformer(): Flow<MainResult> = publish { loadingResult(it.ofType(MainAction.LoadData::class.java)) .merge(loadDescriptionResult(it.ofType(MainAction.LoadDescription::class.java)), pullToRefreshResult(it.ofType(MainAction.PullToRefresh::class.java))) } Flow
61. Merge Operator Actions to Results fun Flow<MainAction>.actionsToResultTransformer(): Flow<MainResult> = publish { loadingResult(it.ofType(MainAction.LoadData::class.java)) .merge(loadDescriptionResult(it.ofType(MainAction.LoadDescription::class.java)), pullToRefreshResult(it.ofType(MainAction.PullToRefresh::class.java))) } Flow
62. Merge Operator • No merge operator at the time • Creating operators with coroutines is comparatively easy • Let's create one
63. fun <T> Flow<T>.merge(other: Flow<T>): Flow<T> = channelFlow { launch { collect { send(it) } } launch { other.collect { send(it) } } } FlowExtensions.kt // collect from this coroutine and send it // collect and send from this coroutine, too, concurrently Merge Operator
64. fun Flow<MainAction>.actionsToResultTransformer(): Flow<MainResult> = publish { loadingResult(it.ofType(MainAction.LoadData::class.java)) .merge(loadDescriptionResult(it.ofType(MainAction.LoadDescription::class.java)), pullToRefreshResult(it.ofType(MainAction.PullToRefresh::class.java))) } Actions to Results - publish Flow
65. fun Flow<MainAction>.actionsToResultTransformer(): Flow<MainResult> = publish { loadingResult(it.ofType(MainAction.LoadData::class.java)) .merge(loadDescriptionResult(it.ofType(MainAction.LoadDescription::class.java)), pullToRefreshResult(it.ofType(MainAction.PullToRefresh::class.java))) } Actions to Results - publish Flow kotlin-flow-extensions: https://github.com/akarnokd/kotlin-flow-extensions
66. Merge Operator Actions to Results - broadcastIn inputActionsFlow .broadcastIn(viewModelScope).asFlow() .actionToResultTransformer() ... Flow
67. Actions to Results fun Flow<MainAction>.actionsToResultTransformer(): Flow<MainResult> = flatMapMerge { flow { when(it) { is MainAction.PullToRefresh -> { ... } is MainAction.LoadData -> { try { emit(MainResult.Loading) val characters = mainRepository.fetchCharacters() emit(MainResult.LoadingComplete(characters)) } catch (exception: Exception) { navigate(MainNavigation.Snackbar(exception.message)) emit(MainResult.LoadingError(exception)) } } is MainAction.LoadDescription -> { … } } } }
68. View ViewModel Data Intent newState Action Result previousState Reducer State Repository Network State View Intention (Rx)
69. View ViewModel Data Intent newState Action Result previousState Reducer State View Intention (Co-routines) State Repository Network
70. View ViewModel Data Intent newState Action Result previousState Reducer State View Intention (Co-routines) State Repository Network Room 2.2.0- alpha
71. State View Intention 3 2 4 5 Actions to Results Reduce Results to new State State stream Plugging-in the pieces 1 Action Events same
72. State View Intention 3 2 4 5 Actions to Results Reduce Results to new State State stream Plugging-in the pieces 1 Action Events same
73. Reduce Results to new State data class MainViewState(val characters: List<CharacterItemState>, val emptyState: EmptyState, val loadingState: LoadingState): MviState sealed class EmptyState { object None: EmptyState() object NoData: EmptyState() object NoInternet: EmptyState() } sealed class LoadingState { object None: LoadingState() object Loading: LoadingState() object PullToRefreshing: LoadingState() } } State
74. Reduce Results to new State fun reduce(result: MainResult) { return when (result) { is MainResult.Loading -> copy(loadingState = LoadingState.Loading) is MainResult.LoadingError -> copy(loadingState = LoadingState.None, emptyState = EmptyState.NoData) is MainResult.LoadingComplete -> { val characterStates = reduceCharactersList(null, result.characters, resources) copy(characterStates, loadingState = LoadingState.None, emptyState = EmptyState.None) } ... } RxReducer
75. Reduce Results to new State FlowReducer fun reduce(result: MainResult) { return when (result) { is MainResult.Loading -> copy(loadingState = LoadingState.Loading) is MainResult.LoadingError -> copy(loadingState = LoadingState.None, emptyState = EmptyState.NoData) is MainResult.LoadingComplete -> { val characterStates = reduceCharactersList(null, result.characters, resources) copy(characterStates, loadingState = LoadingState.None, emptyState = EmptyState.None) } ... }
76. State View Intention 1 Action Events 3 2 4 5 Actions to Results Reduce Results to new State State stream Plugging-in the pieces same
77. State View Intention 3 2 4 5 Actions to Results Reduce Results to new State Plugging-in the pieces State stream 1 Action Events same
78. State View Intention 3 2 4 5 Actions to Results Reduce Results to new State Plugging-in the pieces State stream 1 Action Events same same
79. State State stream data class MainViewState(val characters: List<CharacterItemState>, val emptyState: EmptyState, val loadingState: LoadingState): MviState sealed class EmptyState { object None: EmptyState() object NoData: EmptyState() object NoInternet: EmptyState() } sealed class LoadingState { object None: LoadingState() object Loading: LoadingState() object PullToRefreshing: LoadingState() } }
80. State LiveData fun stateLiveData(): LiveData<MainViewState> = stateLiveData private val stateLiveData = MutableLiveData<MainViewState>() … stateLiveData.postValue(it) ViewModel
81. State LiveData fun stateLiveData(): LiveData<MainViewState> = stateLiveData private val stateLiveData = MutableLiveData<MainViewState>() … stateLiveData.postValue(it) ViewModel viewModel.stateLiveData() .observe(viewLifecycleOwner, Observer { render(it) }) View
82. State Flow private val stateChannel = ConflatedBroadcastChannel<MainViewState>() val stateFlow = stateChannel.asFlow() … stateChannel.offer(it) ViewModel
83. State Flow private val stateChannel = ConflatedBroadcastChannel<MainViewState>() val stateFlow = stateChannel.asFlow() … stateChannel.offer(it) ViewModel View viewModel.stateFlow.collect { render(it) }
84. ConflatedBroadcastChannel State Flow private val stateChannel = ConflatedBroadcastChannel<MainViewState>() val stateFlow = stateChannel.asFlow() … stateChannel.offer(it) ViewModel lifecycleScope.launch { viewModel.stateFlow.collect { render(it) } } View 2.2.0- alpha02
85. ConflatedBroadcastChannel • BroadcastChannel - multiple receivers • Conflate - combine • Recent value is emitted • Like RxJava BehaviorSubject
86. Render override fun render(state: MainViewState) { binding.refreshing = state.loadingState == MainViewState.LoadingState.PullToRefreshing binding.loading = state.loadingState == MainViewState.LoadingState.Loading val characterModelList = state.characters.map { CharacterItem(it, this) } adapter.replaceItems(characterModelList, true) } View
87. State View Intention 4 5 Plugging-in the pieces State stream 3 2 Actions to Results Reduce Results to new State 1 Action Events same same
88. State View Intention 1 Action Events 3 2 4 5 Actions to Results Reduce Results to new State Plugging-in the pieces State stream 3 2 Actions to Results Reduce Results to new State 1 Action Events same same
89. State View Intention 1 Action Events 3 2 4 5 Actions to Results Reduce Results to new State Plugging-in the pieces State stream 3 2 Actions to Results Reduce Results to new State 1 Action Events same same same
90. ViewModel Plugging-in the pieces interface ViewModel { fun onAction(action: MainAction) ... }
91. RxViewModel Plugging-in the pieces val actionsProcessor: PublishProcessor<Action> = PublishProcessor.create() fun onAction(action: Action) { actionsProcessor.onNext(action) }
92. FlowViewModel Plugging-in the pieces var broadcastChannel = ConflatedBroadcastChannel<MainAction>() fun onAction(action: MainAction) = broadcastChannel.offer(action) var actionsFlow = broadcastChannel.asFlow()
93. fun processActions() { actionsProcessor .compose(actionToResultTransformer) .scan(initialState) { state, result -> reduce(state, result)} .subscribe({ stateLiveData.postValue(it) }, Timber::e) .bindToLifecycle() } fun processActions() { actionsProcessor .compose(actionToResultTransformer) .scan(initialState) { state, result: Result -> reduce(state, result)} .subscribe({ stateLiveData.postValue(it) }, Timber::e) .bindToLifecycle() } Plugging-in the pieces RxViewModel
94. Plugging-in the pieces fun processActions() { actionsProcessor .compose(actionToResultTransformer) .scan(initialState) { state, result -> reduce(state, result)} .subscribe({ stateLiveData.postValue(it) }, Timber::e) .bindToLifecycle() } fun processActions() { viewModelScope.launch { actionsFlow .broadcastIn(viewModelScope).asFlow() .actionToResultTransformer() .scan(initialState) { state, result -> reduce(state, result) } .collect { stateLiveData.postValue(it) } } } RxViewModel FlowViewModel
95. State View Intention 5 Plugging-in the pieces 1 Action Events 3 2 4 Actions to Results Reduce Results to new State State stream 3 2 Actions to Results Reduce Results to new State 1 Action Events same same same
96. State View Intention 5 Plugging-in the pieces 1 Action Events 3 2 4 Actions to Results Reduce Results to new State State stream 3 2 Actions to Results Reduce Results to new State 1 Action Events same same same
97. State View Intention Congratulations !!
98. @ragdroid #droidconNYC Summary
99. What Did we learn? Rx Coroutines Single / Completable suspend function Flowable/Observable Flow BehaviorSubject ConflatedBroadcastChannel (DataFlow proposal) Schedulers Dispatchers Disposables Scopes
100. What Did we learn? Rx Flow just(“Hello”) flowOf(“Hello”) flatmap() flatmapMerge() subscribe() collect() publish() broadcastIn() startWith() onStart() onErrorReturn() catch() map() map() scan() scan() ... ...
101. What Did we learn? Rx Flow subscribeOn() flowOn() observeOn() flowOn() ❌ ❌
102. @ragdroid #droidconNYC Common Questions
103. Why Flow? - Current State
104. Why Flow? • First Party support • Android libraries have Flow support now like Room • Reducing complexity of business logic • Hopefully share more business logic
105. RxJava Coroutines Interop • Yes org.jetbrains.kotlinx:kotlinx-coroutines-rx2:1.3.0 • Single.await(), Flowable.awaitX() • rxSingle { }, rxFlowable { } …
106. Non coroutine APIs to Coroutine APIs • Yes • suspendCoroutine { } • Continuation.resumeWith( ) Continuation.resumeWithException()
107. Testing? • Yes • TestDispatcher • Dispatchers.setMain() • Inject a DispatcherProvider?
108. What Next? • kotlin-flow-extensions: https://github.com/akarnokd/kotlin-flow-extensions • CoroutineBinding: CoroutineBinding: https://github.com/satoshun/CoroutineBinding • Flowing in the Deep: https://www.droidcon.com/media-detail?video=352670453 • DataFlow: https://github.com/Kotlin/kotlinx.coroutines/pull/1354/files • Flow Guide: https://github.com/Kotlin/kotlinx.coroutines/blob/1.3.0/docs/flow.md • Flow vs Channel: https://github.com/manuelvicnt/MathCoroutinesFlow • Demo Project: https://github.com/ragdroid/klayground/tree/kotlin-flow
109. References & Acknowledgements • https://github.com/brewin/mvi-coroutines • Manuel Vivo @manuelvicnt • Hannes Dorfmann @sockeqwe • Android Team @Over • Ritesh Gupta @_riteshhh • https://medium.com/@elizarov - Roman Elizarov • https://medium.com/@objcode - Sean McQuillan • https://www.youtube.com/watch?v=PXBXcHQeDLE - MVI for Android, Benoît Quenaudon
110. @ragdroid #droidconNYC
111. Questions?