Flowing Things, not so strange in the MVI world (BlrDroid meetup)

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 @blrdroid Who?
4. Who? • Rx background • Coroutines • MVI • Stranger Things?
5. @ragdroid @blrdroid What?
6. What? • Chapter One : Coroutines and Flow • Chapter Two : MVI • Chapter Three : 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 • Like Reactive Observable / Flowable • Experimental Stable APIs since 1.3.0 • Now 1.3.2
12. Flow Builders ★ flowOf( ) ★ flow { }
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
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 THE BATTLE OF ACTIONS MVI
18. Model View Intent
19. Model View Intent State View Intention
20. State View Intention User Intent State View Changes Updates Seen Interacts
21. View ViewModel Data Intent newState Action Result previousState Reducer State View Intention State MVI on top of MVVM
22. View ViewModel Data Intent newState Action Result previousState Reducer State View Intention State 4 3 5 1 2 MVI on top of MVVM
23. State View Intention 1 3 2 4 5 Action Events Actions to Results Reduce Results to new State Plugging-in the pieces State stream
24. CHAPTER THREE VANISHING OF Rx FLOWABLES Migrating an Rx MVI pattern to use Flow
25. Demo
26. Actions sealed class MainAction { object PullToRefresh object LoadData data class LoadDescription(val characterId: Long) }
27. Actions - ViewModel interface ViewModel { fun onAction(action: MainAction) ... } Rx
28. Actions - ViewModel interface ViewModel { fun onAction(action: MainAction) ... } Flow
29. Action Events viewModel.onAction(MainAction.LoadData) viewModel.onAction(..)
30. viewModel.onAction(MainAction.LoadData) viewModel.onAction(..) refreshLayout.setOnRefreshListener { viewModel.onAction(MainAction.PullToRefresh) } Action Events
31. viewModel.onAction(MainAction.LoadData) viewModel.onAction(..) override fun onCharacterDescriptionClicked(itemId: Long) { viewModel.onAction(MainAction.LoadDescription(itemId)) } refreshLayout.setOnRefreshListener { viewModel.onAction(MainAction.PullToRefresh) } Action Events
32. State View Intention 1 Action Events 3 2 4 5 Actions to Results Reduce Results to new State State stream Plugging-in the pieces
33. State View Intention 1 Action Events 3 2 4 5 Actions to Results Reduce Results to new State State stream Plugging-in the pieces
34. 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
35. 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) } }
36. View ViewModel Data Intent newState Action Result previousState Reducer State View Intention State
37. View ViewModel Data Intent newState Action Result previousState Reducer State View Intention State Repository Network
38. Data Layer @GET("characters/{characterId}") fun getCharactersSingle(...): Single<List<CharacterMarvel>> fun fetchCharactersStream(): Flowable<List<CharacterMarvel>> = marvelApi.getCharactersSingle( ... ).mergeWith(cache...) MarvelApi MainRepository
39. Data Layer @GET("characters/{characterId}") fun getCharactersSingle(...): Single<List<CharacterMarvel>> MarvelApi - Rx
40. Data Layer @GET("characters/{characterId}") suspend fun getCharacters(…): List<CharacterMarvel> MarvelApi - Coroutines 2.6.0
41. Data Layer fun fetchCharactersStream(): Flowable<List<CharacterMarvel>> = marvelApi.getCharactersSingle( ... ) .mergeWith(cache...) MainRepository - Rx
42. Data Layer fun fetchCharactersFlow(): Flow<List<CharacterMarvel>> = flow { val characters = marvelApi.getCharacters( ... ) emit(characters) } .merge(cacheFlow) MainRepository - Flow Room 2.2.0- alpha
43. Merge Operator fun fetchCharactersFlow(): Flow<List<CharacterMarvel>> = flow { val characters = marvelApi.getCharacters( ... ) emit(characters) } .merge(cacheFlow) Data Layer MainRepository - Flow
44. Merge Operator • No merge operator at the time • Creating operators with coroutines is comparatively easy • Home Work (Hint Look into channelFlow)
45. fun loadingResult(actions: Flowable<...>) : Flowable<MainResult> = actions .observeOn(Schedulers.io()) .flatMap { mainRepository.fetchCharactersStream() .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.fetchCharactersStream() .map { states -> MainResult.LoadingComplete(states) } .startWith(MainResult.Loading) .onErrorReturn { error -> navigate(MainNavigation.Snackbar(error.message) MainResult.LoadingError } } Actions to Results - Load Data RxViewModel
46. fun loadingResult(actions: Flowable<...>) : Flowable<MainResult> = actions .observeOn(Schedulers.io()) .flatMap { mainRepository.fetchCharactersStream() .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 { mainRepository.fetchCharactersFlow()) .map { MainResult.LoadingComplete(characters) } .onStart { emit(MainResult.Loading) } .catch { navigate(MainNavigation.Snackbar(it.message) emit(MainResult.LoadingError) } } Actions to Results - Load Data
47. fun pullToRefreshResult(actions: Flowable<MainAction.PullToRefresh>) : Flowable<MainResult> = ... Actions to Results - Pull To Refresh RxViewModel
48. fun pullToRefreshResult(actions: Flowable<MainAction.PullToRefresh>) : Flow<MainResult> = ... Actions to Results - Pull To Refresh FlowViewModel
49. fun loadDesciptionResult(actions: Flowable<MainAction.LoadDescription>) : Flowable<MainResult> = ... Actions to Results - Load Desciption RxViewModel
50. fun loadDesciptionResult(actions: Flowable<MainAction.LoadDescription>) : Flow<MainResult> = ... Actions to Results - Load Desciption FlowViewModel
51. Repository emits Single? fun fetchCharactersStream(): Flowable<List<CharacterMarvel>> = marvelApi.getCharactersSingle( ... ) .mergeWith(cache...) MainRepository - Rx
52. fun fetchCharactersSingle(): Single<List<CharacterMarvel>> = marvelApi.getCharactersSingle( ... ) MainRepository - Rx Repository emits Single?
53. Repository exposes suspend function suspend fun fetchCharacters(): List<CharacterMarvel> = marvelApi.getCharacters( ... ) MainRepository - Coroutines
54. 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 -> { … } } } }
55. View ViewModel Data Intent newState Action Result previousState Reducer State Repository Network State View Intention (Rx)
56. View ViewModel Data Intent newState Action Result previousState Reducer State View Intention (Co-routines) State Repository Network
57. View ViewModel Data Intent newState Action Result previousState Reducer State View Intention (Co-routines) State Repository Network
58. 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
59. 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
60. 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
61. Reduce Results to new State fun reduce(result: MainResult): MainViewState { 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
62. Reduce Results to new State FlowReducer fun reduce(result: MainResult): MainViewState { 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) } ... }
63. 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
64. 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
65. 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
66. 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() } }
67. State LiveData fun stateLiveData(): LiveData<MainViewState> = stateLiveData private val stateLiveData = MutableLiveData<MainViewState>() … stateLiveData.postValue(it) ViewModel
68. State LiveData fun stateLiveData(): LiveData<MainViewState> = stateLiveData private val stateLiveData = MutableLiveData<MainViewState>() … stateLiveData.postValue(it) ViewModel viewModel.stateLiveData() .observe(viewLifecycleOwner, Observer { render(it) }) View
69. State Flow private val stateChannel = ConflatedBroadcastChannel<MainViewState>() val stateFlow = stateChannel.asFlow() … stateChannel.offer(it) ViewModel
70. State Flow private val stateChannel = ConflatedBroadcastChannel<MainViewState>() val stateFlow = stateChannel.asFlow() … stateChannel.offer(it) ViewModel View viewModel.stateFlow.collect { render(it) }
71. 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
72. ConflatedBroadcastChannel • BroadcastChannel - multiple receivers • Conflate - combine • Recent value is emitted • Like RxJava BehaviorSubject
73. 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
74. 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
75. 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
76. 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
77. ViewModel Plugging-in the pieces interface ViewModel { fun onAction(action: MainAction) ... }
78. RxViewModel Plugging-in the pieces val actionsProcessor: PublishProcessor<Action> = PublishProcessor.create() fun onAction(action: Action) { actionsProcessor.onNext(action) }
79. FlowViewModel Plugging-in the pieces var broadcastChannel = ConflatedBroadcastChannel<MainAction>() fun onAction(action: MainAction) = broadcastChannel.offer(action) var actionsFlow = broadcastChannel.asFlow()
80. fun processActions() { actionsProcessor .compose(actionToResultTransformer) .scan(initialState) { state, result -> reduce(state, result)} .subscribe({ stateLiveData.postValue(it) }, Timber::e) .bindToLifecycle() } Plugging-in the pieces RxViewModel fun processActions() { actionsProcessor .compose(actionToResultTransformer) .scan(initialState) { state, result: Result -> reduce(state, result)} .subscribe({ stateLiveData.postValue(it) }, Timber::e) .bindToLifecycle() }
81. 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 .actionToResultTransformer() .scan(initialState) { state, result -> reduce(state, result) } .collect { stateLiveData.postValue(it) } } } RxViewModel FlowViewModel
82. 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
83. 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
84. State View Intention Congratulations !!
85. @ragdroid @blrdroid Summary
86. What Did we learn? Rx Coroutines Single / Completable suspend function Flowable/Observable Flow BehaviorSubject ConflatedBroadcastChannel (DataFlow proposal) Schedulers Dispatchers Disposables Scopes
87. What Did we learn? Rx Flow just(“Hello”) flowOf(“Hello”) flatmap() flatmapMerge() subscribe() collect() publish() broadcastIn() startWith() onStart() onErrorReturn() catch() map() map() scan() scan() ... ...
88. What Did we learn? Rx Flow subscribeOn() flowOn() observeOn() flowOn() ❌ ❌
89. @ragdroid @blrdroid Common Questions
90. Why Flow? - Current State
91. Why Flow? • First Party support • Android libraries have Flow support now like Room • Reducing complexity of business logic • Hopefully share more business logic
92. RxJava Coroutines Interop • Yes org.jetbrains.kotlinx:kotlinx-coroutines-rx2:1.3.0 • Single.await(), Flowable.awaitX() • rxSingle { }, rxFlowable { } …
93. Non coroutine APIs to Coroutine APIs • Yes • suspendCoroutine { } • Continuation.resumeWith( ) Continuation.resumeWithException()
94. Testing? • Yes • TestDispatcher • Dispatchers.setMain() • Inject a DispatcherProvider?
95. 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 • DroidconNYC: https://www.droidcon.com/media-detail?video=362742238
96. 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
97. @ragdroid @blrdroid
98. Questions?