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
   

   View Slide

2. Garima Jain
   @ragdroid
   

   View Slide

3. Garima Jain
   @ragdroid
   

   View Slide

4. @ragdroid
   @AndroidMakersFR
   Who?
   

   View Slide

5. Who?
   • Rx background
   • Coroutines
   • MVI
   • Stranger Things?
   

   View Slide

6. @ragdroid
   @AndroidMakersFR
   What?
   

   View Slide

7. What?
   • Chapter One : Coroutines and Flow
   • Chapter Two : MVI
   • Chapter Three : Rx to Flow
   • Summary
   

   View Slide

8. CHAPTER ONE
   THE FLOW
   

   View Slide

9. Coroutines
   • Lightweight thread
   • Run computations without blocking
   • Can be suspended
   

   View Slide

10. Coroutines
    • Coroutine Scope
    ★ ViewModel Scope
    ★ Lifecycle Scope
    • Coroutine Builders
    ★ launch { } - fire and forget
    ★ async { } - await() result
    ★ within a scope, Structured Concurrency
    

    View Slide

11. Channels
    • Communication between different coroutines
    • Similar to BlockingQueue
    

    View Slide

12. Flow
    • Cold asynchronous stream that sequentially emits values
    • Utilizes coroutines and channels
    • Experimental Stable APIs since 1.3.0
    • Now 1.3.5
    

    View Slide

13. Flow Builders
    ★ flowOf( )
    ★ flow { }
    ★ channelFlow { }
    

    View Slide

14. Flow Operators
    • Intermediate :
    ★ map, filter, take, zip, etc.
    • Terminal :
    ★ collect, single, reduce, etc.
    

    View Slide

15. 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
    

    View Slide

16. Flow Constraints
    ★ Context Preservation
    ★ Exception Transparency
    

    View Slide

17. 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
    

    View Slide

18. CHAPTER TWO
    THE BATTLE OF ACTIONS
    MVI
    

    View Slide

19. Model View Intent
    

    View Slide

20. Model View Intent
    State View Intention
    

    View Slide

21. State View Intention
    User
    Intent
    State
    View
    Changes
    Updates
    Seen Interacts
    

    View Slide

22. View
    ViewModel Data
    Intent
    newState
    Action
    Result
    previousState
    Reducer
    State View Intention
    State
    MVI on top of MVVM
    

    View Slide

23. View
    ViewModel Data
    Intent
    newState
    Action
    Result
    previousState
    Reducer
    State View Intention
    State
    4
    3
    5
    1 2
    MVI on top of MVVM
    

    View Slide

24. State View Intention
    1
    3
    2
    4
    5
    Action Events
    Actions to Results
    Reduce Results to new State
    Plugging-in the pieces
    State stream
    

    View Slide

25. CHAPTER THREE
    VANISHING OF Rx FLOWABLES
    Migrating an Rx MVI pattern to use Flow
    

    View Slide

26. Demo
    

    View Slide

27. Actions
    sealed class MainAction {
    object PullToRefresh
    object LoadData
    data class LoadDescription(val characterId: Long)
    }
    

    View Slide

28. Actions - ViewModel
    interface ViewModel {
    fun onAction(action: MainAction)
    ...
    }
    Rx
    

    View Slide

29. Actions - ViewModel
    interface ViewModel {
    fun onAction(action: MainAction)
    ...
    }
    Flow
    

    View Slide

30. Action Events
    viewModel.onAction(MainAction.LoadData)
    viewModel.onAction(..)
    

    View Slide

31. viewModel.onAction(MainAction.LoadData)
    viewModel.onAction(..)
    refreshLayout.setOnRefreshListener {
    viewModel.onAction(MainAction.PullToRefresh)
    }
    Action Events
    

    View Slide

32. viewModel.onAction(MainAction.LoadData)
    viewModel.onAction(..)
    override fun onCharacterDescriptionClicked(itemId: Long) {
    viewModel.onAction(MainAction.LoadDescription(itemId))
    }
    refreshLayout.setOnRefreshListener {
    viewModel.onAction(MainAction.PullToRefresh)
    }
    Action Events
    

    View Slide

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
    

    View Slide

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
    

    View Slide

35. 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
    

    View Slide

36. Actions to Results
    sealed class MainResult {
    object Loading
    object LoadingError
    data class LoadingComplete(val characters: List)
    object PullToRefreshing: MainResult()
    object PullToRefreshError
    data class PullToRefreshComplete(val characters: List)
    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)
    }
    }
    

    View Slide

37. View
    ViewModel Data
    Intent
    newState
    Action
    Result
    previousState
    Reducer
    State View Intention
    State
    

    View Slide

38. View ViewModel
    Data
    Intent
    newState
    Action Result
    previousState
    Reducer
    State View Intention
    State
    Repository Network
    

    View Slide

39. Data Layer
    @GET("characters/{characterId}")
    fun getCharactersSingle(...): Single>
    fun fetchCharactersSingle(): Single> =
    marvelApi.getCharactersSingle( ... )
    MarvelApi
    MainRepository
    

    View Slide

40. Actions to Results - LoadData
    fun loadingResult(loadDataActionStream: Flowable)
    : Flowable {
    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
    

    View Slide

41. Data Layer
    @GET("characters/{characterId}")
    fun getCharactersSingle(...): Single>
    MarvelApi - Rx
    

    View Slide

42. Data Layer
    @GET("characters/{characterId}")
    suspend fun getCharacters(…): List
    MarvelApi - Coroutines
    2.6.0
    

    View Slide

43. Data Layer
    fun fetchCharactersSingle(): Single> =
    marvelApi.getCharactersSingle( ... )
    MainRepository - Rx
    

    View Slide

44. Data Layer
    suspend fun fetchCharacters(): List =
    marvelApi.getCharacters( ... )
    MainRepository - Coroutines
    

    View Slide

45. fun loadingResult(actions: Flowable<...>)
    : Flowable =
    actions
    .observeOn(Schedulers.io())
    .flatMap {
    mainRepository.fetchCharactersFlowable()
    .map { MainResult.LoadingComplete }
    .startWith(MainResult.Loading)
    .onErrorReturn { error ->
    navigate(MainNavigation.Snackbar)
    MainResult.LoadingError
    }
    }
    fun loadingResult(actions: Flowable)
    : Flowable =
    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
    

    View Slide

46. fun loadingResult(actions: Flowable<...>)
    : Flowable =
    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)
    : Flow =
    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
    

    View Slide

47. fun pullToRefreshResult(actions: Flowable)
    : Flowable =
    ...
    Actions to Results - Pull To Refresh
    RxViewModel
    

    View Slide

48. fun pullToRefreshResult(actions: Flowable)
    : Flow =
    ...
    Actions to Results - Pull To Refresh
    FlowViewModel
    

    View Slide

49. fun loadDesciptionResult(actions: Flowable)
    : Flowable =
    ...
    Actions to Results - Load Desciption
    RxViewModel
    

    View Slide

50. fun loadDesciptionResult(actions: Flow)
    : Flow =
    ...
    Actions to Results - Load Desciption
    FlowViewModel
    

    View Slide

51. Actions to Results
    fun Flowable<…>.actionsToResultTransformer(): Flowable<…> =
    flatMap {
    Flowable.merge(loadingResult(ofType(MainAction.LoadData::class.java)),
    loadDescriptionResult(ofType(MainAction.LoadDescription::class.java)),
    pullToRefreshResult(ofType(MainAction.PullToRefresh::class.java)))
    }
    RxViewModel
    

    View Slide

52. Merge Operator
    Actions to Results
    fun Flow.actionsToResultTransformer(): Flow =
    flatMapMerge {
    loadingResult(ofType(MainAction.LoadData::class.java))
    .merge(loadDescriptionResult(ofType(MainAction.LoadDescription::class.java)),
    pullToRefreshResult(ofType(MainAction.PullToRefresh::class.java)))
    }
    Flow
    

    View Slide

53. Merge Operator
    Actions to Results
    fun Flow.actionsToResultTransformer(): Flow =
    flatMapMerge {
    loadingResult(ofType(MainAction.LoadData::class.java))
    .merge(loadDescriptionResult(ofType(MainAction.LoadDescription::class.java)),
    pullToRefreshResult(ofType(MainAction.PullToRefresh::class.java)))
    }
    Flow
    

    View Slide

54. Merge Operator
    • No merge operator at the time
    • Creating operators with coroutines is comparatively easy
    • Let's create one
    

    View Slide

55. fun Flow.merge(other: Flow): Flow = 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
    

    View Slide

56. Merge Operator
    Actions to Results
    fun Flow.actionsToResultTransformer(): Flow =
    flatMapMerge {
    loadingResult(ofType(MainAction.LoadData::class.java))
    .merge(loadDescriptionResult(ofType(MainAction.LoadDescription::class.java)),
    pullToRefreshResult(ofType(MainAction.PullToRefresh::class.java)))
    }
    Flow
    

    View Slide

57. View ViewModel
    Data
    Intent
    newState
    Action Result
    previousState
    Reducer
    State
    Repository Network
    State View Intention (Rx)
    

    View Slide

58. Actions to Results
    fun Flow.actionsToResultTransformer(): Flow =
    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 -> { … }
    }
    }
    }
    

    View Slide

59. View ViewModel
    Data
    Intent
    newState
    Action Result
    previousState
    Reducer
    State
    Repository Network
    State View Intention (Rx)
    

    View Slide

60. View ViewModel
    Data
    Intent
    newState
    Action Result
    previousState
    Reducer
    State View Intention (Co-routines)
    State
    Repository Network
    

    View Slide

61. View ViewModel
    Data
    Intent
    newState
    Action Result
    previousState
    Reducer
    State View Intention (Co-routines)
    State
    Repository Network
    Room
    2.2.0-
    alpha
    

    View Slide

62. 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
    

    View Slide

63. 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
    

    View Slide

64. Reduce Results to new State
    data class MainViewState(val characters: List,
    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
    

    View Slide

65. 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
    

    View Slide

66. 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)
    }
    ...
    }
    

    View Slide

67. 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
    

    View Slide

68. 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
    

    View Slide

69. 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
    

    View Slide

70. State
    State stream
    data class MainViewState(val characters: List,
    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()
    }
    }
    

    View Slide

71. State LiveData
    fun stateLiveData(): LiveData = stateLiveData
    private val stateLiveData = MutableLiveData()
    …
    stateLiveData.postValue(it)
    ViewModel
    

    View Slide

72. State LiveData
    fun stateLiveData(): LiveData = stateLiveData
    private val stateLiveData = MutableLiveData()
    …
    stateLiveData.postValue(it)
    ViewModel
    viewModel.stateLiveData()
    .observe(viewLifecycleOwner,
    Observer { render(it) })
    View
    

    View Slide

73. State Flow
    private val stateChannel = ConflatedBroadcastChannel()
    val stateFlow = stateChannel.asFlow()
    …
    stateChannel.offer(it)
    ViewModel
    

    View Slide

74. State Flow
    private val stateChannel = ConflatedBroadcastChannel()
    val stateFlow = stateChannel.asFlow()
    …
    stateChannel.offer(it)
    ViewModel
    View
    viewModel.stateFlow.collect {
    render(it)
    }
    

    View Slide

75. ConflatedBroadcastChannel
    State Flow
    private val stateChannel = ConflatedBroadcastChannel()
    val stateFlow = stateChannel.asFlow()
    …
    stateChannel.offer(it)
    ViewModel
    lifecycleScope.launch {
    viewModel.stateFlow.collect {
    render(it)
    }
    }
    View
    2.2.0-
    alpha02
    

    View Slide

76. ConflatedBroadcastChannel
    • BroadcastChannel - multiple receivers
    • Conflate - combine
    • Recent value is emitted
    • Like RxJava BehaviorSubject
    

    View Slide

77. 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
    

    View Slide

78. 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
    

    View Slide

79. 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
    

    View Slide

80. 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
    

    View Slide

81. ViewModel
    Plugging-in the pieces
    interface ViewModel {
    fun onAction(action: MainAction)
    ...
    }
    

    View Slide

82. RxViewModel
    Plugging-in the pieces
    val actionsProcessor: PublishProcessor =
    PublishProcessor.create()
    fun onAction(action: Action) {
    actionsProcessor.onNext(action)
    }
    

    View Slide

83. FlowViewModel
    Plugging-in the pieces
    var broadcastChannel = ConflatedBroadcastChannel()
    fun onAction(action: MainAction) = broadcastChannel.offer(action)
    var actionsFlow = broadcastChannel.asFlow()
    

    View Slide

84. fun processActions() {
    actionsProcessor
    .actionToResultTransformer()
    .scan(initialState) { state, result ->
    reduce(state, result)}
    .subscribe({
    stateLiveData.postValue(it)
    }, Timber::e)
    .bindToLifecycle()
    }
    Plugging-in the pieces
    RxViewModel
    fun processActions() {
    actionsProcessor
    .actionToResultTransformer()
    .scan(initialState) { state, result: Result ->
    reduce(state, result)}
    .subscribe({
    stateLiveData.postValue(it)
    }, Timber::e)
    .bindToLifecycle()
    }
    

    View Slide

85. Plugging-in the pieces
    fun processActions() {
    actionsProcessor
    .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
    

    View Slide

86. 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
    

    View Slide

87. 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
    

    View Slide

88. State View Intention
    Congratulations !!
    

    View Slide

89. @ragdroid
    @AndroidMakersFR
    Summary
    

    View Slide

90. What Did we learn?
    Rx Coroutines
    Single / Completable suspend function
    Flowable/Observable Flow
    BehaviorSubject
    ConflatedBroadcastChannel
    (DataFlow proposal)
    Schedulers Dispatchers
    Disposables Scopes
    

    View Slide

91. What Did we learn?
    Rx Flow
    just(“Hello”) flowOf(“Hello”)
    flatmap() flatmapMerge()
    subscribe() collect()
    publish() broadcastIn()
    startWith() onStart()
    onErrorReturn() catch()
    map() map()
    scan() scan()
    ... ...
    

    View Slide

92. What Did we learn?
    Rx Flow
    subscribeOn() flowOn()
    observeOn() flowOn()
    ❌
    ❌
    

    View Slide

93. @ragdroid
    @AndroidMakersFR
    Common Questions
    

    View Slide

94. Why Flow? - Current State
    

    View Slide

95. Why Flow?
    • First Party support
    • Android libraries have Flow support now like Room
    • Reducing complexity of business logic
    • Hopefully share more business logic
    

    View Slide

96. RxJava Coroutines Interop
    • Yes org.jetbrains.kotlinx:kotlinx-coroutines-rx2:1.3.0
    • Single.await(), Flowable.awaitX()
    • rxSingle { }, rxFlowable { } …
    

    View Slide

97. Non coroutine APIs to Coroutine APIs
    • Yes
    • suspendCoroutine { }
    • Continuation.resumeWith( ) Continuation.resumeWithException()
    

    View Slide

98. Testing?
    • Yes
    • TestDispatcher
    • Dispatchers.setMain()
    • Inject a DispatcherProvider?
    

    View Slide

99. 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
    

    View Slide

100. 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
     

     View Slide

101. @ragdroid
     @AndroidMakersFR
     

     View Slide

102. Questions?
     

     View Slide