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
   

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2. Garima Jain
   @ragdroid
   

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3. @ragdroid
   @blrdroid
   Who?
   

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

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5. @ragdroid
   @blrdroid
   What?
   

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

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7. CHAPTER ONE
   THE FLOW
   

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

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

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

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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
    

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12. Flow Builders
    ★ flowOf( )
    ★ flow { }
    

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

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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
    

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15. Flow Constraints
    ★ Context Preservation
    

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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
    

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17. CHAPTER TWO
    THE BATTLE OF ACTIONS
    MVI
    

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18. Model View Intent
    

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19. Model View Intent
    State View Intention
    

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

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

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

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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
    

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

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25. Demo
    

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

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

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

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

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

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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
    

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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
    

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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
    

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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
    

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35. 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)
    }
    }
    

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

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

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38. Data Layer
    @GET("characters/{characterId}")
    fun getCharactersSingle(...): Single>
    fun fetchCharactersStream(): Flowable> =
    marvelApi.getCharactersSingle( ... ).mergeWith(cache...)
    MarvelApi
    MainRepository
    

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

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

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41. Data Layer
    fun fetchCharactersStream(): Flowable> =
    marvelApi.getCharactersSingle( ... )
    .mergeWith(cache...)
    MainRepository - Rx
    

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42. Data Layer
    fun fetchCharactersFlow(): Flow> = flow {
    val characters = marvelApi.getCharacters( ... )
    emit(characters)
    }
    .merge(cacheFlow)
    MainRepository - Flow
    Room
    2.2.0-
    alpha
    

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43. Merge Operator
    fun fetchCharactersFlow(): Flow> = flow {
    val characters = marvelApi.getCharacters( ... )
    emit(characters)
    }
    .merge(cacheFlow)
    Data Layer
    MainRepository - Flow
    

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44. Merge Operator
    • No merge operator at the time
    • Creating operators with coroutines is comparatively easy
    • Home Work (Hint Look into channelFlow)
    

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

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

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

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

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

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

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51. Repository emits Single?
    fun fetchCharactersStream(): Flowable> =
    marvelApi.getCharactersSingle( ... )
    .mergeWith(cache...)
    MainRepository - Rx
    

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52. fun fetchCharactersSingle(): Single> =
    marvelApi.getCharactersSingle( ... )
    MainRepository - Rx
    Repository emits Single?
    

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53. Repository exposes suspend function
    suspend fun fetchCharacters(): List =
    marvelApi.getCharacters( ... )
    MainRepository - Coroutines
    

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54. 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 -> { … }
    }
    }
    }
    

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

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

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

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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
    

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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
    

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60. 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
    

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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
    

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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)
    }
    ...
    }
    

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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
    

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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
    

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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
    

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66. 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()
    }
    }
    

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

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

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

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

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71. 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
    

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

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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
    

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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
    

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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
    

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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
    

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

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

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

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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()
    }
    

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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
    

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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
    

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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
    

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84. State View Intention
    Congratulations !!
    

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85. @ragdroid
    @blrdroid
    Summary
    

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

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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()
    ... ...
    

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

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89. @ragdroid
    @blrdroid
    Common Questions
    

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90. Why Flow? - Current State
    

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

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

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

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

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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
    

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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
    

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97. @ragdroid
    @blrdroid
    

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98. Questions?
    

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