Unidirectional Data Flow on Android

The introduction of Android Architecture Components made things are easier for the less experienced developers. However, In order to build robust, testable and scalable applications we need more. That's where the Unidirectional Data Flow architecture comes in, borrowed from Flex and Redux on the web.

The approach is based on the idea of an immutable state that represents the state of our app. All the components are decoupled from each other, and they work together taking advantage of Kotlin's language features. We solve the asynchrony problem using Coroutines, specifically Channels and Actors.

Do you want to know more? This talk will guide you through our implementation of this paradigm, with clear steps and a detailed explanation that will help you understand its value and spark your curiosity!

Transcript

1. Unidirectional Data Flow on Android
   David González
   @dggonzalez
   malmstein@gmail.com
   Droidcon Italy 2019
   

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2. Credit when credit is due…
   Fernando Cejas Cesar Valiente Fabio Collini
   

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3. View ViewModel
   Store
   Reducer
   Action
   State
   send
   interpret
   dispatch
   notify
   

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4. View ViewModel
   Action
   

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5. sealed class OrdersViewAction {
   object NavigateToChatScreen : OrdersViewAction()
   data class SelectAddress(val orderId: String) : OrdersViewAction()
   data class CancelOrder(val orderId: String) : OrdersViewAction()
   data class TrackShipment(val url: String) : OrdersViewAction()
   }
   

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6. open class MainViewModel : ViewModel() {
   val store: Store = Store()
   fun observe(owner: LifecycleOwner, stateObserver: (ViewState) -> Unit) =
   store.stateLiveData.observe(owner, Observer {
   observer(it!!)
   })
   fun interpret(action: ViewAction)
   override fun onCleared() {
   store.cancel()
   }
   }
   

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7. open class MainViewModel : ViewModel() {
   val store: Store = Store()
   fun observe(owner: LifecycleOwner,stateObserver: (ViewState) -> Unit) =
   store.stateLiveData.observe(owner, Observer {
   observer(it!!)
   })
   fun interpret(action: ViewAction)
   override fun onCleared() {
   store.cancel()
   }
   }
   

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8. open class MainViewModel : ViewModel() {
   val store: Store = Store()
   fun observe(owner: LifecycleOwner, stateObserver: (ViewState) -> Unit) =
   store.stateLiveData.observe(owner, Observer {
   observer(it!!)
   })
   fun interpret(action: ViewAction)
   override fun onCleared() {
   store.cancel()
   }
   }
   

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9. val store: Store = Store()
   fun observe(owner: LifecycleOwner, stateObserver: (ViewState) -> Unit) =
   store.stateLiveData.observe(owner, Observer {
   observer(it!!)
   })
   fun interpret(action: ViewAction)
   override fun onCleared() {
   store.cancel()
   }
   }
   

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10. store.stateLiveData.observe(owner, Observer {
    observer(it!!)
    })
    fun interpret(action: ViewAction)
    override fun onCleared() {
    store.cancel()
    }
    }
    

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11. ViewModel
    Store
    Reducer
    Action
    State
    

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12. class Store(initialState: ViewState = ViewState.Idle){
    val stateLiveData = MutableLiveData()
    .apply {
    value = initialState
    }
    @MainThread
    fun dispatchState(state: ViewState) {
    stateLiveData.value = state
    }
    }
    

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13. class Store(initialState: ViewState = ViewState.Idle){
    val stateLiveData = MutableLiveData()
    .apply {
    value = initialState
    }
    @MainThread
    fun dispatchState(state: ViewState) {
    stateLiveData.value = state
    }
    }
    

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14. class Store(initialState: ViewState = ViewState.Idle){
    val stateLiveData = MutableLiveData()
    .apply {
    value = initialState
    }
    @MainThread
    fun dispatchState(state: ViewState) {
    stateLiveData.value = state
    }
    }
    

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15. class Store(initialState: ViewState = ViewState.Idle) : CoroutineScope {
    private val job = Job()
    fun cancel() = job.cancel()
    override val coroutineContext: CoroutineContext = job + Dispatchers.IO
    private fun state() = stateLiveData.value!!
    fun dispatchState(f: suspend (ViewState) -> State) {
    launch {
    val action = f(state())
    withContext(Dispatchers.Main) {
    dispatchState(action(state()))
    }
    

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16. class Store(initialState: ViewState = ViewState.Idle) : CoroutineScope {
    private val job = Job()
    fun cancel() = job.cancel()
    override val coroutineContext: CoroutineContext = job + Dispatchers.IO
    private fun state() = stateLiveData.value!!
    fun dispatchState(f: suspend (ViewState) -> State) {
    launch {
    val action = f(state())
    withContext(Dispatchers.Main) {
    dispatchState(action(state()))
    }
    

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17. class Store(initialState: ViewState = ViewState.Idle) : CoroutineScope {
    private val job = Job()
    fun cancel() = job.cancel()
    override val coroutineContext: CoroutineContext = job + Dispatchers.IO
    private fun state() = stateLiveData.value!!
    fun dispatchState(f: suspend (ViewState) -> State) {
    launch {
    val action = f(state())
    withContext(Dispatchers.Main) {
    dispatchState(action(state()))
    }
    

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18. class Store(initialState: ViewState = ViewState.Idle) : CoroutineScope {
    private val job = Job()
    fun cancel() = job.cancel()
    override val coroutineContext: CoroutineContext = job + Dispatchers.IO
    private fun state() = stateLiveData.value!!
    fun dispatchState(f: suspend (ViewState) -> State) {
    launch {
    val reducer = f(state())
    withContext(Dispatchers.Main) {
    dispatchState(reducer(state()))
    }
    }
    

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19. private val job = Job()
    fun cancel() = job.cancel()
    override val coroutineContext: CoroutineContext = job + Dispatchers.IO
    private fun state() = stateLiveData.value!!
    fun dispatchState(f: suspend (ViewState) -> State) {
    launch {
    val reducer = f(state())
    withContext(Dispatchers.Main) {
    dispatchState(reducer(state()))
    }
    }
    }
    }
    

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20. fun cancel() = job.cancel()
    override val coroutineContext: CoroutineContext = job + Dispatchers.IO
    private fun state() = stateLiveData.value!!
    fun dispatchState(f: suspend (ViewState) -> State) {
    launch {
    val reducer = f(state())
    withContext(Dispatchers.Main) {
    dispatchState(reducer(state()))
    }
    }
    }
    }
    

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21. private fun state() = stateLiveData.value!!
    fun dispatchState(f: suspend (ViewState) -> State) {
    launch {
    val reducer = f(state())
    withContext(Dispatchers.Main) {
    dispatchState(reducer(state()))
    }
    }
    }
    }
    

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22. private fun state() = stateLiveData.value!!
    fun dispatchState(f: suspend (ViewState) -> State) {
    launch {
    val reducer = f(state())
    withContext(Dispatchers.Main) {
    dispatchState(reducer(state()))
    }
    }
    }
    }
    

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23. class PitchReducer
    @Inject constructor(val testimonialsRepo: TestimonialsRepository,
    val pricingRepo: PricingRepository){
    operator fun invoke() = loadPitch()
    private fun loadPitch(): State {
    val testimonials = testimonialsRepo.all()
    val pricing = pricingRepo.current()
    return State {
    Pitch(testimonials, pricing)
    }
    }
    }
    

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24. class PitchReducer
    @Inject constructor(val testimonialsRepo: TestimonialsRepository,
    val pricingRepo: PricingRepository){
    operator fun invoke() = loadPitch()
    private fun loadPitch(): State {
    val testimonials = testimonialsRepo.all()
    val pricing = pricingRepo.current()
    return State {
    Pitch(testimonials, pricing)
    }
    }
    }
    

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25. class PitchReducer
    @Inject constructor(val testimonialsRepo: TestimonialsRepository,
    val pricingRepo: PricingRepository){
    operator fun invoke() = loadPitch()
    private fun loadPitch(): State {
    val testimonials = testimonialsRepo.all()
    val pricing = pricingRepo.current()
    return State {
    Pitch(testimonials, pricing)
    }
    }
    }
    

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26. class PitchReducer
    @Inject constructor(val testimonialsRepo: TestimonialsRepository,
    val pricingRepo: PricingRepository){
    operator fun invoke() = loadPitch()
    private fun loadPitch(): State {
    val testimonials = testimonialsRepo.all()
    val pricing = pricingRepo.current()
    return State {
    Pitch(testimonials, pricing)
    }
    }
    }
    

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27. private fun initViewModel() {
    viewModel=ViewModelProviders.of(this,vmf)[…]
    viewModel.observe(this, ::renderViewState)
    }
    override fun renderViewState(viewState: ViewState) {
    when (viewState) {
    is Pitch -> renderPitch(viewState)
    }
    }
    PitchActivity
    

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28. class PitchViewModel
    @Inject constructor(private val reducer: PitchReducer): MainViewModel(){
    fun interpret(action: ViewAction) {
    return when (action) {
    is LoadPitch -> store.dispatchState(reducer())
    else -> throw IncompatibleActionException()
    }
    }
    PitchViewModel
    

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29. class PitchViewModel
    @Inject constructor(private val reducer: PitchReducer): MainViewModel(){
    fun interpret(action: ViewAction) {
    return when (action) {
    is LoadPitch -> store.dispatchState(reducer())
    else -> throw IncompatibleActionException()
    }
    }
    PitchViewModel
    

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30. class PitchViewModel
    @Inject constructor(private val reducer: PitchReducer): MainViewModel(){
    fun interpret(action: ViewAction) {
    return when (action) {
    is LoadPitch -> store.dispatchState(reducer())
    else -> throw IncompatibleActionException()
    }
    }
    PitchViewModel
    

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31. class PitchViewModel
    @Inject constructor(private val reducer: PitchReducer): MainViewModel(){
    fun interpret(action: ViewAction) {
    return when (action) {
    is LoadPitch -> store.dispatchState(reducer())
    else -> throw IncompatibleActionException()
    }
    }
    PitchViewModel
    

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32. class PitchViewModel
    @Inject constructor(private val reducer: PitchReducer): MainViewModel(){
    fun interpret(action: ViewAction) {
    return when (action) {
    is LoadPitch -> store.dispatchState(reducer())
    else -> throw IncompatibleActionException()
    }
    }
    PitchViewModel
    

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33. Problems going forward
    How do you deal with a more complicated ViewState?
    How do you deal with an error?
    How do you deal with multiple errors that must be displayed in screen?
    

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34. data class
    Address(val order: Order = Order.empty(),
    val wrongCity: Boolean,
    val wrongCountry: Boolean,
    val wrongAddress: Boolean,
    val isLoading: Boolean,
    val isCompleted: Boolean,
    val exception: Exception) : ViewState()
    }
    ViewState
    

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35. private fun render(viewState: Address) {
    if (viewState.isCompleted){
    finish()
    else {
    progress.isVisible = viewState.isLoading
    viewState.exception?.let{
    renderException(it)
    }
    cityError.isVisible = viewState.wrongCity
    countryError.isVisible = viewState.wrongCountry
    addressError.isVisible = viewState.wrongAddress
    renderOrder(order)
    }
    }
    Activity
    

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36. private fun render(viewState: Address) {
    if (viewState.isCompleted){
    renderCompleted()
    else {
    progress.isVisible = viewState.isLoading
    viewState.exception?.let{
    renderException(it)
    }
    cityError.isVisible = viewState.wrongCity
    countryError.isVisible = viewState.wrongCountry
    addressError.isVisible = viewState.wrongAddress
    renderOrder(order)
    }
    }
    Activity
    

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37. private fun render(viewState: Address) {
    if (viewState.isCompleted){
    renderCompleted()
    else {
    progress.isVisible = viewState.isLoading
    viewState.exception?.let{
    renderException(it)
    }
    cityError.isVisible = viewState.wrongCity
    countryError.isVisible = viewState.wrongCountry
    addressError.isVisible = viewState.wrongAddress
    renderOrder(order)
    }
    }
    

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38. private fun render(viewState: Address) {
    if (viewState.isCompleted){
    renderCompleted()
    else {
    progress.isVisible = viewState.isLoading
    viewState.exception?.let{
    renderException(it)
    }
    cityError.isVisible = viewState.wrongCity
    countryError.isVisible = viewState.wrongCountry
    addressError.isVisible = viewState.wrongAddress
    renderOrder(order)
    }
    }
    

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39. renderCompleted()
    else {
    progress.isVisible = viewState.isLoading
    viewState.exception?.let{
    renderException(it)
    }
    cityError.isVisible = viewState.wrongCity
    countryError.isVisible = viewState.wrongCountry
    addressError.isVisible = viewState.wrongAddress
    renderOrder(order)
    }
    }
    

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40. progress.isVisible = viewState.isLoading
    viewState.exception?.let{
    renderException(it)
    }
    cityError.isVisible = viewState.wrongCity
    countryError.isVisible = viewState.wrongCountry
    addressError.isVisible = viewState.wrongAddress
    renderOrder(order)
    }
    }
    

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41. sealed class ChangeAddressViewAction {
    object TalkToUs : ChangeAddressViewAction()
    data class LoadOrder(val orderId: String): ChangeAddressViewAction()
    data class ChangeAddress(
    val orderId: String,
    val shippingAddress: ShippingAddress): ChangeAddressViewAction()
    }
    Action
    

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42. fun interpret(action: ChangeAddressViewAction) {
    when (action) {
    is LoadOrder -> loadOrder(action.orderId)
    is ChangeAddress -> changeAddress(action)
    is TalkToUs -> navigator.toChat()
    }
    }
    private fun changeAddress(orderId: String, address: ShippingAddress) {
    store.dispatchState { State { Loading } }
    store.dispatchState (changeOrderAddress(orderId, shippingAddress))
    }
    ViewModel
    

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43. when (action) {
    is LoadOrder -> loadOrder(action.orderId)
    is ChangeAddress -> changeAddress(action)
    is TalkToUs -> navigator.toChat()
    }
    }
    private fun changeAddress(orderId: String, address: ShippingAddress) {
    store.dispatchState { State { Loading } }
    store.dispatchState { changeOrderAddress(orderId, shippingAddress)}
    }
    

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44. Moar problems…
    Navigation Event vs View State
    ViewModel is calling store twice, once per state
    

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45. sealed class Either {
    // Represents the left side of Either class
    // which by convention is a "Failure"
    data class Left(val a: L) : Either()
    //Represents the right side of Either class
    //which by convention is a "Success"
    data class Right(val b: R) : Either()
    }
    https://arrow-kt.io/docs/arrow/core/either/
    

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46. sealed class Failure {
    // App wide failures
    data class ServerError(val error: String) : Failure()
    object NetworkConnection : Failure()
    data class GenericError(val exception: Exception) : Failure()
    // Extend this class for feature specific failures
    abstract class FeatureFailure : Failure()
    }
    

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47. sealed class Success {
    // Extend this classes for feature specific success
    abstract class ViewState : Success()
    abstract class ViewEvent : Success()
    // App wide success view states
    object Idle : ViewState()
    object Loading : ViewState()
    }
    

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48. class Store(initialState: ViewState) : CoroutineScope {
    val stateLiveData = MutableLiveData>()
    .apply { value = initialState }
    fun dispatchState(f: suspend (Either)
    -> State>) {
    launch {
    val reducer = f(state())
    withContext(Dispatchers.Main) {
    dispatchState(reducer(state()))
    }
    

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49. class Store(initialState: ViewState) : CoroutineScope {
    val stateLiveData = MutableLiveData>()
    .apply { value = initialState }
    fun dispatchState(f: suspend (Either)
    -> State>) {
    launch {
    val reducer = f(state())
    withContext(Dispatchers.Main) {
    dispatchState(reducer(state()))
    }
    

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50. class Store(initialState: ViewState) : CoroutineScope {
    val stateLiveData = MutableLiveData>()
    .apply { value = initialState }
    fun dispatchState(f: suspend (Either)
    -> State>) {
    launch {
    val reducer = f(state())
    withContext(Dispatchers.Main) {
    dispatchState(reducer(state()))
    }
    

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51. val stateLiveData = MutableLiveData>()
    .apply { value = initialState }
    fun dispatchState(f: suspend (Either)
    -> State>) {
    launch {
    val reducer = f(state())
    withContext(Dispatchers.Main) {
    dispatchState(reducer(state()))
    }
    

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52. open class MainViewModel : ViewModel() {
    private val successLiveData = LiveData()
    private val failureLiveData = LiveData()
    fun observe(owner: LifecycleOwner,
    successObserver: (Success) -> Unit,
    failureObserver: (Failure) -> Unit) {
    successLiveData.observe(owner,
    Observer { successObserver(it!!) })
    failureLiveData.observe(owner,
    Observer { failureObserver(it) })
    store.observeState(owner) {
    when (it) {
    

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53. open class MainViewModel : ViewModel() {
    private val successLiveData = LiveData()
    private val failureLiveData = LiveData()
    fun observe(owner: LifecycleOwner,
    successObserver: (Success) -> Unit,
    failureObserver: (Failure) -> Unit) {
    successLiveData.observe(owner,
    Observer { successObserver(it!!) })
    failureLiveData.observe(owner,
    Observer { failureObserver(it) })
    store.observeState(owner) {
    when (it) {
    

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54. open class MainViewModel : ViewModel() {
    private val successLiveData = LiveData()
    private val failureLiveData = LiveData()
    fun observe(owner: LifecycleOwner,
    successObserver: (Success) -> Unit,
    failureObserver: (Failure) -> Unit) {
    successLiveData.observe(owner,
    Observer { successObserver(it!!) })
    failureLiveData.observe(owner,
    Observer { failureObserver(it) })
    store.observeState(owner) {
    when (it) {
    is Either.Left -> failureLiveData.value = it.a
    is Either.Right -> successLiveData.value = it.b
    

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55. private val failureLiveData = LiveData()
    fun observe(owner: LifecycleOwner,
    successObserver: (Success) -> Unit,
    failureObserver: (Failure) -> Unit) {
    successLiveData.observe(owner,
    Observer { successObserver(it!!) })
    failureLiveData.observe(owner,
    Observer { failureObserver(it) })
    store.observeState(owner) {
    when (it) {
    is Either.Left -> failureLiveData.value = it.a
    is Either.Right -> successLiveData.value = it.b
    }
    }
    

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56. failureObserver: (Failure) -> Unit) {
    successLiveData.observe(owner,
    Observer { successObserver(it!!) })
    failureLiveData.observe(owner,
    Observer { failureObserver(it) })
    store.observeState(owner) {
    when (it) {
    is Either.Left -> failureLiveData.value = it.a
    is Either.Right -> successLiveData.value = it.b
    }
    }
    

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57. data class
    Address(val order: Order = Order.empty(),
    val wrongCity: Boolean,
    val wrongCountry: Boolean,
    val wrongAddress: Boolean,
    val isLoading: Boolean,
    val isCompleted: Boolean,
    val exception: Exception) : ViewState()
    }
    

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58. data class
    Address(val order: Order = Order.empty()) : Success.ViewState()
    object TalkToUs : Success.ViewEvent()
    object AddressChanged : Success.ViewEvent()
    data class InvalidAddress(val wrongCity: Boolean,
    val wrongCountry: Boolean,
    val wrongAddress: Boolean) : FeatureFailure()
    

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59. viewModel = ViewModelProviders.of(this,
    viewModelFactory)[MyViewModel::class.java]
    viewModel.observe(this, ::renderSuccess, ::renderFailure)
    protected fun renderSuccess(success: Success) {
    when (success) {
    is Success.ViewEvent -> reactToEvent(success)
    is Success.ViewState -> renderViewState(success)
    }
    }
    override fun renderViewState(viewState: Success.ViewState) {
    when (viewState) {
    Activity
    

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60. viewModel = ViewModelProviders.of(this,
    viewModelFactory)[MyViewModel::class.java]
    viewModel.observe(this, ::renderSuccess, ::renderFailure)
    protected fun renderSuccess(success: Success) {
    when (success) {
    is Success.ViewEvent -> reactToEvent(success)
    is Success.ViewState -> renderViewState(success)
    }
    }
    override fun renderViewState(viewState: Success.ViewState) {
    when (viewState) {
    is Orders -> renderOrders(viewState)
    }
    }
    

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61. when (success) {
    is Success.ViewEvent -> reactToEvent(success)
    is Success.ViewState -> renderViewState(success)
    }
    }
    override fun renderViewState(viewState: Success.ViewState) {
    when (viewState) {
    is Orders -> renderOrders(viewState)
    }
    }
    override fun reactToEvent(event: Success.ViewEvent) {
    when (event) {
    is Success.TalkToUs -> navigator.toSupportChat()
    is SelectAddress -> navigator.toChangeAddress(this, event.orderId)
    is OrderCancelled -> displaySnackbar(event.failureMessage)
    is CancelOrder -> navigator.toCancelOrder(this, event.orderId)
    

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62. override fun renderViewState(viewState: Success.ViewState) {
    when (viewState) {
    is Orders -> renderOrders(viewState)
    }
    }
    override fun reactToEvent(event: Success.ViewEvent) {
    when (event) {
    is Success.TalkToUs -> navigator.toSupportChat()
    is SelectAddress -> navigator.toChangeAddress(this, event.orderId)
    is OrderCancelled -> displaySnackbar(event.failureMessage)
    is CancelOrder -> navigator.toCancelOrder(this, event.orderId)
    is TrackShipment -> navigator.toTrackingShipment(event.url)
    }
    }
    override fun renderFailure(failure: Failure) {
    ordersList.gone()
    

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63. is Success.TalkToUs -> navigator.toSupportChat()
    is SelectAddress -> navigator.toChangeAddress(this, event.orderId)
    is OrderCancelled -> displaySnackbar(event.failureMessage)
    is CancelOrder -> navigator.toCancelOrder(this, event.orderId)
    is TrackShipment -> navigator.toTrackingShipment(event.url)
    }
    }
    override fun renderFailure(failure: Failure) {
    ordersList.gone()
    loading.gone()
    empty.show()
    }
    

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64. The final step…
    

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66. public interface SendChannel {
    public suspend fun send(element: E)
    public fun offer(element: E)
    public fun close(cause: Throwable? = null): Boolean
    }
    public interface ReceiveChannel {
    public suspend fun receive(): E
    public fun close(cause: Throwable? = null): Boolean
    }
    Deferred vs Channel
    

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67. fun main() = runBlocking {
    val channel = Channel()
    launch {
    for (i in 1..5) {
    channel.send(i)
    }
    launch {
    for (value in channel) {
    println("received: $value")
    }
    //console output
    received: 1
    received: 2
    received: 3
    received: 4
    received: 5
    Channel
    

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68. fun main() = runBlocking {
    val channel = Channel()
    launch {
    for (i in 1..5) {
    channel.send(i)
    }
    launch {
    for (value in channel) {
    println("received: $value")
    }
    //console output
    received: 1
    received: 2
    received: 3
    received: 4
    received: 5
    Channel
    

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69. fun main() = runBlocking {
    val channel = Channel()
    launch {
    for (i in 1..5) {
    channel.send(i)
    }
    launch {
    for (value in channel) {
    println("received: $value")
    }
    //console output
    received: 1
    received: 2
    received: 3
    received: 4
    received: 5
    Channel
    

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70. fun main() = runBlocking {
    val channel = Channel()
    launch {
    for (i in 1..5) {
    channel.send(i)
    }
    launch {
    for (value in channel) {
    println("received: $value")
    }
    //console output
    received: 1
    received: 2
    received: 3
    received: 4
    received: 5
    Channel
    

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71. 

    public fun CoroutineScope.produce(
    context: CoroutineContext = EmptyCoroutineContext,
    capacity: Int = 0,
    block: suspend ProducerScope.() -> Unit
    ): ReceiveChannel
    Producer
    

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72. val publisher = produce(capacity = 2){
    for (i in 1..5){
    send(i)
    }
    launch {
    publisher.consumeEach {
    println("received: $it”)
    }
    }
    //console output
    received: 1
    received: 2
    received: 3
    received: 4
    received: 5
    Producer
    

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73. val publisher = produce(capacity = 2){
    for (i in 1..5){
    send(i)
    }
    launch {
    publisher.consumeEach {
    println("received: $it”)
    }
    }
    //console output
    received: 1
    received: 2
    received: 3
    received: 4
    received: 5
    Producer
    

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74. val publisher = produce(capacity = 2){
    for (i in 1..5){
    send(i)
    }
    launch {
    publisher.consumeEach {
    println("received: $it”)
    }
    }
    //console output
    received: 1
    received: 2
    received: 3
    received: 4
    received: 5
    Producer
    

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75. val publisher = produce(capacity = 2){
    for (i in 1..5){
    send(i)
    }
    launch {
    publisher.consumeEach {
    println("received: $it”)
    }
    }
    //console output
    received: 1
    received: 2
    received: 3
    received: 4
    received: 5
    Producer
    

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76. open class Reducer {
    suspend fun ProducerScope>.send(f: T.() -> T)
    = send(State(f))
    fun produceStates(f: suspend ProducerScope>.() -> Unit)
    : ReceiveChannel> =
    GlobalScope.produce(block = f)
    }
    

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77. open class Reducer {
    suspend fun ProducerScope>.send(f: T.() -> T)
    = send(State(f))
    fun produceStates(f: suspend ProducerScope>.() -> Unit)
    : ReceiveChannel> =
    GlobalScope.produce(block = f)
    }
    

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78. fun dispatchStates(channel: ReceiveChannel>>) {
    launch {
    channel.consumeEach { reducer ->
    withContext(Dispatchers.Main) {
    dispatchState(reducer(state()))
    }
    }
    }
    }
    fun loadOrders() {
    store.dispatchStates(getOrders())
    }
    Store
    

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79. fun dispatchStates(channel: ReceiveChannel>>) {
    launch {
    channel.consumeEach { reducer ->
    withContext(Dispatchers.Main) {
    dispatchState(reducer(state()))
    }
    }
    }
    }
    fun loadOrders() {
    store.dispatchStates(getOrders())
    }
    Store
    

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80. launch {
    channel.consumeEach { action ->
    withContext(Dispatchers.Main) {
    dispatchState(action(state()))
    }
    }
    }
    }
    fun loadOrders() {
    store.dispatchStates(getOrders())
    }
    

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81. class GetOrders
    @Inject constructor(private val repository: OrdersRepository)
    : Reducer() {
    operator fun invoke() = getOrders()
    private fun getOrders()
    : ReceiveChannel>> = produceActions
    {
    try {
    send { Either.Right(Success.Loading) }
    val orders = repository.allOrders()
    send { Either.Right(Orders(orders)) }
    } catch (e: Exception) {
    GetOrders
    

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82. @Inject constructor(private val repository: OrdersRepository)
    : Reducer() {
    operator fun invoke() = getOrders()
    private fun getOrders()
    : ReceiveChannel>> = produceStates
    {
    try {
    send { Either.Right(Success.Loading) }
    val orders = repository.allOrders()
    send { Either.Right(Orders(orders)) }
    } catch (e: Exception) {
    send { Either.Left(OrdersFailure(e)) }
    }
    }
    

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83. private fun getOrders()
    : ReceiveChannel>> = produceStates
    {
    try {
    send { Either.Right(Success.Loading) }
    val orders = repository.allOrders()
    send { Either.Right(Orders(orders)) }
    } catch (e: Exception) {
    send { Either.Left(OrdersFailure(e)) }
    }
    }
    

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84. {
    try {
    send { Either.Right(Success.Loading) }
    val orders = repository.allOrders()
    send { Either.Right(Orders(orders)) }
    } catch (e: Exception) {
    send { Either.Left(OrdersFailure(e)) }
    }
    }
    

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85. Testing
    UI tests on Activity / Fragments
    

    Unit + Integration tests on ViewModels
    Unit + instrumentation tests on Reducers
    

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86. suspend inline fun ReceiveChannel>.states(initialState: T): List
    {
    return fold(emptyList()) { states, action ->
    states + action(states.lastOrNull() ?: initialState)
    }
    }
    ReducerTest
    

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87. @Test
    fun `changes state on successful loading`() {
    val expectedState = Orders(listOf(ORDER_1, ORDER_2))
    coEvery { ordersRepository.all() } returns listOf(ORDER_1, ORDER_2)
    val states = runBlocking {
    useCase.invoke().states(Either.Right(Success.Idle))
    }
    assert(states).hasSize(2)
    assert(states[0]).isEqualTo(Either.Right(Success.Loading))
    assert(states[1]).isEqualTo(Either.Right(expectedState))
    }
    ReducerTest
    

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88. @Test
    fun `changes state on error`() {
    val exception = IOException()
    coEvery { ordersRepository.allOrders() } throws exception
    val states = runBlocking {
    useCase.invoke().states(Either.Right(Success.Idle))
    }
    assert(states).hasSize(2)
    assert(states[0]).isEqualTo(Either.Right(Success.Loading))
    assert(states[1]).isEqualTo(Either.Left(OrdersFailure(exception)))
    }
    ReducerTest
    

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89. What’s next?
    Explore Data Binding - Watch Jose’s talk
    This API will become obsolete in future updates with
    introduction of lazy asynchronous streams
    (https://github.com/Kotlin/kotlinx.coroutines/issues/254).
    

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90. View Slide

91. Thank you!
    David González
    @dggonzalez
    malmstein@gmail.com
    

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92. app/build.gradle
    buildscript {
    repositories {
    jcenter()
    }
    dependencies {
    classpath 'com.android.tools.build:gradle:1.5.0'
    classpath 'com.getkeepsafe.dexcount:dexcount-gradle-plugin:0.2.1'
    }
    }
    ../build.gradle
    dependencies {
    compile 'com.android.support:herbs-v13:23.1.1'
    compile 'com.android.support:meat:23.1.1'
    compile ‘com.android.support:cookware-v7:23.1.1'
    compile ‘com.google.android.gms:play-services:8.4.0'
    }
    

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93. CookingMethod cookingMethod = new CookingMethod.Builder()
    .withFlavour(Flavours.MediumRare)
    .withSide(Sides.Fries)
    .fromButcher(Butchers.GingerPig)
    .build();
    private Steak from(CookingMethod cookingMethod){
    Steak steak = Steak.from(cookingMethod.getButcher());
    steak.addOil(new Oil());
    steak.addSalt(new Salt());
    steak.addPepper(new Pepper());
    return steak;
    }
    Steak.java
    

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