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

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

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View ViewModel Store Reducer Action State send interpret dispatch notify

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View ViewModel Action

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

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ViewModel Store Reducer Action State

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

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

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

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

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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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The final step…

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

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

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

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

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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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@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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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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{ 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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Testing UI tests on Activity / Fragments 
 Unit + Integration tests on ViewModels Unit + instrumentation tests on Reducers

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

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

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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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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