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Riding the State Flow AndroidMakers

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! Software Eng @ BlaBlaCar Mobile First Kotlin Open Source Arnaud GIULIANI

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Building mobile applications…

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View Model Presenter ViewModel …

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business logic & data layers (database, networking…) Model Presenter ViewModel … Request data to Model and prepare it for UI

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Why are we still having issues with MVP, MVVM?

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View, State, Action … … to help us solve them

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MVP

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View: display data, notified by the presenter & notify for actions Presenter: manage UI logic, data & interactions View Presenter 1 .. 1 - Contract MVP

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Problems with MVP ⚠

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Tight coupling logic View/Presenter Data inconsistency Not reactive approach?

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MVVM

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View: observe streams to display data & uses actions ViewModel: exposes actions & expose streams of data View ViewModel actions data MVVM

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From MVP to MVVM

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Architecture Components Lifecycle ViewModel LiveData

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Just use streams, what else?

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Still got some problems ⚠

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View & business logic coupling Data streams inconsistency

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Why is it hard to write? UI binding with incomplete data? Bad separation of concerns? is MVVM less formal?

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Let’s think states over interactions

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Unidirectional data flow ♻

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•- the state change is passed to the view •- actions can update the state •- actions are triggered by the view •- state is passed to the view * aka Redux

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Single source of truth Immutable data Testability, Logging & Replayability

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Redux-ing UI bugs - Christina Lee @RunChristinaRun

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MVI - Mosby http:/ /hannesdorfmann.com/mosby/

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@dggonzalez

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@dggonzalez

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…well

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View ViewModel Trigger action Dispatch action Action Reducer State Store Run action & get new state Apply & Store new state Notify new state Notify state

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View ViewModel Trigger action Dispatch action Action Reducer State Store Run action & Got event No state Notify new event Notify event

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Can we avoid to write Reducer & Store?

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Just focus on writing Actions?

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ViewModel Event State View action State - UI State (Main Data) Event - Side events

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ViewModel offers Actions

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Action emits States

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Testing—action’s sequence of states & events

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Coroutines inside ✅

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Simplify Async Code

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View ViewModel Trigger action Dispatch action Action Reducer State Store Run action on current state (Actor & FlowAction) Apply & Store new state (LiveData) Notify new state Notify state

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View AndroidDataFlow Trigger action Dispatch action Action Reducer State Store Run action on current state (Actor & FlowAction) Apply & Store new state (LiveData) Notify new state Notify state Provided by Uniflow

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Let’s go

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#1 - A simple “Action”

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#1 - A simple “Action”

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Day Weather Icon Weather Description Temperature Humidity Wind

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Day Weather Icon Weather Description Temperature Humidity Wind Empty WeatherDetail

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Immutable Data

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object class Empty() data class WeatherDetail(val day: String, val icon: String, val description: String, val wind: String, val temperature: String, val humidity: String)

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object class Empty() : UIState() data class WeatherDetail(val day: String, val icon: String, val description: String, val wind: String, val temperature: String, val humidity: String) : UIState()

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sealed class GetWeatherState : UIState(){ object class Empty() : GetWeatherState() data class WeatherDetail(val day: String, val icon: String, val description: String, val wind: String, val temperature: String, val humidity: String) : GetWeatherState() }

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sealed class GetWeatherState : UIState(){ object class Empty() : GetWeatherState() data class WeatherDetail(val day: String, val icon: String, val description: String, val wind: String, val temperature: String, val humidity: String) : GetWeatherState() }

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ViewModel

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class WeatherDetailViewModel( ) : ViewModel() { }

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class WeatherDetailViewModel( ) : ViewModel() { }

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : ViewModel() { }

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : ViewModel() { fun getDetail() { } }

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Action as function

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : ViewModel() { fun getDetail() { } }

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow() { fun getDetail() { } } UDF ViewModel

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow() { fun getDetail() = action { } }

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow() { fun getDetail() = action { } } Emit states & events ♻

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow() { fun getDetail() = action { } } Run coroutines on IO ⚡

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow() { fun getDetail() = action { currentState : UIState -> } } Run on current state ♻

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow() { fun getDetail() = action { val dailyForecast = repo.getDailyForecast(id) setState { dailyForecast.mapToWeatherState() } } }

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow() { fun getDetail() = action { val dailyForecast = repo.getDailyForecast(id) setState { dailyForecast.mapToWeatherState() } } } async call ⚡

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow() { fun getDetail() = action { val dailyForecast = repo.getDailyForecast(id) setState { dailyForecast.mapToWeatherState() } } }

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow() { fun getDetail() = action { val dailyForecast = repo.getDailyForecast(id) setState { dailyForecast.mapToWeatherState() } } } emit new state ♻

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow() { fun getDetail() = action { val dailyForecast = repo.getDailyForecast(id) setState { dailyForecast.mapToWeatherState() } } }

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow() { fun getDetail() = action { val dailyForecast = repo.getDailyForecast(id) setState { dailyForecast.mapToWeatherState() } } } fun DailyForecast.mapToWeatherState() = WeatherDetail(...)

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow() { fun getDetail() = action { val dailyForecast = repo.getDailyForecast(id) setState { dailyForecast.mapToWeatherState() } } }

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow(defaultState = Empty) { fun getDetail() = action { val dailyForecast = repo.getDailyForecast(id) setState { dailyForecast.mapToWeatherState() } } }

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Error Handling

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object class Empty() : UIState() data class Failed(val message: String, val error: Exception) : UIState() data class WeatherDetail(val day: String, val icon: String, val description: String, val wind: String, val temperature: String, val humidity: String) : UIState()

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object class Empty() : UIState() data class Failed(val message: String, val error: Exception) : UIState() data class WeatherDetail(val day: String, val icon: String, val description: String, val wind: String, val temperature: String, val humidity: String) : UIState()

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow() { fun getDetail() = action { val dailyForecast = repo.getDailyForecast(id) setState { dailyForecast.mapToWeatherState() } } }

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow() { fun getDetail() = action { try { val dailyForecast = repo.getDailyForecast(id) setState { dailyForecast.mapToWeatherState() } } catch (error: Exception) { setState { Failed(“got error”, error) } } } }

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow() { fun getDetail() = action { val dailyForecast = repo.getDailyForecast(id) setState { dailyForecast.mapToWeatherState() } } }

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow() { fun getDetail() = action( onAction = { val dailyForecast = repo.getDailyForecast(id) setState { dailyForecast.mapToWeatherState() } } ) }

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow() { fun getDetail() = action( onAction = { val dailyForecast = repo.getDailyForecast(id) setState { dailyForecast.mapToWeatherState() } }, onError = { error , state -> setState { Failed("got error", error) } } ) }

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow() { fun getDetail() = action( onAction = { val dailyForecast = repo.getDailyForecast(id) setState { dailyForecast.mapToWeatherState() } }, onError = { error , state -> setState { Failed("got error", error) } } ) }

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UI Binding

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class WeatherDetailActivity : AppCompatActivity() { private val detailViewModel: DetailViewModel by viewModel() override fun onCreate(savedInstanceState: Bundle?) { //... onStates(detailViewModel) { state -> when (state) { is Empty -> {} // do nothing is WeatherDetail -> showDetail(state) is Failed -> showError(state.error) } } detailViewModel.getDetail() }

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class WeatherDetailActivity : AppCompatActivity() { private val detailViewModel: DetailViewModel by viewModel() override fun onCreate(savedInstanceState: Bundle?) { //... onStates(detailViewModel) { state -> when (state) { is Empty -> {} // do nothing is WeatherDetail -> showDetail(state) is Failed -> showError(state.error) } } detailViewModel.getDetail() }

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class WeatherDetailActivity : AppCompatActivity() { private val detailViewModel: DetailViewModel by viewModel() override fun onCreate(savedInstanceState: Bundle?) { //... onStates(detailViewModel) { state -> when (state) { is Empty -> {} // do nothing is WeatherDetail -> showDetail(state) is Failed -> showError(state.error) } } detailViewModel.getDetail() }

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class WeatherDetailActivity : AppCompatActivity() { private val detailViewModel: DetailViewModel by viewModel() override fun onCreate(savedInstanceState: Bundle?) { //... onStates(detailViewModel) { state -> when (state) { is Empty -> {} // do nothing is WeatherDetail -> showDetail(state) is Failed -> showError(state.error) } } detailViewModel.getDetail() }

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class WeatherDetailActivity : AppCompatActivity() { private val detailViewModel: DetailViewModel by viewModel() override fun onCreate(savedInstanceState: Bundle?) { //... onStates(detailViewModel) { state -> when (state) { is Empty -> {} // do nothing is WeatherDetail -> showDetail(state) is Failed -> showError(state.error) } } detailViewModel.getDetail() }

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class WeatherDetailActivity : AppCompatActivity() { private val detailViewModel: DetailViewModel by viewModel() override fun onCreate(savedInstanceState: Bundle?) { //... onStates(detailViewModel) { state -> when (state) { is Empty -> {} // do nothing is WeatherDetail -> showDetail(state) is Failed -> showError(state.error) } } detailViewModel.getDetail() }

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class WeatherDetailActivity : AppCompatActivity() { private val detailViewModel: DetailViewModel by viewModel() override fun onCreate(savedInstanceState: Bundle?) { //... onEvents(detailViewModel) { event -> when (event) { } } detailViewModel.getDetail() }

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Testing

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@Test fun `get a daily forecast`() { // prepare detailViewModel = DetailViewModel(id, repo) view = detailViewModel.mockObservers() // mocks val weather = MockedData.dailyForecasts.first() coEvery { repo.getDailyForecast(id) } returns weather // call detailViewModel.getDetail() // sequence of states & events verifySequence { view.states.onChanged(UIState.Empty) view.states.onChanged(weather.mapToDetailState()) } }

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@Test fun `get a daily forecast`() { // prepare detailViewModel = DetailViewModel(id, repo) view = detailViewModel.mockObservers() // mocks val weather = MockedData.dailyForecasts.first() coEvery { repo.getDailyForecast(id) } returns weather // call detailViewModel.getDetail() // sequence of states & events verifySequence { view.states.onChanged(UIState.Empty) view.states.onChanged(weather.mapToDetailState()) } }

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@Test fun `get a daily forecast`() { // prepare detailViewModel = DetailViewModel(id, repo) view = detailViewModel.mockObservers() // mocks val weather = MockedData.dailyForecasts.first() coEvery { repo.getDailyForecast(id) } returns weather // call detailViewModel.getDetail() // sequence of states & events verifySequence { view.states.onChanged(UIState.Empty) view.states.onChanged(weather.mapToDetailState()) } }

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@Test fun `get a daily forecast`() { // prepare detailViewModel = DetailViewModel(id, repo) view = detailViewModel.mockObservers() // mocks val weather = MockedData.dailyForecasts.first() coEvery { repo.getDailyForecast(id) } returns weather // call detailViewModel.getDetail() // sequence of states & events verifySequence { view.states.onChanged(UIState.Empty) view.states.onChanged(weather.mapToDetailState()) } }

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@Test fun `get a daily forecast`() { // prepare detailViewModel = DetailViewModel(id, repo) view = detailViewModel.mockObservers() // mocks val weather = MockedData.dailyForecasts.first() coEvery { repo.getDailyForecast(id) } returns weather // call detailViewModel.getDetail() // sequence of states & events verifySequence { view.states.onChanged(UIState.Empty) view.states.onChanged(weather.mapToDetailState()) } }

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@Test fun `get a daily forecast`() { // prepare detailViewModel = DetailViewModel(id, repo) view = detailViewModel.mockObservers() // mocks val weather = MockedData.dailyForecasts.first() coEvery { repo.getDailyForecast(id) } returns weather // call detailViewModel.getDetail() // sequence of states & events verifySequence { view.states.onChanged(Empty) view.states.onChanged(weather.mapToDetailState()) } }

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One Coding Convention

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fun myAction() = action { }

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View AndroidDataFlow Trigger action Dispatch action Action Reducer State Store Run action on current state (Actor & FlowAction) Apply & Store new state (LiveData) Notify new state Notify state Provided by Uniflow

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#2 - State Guard

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#2 - State Guard

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NoSuggestion NewSuggestions The user has no company or n/a The user can select a company (can’t select anything) (select an existing or provide a new one)

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One View - Multiple States

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sealed class SelectCompanyState : UIState() { object Empty : SelectCompanyState() data class CompanyList(val list: List) : SelectCompanyState() data class CompanySelected(val selectedCompany: CompanyItem) : SelectCompanyState() object NoCompanySelected : SelectCompanyState() }

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sealed class SelectCompanyState : UIState() { object Empty : SelectCompanyState() data class CompanyList(val list: List) : SelectCompanyState() data class CompanySelected(val selectedCompany: CompanyItem) : SelectCompanyState() object NoCompanySelected : SelectCompanyState() }

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sealed class SelectCompanyState : UIState() { object Empty : SelectCompanyState() data class CompanyList(val list: List) : SelectCompanyState() data class CompanySelected(val selectedCompany: CompanyItem) : SelectCompanyState() object NoCompanySelected : SelectCompanyState() }

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sealed class SelectCompanyState : UIState() { object Empty : SelectCompanyState() data class CompanyList(val list: List) : SelectCompanyState() data class CompanySelected(val selectedCompany: CompanyItem) : SelectCompanyState() object NoCompanySelected : SelectCompanyState() }

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sealed class SelectCompanyState : UIState() { object Empty : SelectCompanyState() data class CompanyList(val list: List) : SelectCompanyState() data class CompanySelected(val selectedCompany: CompanyItem) : SelectCompanyState() object NoCompanySelected : SelectCompanyState() }

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class SelectCompanyViewModel( private val repo: CompanySelectionRepository ) : AndroidDataFlow(defaultState = Empty) { // Search for campanies fun searchForName(name: String) = action { // look for new suggestions ... val suggestions = repo.searchForName(name) setState { CompanyList(suggestions.mapToItems()) } } // Select No Company fun selectNoCompany() = singleActionOn { repo.setNoCompany() setState { NoCompanySelected } } // Select a Company fun selectCompany(item: CompanyItem) = singleActionOn {

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class SelectCompanyViewModel( private val repo: CompanySelectionRepository ) : AndroidDataFlow(defaultState = Empty) { // Search for campanies fun searchForName(name: String) = action { // look for new suggestions ... val suggestions = repo.searchForName(name) setState { CompanyList(suggestions.mapToItems()) } } // Select No Company fun selectNoCompany() = singleActionOn { repo.setNoCompany() setState { NoCompanySelected } } // Select a Company fun selectCompany(item: CompanyItem) = singleActionOn {

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class SelectCompanyViewModel( private val repo: CompanySelectionRepository ) : AndroidDataFlow(defaultState = Empty) { // Search for campanies fun searchForName(name: String) = action { // look for new suggestions ... val suggestions = repo.searchForName(name) setState { CompanyList(suggestions.mapToItems()) } } // Select No Company fun selectNoCompany() = singleActionOn { repo.setNoCompany() setState { NoCompanySelected } } // Select a Company fun selectCompany(item: CompanyItem) = singleActionOn {

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class SelectCompanyViewModel( private val repo: CompanySelectionRepository ) : AndroidDataFlow(defaultState = Empty) { // Search for campanies fun searchForName(name: String) = action { // look for new suggestions ... val suggestions = repo.searchForName(name) setState { CompanyList(suggestions.mapToItems()) } } // Select No Company fun selectNoCompany() = singleActionOn { repo.setNoCompany() setState { NoCompanySelected } } // Select a Company fun selectCompany(item: CompanyItem) = singleActionOn {

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) : AndroidDataFlow(default state = Empty) { // Search for campanies fun searchForName(name: String) = action { // look for new suggestions ... val suggestions = repo.searchForName(name) setState { CompanyList(suggestions.mapToItems()) } } // Select No Company fun selectNoCompany() = actionOn { repo.setNoCompany() setState { NoCompanySelected } } // Select a Company fun selectCompany(item: CompanyItem) = actionOn { repo.setCompany()(item) setState { CompanySelected(item) } }

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) : AndroidDataFlow(default state = Empty) { // Search for campanies fun searchForName(name: String) = action { // look for new suggestions ... val suggestions = repo.searchForName(name) setState { CompanyList(suggestions.mapToItems()) } } // Select No Company fun selectNoCompany() = actionOn { repo.setNoCompany() setState { NoCompanySelected } } // Select a Company fun selectCompany(item: CompanyItem) = actionOn { repo.setCompany()(item) setState { CompanySelected(item) } } Guard on state

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) : AndroidDataFlow(default state = Empty) { // Search for campanies fun searchForName(name: String) = action { // look for new suggestions ... val suggestions = repo.searchForName(name) setState { CompanyList(suggestions.mapToItems()) } } // Select No Company fun selectNoCompany() = actionOn { currentState: Empty -> repo.setNoCompany() setState { NoCompanySelected } } // Select a Company fun selectCompany(item: CompanyItem) = actionOn { repo.setCompany()(item) setState { CompanySelected(item) } } current state is

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val suggestions = repo.searchForName(name) setState { CompanyList(suggestions.mapToItems()) } } // Select No Company fun selectNoCompany() = actionOn { repo.setNoCompany() setState { NoCompanySelected } } // Select a Company fun selectCompany(item: CompanyItem) = actionOn { repo.setCompany(item) setState { CompanySelected(item) } }

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val suggestions = repo.searchForName(name) setState { CompanyList(suggestions.mapToItems()) } } // Select No Company fun selectNoCompany() = actionOn { repo.setNoCompany() setState { NoCompanySelected } } // Select a Company fun selectCompany(item: CompanyItem) = actionOn { repo.setCompany(item) setState { CompanySelected(item) } } Guard on state

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View ViewModel Trigger action Dispatch action Action Reducer State Store Run action on current state State is Wrong. Notify with BadOrWrongState event Notify BoW Notify BoW

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@Test fun `select a company`() { // mock ... viewModel = SelectCompanyViewModel(repo) // call viewModel.selectCompany(mockItem) // sequence of states & events verifySequence { view.states.onChanged(Empty) view.events.onChanged(BadOrWrongState(state = Empty)) } }

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@Test fun `select a company`() { // mock ... viewModel = SelectCompanyViewModel(repo) // call viewModel.selectCompany(mockItem) // sequence of states & events verifySequence { view.states.onChanged(Empty) view.events.onChanged(BadOrWrongState(state = Empty)) } }

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NoSuggestion NewSuggestions

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#3 - Rooting States & Views

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Activity -> View rooting Shared ViewModel - ViewModelStoreOwner

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One stream -> Multiple Views

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WeatherActivity WeatherHeader Fragment WeatherList Fragment ViewModel Weather State Create Create Weather State Weather State ⚠ no arg passed

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onStates(viewModel) { state -> when (state) { is Loading -> { // show loading animation } is WeatherState -> { // create Fragments } is Failed -> { // display error } } } viewModel.getWeather() onStates(viewModel) { state -> when (state) { is WeatherState -> { // display data } } }

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One stream -> Multiple Views*

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No content

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NewCardActivity NewCardViewModel StartFragment Start State Start State

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NewCardActivity NewCardViewModel EditCardFragment EditCard State EditCard State

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NewCardActivity NewCardViewModel VerifyCardFragment VerifyCard State VerifyCard State

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NewCardActivity NewCardViewModel CheckedCardFragment CheckedCard State CheckedCard State

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onStates(viewModel) { state -> when (state) { is StartState -> openFragment() is EditCardState -> openFragment() is VerifyCardState -> openFragment() is CheckedCardState -> openFragment() } } viewModel.startAddCardFlow() onStates(viewModel) { state -> when (state) { is -> { // display data } } }

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#4 - Safe & Functional

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No content

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Better Unsafe Values Handling With Arrow.io

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow() { fun getDetail() = action { val dailyForecast = repo.getDailyForecast(id) setState { dailyForecast.mapToWeatherState() } } }

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow() { fun getDetail() = action { val dailyForecast = repo.getDailyForecast(id) setState { dailyForecast.mapToWeatherState() } } }

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow() { fun getDetail() = action { val safeState = Either.catch { repo.getDailyForecast(id) } .map { it.mapToWeatherState() } .getOrHandle { error -> Failed(“got error", error) } setState { safeState } } }

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow() { fun getDetail() = action { val safeState = Either.catch { repo.getDailyForecast(id) } .map { it.mapToWeatherState() } .getOrHandle { error -> Failed(“got error", error) } setState { safeState } } }

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow() { fun getDetail() = action { val safeState = Either.catch { repo.getDailyForecast(id) } .map { it.mapToWeatherState() } .getOrHandle { error -> Failed("got error", error) } setState { safeState } } }

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow() { fun getDetail() = action { val safeState = Either.catch { repo.getDailyForecast(id) } .map { it.mapToWeatherState() } .getOrHandle { error -> Failed(“got error", error) } setState { safeState } } }

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class WeatherDetailViewModel( private val id: DailyForecastId, private val repo: WeatherRepository ) : AndroidDataFlow() { fun getDetail() = action { val safeState = Either.catch { repo.getDailyForecast(id) } .map { it.mapToWeatherState() } .getOrHandle { error -> Failed(“got error", error) } setState { safeState } } }

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Ready to ride the State Flow?

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Make your own Experience

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Think UI as States

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Unidirectional Data Flow

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Architecture — Decisions which are both important and hard to change Martin Fowler

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One way of thinking But many ways to write it

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~5 classes

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Uniflow https:/ /github.com/uniflow-kt/

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Arnaud Giuliani @arnogiu Thank you!

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Any question? 3 sli.do