A State Container Architecture for mobile applications

Transcript

1. A State Container based
   approach to
   Architecting Mobile
   applications
   David González
   @dggonzalez
   davidgonzalez@helpscout.com
   

   View Slide

2. Beacon Mobile SDK
   

   View Slide

3. Beacon Mobile SDK
   

   View Slide

4. Beacon Mobile SDK
   

   View Slide

5. Beacon Mobile SDK
   

   View Slide

6. Motivation
   

   View Slide

7. Page heading
   Motivation
   Explore a common (and
   shared) architecture
   language (and code)
   between iOS and Android
   

   View Slide

8. Page heading
   Three traits of good design
   Balanced
   distribution of
   responsibilities
   

   View Slide

9. Page heading
   Three traits of good design
   Testability
   

   View Slide

10. Page heading
    Three traits of good design
    Easy to use
    

    View Slide

11. Good code is the one
    that you can easily
    delete
    - David González
    

    View Slide

12. Page heading
    Motivation
    Greenfield
    

    View Slide

13. Page heading
    Motivation
    Brownfield
    

    View Slide

14. A good design is easier
    to change than a bad
    design
    - Dave Thomas
    

    View Slide

15. Beacon Mobile SDK
    

    View Slide

16. MVC
    

    View Slide

17. MVP
    

    View Slide

18. Page heading
    The assembly problem
    let viewController = UIViewController()
    let presenter = Presenter(viewController, model)
    viewController.presenter = presenter
    

    View Slide

19. MVVM
    

    View Slide

20. Conflictive states
    Invisible outputs
    - MVP: Command from
    presenter to View
    - MVVM: Observable property
    No traceability
    Hard to follow the course of events”
    - showLoading(), showData(),
    refresh(), showMoreData()
    

    View Slide

21. Page heading
    Motivation
    Null references
    The billion dollar mistake
    

    View Slide

22. A State Container
    approach
    

    View Slide

23. Page heading
    Implementation
    View State
    

    View Slide

24. Page heading
    ViewState
    sealed class ReplyViewState : ViewState() {
    data class Form(val formOptions: BeaconContactForm,
    val message: String,
    val attachments: Map,
    val formValid: Boolean,
    val draft: String) : ReplyViewState()
    object SendingReply : ReplyViewState()
    object ReplySent : ReplyViewState()
    class SendReplyError(error: Throwable) : ViewState.Error(error)
    }
    

    View Slide

25. Page heading
    ViewState
    sealed class ReplyViewState : ViewState() {
    data class Form(val formOptions: BeaconContactForm,
    val message: String,
    val attachments: Map,
    val formValid: Boolean,
    val draft: String) : ReplyViewState()
    object SendingReply : ReplyViewState()
    object ReplySent : ReplyViewState()
    class SendReplyError(error: Throwable) : ViewState.Error(error)
    }
    

    View Slide

26. Page heading
    ViewState
    sealed class ReplyViewState : ViewState() {
    data class Form(val formOptions: BeaconContactForm,
    val message: String,
    val attachments: Map,
    val formValid: Boolean,
    val draft: String) : ReplyViewState()
    object SendingReply : ReplyViewState()
    object ReplySent : ReplyViewState()
    class SendReplyError(error: Throwable) : ViewState.Error(error)
    }
    

    View Slide

27. Page heading
    ViewState
    sealed class ReplyViewState : ViewState() {
    data class Form(val formOptions: BeaconContactForm,
    val message: String,
    val attachments: Map,
    val formValid: Boolean,
    val draft: String) : ReplyViewState()
    object SendingReply : ReplyViewState()
    object ReplySent : ReplyViewState()
    class SendReplyError(error: Throwable) : ViewState.Error(error)
    }
    

    View Slide

28. Page heading
    ViewState
    sealed class ReplyViewState : ViewState() {
    data class Form(val formOptions: BeaconContactForm,
    val message: String,
    val attachments: Map,
    val formValid: Boolean,
    val draft: String) : ReplyViewState()
    object SendingReply : ReplyViewState()
    object ReplySent : ReplyViewState()
    class SendReplyError(error: Throwable) : ViewState.Error(error)
    }
    

    View Slide

29. Page heading
    Pattern matching
    override fun render(state: ViewState) {
    when (state) {
    is ReplyViewState.Form -> renderForm(state)
    is ReplyViewState.ReplySent -> renderReplySent()
    is ReplyViewState.MissingMessage -> renderMissingMessage()
    is ReplyViewState.SendingReply -> renderLoading()
    is ReplyViewState.SendReplyError -> renderError(state)
    }
    }
    

    View Slide

30. Page heading
    Pattern matching
    override fun render(state: ViewState) {
    when (state) {
    is ReplyViewState.Form -> renderForm(state)
    is ReplyViewState.ReplySent -> renderReplySent()
    is ReplyViewState.MissingMessage -> renderMissingMessage()
    is ReplyViewState.SendingReply -> renderLoading()
    is ReplyViewState.SendReplyError -> renderError(state)
    }
    }
    

    View Slide

31. Page heading
    Pattern matching
    override fun render(state: ViewState) {
    when (state) {
    is ReplyViewState.Form -> renderForm(state)
    is ReplyViewState.ReplySent -> renderReplySent()
    is ReplyViewState.MissingMessage -> renderMissingMessage()
    is ReplyViewState.SendingReply -> renderLoading()
    is ReplyViewState.SendReplyError -> renderError(state)
    }
    }
    

    View Slide

32. Page heading
    Pattern matching
    override fun render(state: ViewState) {
    when (state) {
    is ReplyViewState.Form -> renderForm(state)
    is ReplyViewState.ReplySent -> renderReplySent()
    is ReplyViewState.MissingMessage -> renderMissingMessage()
    is ReplyViewState.SendingReply -> renderLoading()
    is ReplyViewState.SendReplyError -> renderError(state)
    }
    }
    

    View Slide

33. Page heading
    Pattern matching
    override fun render(state: ViewState) {
    when (state) {
    is ReplyViewState.Form -> renderForm(state)
    is ReplyViewState.ReplySent -> renderReplySent()
    is ReplyViewState.MissingMessage -> renderMissingMessage()
    is ReplyViewState.SendingReply -> renderLoading()
    is ReplyViewState.SendReplyError -> renderError(state)
    }
    }
    

    View Slide

34. Page heading
    Pattern matching
    override fun render(state: ViewState) {
    when (state) {
    is ReplyViewState.Form -> renderForm(state)
    is ReplyViewState.ReplySent -> renderReplySent()
    is ReplyViewState.MissingMessage -> renderMissingMessage()
    is ReplyViewState.SendingReply -> renderLoading()
    is ReplyViewState.SendReplyError -> renderError(state)
    }
    }
    

    View Slide

35. Page heading
    Implementation
    UI as a State
    Machine
    

    View Slide

36. Page heading
    UI as a State Machine
    sealed class ViewState {
    object Idle : ViewState()
    object Loading : ViewState()
    object Empty : ViewState()
    open class Error(val exception: Throwable) : ViewState()
    fun toLoading(viewState: ViewState): ViewState {
    return when(viewState){
    is Loading -> Error(Throwable("Wrong state transition"))
    else -> Loading
    }
    }
    }
    

    View Slide

37. Page heading
    UI as a State Machine
    sealed class ViewState {
    object Idle : ViewState()
    object Loading : ViewState()
    object Empty : ViewState()
    open class Error(val exception: Throwable) : ViewState()
    fun toLoading(viewState: ViewState): ViewState {
    return when(viewState){
    is Loading -> Error(Throwable("Wrong state transition"))
    else -> Loading
    }
    }
    }
    

    View Slide

38. Page heading
    UI as a State Machine
    sealed class ViewState {
    object Idle : ViewState()
    object Loading : ViewState()
    object Empty : ViewState()
    open class Error(val exception: Throwable) : ViewState()
    fun toLoading(viewState: ViewState): ViewState {
    return when(viewState){
    is Loading -> Error(Throwable("Wrong state transition"))
    else -> Loading
    }
    }
    }
    

    View Slide

39. Page heading
    UI as a State Machine
    sealed class ViewState {
    object Idle : ViewState()
    object Loading : ViewState()
    object Empty : ViewState()
    open class Error(val exception: Throwable) : ViewState()
    fun toLoading(viewState: ViewState): ViewState {
    return when(viewState){
    is Loading -> Error(Throwable("Wrong state transition"))
    else -> Loading
    }
    }
    }
    

    View Slide

40. Page heading
    Implementation
    Action (Command)
    Identifies actions from UI layer
    

    View Slide

41. Page heading
    Implementation
    sealed class SendMessageAction : BeaconAction() {
    object Cleanup : SendMessageAction()
    object SelectAttachment : SendMessageAction()
    object InitialAction : SendMessageAction()
    object LoadForm : SendMessageAction()
    data class SendMessage(val formValues: FormValues) : SendMessageAction()
    data class ValidateForm(val formValues: FormValues) : SendMessageAction()
    }
    

    View Slide

42. Page heading
    Implementation
    sealed class SendMessageAction : BeaconAction() {
    object Cleanup : SendMessageAction()
    object SelectAttachment : SendMessageAction()
    object InitialAction : SendMessageAction()
    object LoadForm : SendMessageAction()
    data class SendMessage(val formValues: FormValues) : SendMessageAction()
    data class ValidateForm(val formValues: FormValues) : SendMessageAction()
    }
    

    View Slide

43. Page heading
    Implementation
    sealed class SendMessageAction : BeaconAction() {
    object Cleanup : SendMessageAction()
    object SelectAttachment : SendMessageAction()
    object InitialAction : SendMessageAction()
    object LoadForm : SendMessageAction()
    data class SendMessage(val formValues: FormValues) : SendMessageAction()
    data class ValidateForm(val formValues: FormValues) : SendMessageAction()
    }
    

    View Slide

44. Page heading
    Implementation
    ViewModel
    

    View Slide

45. Page heading
    ViewModel
    abstract class BaseViewModel : ViewModel() {
    val state = MediatorLiveData()
    protected fun stateDidChange(viewState: ViewState){
    state.value = viewState
    }
    abstract fun interpret(action: Action)
    }
    

    View Slide

46. Page heading
    ViewModel
    abstract class BaseViewModel : ViewModel() {
    val state = MediatorLiveData()
    protected fun stateDidChange(viewState: ViewState){
    state.value = viewState
    }
    abstract fun interpret(action: Action)
    }
    

    View Slide

47. Page heading
    ViewModel
    abstract class BaseViewModel : ViewModel() {
    val state = MediatorLiveData()
    protected fun stateDidChange(viewState: ViewState){
    state.value = viewState
    }
    abstract fun interpret(action: Action)
    }
    

    View Slide

48. Page heading
    ViewModel
    abstract class BaseViewModel : ViewModel() {
    val state = MediatorLiveData()
    protected fun stateDidChange(viewState: ViewState){
    state.value = viewState
    }
    abstract fun interpret(action: Action)
    }
    

    View Slide

49. Page heading
    ViewModel
    class MoviesViewModel: BaseViewModel() {
    init {
    stateDidChange(ViewState.Idle)
    …
    }
    }
    

    View Slide

50. Page heading
    Implementation
    The glue
    Observing the view state from the Activity
    

    View Slide

51. Page heading
    Implementation
    viewModel.viewState.observe(this, Observer {
    it?.let { render(it) }
    })
    override fun render(state: BeaconViewState) {
    when (state) {
    is ReplyViewState.Form -> renderForm(state)
    is ReplyViewState.ReplySent -> renderReplySent()
    is ReplyViewState.MissingMessage -> renderMissingMessage()
    is ReplyViewState.SendingReply -> renderLoading()
    is ReplyViewState.SendReplyError -> renderError(state)
    }
    }
    

    View Slide

52. Page heading
    Implementation
    viewModel.viewState.observe(this, Observer {
    it?.let { render(it) }
    })
    override fun render(state: BeaconViewState) {
    when (state) {
    is ReplyViewState.Form -> renderForm(state)
    is ReplyViewState.ReplySent -> renderReplySent()
    is ReplyViewState.MissingMessage -> renderMissingMessage()
    is ReplyViewState.SendingReply -> renderLoading()
    is ReplyViewState.SendReplyError -> renderError(state)
    }
    }
    

    View Slide

53. Page heading
    Implementation
    viewModel.viewState.observe(this, Observer {
    it?.let { render(it) }
    })
    override fun render(state: BeaconViewState) {
    when (state) {
    is ReplyViewState.Form -> renderForm(state)
    is ReplyViewState.ReplySent -> renderReplySent()
    is ReplyViewState.MissingMessage -> renderMissingMessage()
    is ReplyViewState.SendingReply -> renderLoading()
    is ReplyViewState.SendReplyError -> renderError(state)
    }
    }
    

    View Slide

54. Page heading
    Implementation
    ViewState vs DomainState
    

    View Slide

55. Page heading
    Implementation
    App = UI(state)
    

    View Slide

56. Page heading
    Implementation
    UI(viewState(domainState))
    

    View Slide

57. Page heading
    Implementation
    Store (Reducers)
    

    View Slide

58. Page heading
    The missing piece
    interface Store {
    fun reduce(action: Action)
    fun subscribe() LiveData
    }
    

    View Slide

59. Page heading
    The missing piece
    sealed class DomainState {
    object LoadingMovies : DomainState()
    data class MoviesLoaded(movies: List)
    }
    

    View Slide

60. Page heading
    The missing piece
    class MoviesViewModel(private val store: MoviesStore) : BaseViewModel() {
    init {
    stateDidChange(ViewState.Idle)
    state.addSource(store.subscribe(), {
    when (it) {
    is DomainState.LoadingMovies -> stateDidChange(ViewState.Loading)
    }
    })
    }
    override fun interpret(action: Action) {
    store.reduce(action)
    }
    }
    

    View Slide

61. Page heading
    The missing piece
    class MoviesViewModel(private val store: MoviesStore) : BaseViewModel() {
    init {
    stateDidChange(ViewState.Idle)
    state.addSource(store.subscribe(), {
    when (it) {
    is DomainState.LoadingMovies -> stateDidChange(ViewState.Loading)
    }
    })
    }
    override fun interpret(action: Action) {
    store.reduce(action)
    }
    }
    

    View Slide

62. Page heading
    The missing piece
    class MoviesViewModel(private val store: MoviesStore) : BaseViewModel() {
    init {
    stateDidChange(ViewState.Idle)
    state.addSource(store.subscribe(), {
    when (it) {
    is DomainState.LoadingMovies -> stateDidChange(ViewState.Loading)
    }
    })
    }
    override fun interpret(action: Action) {
    store.reduce(action)
    }
    }
    

    View Slide

63. Page heading
    The missing piece
    class MoviesViewModel(private val store: MoviesStore) : BaseViewModel() {
    init {
    stateDidChange(ViewState.Idle)
    state.addSource(store.subscribe(), {
    when (it) {
    is DomainState.LoadingMovies -> stateDidChange(ViewState.Loading)
    }
    })
    }
    override fun interpret(action: Action) {
    store.reduce(action)
    }
    }
    

    View Slide

64. Page heading
    The missing piece
    class MoviesViewModel(private val store: MoviesStore) : BaseViewModel() {
    init {
    stateDidChange(ViewState.Idle)
    state.addSource(store.subscribe(), {
    when (it) {
    is DomainState.LoadingMovies -> stateDidChange(ViewState.Loading)
    }
    })
    }
    override fun interpret(action: Action) {
    store.reduce(action)
    }
    }
    

    View Slide

65. Page heading
    The missing piece
    

    View Slide

66. Page heading
    The missing piece
    Functional Use Cases
    

    View Slide

67. Page heading
    The missing piece
    abstract class UseCase where Type : Any {
    abstract suspend fun run(params: Params): Either
    fun execute(onResult: (Either) -> Unit, params: Params) {
    val job = async(CommonPool) { run(params) }
    launch(UI) { onResult.invoke(job.await()) }
    }
    class None
    }
    

    View Slide

68. Page heading
    The missing piece
    abstract class UseCase where Type : Any {
    abstract suspend fun run(params: Params): Either
    fun execute(onResult: (Either) -> Unit, params: Params) {
    val job = async(CommonPool) { run(params) }
    launch(UI) { onResult.invoke(job.await()) }
    }
    class None
    }
    

    View Slide

69. Page heading
    The missing piece
    abstract class UseCase where Type : Any {
    abstract suspend fun run(params: Params): Either
    fun execute(onResult: (Either) -> Unit, params: Params) {
    val job = async(CommonPool) { run(params) }
    launch(UI) { onResult.invoke(job.await()) }
    }
    class None
    }
    

    View Slide

70. Page heading
    The missing piece
    abstract class UseCase where Type : Any {
    abstract suspend fun run(params: Params): Either
    fun execute(onResult: (Either) -> Unit, params: Params) {
    val job = async(CommonPool) { run(params) }
    launch(UI) { onResult.invoke(job.await()) }
    }
    class None
    }
    

    View Slide

71. Page heading
    The missing piece
    abstract class UseCase where Type : Any {
    abstract suspend fun run(params: Params): Either
    fun execute(onResult: (Either) -> Unit, params: Params) {
    val job = async(CommonPool) { run(params) }
    launch(UI) { onResult.invoke(job.await()) }
    }
    class None
    }
    

    View Slide

72. Page heading
    The missing piece
    abstract class UseCase where Type : Any {
    abstract suspend fun run(params: Params): Either
    fun execute(onResult: (Either) -> Unit, params: Params) {
    val job = async(CommonPool) { run(params) }
    launch(UI) { onResult.invoke(job.await()) }
    }
    class None
    }
    

    View Slide

73. Page heading
    The missing piece
    abstract class UseCase where Type : Any {
    abstract suspend fun run(params: Params): Either
    fun execute(onResult: (Either) -> Unit, params: Params) {
    val job = async(CommonPool) { run(params) }
    launch(UI) { onResult.invoke(job.await()) }
    }
    class None
    }
    

    View Slide

74. Page heading
    The missing piece
    class GetMovies(private val repository: MoviesRepository) :
    UseCase() {
    override suspend fun run(params: None): Either {
    return repository.getAllMovies()
    }
    }
    

    View Slide

75. Page heading
    The missing piece
    Error Handling
    

    View Slide

76. Page heading
    The missing piece
    sealed class Either {
    data class Left(val a: L) : Either()
    data class Right(val b: R) : Either()
    val isRight get() = this is Right
    val isLeft get() = this is Left
    fun either(fnL: (L) -> Any, fnR: (R) -> Any): Any =
    when (this) {
    is Either.Left -> fnL(a)
    is Either.Right -> fnR(b)
    }
    fun flatMap(fn: (R) -> Either): Either {...}
    fun map(fn: (R) -> (T)): Either {...}
    }
    

    View Slide

77. Page heading
    The missing piece
    sealed class Failure {
    class NetworkConnection: Failure()
    class ServerError: Failure()
    /** Feature specific failures */
    abstract class FeatureFailure: Failure()
    }
    

    View Slide

78. Page heading
    The missing piece
    sealed class Failure {
    class NetworkConnection: Failure()
    class ServerError: Failure()
    /** Feature specific failures */
    abstract class FeatureFailure: Failure()
    }
    

    View Slide

79. Page heading
    The missing piece
    sealed class Failure {
    class NetworkConnection: Failure()
    class ServerError: Failure()
    /** Feature specific failures */
    abstract class FeatureFailure: Failure()
    }
    

    View Slide

80. Page heading
    The missing piece
    sealed class Failure {
    class NetworkConnection: Failure()
    class ServerError: Failure()
    /** Feature specific failures */
    abstract class FeatureFailure: Failure()
    }
    

    View Slide

81. Page heading
    The missing piece
    private fun handleFailure(failure: Failure?) {
    when (failure) {
    is NetworkConnection -> renderFailure(R.string.failure_network)
    is ServerError -> renderFailure(R.string.failure_server)
    is ListNotAvailable -> renderFailure(R.string.failure_movies_error
    }
    }
    

    View Slide

82. Page heading
    Error Handling
    class MoviesViewModel
    @Inject constructor(private val store: MoviesStore) {
    var movies: MutableLiveData = MutableLiveData()
    var failure: MutableLiveData = MutableLiveData()
    fun loadMovies() =
    getMovies.reduce({ it.either(::handleFailure, ::handleMovieList) }, None())
    private fun handleMovieList(domainState: DomainState) {
    this.movies.value = MoviesViewState(domainState)
    }
    private fun handleFailure(failure: Failure) {
    this.failure.value = failure
    }
    }
    

    View Slide

83. Page heading
    Error Handling
    class MoviesViewModel
    @Inject constructor(private val store: MoviesStore) {
    var movies: MutableLiveData = MutableLiveData()
    var failure: MutableLiveData = MutableLiveData()
    fun loadMovies() =
    getMovies.reduce({ it.either(::handleFailure, ::handleMovieList) }, None())
    private fun handleMovieList(domainState: DomainState) {
    this.movies.value = MoviesViewState(domainState)
    }
    private fun handleFailure(failure: Failure) {
    this.failure.value = failure
    }
    }
    

    View Slide

84. Page heading
    Error Handling
    class MoviesViewModel
    @Inject constructor(private val store: MoviesStore) {
    var movies: MutableLiveData = MutableLiveData()
    var failure: MutableLiveData = MutableLiveData()
    fun loadMovies() =
    getMovies.reduce({ it.either(::handleFailure, ::handleMovieList) }, None())
    private fun handleMovieList(domainState: DomainState) {
    this.movies.value = MoviesViewState(domainState)
    }
    private fun handleFailure(failure: Failure) {
    this.failure.value = failure
    }
    }
    

    View Slide

85. Page heading
    Error Handling
    class MoviesViewModel
    @Inject constructor(private val store: MoviesStore) {
    var movies: MutableLiveData = MutableLiveData()
    var failure: MutableLiveData = MutableLiveData()
    fun loadMovies() =
    getMovies.reduce({ it.either(::handleFailure, ::handleMovieList) }, None())
    private fun handleMovieList(domainState: DomainState) {
    this.movies.value = MoviesViewState(domainState)
    }
    private fun handleFailure(failure: Failure) {
    this.failure.value = failure
    }
    }
    

    View Slide

86. Page heading
    Error Handling
    class MoviesViewModel
    @Inject constructor(private val store: MoviesStore) {
    var movies: MutableLiveData = MutableLiveData()
    var failure: MutableLiveData = MutableLiveData()
    fun loadMovies() =
    getMovies.reduce({ it.either(::handleFailure, ::handleMovieList) }, None())
    private fun handleMovieList(domainState: DomainState) {
    this.movies.value = MoviesViewState(domainState)
    }
    private fun handleFailure(failure: Failure) {
    this.failure.value = failure
    }
    }
    

    View Slide

87. Page heading
    Error Handling
    class MoviesViewModel
    @Inject constructor(private val store: MoviesStore) {
    var movies: MutableLiveData = MutableLiveData()
    var failure: MutableLiveData = MutableLiveData()
    fun loadMovies() =
    getMovies.reduce({ it.either(::handleFailure, ::handleMovieList) }, None())
    private fun handleMovieList(domainState: DomainState) {
    this.movies.value = MoviesViewState(domainState)
    }
    private fun handleFailure(failure: Failure) {
    this.failure.value = failure
    }
    }
    

    View Slide

88. Page heading
    Implementation
    Taking advantage of Kotlin
    

    View Slide

89. Page heading
    The missing piece
    inline fun
    Activity.viewModel(factory: Factory, body: T.() -> Unit): T {
    val vm = ViewModelProviders.of(this, factory)[T::class.java]
    vm.body()
    return vm
    }
    

    View Slide

90. Page heading
    The missing piece
    fun >
    LifecycleOwner.observe(liveData: L, body: (T?) -> Unit) =
    liveData.observe(this, Observer(body))
    fun >
    LifecycleOwner.failure(liveData: L, body: (Failure?) -> Unit) =
    liveData.observe(this, Observer(body))
    

    View Slide

91. Page heading
    The missing piece
    fun >
    LifecycleOwner.observe(liveData: L, body: (T?) -> Unit) =
    liveData.observe(this, Observer(body))
    fun >
    LifecycleOwner.failure(liveData: L, body: (Failure?) -> Unit) =
    liveData.observe(this, Observer(body))
    

    View Slide

92. Page heading
    The missing piece
    fun >
    LifecycleOwner.observe(liveData: L, body: (T?) -> Unit) =
    liveData.observe(this, Observer(body))
    fun >
    LifecycleOwner.failure(liveData: L, body: (Failure?) -> Unit) =
    liveData.observe(this, Observer(body))
    

    View Slide

93. Page heading
    The missing piece
    //subscription to LiveData in MoviesViewModel
    moviesViewModel = viewModel(viewModelFactory) {
    observe(movies, ::renderMoviesList)
    failure(failure, ::handleFailure)
    }
    

    View Slide

94. What’s next?
    

    View Slide

95. Page heading
    The missing piece
    ViewState vs Event
    

    View Slide

96. Page heading
    ViewState vs ViewEvent
    class BeaconViewModel(val store: BeaconStore) : ViewModel() {
    val viewState = MediatorLiveData()
    val viewEvents = MediatorLiveData()
    init {
    viewState.value = BeaconViewState.Idle
    viewState.addSource(reducer.subscribeToViewStates(), {
    if (viewState is BeaconViewState.Error) {
    viewState.shouldReThrow().let { throw viewState.exception }
    }
    viewState.setValue(it)
    })
    viewEvents.addSource(reducer.subscribeToEvents(), {
    viewEvents.setValue(it)
    })
    }
    fun interpret(action: BeaconAction) {
    reducer.reduce(action, viewState.value!!)
    }
    }
    

    View Slide

97. Page heading
    ViewState vs ViewEvent
    class BeaconViewModel(val store: BeaconStore) : ViewModel() {
    val viewState = MediatorLiveData()
    val viewEvents = MediatorLiveData()
    init {
    viewState.value = BeaconViewState.Idle
    viewState.addSource(reducer.subscribeToViewStates(), {
    if (viewState is BeaconViewState.Error) {
    viewState.shouldReThrow().let { throw viewState.exception }
    }
    viewState.setValue(it)
    })
    viewEvents.addSource(reducer.subscribeToEvents(), {
    viewEvents.setValue(it)
    })
    }
    fun interpret(action: BeaconAction) {
    reducer.reduce(action, viewState.value!!)
    }
    }
    

    View Slide

98. Page heading
    ViewState vs ViewEvent
    override fun reactTo(event: BeaconEvent) {
    when (event) {
    is BeaconEvent.SelectAttachment -> openFileSelector()
    is BeaconEvent.AttachmentDownloaded -> open(event.File)
    is BeaconEvent.Close -> closeActivity()
    }
    }
    

    View Slide

99. What’s next?
    Kotlin Multiplatform
    DomainState persistence
    App as a State Machine
    

    View Slide

100. Thank you!
     David González
     @dggonzalez
     davidgonzalez@helpscout.com
     

     View Slide