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typealias Reducer<T> = (T) -> T

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// User changes text entry
data class TextChangedEvent(val newText: String)

// User clicks create 
data class CreateTodoEvent(val text: String)

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data class TodoFormState(
  val enteredText: String,
  // After user clicks create, we send a post request.
  val createTodoRequest: CreateTodoRequestState?)

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// User can submit the comment
data class SubmitCommentEvent(val comment: String)

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sealed class UserEvent {
  data class TextChangedEvent(...): UserEvent()
  data class SubmitCommentRequestEvent(...): UserEvent()
}

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sealed class CommentFormUserEvent {
  data class TextChangedEvent(..): CommentFormUserEvent()
  data class SubmitCommentRequestEvent(..): CommentFormUserEvent()
}

sealed class LoginFormUserEvent {
  // We cannot use TextChangedEvent from CommentFormUserEvent
  data class TextChangedEvent(..): LoginFormUserEvent() 
  data class LoginRequestEvent(..): LoginFormUserEvent()
}

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data class CommentFormData(
  // defines what user has entered
  val comment: String,
  // defines if the entered comment is valid for submission
  val isCommentValid: Boolean,
  // defines if there is a submit request in progress
  val submitRequest: Lce<Comment>?
)

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// User modifies the comment text field
data class TextChangedEvent(val enteredText: String)

// User clicks submit
data class SubmitCommentEvent(val comment: String)

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class CommentFormViewModel {
  // Let's create a view state from the data provided.
  fun createViewState(
    data: CommentFormData,
    onTextChanged: (TextChangedEvent) -> Unit,
    onSubmitSelected: (SubmitCommentEvent) -> Unit
  ): CommentFormViewState{
    val isSubmitting = data.submitRequest?.let { it.isLoading} ?: false
    return CommentFormViewState(
      textEntered = data.comment,

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fun dataStream(
  textChangedEvents: Flowable<TextChangedEvent>,
  submitCommentEvents: Flowable<SubmitCommentEvent>
): Flowable<CommentFormData> {
  // We need to start with something
  val initialFormState = CommentFormData(comment = "", isCommentValid = false, submitRequest = null)
  // RxJava scan operator takes initial state and a reduce function
  // This operator manages the state for us, where it will keep track 
  // of the latest state and provide it for next incoming event 
  return textChangedEvents.scan(initialFormState) { event, currentState ->