UdfViewModel

About

A view model base class that enforces a unidirectional data flow pattern.

All changes to the view state must be executed via a call to dispatchEvent with a specific MutationEvent. Also exposes an EffectEvent which can be used for the view model to action on events which do not result in changes to the view state itself (ie. side effects like screen navigation).

If your use case does not require either a MutationEvent or an EffectEvent, you can use NoOpMutation and NoOpEffect as generic parameters.

onViewStateInitialized will be called after a view starts subscribing to the viewState, but before the first emission. This could be used to do some asynchronous initialization of the view state, but be aware that this will delay the first view state emission. In most cases it is probably better to have an initial empty (or loading) state and dispatch an event from onViewStateInitialized to do the async setup.

Mutations will always be applied in sequence, in the order they were dispatched. Duplicate view states are not emitted.

Read more about the UDF pattern here.

Download

repositories {
  mavenCentral()
}
dependencies {
  implementation("io.github.howettl:udfviewmodel:<latest-version>")
}

Example

class UdfViewModelImpl : UdfViewModel<ViewStateImpl, Mutation, Effect>() {
    override fun createViewState() = ViewStateImpl()

    override suspend fun onViewStateInitialized() {
        // Start loading some data for the screen
    }

    override suspend fun mutateViewState(mutation: Mutation): ViewStateImpl {
        return when (mutation) {
            OnSignInPressed -> {
                // authenticate with a backend service ...
                currentViewState.copy(isSignedIn = true)
            }
            OnSignOutPressed -> {
                // log out of the backend service ...
                currentViewState.copy(isSignedIn = false)
            }
        }
    }

    override suspend fun handleEffect(effect: Effect) {
        when (effect) {
            OnContinuePressed -> {
                // Take some action, such as navigating to another screen
            }
        }
    }
}

data class ViewStateImpl(
    val isSignedIn: Boolean = false
) : ViewState

sealed interface Mutation : MutationEvent {
    data object OnSignInPressed : Mutation
    data object OnSignOutPressed : Mutation
}

sealed interface Effect : EffectEvent {
    data object OnContinuePressed : Effect
}