A new way to implement One-Time-UI-Events (former SingleLiveEvent) in a Compose world.

This library will help you to avoid implementing any antipatterns regarding One-Time-UI-Events as despribed by Manuel Vivo's article.

See the samples below on how to effectively use StateEvent in your view's state and EventEffect in your composables.

To get an in depth idea on how to migrate see this article from Yanneck Reiß.

Imagine a simple usecase where you need to fetch some list data from an API and display the result on the screen.

data class FlowerViewState(
    val flowers: List<Flower> = emptyList(),
    val isLoadingFlowers: Boolean = false,
    val downloadSucceededEvent: StateEvent = consumed,
    val downloadFailedEvent: StateEventWithContent<Int> = consumed()
)

Imagine we would like to show a green success snackbar or a red failure snackbar after the loading has finished. These two events would be represented with the two new fields downloadSucceededEvent and downloadFailedEvent in our view state object.

Use the StateEventWithContent<T> when you need to pass some data to the consumer (the composable). In the example above a StringRes with a fitting error description is passed.

private val _stateStream = MutableStateFlow(FlowerViewState())
val stateStream = _stateStream.asStateFlow()

private var state: FlowerViewState
    get() = _stateStream.value
    set(newState) {
        _stateStream.update { newState }
    }
    
fun loadFlowers(){
  viewModelScope.launch {
    state = state.copy(isLoading = true)
    state = when (val apiResult = loadAllFlowersFromApiUseCase.call()) {
        is Success -> state.copy(flowers = apiResult, downloadSucceededEvent = triggered)
        is Failure -> state.copy(downloadFailedEvent = triggered(R.string.error_load_flowers))
    }
    state = state.copy(isLoading = false)
  }
}

fun onConsumedDownloadSucceededEvent(){
  state = state.copy(downloadSucceededEvent = consumed)
}

fun onConsumedDownloadFailedEvent(){
  state = state.copy(downloadFailedEvent = consumed())
}

To trigger an event without any data just use the triggered value, otherwise use the triggered(content: T) function. To consume an event without any data just use the consumed value, otherwise use the consumed() function.

val viewModel: MainViewModel = viewModel()
val viewState: MainViewState by viewModel.stateStream.collectAsStateLifecycleAware()

EventEffect(
    event = viewState.downloadSucceededEvent, 
    onConsumed = viewModel::onConsumedDownloadSucceededEvent
) {
    scaffoldState.snackbarHostState.showSnackbar("Download succeeded.")
}

EventEffect(
    event = viewState.downloadFailedEvent, 
    onConsumed = viewModel::onConsumedDownloadFailedEvent
) { stringRes ->
    scaffoldState.snackbarHostState.showSnackbar(context.resources.getString(stringRes))
}

The EventEffect is a LaunchedEffect that will be executed, when the event is in its triggered state. When the event action was executed the effect calls the passed onConsumed callback to force you to set the view state field to be consumed.

In the regular way you only want to consume your event when it really got processed. However, in some special cases you want to make sure that the StateEvent gets consumed no matter what the outcome of your code that gets triggered will be. One case in which that gets relevant is when you want to navigate to another screen, if a StateEvent or StateEventWithContent<T> got invoked.

For this special case, instead of EventEffect, you can use the NavigationEventEffect.

NavigationEventEffect(  
  event = viewState.downloadFailedEvent,  
  onConsumed = viewModel::onConsumedDownloadFailedEvent  
) {
  navigator.navigateBack()  
}

allprojects {
   repositories {
       ...
       maven { url "https://jitpack.io" }
   }
}
dependencies {
   implementation 'com.github.leonard-palm:compose-state-events:2.2.0'
}