Compose state management

Compose State Management (ViewModel + StateFlow + UDF)

Instructions

Follow Unidirectional Data Flow (UDF): state flows down from a single source of truth (ViewModel), events flow up.

1. UiState as a Single Immutable Class

@Immutable
data class ArticlesUiState(
    val isLoading: Boolean = false,
    val articles: ImmutableList<Article> = persistentListOf(),
    val error: String? = null,
)

Use kotlinx.collections.immutable (ImmutableList, PersistentList) so Compose treats list fields as stable and skips unnecessary recompositions.

2. ViewModel Exposes StateFlow

@HiltViewModel
class ArticlesViewModel @Inject constructor(
    private val getArticles: GetArticlesUseCase,
) : ViewModel() {

    private val _state = MutableStateFlow(ArticlesUiState(isLoading = true))
    val state: StateFlow<ArticlesUiState> = _state.asStateFlow()

    private val _events = Channel<ArticlesEvent>(Channel.BUFFERED)
    val events = _events.receiveAsFlow()

    init { load() }

    fun onAction(action: ArticlesAction) = when (action) {
        ArticlesAction.Refresh -> load()
        is ArticlesAction.Open  -> viewModelScope.launch { _events.send(ArticlesEvent.Navigate(action.id)) }
    }

    private fun load() = viewModelScope.launch {
        _state.update { it.copy(isLoading = true, error = null) }
        runCatching { getArticles() }
            .onSuccess { list -> _state.update { it.copy(isLoading = false, articles = list.toPersistentList()) } }
            .onFailure { e -> _state.update { it.copy(isLoading = false, error = e.message) } }
    }
}

sealed interface ArticlesAction {
    data object Refresh : ArticlesAction
    data class Open(val id: String) : ArticlesAction
}

sealed interface ArticlesEvent {
    data class Navigate(val id: String) : ArticlesEvent
}

3. Collecting in Composables (Lifecycle-Aware)

@Composable
fun ArticlesRoute(vm: ArticlesViewModel = hiltViewModel(), onOpen: (String) -> Unit) {
    val state by vm.state.collectAsStateWithLifecycle()

    LaunchedEffect(Unit) {
        vm.events.collect { e ->
            when (e) { is ArticlesEvent.Navigate -> onOpen(e.id) }
        }
    }

    ArticlesScreen(state = state, onAction = vm::onAction)
}

Always use collectAsStateWithLifecycle() (from lifecycle-runtime-compose) — never raw collectAsState() for Flows that stay hot, to avoid work while the UI is stopped.

4. State Hoisting

Composables render state and raise events. They should not own business state.

@Composable
fun ArticlesScreen(state: ArticlesUiState, onAction: (ArticlesAction) -> Unit) {
    when {
        state.isLoading -> LoadingIndicator()
        state.error != null -> ErrorView(state.error, onRetry = { onAction(ArticlesAction.Refresh) })
        else -> ArticleList(state.articles, onClick = { onAction(ArticlesAction.Open(it.id)) })
    }
}

5. remember vs rememberSaveable

• remember { mutableStateOf(...) } — ephemeral UI state lost across config changes. Use for scroll position on a short-lived screen, hover state, etc.
• rememberSaveable { mutableStateOf(...) } — survives config change and process death. Use for search text fields, dialog open/closed flags.
• For complex saved state use a custom Saver:

val FilterSaver = Saver<Filter, List<String>>(
    save = { listOf(it.query, it.category) },
    restore = { Filter(query = it[0], category = it[1]) },
)
val filter = rememberSaveable(saver = FilterSaver) { Filter("", "all") }

6. Derived and Produced State

val canSubmit by remember(state.articles) {
    derivedStateOf { state.articles.isNotEmpty() && !state.isLoading }
}

val temperature by produceState(initialValue = 0f, key1 = sensorId) {
    sensorFlow(sensorId).collect { value = it }
}

derivedStateOf prevents recomposition when the derived value is unchanged even though inputs changed.

7. Anti-Patterns

• Do not hold Context, View, or NavController in ViewModel.
• Do not mutate MutableStateFlow.value from the UI thread directly from a composable body; route through onAction.
• Do not expose MutableStateFlow publicly — always .asStateFlow().
• Do not use LiveData in new Compose code (see livedata-to-flow skill for migration).

Checklist

• ViewModel exposes a single immutable UiState via StateFlow.
• UI collects with collectAsStateWithLifecycle().
• One-shot events use Channel/SharedFlow, not StateFlow.
• No Context or Android framework types stored in ViewModel fields.
• rememberSaveable is used for any state that must survive config change.
• List fields use ImmutableList / persistentListOf() for stability.