compose-side-effects

Compose: side effects

Core principle

Composable bodies describe UI. They can be recomposed, skipped, or abandoned. Work that changes the outside world belongs in an effect API whose lifecycle matches the work.

Pick the smallest effect

Effect keys

Keys define restart identity. When any key changes, the old effect is cancelled/disposed and a new one starts.

// ✅ Restart collection when userId changes
LaunchedEffect(userId) {
    repository.events(userId).collect { event -> handle(event) }
}

// ❌ Unit hides a changing input; collection keeps using the first userId
LaunchedEffect(Unit) {
    repository.events(userId).collect { event -> handle(event) }
}

Use stable, semantic keys:

• Use the thing whose lifecycle the effect follows: userId, screenId, lifecycleOwner, focusRequester.
• Do not use broad objects (state, viewModel) when only one property matters.
• Do not add changing lambdas as keys unless you really want restarts on every lambda change.

Avoid stale captures

For long-running effects that should not restart but need the latest callback or value, use rememberUpdatedState.

@Composable
fun Timeout(onTimeout: () -> Unit) {
    val latestOnTimeout by rememberUpdatedState(onTimeout)

    LaunchedEffect(Unit) {
        delay(1_000)
        latestOnTimeout()
    }
}

Use this when the lifecycle is "start once" but the invoked lambda should stay fresh. Common cases:

• A timeout or splash effect should not restart when onTimeout changes, but it should call the latest callback.
• A lifecycle observer should stay registered to the same owner, but invoke the latest onStart / onStop lambdas.
• A long-running collector should keep its collection lifecycle, but call the latest event handler.

Do not use rememberUpdatedState to avoid choosing proper keys. If the changed value should restart the work, make it a key instead:

// BAD: userId changes should restart the collection, not update a captured value.
val latestUserId by rememberUpdatedState(userId)
LaunchedEffect(Unit) {
    repository.events(latestUserId).collect { event -> handle(event) }
}

// GOOD: the collection lifecycle follows userId.
LaunchedEffect(userId) {
    repository.events(userId).collect { event -> handle(event) }
}

rememberUpdatedState also does not make render state "non-recomposing." If the UI needs to display a changing value, read normal State in composition or use the deferred-read patterns in compose-state-deferred-reads for frame-rate values.

Collecting Flow

Use LaunchedEffect for side-effect/event flows: snackbars, navigation events, analytics events, focus commands, or other streams where each emission triggers imperative work.

LaunchedEffect(events) {
    events.collect { event ->
        snackbarHostState.showSnackbar(event.message)
    }
}

Do not collect render state imperatively just to mutate local state. For UI state, collect near the state holder and pass plain values into the UI composable—the state-holder vs UI split, collectAsStateWithLifecycle() / collectAsState(), and preview-friendly wiring are covered in compose-state-holder-ui-split. Do not duplicate that architecture here.

On Android, prefer lifecycle-aware collection where available; use collectAsState() on targets without lifecycle-aware APIs.

For Compose state reads, use snapshotFlow:

LaunchedEffect(listState) {
    snapshotFlow { listState.firstVisibleItemIndex }
        .distinctUntilChanged()
        .collect { index -> analytics.visibleIndex(index) }
}

snapshotFlow { ... }.map { ... } without a terminal collect does nothing.

User events

Use rememberCoroutineScope() when a click or gesture starts suspending work:

@Composable
fun SaveButton(snackbarHostState: SnackbarHostState) {
    val scope = rememberCoroutineScope()

    Button(
        onClick = {
            scope.launch {
                snackbarHostState.showSnackbar("Saved")
            }
        },
    ) {
        Text("Save")
    }
}

Avoid "event flag" state just to trigger a LaunchedEffect. The click already is the event.

Registration and cleanup

Use DisposableEffect for paired setup/teardown:

@Composable
fun ObserveLifecycle(owner: LifecycleOwner, observer: LifecycleObserver) {
    DisposableEffect(owner, observer) {
        owner.lifecycle.addObserver(observer)
        onDispose {
            owner.lifecycle.removeObserver(observer)
        }
    }
}

Every registration path should have a matching onDispose cleanup path.

Common mistakes

Red flags during review

• "This only runs once" about code in a composable body.
• LaunchedEffect(Unit) in a function with changing parameters.
• A flow chain inside an effect with no terminal collection.
• Effects whose keys are chosen to silence lint instead of model lifecycle.
• Callback lambdas used from long-lived effects without either a key or rememberUpdatedState.