Side-effects in Compose

Composables should be side-effect free. However, when they're necessary to mutate the state of the app, they should be called from a controlled environment that is aware of the lifecycle of the composable. In this page, you'll learn about the different side-effect APIs Jetpack Compose offers.

State and effect use cases

As covered in the Thinking in Compose documentation, composables should be side-effect free. When you need to make changes to the state of the app (as described in the Managing state documentation doc), you should use the Effect APIs so that those side effects are executed in a predictable manner.

Due to the different possibilities effects open up in Compose, they can be easily overused. Make sure that the work you do in them is UI related and doesn't break unidirectional data flow as explained in the Managing state documentation.

LaunchedEffect: run suspend functions in the scope of a composable

To call suspend functions safely from inside a composable, use the LaunchedEffect composable. When LaunchedEffect enters the Composition, it launches a coroutine with the block of code passed as a parameter. The coroutine will be cancelled if LaunchedEffect leaves the composition. If LaunchedEffect is recomposed with different keys (see the Restarting Effects section below), the existing coroutine will be cancelled and the new suspend function will be launched in a new coroutine.

For example, showing a Snackbar in a Scaffold is done with the SnackbarHostState.showSnackbar function, which is a suspend function.

@Composable
fun MyScreen(
    state: UiState<List<Movie>>,
    scaffoldState: ScaffoldState = rememberScaffoldState()
) {

    // If the UI state contains an error, show snackbar
    if (state.hasError) {

        // `LaunchedEffect` will cancel and re-launch if
        // `scaffoldState.snackbarHostState` changes
        LaunchedEffect(scaffoldState.snackbarHostState) {
            // Show snackbar using a coroutine, when the coroutine is cancelled the
            // snackbar will automatically dismiss. This coroutine will cancel whenever
            // `state.hasError` is false, and only start when `state.hasError` is true
            // (due to the above if-check), or if `scaffoldState.snackbarHostState` changes.
            scaffoldState.snackbarHostState.showSnackbar(
                message = "Error message",
                actionLabel = "Retry message"
            )
        }
    }

    Scaffold(scaffoldState = scaffoldState) {
        /* ... */
    }
}

In the code above, a coroutine is triggered if the state contains an error and it'll be cancelled when it doesn't. As the LaunchedEffect call site is inside an if statement, when the statement is false, if LaunchedEffect was in the Composition, it'll be removed, and therefore, the coroutine will be cancelled.

rememberCoroutineScope: obtain a composition-aware scope to launch a coroutine outside a composable

As LaunchedEffect is a composable function, it can only be used inside other composable functions. In order to launch a coroutine outside of a composable, but scoped so that it will be automatically canceled once it leaves the composition, use rememberCoroutineScope. Also use rememberCoroutineScope whenever you need to control the lifecycle of one or more coroutines manually, for example, cancelling an animation when a user event happens.

rememberCoroutineScope is a composable function that returns a CoroutineScope bound to the point of the Composition where it's called. The scope will be cancelled when the call leaves the Composition.

Following the previous example, you could use this code to show a Snackbar when the user taps on a Button:

@Composable
fun MoviesScreen(scaffoldState: ScaffoldState = rememberScaffoldState()) {

    // Creates a CoroutineScope bound to the MoviesScreen's lifecycle
    val scope = rememberCoroutineScope()

    Scaffold(scaffoldState = scaffoldState) {
        Column {
            /* ... */
            Button(
                onClick = {
                    // Create a new coroutine in the event handler to show a snackbar
                    scope.launch {
                        scaffoldState.snackbarHostState.showSnackbar("Something happened!")
                    }
                }
            ) {
                Text("Press me")
            }
        }
    }
}

rememberUpdatedState: reference a value in an effect that shouldn't restart if the value changes

LaunchedEffect restarts when one of the key parameters changes. However, in some situations you might want to capture a value in your effect that, if it changes, you do not want the effect to restart. In order to do this, it is required to use rememberUpdatedState to create a reference to this value which can be captured and updated. This approach is helpful for effects that contain long-lived operations that may be expensive or prohibitive to recreate and restart.

For example, suppose your app has a LandingScreen that disappears after some time. Even if LandingScreen is recomposed, the effect that waits for some time and notifies that the time passed shouldn't be restarted:

@Composable
fun LandingScreen(onTimeout: () -> Unit) {

    // This will always refer to the latest onTimeout function that
    // LandingScreen was recomposed with
    val currentOnTimeout by rememberUpdatedState(onTimeout)

    // Create an effect that matches the lifecycle of LandingScreen.
    // If LandingScreen recomposes, the delay shouldn't start again.
    LaunchedEffect(true) {
        delay(SplashWaitTimeMillis)
        currentOnTimeout()
    }

    /* Landing screen content */
}

To create an effect that matches the lifecycle of the call site, a never-changing constant like Unit or true is passed as a parameter. In the code above, LaunchedEffect(true) is used. To make sure that the onTimeout lambda always contains the latest value that LandingScreen was recomposed with, onTimeout needs to be wrapped with the rememberUpdatedState function. The returned State, currentOnTimeout in the code, should be used in the effect.

DisposableEffect: effects that require cleanup

For side effects that need to be cleaned up after the keys change or if the composable leaves the Composition, use DisposableEffect. If the DisposableEffect keys change, the composable needs to dispose (do the cleanup for) its current effect, and reset by calling the effect again.

As an example, you might want to send analytics events based on Lifecycle events by using a LifecycleObserver. To listen for those events in Compose, use a DisposableEffect to register and unregister the observer when needed.

@Composable
fun HomeScreen(
    lifecycleOwner: LifecycleOwner = LocalLifecycleOwner.current,
    onStart: () -> Unit, // Send the 'started' analytics event
    onStop: () -> Unit // Send the 'stopped' analytics event
) {
    // Safely update the current lambdas when a new one is provided
    val currentOnStart by rememberUpdatedState(onStart)
    val currentOnStop by rememberUpdatedState(onStop)

    // If `lifecycleOwner` changes, dispose and reset the effect
    DisposableEffect(lifecycleOwner) {
        // Create an observer that triggers our remembered callbacks
        // for sending analytics events
        val observer = LifecycleEventObserver { _, event ->
            if (event == Lifecycle.Event.ON_START) {
                currentOnStart()
            } else if (event == Lifecycle.Event.ON_STOP) {
                currentOnStop()
            }
        }

        // Add the observer to the lifecycle
        lifecycleOwner.lifecycle.addObserver(observer)

        // When the effect leaves the Composition, remove the observer
        onDispose {
            lifecycleOwner.lifecycle.removeObserver(observer)
        }
    }

    /* Home screen content */
}

In the code above, the effect will add the observer to the lifecycleOwner. If lifecycleOwner changes, the effect is disposed and restarted with the new lifecycleOwner.

A DisposableEffect must include an onDispose clause as the final statement in its block of code. Otherwise, the IDE displays a build-time error.

SideEffect: publish Compose state to non-compose code

To share Compose state with objects not managed by compose, use the SideEffect composable, as it's invoked on every successful recomposition.

For example, your analytics library might allow you to segment your user population by attaching custom metadata ("user properties" in this example) to all subsequent analytics events. To communicate the user type of the current user to your analytics library, use SideEffect to update its value.

@Composable
fun rememberAnalytics(user: User): FirebaseAnalytics {
    val analytics: FirebaseAnalytics = remember {
        /* ... */
    }

    // On every successful composition, update FirebaseAnalytics with
    // the userType from the current User, ensuring that future analytics
    // events have this metadata attached
    SideEffect {
        analytics.setUserProperty("userType", user.userType)
    }
    return analytics
}

produceState: convert non-Compose state into Compose state

produceState launches a coroutine scoped to the Composition that can push values into a returned State. Use it to convert non-Compose state into Compose state, for example bringing external subscription-driven state such as Flow, LiveData, or RxJava into the Composition.

The producer is launched when produceState enters the Composition, and will be cancelled when it leaves the Composition. The returned State conflates; setting the same value won't trigger a recomposition.

Even though produceState creates a coroutine, it can also be used to observe non-suspending sources of data. To remove the subscription to that source, use the awaitDispose function.

The following example shows how to use produceState to load an image from the network. The loadNetworkImage composable function returns a State that can be used in other composables.

@Composable
fun loadNetworkImage(
    url: String,
    imageRepository: ImageRepository
): State<Result<Image>> {

    // Creates a State<T> with Result.Loading as initial value
    // If either `url` or `imageRepository` changes, the running producer
    // will cancel and will be re-launched with the new inputs.
    return produceState<Result<Image>>(initialValue = Result.Loading, url, imageRepository) {

        // In a coroutine, can make suspend calls
        val image = imageRepository.load(url)

        // Update State with either an Error or Success result.
        // This will trigger a recomposition where this State is read
        value = if (image == null) {
            Result.Error
        } else {
            Result.Success(image)
        }
    }
}

derivedStateOf: convert one or multiple state objects into another state

Use derivedStateOf when a certain state is calculated or derived from other state objects. Using this function guarantees that the calculation will only occur whenever one of the states used in the calculation changes.

The following example shows a basic To Do list whose tasks with user-defined high priority keywords appear first:

@Composable
fun TodoList(highPriorityKeywords: List<String> = listOf("Review", "Unblock", "Compose")) {

    val todoTasks = remember { mutableStateListOf<String>() }

    // Calculate high priority tasks only when the todoTasks or highPriorityKeywords
    // change, not on every recomposition
    val highPriorityTasks by remember {
        derivedStateOf { todoTasks.filter { it.containsWord(highPriorityKeywords) } }
    }

    Box(Modifier.fillMaxSize()) {
        LazyColumn {
            items(highPriorityTasks) { /* ... */ }
            items(todoTasks) { /* ... */ }
        }
        /* Rest of the UI where users can add elements to the list */
    }
}

In the code above, derivedStateOf guarantees that whenever todoTasks or highPriorityKeywords changes, the highPriorityTasks calculation will occur and the UI will be updated accordingly. As the filtering to calculate highPriorityTasks can be expensive, it should only be executed when any of the lists change, not on every recomposition.

Furthermore, an update to the state produced by derivedStateOf doesn't cause the composable where it's declared to recompose, Compose only recomposes those composables where its returned state is read, inside LazyColumn in the example.

snapshotFlow: convert Compose's State into Flows

Use snapshotFlow to convert State<T> objects into a cold Flow. snapshotFlow runs its block when collected and emits the result of the State objects read in it. When one of the State objects read inside the snapshotFlow block mutates, the Flow will emit the new value to its collector if the new value is not equal to the previous emitted value (this behavior is similar to that of Flow.distinctUntilChanged).

The following example shows a side effect that records when the user scrolls past the first item in a list to analytics:

val listState = rememberLazyListState()

LazyColumn(state = listState) {
    // ...
}

LaunchedEffect(listState) {
    snapshotFlow { listState.firstVisibleItemIndex }
        .map { index -> index > 0 }
        .distinctUntilChanged()
        .filter { it == true }
        .collect {
            MyAnalyticsService.sendScrolledPastFirstItemEvent()
        }
}

In the code above, listState.firstVisibleItemIndex is converted to a Flow that can benefit from the power of Flow's operators.

Restarting effects

Some effects in Compose, like LaunchedEffect, produceState, or DisposableEffect, take a variable number of arguments, keys, that are used to cancel the running effect and start a new one with the new keys.

The typical form for these APIs is:

EffectName(restartIfThisKeyChanges, orThisKey, orThisKey, ...) { block }

Due to the subtleties of this behavior, problems can occur if the parameters used to restart the effect are not the right ones:

  • Restarting effects less than they should could cause bugs in your app.
  • Restarting effects more than they should could be inefficient.

As a rule of thumb, mutable and immutable variables used in the effect block of code should be added as parameters to the effect composable. Apart from those, more parameters can be added to force restarting the effect. If the change of a variable shouldn't cause the effect to restart, the variable should be wrapped in rememberUpdatedState. If the variable never changes because it's wrapped in a remember with no keys, you don't need to pass the variable as a key to the effect.

In the DisposableEffect code shown above, the effect takes as a parameter the lifecycleOwner used in its block because any change to them should cause the effect to restart.

@Composable
fun HomeScreen(
    lifecycleOwner: LifecycleOwner = LocalLifecycleOwner.current,
    onStart: () -> Unit, // Send the 'started' analytics event
    onStop: () -> Unit // Send the 'stopped' analytics event
) {
    // These values never change in Composition
    val currentOnStart by rememberUpdatedState(onStart)
    val currentOnStop by rememberUpdatedState(onStop)

    DisposableEffect(lifecycleOwner) {
        val observer = LifecycleEventObserver { _, event ->
            /* ... */
        }

        lifecycleOwner.lifecycle.addObserver(observer)
        onDispose {
            lifecycleOwner.lifecycle.removeObserver(observer)
        }
    }
}

currentOnStart and currentOnStop are not needed as a DisposableEffect key because its value never changes in Composition due to the usage of rememberUpdatedState. If lifecycleOwner is not passed as a parameter and it changes, HomeScreen recomposes but the DisposableEffect isn't disposed of and restarted. That causes problems because the wrong lifecycleOwner is used from that point forward.

Constants as keys

You can use a constant like true as an effect key to make it follow the lifecycle of the call site. There are valid use cases for it, like the LaunchedEffect example shown above. However, before doing that, think twice and make sure that's what you need.