Lifecycle and side-effects

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 lifecycle of a composable and the different side-effect APIs Jetpack Compose offers.

Lifecycle of a composable

As mentioned in the Managing state documentation, a Composition describes the UI of your app and is produced by running composables. A Composition is a tree-structure of the composables that describe your UI.

When Jetpack Compose runs your composables for the first time, during initial composition, it will keep track of the composables that you call to describe your UI in a Composition. Then, when the state of your app changes, Jetpack Compose schedules a recomposition. Recomposition is when Jetpack Compose re-executes the composables that may have changed in response to state changes, and then updates the Composition to reflect any changes.

A Composition can only be produced by an initial composition and updated by recomposition. The only way to modify a Composition is through recomposition.

Figure 1. Lifecycle of a composable in the Composition. It enters the Composition, gets recomposed 0 or more times, and leaves the Composition.

Recomposition is typically triggered by a change to a State<T> object. Compose tracks these and runs all composables in the Composition that read that particular State<T>, and any composables that they call that cannot be skipped.

If a composable is called multiple times, multiple instances are placed in the Composition. Each call has its own lifecycle in the Composition.

@Composable
fun MyComposable() {
    Column {
        Text("Hello")
        Text("World")
    }
}

Figure 2. Representation of MyComposable in the Composition. If a composable is called multiple times, multiple instances are placed in the Composition. An element having a different color is indicative of it being a separate instance.

Anatomy of a composable in Composition

The instance of a composable in Composition is identified by its call site. The Compose compiler considers each call site as distinct. Calling composables from multiple call sites will create multiple instances of the composable in Composition.

If during a recomposition a composable calls different composables than it did during the previous composition, Compose will identify which composables were called or not called and for the composables that were called in both compositions, Compose will avoid recomposing them if their inputs haven't changed.

Preserving identity is crucial to associate side effects with their composable, so that they can complete successfully rather than restart for every recomposition.

Consider the following example:

@Composable
fun LoginScreen(showError: Boolean) {
    if (showError) {
        LoginError()
    }
    LoginInput() // This call site affects where LoginInput is placed in Composition
}

@Composable
fun LoginInput() { /* ... */ }

In the code snippet above, LoginScreen will conditionally call the LoginError composable and will always call the LoginInput composable. Each call has a unique call site and source position, which the compiler will use to uniquely identify it.

Figure 3. Representation of LoginScreen in the Composition when the state changes and a recomposition occurs. Same color means it hasn't been recomposed.

Even though LoginInput went from being called first to being called second, the LoginInput instance will be preserved across recompositions. Additionally, because LoginInput doesn’t have any parameters that have changed across recomposition, the call to LoginInput will be skipped by Compose.

Add extra information to help smart recompositions

Calling a composable multiple times will add it to Composition multiple times as well. When calling a composable multiple times from the same call site, Compose doesn’t have any information to uniquely identify each call to that composable, so the execution order is used in addition to the call site in order to keep the instances distinct. This behavior is sometimes all that is needed, but in some cases it can cause unwanted behavior.

@Composable
fun MoviesScreen(movies: List<Movie>) {
    Column {
        for (movie in movies) {
            // MovieOverview composables are placed in Composition given its
            // index position in the for loop
            MovieOverview(movie)
        }
    }
}

In the example above, Compose uses the execution order in addition to the call site to keep the instance distinct in the Composition. If a new movie is added to the bottom of the list, Compose can reuse the instances already in the Composition since their location in the list haven't changed and therefore, the movie input is the same for those instances.

Figure 4. Representation of MoviesScreen in the Composition when a new element is added to the bottom of the list. MovieOverview composables in the Composition can be reused. Same color in MovieOverview means the composable hasn't been recomposed.

However, if the movies list changes by either adding to the top or the middle of the list, removing or reordering items, it'll cause a recomposition in all MovieOverview calls whose input parameter has changed position in the list. That's extremely important if, for example, MovieOverview fetches a movie image using a side effect. If recomposition happens while the effect is in progress, it will be cancelled and will start again.

@Composable
fun MovieOverview(movie: Movie) {
    Column {
        // Side effect explained later in the docs. If MovieOverview
        // recomposes, while fetching the image is in progress,
        // it is cancelled and restarted.
        val image = loadNetworkImage(movie.url)
        MovieHeader(image)

        /* ... */
    }
}

Figure 5. Representation of MoviesScreen in the Composition when a new element is added to the list. MovieOverview composables cannot be reused and all side effects will restart. A different color in MovieOverview means the composable has been recomposed.

Ideally, we want to think of the identity of the MovieOverview instance as linked to the identity of the movie that is passed to it. If we reorder the list of movies, ideally we would similarly reorder the instances in the Composition tree instead of recomposing each MovieOverview composable with a different movie instance. Compose provides a way for you to tell the runtime what values you want to use to identify a given part of the tree: the key composable.

By wrapping a block of code with a call to the key composable with one or more values passed in, those values will be combined to be used to identify that instance in the composition. The value for a key does not need to be globally unique, it needs to only be unique amongst the invocations of composables at the call site. So in this example, each movie needs to have a key that's unique among the movies; it's fine if it shares that key with some other composable elsewhere in the app.

@Composable
fun MoviesScreen(movies: List<Movie>) {
    Column {
        for (movie in movies) {
            key(movie.id) { // Unique ID for this movie
                MovieOverview(movie)
            }
        }
    }
}

With the above, even if the elements on the list change, Compose recognizes individual calls to MovieOverview and can reuse them.

Figure 6. Representation of MoviesScreen in the Composition when a new element is added to the list. Since the MovieOverview composables have unique keys, Compose recognizes which MovieOverview instances haven't changed, and can reuse them; their side effects will continue executing.

Some composables have built-in support for the key composable. For example, LazyColumn accepts specifying a custom key in the items DSL.

@Composable
fun MoviesScreen(movies: List<Movie>) {
    LazyColumn {
        items(movies, key = { movie -> movie.id }) { movie ->
            MovieOverview(movie)
        }
    }
}

Skipping if the inputs haven't changed

If a composable is already in the Composition, it can skip recomposition if all the inputs are stable and haven't changed.

A stable type must comply with the following contract:

• The result of equals for two instances will forever be the same for the same two instances.
• If a public property of the type changes, Composition will be notified.
• All public property types are also stable.

There are some important common types that fall into this contract that the compose compiler will treat as @Stable, even though they are not explicitly marked as @Stable.

• All primitive value types: Boolean, Int, Long, Float, Char, etc.
• Strings
• All Function types (lambdas)

All of these types are able to follow the contract of @Stable because they are immutable. Since immutable types never change, they never have to notify Composition of the change, so it is much easier to follow this contract.

One notable type that is stable but is mutable is Compose’s MutableState type. If a value is held in a MutableState, the state object overall is considered to be stable as Compose will be notified of any changes to the .value property of State.

When all types passed as parameters to a composable are stable, the parameter values are compared for equality based on the composable position in the UI tree. Recomposition is skipped if all the values are unchanged since the previous call.

Compose considers a type stable only if it can prove it. For example, an interface is generally treated as not stable, and types with mutable public properties whose implementation could be immutable are not stable either.

If Compose is not able to infer that a type is stable, but you want to force Compose to treat it as stable, mark it with the @Stable annotation.

// Marking the type as stable to favor skipping and smart recompositions.
@Stable
interface UiState<T : Result<T>> {
    val value: T?
    val exception: Throwable?

    val hasError: Boolean
        get() = exception != null
}

In the code snippet above, since UiState is an interface, Compose could ordinarily consider this type to be not stable. By adding the @Stable annotation, you tell Compose that this type is stable, allowing Compose to favor smart recompositions. This also means that Compose will treat all its implementations as stable if the interface is used as the parameter type.

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` 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` 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 example, an OnBackPressedCallback needs to be registered to listen for the back button being pressed on a OnBackPressedDispatcher. To listen for those events in Compose, use a DisposableEffect to register and unregister the callback when needed.

@Composable
fun BackHandler(backDispatcher: OnBackPressedDispatcher, onBack: () -> Unit) {

    // Safely update the current `onBack` lambda when a new one is provided
    val currentOnBack by rememberUpdatedState(onBack)

    // Remember in Composition a back callback that calls the `onBack` lambda
    val backCallback = remember {
        // Always intercept back events. See the SideEffect for
        // a more complete version
        object : OnBackPressedCallback(true) {
            override fun handleOnBackPressed() {
                currentOnBack()
            }
        }
    }

    // If `backDispatcher` changes, dispose and reset the effect
    DisposableEffect(backDispatcher) {
        // Add callback to the backDispatcher
        backDispatcher.addCallback(backCallback)

        // When the effect leaves the Composition, remove the callback
        onDispose {
            backCallback.remove()
        }
    }
}

In the code above, the effect will add the remembered backCallback to the backDispatcher. If backDispatcher changes, the effect is disposed and restarted again.

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.

Taking the previous BackHandler code as an example, to communicate whether the callback should be enabled or not, use SideEffect to update its value.

@Composable
fun BackHandler(
    backDispatcher: OnBackPressedDispatcher,
    enabled: Boolean = true, // Whether back events should be intercepted or not
    onBack: () -> Unit
) {
    /* ... */
    val backCallback = remember { /* ... */ }

    // On every successful composition, update the callback with the `enabled` value
    // to tell `backCallback` whether back events should be intercepted or not
    SideEffect {
        backCallback.isEnabled = enabled
    }

    /* Rest of the code */
}

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 keys.
    return produceState(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(todoTasks, highPriorityKeywords) {
        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.

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 backDispatcher used in its block as any change to them should cause the effect to restart.

@Composable
fun BackHandler(backDispatcher: OnBackPressedDispatcher, onBack: () -> Unit) {
    /* ... */
    val backCallback = remember { /* ... */ }

    DisposableEffect(backDispatcher) {
        backDispatcher.addCallback(backCallback)
        onDispose {
            backCallback.remove()
        }
    }
}

backCallback is not needed as a DisposableEffect key because its value will never change in Composition; it's wrapped in a remember with no keys. If backDispatcher is not passed as a parameter and it changes, BackHandler will recompose but the DisposableEffect won't dispose and restart again. That'll cause problems as the wrong backDispatcher will be 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.