State and Jetpack Compose

State in an app is any value that can change over time. This is a very broad definition and encompasses everything from a Room database to a variable on a class.

All Android apps display state to the user. A few examples of state in Android apps:

  • A Snackbar that shows when a network connection can't be established.
  • A blog post and associated comments.
  • Ripple animations on buttons that play when a user clicks them.
  • Stickers that a user can draw on top of an image.

Jetpack Compose helps you be explicit about where and how you store and use state in an Android app. This guide focuses on the connection between state and composables, and on the APIs that Jetpack Compose offers to work with state more easily.

State and composition

Compose is declarative and as such the only way to update it is by calling the same composable with new arguments. These arguments are representations of the UI state. Any time a state is updated a recomposition takes place. As a result, things like TextField don’t automatically update like they do in imperative XML based views. A composable has to explicitly be told the new state in order for it to update accordingly.

@Composable
private fun HelloContent() {
    Column(modifier = Modifier.padding(16.dp)) {
        Text(
            text = "Hello!",
            modifier = Modifier.padding(bottom = 8.dp),
            style = MaterialTheme.typography.bodyMedium
        )
        OutlinedTextField(
            value = "",
            onValueChange = { },
            label = { Text("Name") }
        )
    }
}

If you run this and try to enter text, you'll see that nothing happens. That's because the TextField doesn't update itself—it updates when its value parameter changes. This is due to how composition and recomposition work in Compose.

To learn more about initial composition and recomposition, see Thinking in Compose.

State in composables

Composable functions can use the remember API to store an object in memory. A value computed by remember is stored in the Composition during initial composition, and the stored value is returned during recomposition. remember can be used to store both mutable and immutable objects.

mutableStateOf creates an observable MutableState<T>, which is an observable type integrated with the compose runtime.

interface MutableState<T> : State<T> {
    override var value: T
}

Any changes to value schedules recomposition of any composable functions that read value. In the case of ExpandingCard, whenever expanded changes, it causes ExpandingCard to be recomposed.

There are three ways to declare a MutableState object in a composable:

  • val mutableState = remember { mutableStateOf(default) }
  • var value by remember { mutableStateOf(default) }
  • val (value, setValue) = remember { mutableStateOf(default) }

These declarations are equivalent, and are provided as syntax sugar for different uses of state. You should pick the one that produces the easiest-to-read code in the composable you're writing.

The by delegate syntax requires the following imports:

import androidx.compose.runtime.getValue
import androidx.compose.runtime.setValue

You can use the remembered value as a parameter for other composables or even as logic in statements to change which composables are displayed. For example, if you don't want to display the greeting if the name is empty, use the state in an if statement:

@Composable
fun HelloContent() {
    Column(modifier = Modifier.padding(16.dp)) {
        var name by remember { mutableStateOf("") }
        if (name.isNotEmpty()) {
            Text(
                text = "Hello, $name!",
                modifier = Modifier.padding(bottom = 8.dp),
                style = MaterialTheme.typography.bodyMedium
            )
        }
        OutlinedTextField(
            value = name,
            onValueChange = { name = it },
            label = { Text("Name") }
        )
    }
}

While remember helps you retain state across recompositions, the state is not retained across configuration changes. For this, you must use rememberSaveable. rememberSaveable automatically saves any value that can be saved in a Bundle. For other values, you can pass in a custom saver object.

Other supported types of state

Compose doesn't require that you use MutableState<T> to hold state; it supports other observable types. Before reading another observable type in Compose, you must convert it to a State<T> so that composables can automatically recompose when the state changes.

Compose ships with functions to create State<T> from common observable types used in Android apps. Before using these integrations, add the appropriate artifact(s) as outlined below:

  • Flow: collectAsStateWithLifecycle()

    collectAsStateWithLifecycle() collects values from a Flow in a lifecycle-aware manner, allowing your app to save unneeded app resources. It represents the latest emitted value via Compose State. Use this API as the recommended way to collect flows on Android apps.

    The following dependency is required in the build.gradle file (it should be 2.6.0-beta01 or newer):

Kotlin

dependencies {
      ...
      implementation("androidx.lifecycle:lifecycle-runtime-compose:2.6.0-beta01")
}

Groovy

dependencies {
      ...
      implementation "androidx.lifecycle:lifecycle-runtime-compose:2.6.0-beta01"
}
  • Flow: collectAsState()

    collectAsState is similar to collectAsStateWithLifecycle, because it also collects values from a Flow and transforms it into Compose State.

    Use collectAsState for platform-agnostic code instead of collectAsStateWithLifecycle, which is Android-only.

    Additional dependencies are not required for collectAsState, because it is available in compose-runtime.

  • LiveData: observeAsState()

    observeAsState() starts observing this LiveData and represents its values via State.

    The following dependency is required in the build.gradle file:

Kotlin

dependencies {
      ...
      implementation("androidx.compose.runtime:runtime-livedata:1.4.3")
}

Groovy

dependencies {
      ...
      implementation "androidx.compose.runtime:runtime-livedata:1.4.3"
}

Kotlin

dependencies {
      ...
      implementation("androidx.compose.runtime:runtime-rxjava2:1.4.3")
}

Groovy

dependencies {
      ...
      implementation "androidx.compose.runtime:runtime-rxjava2:1.4.3"
}

Kotlin

dependencies {
      ...
      implementation("androidx.compose.runtime:runtime-rxjava3:1.4.3")
}

Groovy

dependencies {
      ...
      implementation "androidx.compose.runtime:runtime-rxjava3:1.4.3"
}

Stateful versus stateless

A composable that uses remember to store an object creates internal state, making the composable stateful. HelloContent is an example of a stateful composable because it holds and modifies its name state internally. This can be useful in situations where a caller doesn't need to control the state and can use it without having to manage the state themselves. However, composables with internal state tend to be less reusable and harder to test.

A stateless composable is a composable that doesn't hold any state. An easy way to achieve stateless is by using state hoisting.

As you develop reusable composables, you often want to expose both a stateful and a stateless version of the same composable. The stateful version is convenient for callers that don't care about the state, and the stateless version is necessary for callers that need to control or hoist the state.

State hoisting

State hoisting in Compose is a pattern of moving state to a composable's caller to make a composable stateless. The general pattern for state hoisting in Jetpack Compose is to replace the state variable with two parameters:

  • value: T: the current value to display
  • onValueChange: (T) -> Unit: an event that requests the value to change, where T is the proposed new value

However, you are not limited to onValueChange. If more specific events are appropriate for the composable you should define them using lambdas like ExpandingCard does with onExpand and onCollapse.

State that is hoisted this way has some important properties:

  • Single source of truth: By moving state instead of duplicating it, we're ensuring there's only one source of truth. This helps avoid bugs.
  • Encapsulated: Only stateful composables can modify their state. It's completely internal.
  • Shareable: Hoisted state can be shared with multiple composables. If you wanted to read name in a different composable, hoisting would allow you to do that.
  • Interceptable: callers to the stateless composables can decide to ignore or modify events before changing the state.
  • Decoupled: the state for the stateless ExpandingCard may be stored anywhere. For example, it's now possible to move name into a ViewModel.

In the example case, you extract the name and the onValueChange out of HelloContent and move them up the tree to a HelloScreen composable that calls HelloContent.

@Composable
fun HelloScreen() {
    var name by rememberSaveable { mutableStateOf("") }

    HelloContent(name = name, onNameChange = { name = it })
}

@Composable
fun HelloContent(name: String, onNameChange: (String) -> Unit) {
    Column(modifier = Modifier.padding(16.dp)) {
        Text(
            text = "Hello, $name",
            modifier = Modifier.padding(bottom = 8.dp),
            style = MaterialTheme.typography.bodyMedium
        )
        OutlinedTextField(value = name, onValueChange = onNameChange, label = { Text("Name") })
    }
}

By hoisting the state out of HelloContent, it's easier to reason about the composable, reuse it in different situations, and test. HelloContent is decoupled from how its state is stored. Decoupling means that if you modify or replace HelloScreen, you don't have to change how HelloContent is implemented.

The pattern where the state goes down, and events go up is called a unidirectional data flow. In this case, the state goes down from HelloScreen to HelloContent and events go up from HelloContent to HelloScreen. By following unidirectional data flow, you can decouple composables that display state in the UI from the parts of your app that store and change state.

See the Where to hoist state page to learn more.

Restoring state in Compose

The rememberSaveable API behaves similarly to remember because it retains state across recompositions, and also across activity or process recreation using the saved instance state mechanism. For example, this happens, when the screen is rotated.

Ways to store state

All data types that are added to the Bundle are saved automatically. If you want to save something that cannot be added to the Bundle, there are several options.

Parcelize

The simplest solution is to add the @Parcelize annotation to the object. The object becomes parcelable, and can be bundled. For example, this code makes a parcelable City data type and saves it to the state.

@Parcelize
data class City(val name: String, val country: String) : Parcelable

@Composable
fun CityScreen() {
    var selectedCity = rememberSaveable {
        mutableStateOf(City("Madrid", "Spain"))
    }
}

MapSaver

If for some reason @Parcelize is not suitable, you can use mapSaver to define your own rule for converting an object into a set of values that the system can save to the Bundle.

data class City(val name: String, val country: String)

val CitySaver = run {
    val nameKey = "Name"
    val countryKey = "Country"
    mapSaver(
        save = { mapOf(nameKey to it.name, countryKey to it.country) },
        restore = { City(it[nameKey] as String, it[countryKey] as String) }
    )
}

@Composable
fun CityScreen() {
    var selectedCity = rememberSaveable(stateSaver = CitySaver) {
        mutableStateOf(City("Madrid", "Spain"))
    }
}

ListSaver

To avoid needing to define the keys for the map, you can also use listSaver and use its indices as keys:

data class City(val name: String, val country: String)

val CitySaver = listSaver<City, Any>(
    save = { listOf(it.name, it.country) },
    restore = { City(it[0] as String, it[1] as String) }
)

@Composable
fun CityScreen() {
    var selectedCity = rememberSaveable(stateSaver = CitySaver) {
        mutableStateOf(City("Madrid", "Spain"))
    }
}

State holders in Compose

Simple state hoisting can be managed in the composable functions itself. However, if the amount of state to keep track of increases, or the logic to perform in composable functions arises, it's a good practice to delegate the logic and state responsibilities to other classes: state holders.

See the state hoisting in Compose documentation or, more generally, the State holders and UI State page in the architecture guide to learn more.

Retrigger remember calculations when keys change

The remember API is frequently used together with MutableState:

var name by remember { mutableStateOf("") }

Here, using the remember function makes the MutableState value survive recompositions.

In general, remember takes a calculation lambda parameter. When remember is first run, it invokes the calculation lambda and stores its result. During recomposition, remember returns the value that was last stored.

Apart from caching state, you can also use remember to store any object or result of an operation in the Composition that is expensive to initialize or calculate. You might not want to repeat this calculation in every recomposition. An example is creating this ShaderBrush object, which is an expensive operation:

val brush = remember {
    ShaderBrush(
        BitmapShader(
            ImageBitmap.imageResource(res, avatarRes).asAndroidBitmap(),
            Shader.TileMode.REPEAT,
            Shader.TileMode.REPEAT
        )
    )
}

remember stores the value until it leaves the Composition. However, there is a way to invalidate the cached value. The remember API also takes a key or keys parameter. If any of these keys change, the next time the function recomposes, remember invalidates the cache and executes the calculation lambda block again. This mechanism gives you control over the lifetime of an object in the Composition. The calculation remains valid until the inputs change, instead of until the remembered value leaves the Composition.

The following examples show how this mechanism works.

In this snippet, a ShaderBrush is created and used as the background paint of a Box composable. remember stores the ShaderBrush instance because it is expensive to recreate, as explained earlier. remember takes avatarRes as the key1 parameter, which is the selected background image. If avatarRes changes, the brush recomposes with the new image, and reapplies to the Box. This can occur when the user selects another image to be the background from a picker.

@Composable
private fun BackgroundBanner(
    @DrawableRes avatarRes: Int,
    modifier: Modifier = Modifier,
    res: Resources = LocalContext.current.resources
) {
    val brush = remember(key1 = avatarRes) {
        ShaderBrush(
            BitmapShader(
                ImageBitmap.imageResource(res, avatarRes).asAndroidBitmap(),
                Shader.TileMode.REPEAT,
                Shader.TileMode.REPEAT
            )
        )
    }

    Box(
        modifier = modifier.background(brush)
    ) {
        /* ... */
    }
}

In the next snippet, state is hoisted to a plain state holder class MyAppState. It exposes a rememberMyAppState function to initialize an instance of the class using remember. Exposing such functions to create an instance that survives recompositions is a common pattern in Compose. The rememberMyAppState function receives windowSizeClass, which serves as the key parameter for remember. If this parameter changes, the app needs to recreate the plain state holder class with the latest value. This may occur if, for example, the user rotates the device.

@Composable
private fun rememberMyAppState(
    windowSizeClass: WindowSizeClass
): MyAppState {
    return remember(windowSizeClass) {
        MyAppState(windowSizeClass)
    }
}

@Stable
class MyAppState(
    private val windowSizeClass: WindowSizeClass
) { /* ... */ }

Compose uses the class's equals implementation to decide if a key has changed and invalidate the stored value.

Store state with keys beyond recomposition

The rememberSaveable API is a wrapper around remember that can store data in a Bundle. This API allows state to survive not only recomposition, but also activity recreation and system-initiated process death. rememberSaveable receives input parameters for the same purpose that remember receives keys. The cache is invalidated when any of the inputs change. The next time the function recomposes, rememberSaveable re-executes the calculation lambda block.

In the following example, rememberSaveable stores userTypedQuery until typedQuery changes:

var userTypedQuery by rememberSaveable(typedQuery, stateSaver = TextFieldValue.Saver) {
    mutableStateOf(
        TextFieldValue(text = typedQuery, selection = TextRange(typedQuery.length))
    )
}

Learn more

To learn more about state and Jetpack Compose, consult the following additional resources.

Samples

Codelabs

Videos

Blogs