ViewModel overview Part of Android Jetpack.

The ViewModel class is a business logic or screen level state holder. It exposes state to the UI and encapsulates related business logic. Its principal advantage is that it caches state and persists it through configuration changes. This means that your UI doesn't have to fetch data again when navigating between activities, or following configuration changes, such as when rotating the screen.

For more information on state holders, see the state holders guidance. Similarly, for more information on the UI layer generally, see the UI layer guidance.

ViewModel benefits

The alternative to a ViewModel is a plain class that holds the data you display in your UI. This can become a problem when navigating between activities or Navigation destinations. Doing so destroys that data if you don't store it using the saved instance state mechanism. ViewModel provides a convenient API for data persistence that resolves this issue.

Alternatively, for pure state holders, Compose offers retain capabilities that allow plain classes to survive configuration changes without the full infrastructure of a ViewModel. While both mechanisms help with state retention, it is generally safer to provide a ViewModel to a retained instance rather than the other way around, as their lifecycles and cleanup behaviors differ.

The key benefits of the ViewModel class are essentially two:

It lets you persist UI state.
It provides access to business logic.

Persistence

ViewModel allows persistence through both the state that a ViewModel holds, and the operations that a ViewModel triggers. This caching means that you don't have to fetch data again through common configuration changes, such as a screen rotation.

Scope

When you instantiate a ViewModel, you pass it an object that implements the ViewModelStoreOwner interface. This may be a Navigation destination, Navigation graph, activity, or any other type that implements the interface. You also can scope a ViewModel directly to a composable using the rememberViewModelStoreOwner API. Your ViewModel is then scoped to the Lifecycle of the ViewModelStoreOwner. It remains in memory until its ViewModelStoreOwner goes away permanently (like when the composable owner exits the Composition).

A range of classes are either direct or indirect subclasses of the ViewModelStoreOwner interface. The direct subclasses are ComponentActivity and NavBackStackEntry. For a full list of indirect subclasses, see the ViewModelStoreOwner reference. To scope ViewModels to individual items in a LazyList or Pager, use rememberViewModelStoreProvider() to hoist the owner management to the parent.

When the host activity undergoes a configuration change, asynchronous work continues in the ViewModel, whether it is scoped to the activity or to specific composable. This is the key to persistence.

For more information, see the ViewModel lifecycle section that follows, ViewModel Scoping APIs, and the guide on state hoisting for Jetpack Compose.

SavedStateHandle

SavedStateHandle lets you persist data not just through configuration changes, but across process death. That is, it lets you keep the UI state intact even when the user closes the app and opens it at a later time.

For more information about saving UI state, see Save UI state in Compose.

Access to business logic

Even though the vast majority of business logic is present in the data layer, the UI layer can also contain business logic. This can be the case when combining data from multiple repositories to create the screen UI state, or when a particular type of data doesn't require a data layer.

ViewModel is the right place to handle business logic in the UI layer. The ViewModel is also in charge of handling events and delegating them to other layers of the hierarchy when business logic needs to be applied to modify application data.

Implement a ViewModel

The following is an example implementation of a ViewModel for a screen that allows the user to roll dice.

data class DiceUiState(
    val firstDieValue: Int? = null,
    val secondDieValue: Int? = null,
    val numberOfRolls: Int = 0,
)

class DiceRollViewModel : ViewModel() {

    // Expose screen UI state
    private val _uiState = MutableStateFlow(DiceUiState())
    val uiState: StateFlow<DiceUiState> = _uiState.asStateFlow()

    // Handle business logic
    fun rollDice() {
        _uiState.update { currentState ->
            currentState.copy(
                firstDieValue = Random.nextInt(from = 1, until = 7),
                secondDieValue = Random.nextInt(from = 1, until = 7),
                numberOfRolls = currentState.numberOfRolls + 1,
            )
        }
    }
}

You can then access the ViewModel from a screen-level composable as follows:

import androidx.lifecycle.viewmodel.compose.viewModel

// Use the 'viewModel()' function from the lifecycle-viewmodel-compose artifact
@Composable
fun DiceRollScreen(
    viewModel: DiceRollViewModel = viewModel()
) {
    val uiState by viewModel.uiState.collectAsStateWithLifecycle()
    // Update UI elements
}

Use coroutines with ViewModel

ViewModel includes support for Kotlin coroutines. It is able to persist asynchronous work in the same manner as it persists UI state.

For more information, see Use Kotlin coroutines with Android Architecture Components.

The lifecycle of a ViewModel

The lifecycle of a ViewModel is tied directly to its scope. A ViewModel remains in memory until the ViewModelStoreOwner to which it is scoped disappears. This may occur in the following contexts:

In the case of an activity, when it finishes.
In the case of a Navigation entry, when it's removed from the back stack.
In the case of a composable, when it exits the Composition. You can use rememberViewModelStoreOwner to scope a ViewModel directly to an arbitrary part of your UI (like a Pager or LazyList).

This makes ViewModels a great solution for storing data that survives configuration changes.

Figure 1 illustrates the various lifecycle states of an activity as it undergoes a rotation and then is finished. The illustration also shows the lifetime of the ViewModel next to the associated activity lifecycle. This particular diagram illustrates the states of an activity.

Illustrates the lifecycle of a ViewModel as an activity changes state. — **Figure 1.** Lifecycle states of an activity and a ViewModel.

You usually request a ViewModel the first time the system calls an activity object's onCreate() method. The system may call onCreate() several times throughout the existence of an activity, such as when a device screen is rotated. The ViewModel exists from when you first request a ViewModel until the activity is finished and destroyed.

Clearing ViewModel dependencies

The ViewModel calls the onCleared method when the ViewModelStoreOwner destroys it in the course of its lifecycle. This allows you to clean up any work or dependencies that follow the ViewModel's lifecycle.

The following example shows an alternative to viewModelScope. viewModelScope is a built-in CoroutineScope that automatically follows the ViewModel's lifecycle. The ViewModel uses it to trigger business-related operations. If you want to use a custom scope instead of viewModelScope for easier testing, the ViewModel can receive a CoroutineScope as a dependency in its constructor. When the ViewModelStoreOwner clears the ViewModel at the end of its lifecycle, the ViewModel also cancels the CoroutineScope.

class MyViewModel(
    private val coroutineScope: CoroutineScope =
        CoroutineScope(SupervisorJob() + Dispatchers.Main.immediate)
) : ViewModel() {

    // Other ViewModel logic ...

    override fun onCleared() {
        coroutineScope.cancel()
    }
}

From lifecycle version 2.5 and above, you can pass one or more Closeable objects to the ViewModel's constructor that automatically closes when the ViewModel instance is cleared.

class CloseableCoroutineScope(
    context: CoroutineContext = SupervisorJob() + Dispatchers.Main.immediate
) : Closeable, CoroutineScope {
    override val coroutineContext: CoroutineContext = context
    override fun close() {
        coroutineContext.cancel()
   }
}

class MyViewModel(
    private val coroutineScope: CoroutineScope = CloseableCoroutineScope()
) : ViewModel(coroutineScope) {
    // Other ViewModel logic ...
}

Best practices

The following are several key best practices you should follow when implementing ViewModel:

Because of their scoping, use ViewModels as implementation details of a screen level state holder. Don't use them as state holders of reusable UI components such as chip groups or forms. Otherwise, you'd get the same ViewModel instance in different usages of the same UI component under the same ViewModelStoreOwner unless you use an explicit view model key per chip.
ViewModels shouldn't know about the UI implementation details. Keep the names of the methods the ViewModel API exposes and those of the UI state fields as generic as possible. In this way, your ViewModel can accommodate any type of UI: a mobile phone, foldable, tablet, or even a Chromebook!
As they can potentially live longer than the ViewModelStoreOwner, ViewModels shouldn't hold any references of lifecycle-related APIs such as the Context or Resources to prevent memory leaks.
Don't pass ViewModels to other classes, functions or other UI components. Because the platform manages them, you should keep them as close to it as you can—close to your Activity, screen level composable function, or Navigation destination. This prevents lower level components from accessing more data and logic than they need.

Further information

As your data grows more complex, you might choose to have a separate class just to load the data. The purpose of ViewModel is to encapsulate the data for a UI controller to let the data survive configuration changes. For information about how to load, persist, and manage data across configuration changes, see Saved UI States.

The Guide to Android App Architecture suggests building a repository class to handle these functions.

Additional resources

For further information about the ViewModel class, consult the following resources.