ViewModel lifecycle, scope, factories, and memory

ViewModel is a screen-level business-state holder whose lifetime is controlled by a ViewModelStoreOwner.

Scope determines identity

Owner	Same instance until	Use case
Activity	Activity finishes	app-shell/shared Activity state
Fragment	Fragment permanently removed	one Fragment screen
Nav destination	entry popped	destination state
Navigation graph	graph entry removed	multi-step flow sharing

Rotation recreates Activity/Fragment objects but retains the owner's store. Process death destroys the process and ViewModel; reconstruction needs saved/durable state.

@Composable
fun CheckoutRoute(vm: CheckoutViewModel = hiltViewModel()) {
    val state by vm.state.collectAsStateWithLifecycle()
    CheckoutContent(state, vm::onAction)
}

Keep ViewModel acquisition at the route boundary. Pass values/callbacks to reusable content instead of passing the ViewModel through the tree.

Scope mistakes

❌ Activity-scoping every ViewModel retains finished feature state, couples screens, and makes back-stack behavior surprising.

❌ Creating a ViewModel for every lazy-list row can create hundreds of lifecycle objects and unstable identity unless ownership/keys are deliberately managed.

✅ Choose the narrowest owner that must share and retain the state.

Memory rules

ViewModel can outlive an Activity instance. It should not hold Activity, Fragment, View, NavController, or UI callback references.

// ❌ Activity may be destroyed while ViewModel remains.
class BadViewModel(val activity: Activity) : ViewModel()

Application context is safer technically but still often signals misplaced UI/resource work. Expose semantic errors; let UI resolve localized strings.

viewModelScope cancels when onCleared runs. It does not make work durable beyond the screen/process; use repository/application scope or WorkManager when required.

Creation and runtime inputs

Constructor injection supplies stable dependencies. Navigation/runtime identifiers come from typed route arguments/SavedStateHandle or an assisted factory. Do not use a global service locator inside the ViewModel; hidden dependencies harm tests and lifecycle clarity.

class ProductViewModel(
    savedStateHandle: SavedStateHandle,
    private val products: ProductRepository,
) : ViewModel() {
    private val productId = ProductId(requireNotNull(savedStateHandle["productId"]))
}

Validate arguments once and fail with a useful boundary error or render an invalid-link state.

Sources

• ViewModel overview
• ViewModel scoping APIs

Interview exercises

1. Choose scopes for checkout steps and shared cart state.
2. Explain why ViewModel surviving rotation does not mean process persistence.
3. Find lifecycle leaks in a ViewModel holding callbacks and Resources.