app-lifecycle

App Lifecycle Patterns

Android

SavedStateHandle in ViewModel

SavedStateHandle survives both configuration changes and process death:

class ProductDetailViewModel(
    private val savedStateHandle: SavedStateHandle,
    private val productRepository: ProductRepository
) : ViewModel() {

    // Survives process death
    val productId: String = savedStateHandle.get<String>("productId") ?: ""

    // StateFlow backed by SavedStateHandle
    val searchQuery = savedStateHandle.getStateFlow("searchQuery", "")
    val selectedTab = savedStateHandle.getStateFlow("selectedTab", 0)

    fun updateSearchQuery(query: String) {
        savedStateHandle["searchQuery"] = query
    }

    fun selectTab(index: Int) {
        savedStateHandle["selectedTab"] = index
    }

    // Transient state (lost on process death, OK for derived data)
    private val _uiState = MutableStateFlow<UiState>(UiState.Loading)
    val uiState: StateFlow<UiState> = _uiState.asStateFlow()

    init {
        loadProduct(productId)
    }

    private fun loadProduct(id: String) {
        viewModelScope.launch {
            _uiState.value = UiState.Loading
            productRepository.getProduct(id)
                .onSuccess { _uiState.value = UiState.Success(it) }
                .onFailure { _uiState.value = UiState.Error(it.message ?: "Unknown error") }
        }
    }
}

rememberSaveable in Compose

@Composable
fun SearchScreen() {
    // Survives configuration change AND process death
    var searchQuery by rememberSaveable { mutableStateOf("") }
    var isFilterExpanded by rememberSaveable { mutableStateOf(false) }

    // For complex objects, use a custom Saver
    val selectedFilters = rememberSaveable(
        saver = listSaver(
            save = { it.toList() },
            restore = { it.toMutableStateList() }
        )
    ) { mutableStateListOf<String>() }

    // For Parcelable objects
    var selectedProduct by rememberSaveable(stateSaver = autoSaver()) {
        mutableStateOf<Product?>(null)
    }

    Column {
        TextField(
            value = searchQuery,
            onValueChange = { searchQuery = it },
            placeholder = { Text("Search...") }
        )
    }
}