fabioCollini

@Test
fun `sets the result when an input changes`() {
    Snapshot.withMutableSnapshot {
        viewModel.v1 = "10"
        viewModel.v2 = "20"
    }
    assertThat(viewModel.result).isEqualTo("30")
}

fabioCollini

class ViewModelTest {
    @get:Rule
    val mainCoroutineRule = MainCoroutineRule()

    private val viewModel by lazy {
        CalculatorViewModel(SavedStateHandle(mutableMapOf()))
    }

    @Test
    fun `sets the result when an input changes`() {

fabioCollini

class CalculatorViewModel(
    state: SavedStateHandle,
) : ViewModel() {
    var v1 by state.mutableStateOf("0")
    var v2 by state.mutableStateOf("0")

    val result by state.mutableStateOf("0") { valueLoadedFromState, setter ->
        snapshotFlow { v1 to v2 }
            .drop(if (valueLoadedFromState != null) 1 else 0)
            .mapLatest {

fabioCollini

fun <T> SavedStateHandle.mutableStateOf(
    defaultValue: T,
    initializer: (valueLoadedFromState: T?, setter: (T) -> Unit) -> Unit = { _, _ -> },
) = PropertyDelegateProvider<Any, SavedStateHandleDelegate<T>> { _, property ->
    SavedStateHandleDelegate(
        savedStateHandle = this,
        key = property.name,
        defaultValue = defaultValue,
        initializer = initializer
    )

fabioCollini

class SavedStateHandleDelegate<T>(
    private val savedStateHandle: SavedStateHandle,
    private val key: String,
    defaultValue: T,
    initializer: (valueLoadedFromState: T?, setter: (T) -> Unit) -> Unit
) : ReadWriteProperty<Any, T> {
    private val state: MutableState<T>

    init {
        val savedValue = savedStateHandle.get<T>(key)

fabioCollini

class CalculatorViewModel(
    state: SavedStateHandle,
) : ViewModel() {
    var v1 by state.mutableStateOf("0")
    var v2 by state.mutableStateOf("0")

    var result by state.mutableStateOf("0")
        private set

    init {

fabioCollini

fun <T> SavedStateHandle.mutableStateOf(
    defaultValue: T,
) = PropertyDelegateProvider<Any, SavedStateHandleDelegate<T>> { _, property ->
    SavedStateHandleDelegate(
        savedStateHandle = this,
        key = property.name,
        defaultValue = defaultValue,
    )
}

fabioCollini

class SavedStateHandleDelegate<T>(
    private val savedStateHandle: SavedStateHandle,
    private val key: String,
    defaultValue: T,
) : ReadWriteProperty<Any, T> {
    private val state: MutableState<T>

    init {
        val savedValue = savedStateHandle.get<T>(key)
        state = mutableStateOf(

fabioCollini

@Composable
fun Calculator(state: CalculatorViewModel) {
    Column {
        TextField(value = state.v1, onValueChange = { state.v1 = it })
        TextField(value = state.v2, onValueChange = { state.v2 = it })
        Text(text = state.result)
    }
}

fabioCollini

class CalculatorViewModel : ViewModel() {
    var v1 by mutableStateOf("0")
    var v2 by mutableStateOf("0")
    var result by mutableStateOf("0")
        private set

    init {
        snapshotFlow { v1 to v2 }
            .mapLatest {
                sum(it.first, it.second)