Hello. I'm having trouble testing a simple ViewMod...

# coroutines

[Berkeli Alashov]

Hello. I'm having trouble testing a simple ViewModel that exposes a state by combining MutableStateflows.
Updating MutableStateflow value doesn't cause combined state to emit new value during testing (while works as expected when manually testing on device)
Code with failing test comments in the thread🧵

[Berkeli Alashov]

class StateFlowTestViewModel : ViewModel() {
    data class State(val data: Boolean? = null, val isSaving: Boolean = false, val error: Throwable? = null) {
        val isSaveEnabled = data != null && !isSaving

        companion object {
            val Empty = State()
        }
    }

    @VisibleForTesting val dataState = MutableStateFlow<Boolean?>(null)
    @VisibleForTesting val isSavingState = MutableStateFlow<Boolean>(false)
    @VisibleForTesting val errorState = MutableStateFlow<Throwable?>(null)

    val state = combine(dataState, isSavingState, errorState, ::State).stateIn(viewModelScope, SharingStarted.Lazily, State())

    fun setData(data: Boolean?) {
        dataState.value = data
        // also tried .update { data}, .emit, .tryEmit with viewModelScope.launch
    }

    fun save() = viewModelScope.launch {
        with(state.value) {
            runCatching {
                require(isSaveEnabled) { "Save is not enabled, is UI out of sync?" }
            }.onFailure {
                errorState.value = it
                return@launch
            }

            isSavingState.value = true
            // delay(100)
            isSavingState.value = false
            errorState.value = Throwable("TestError")
        }
    }
}

@RunWith(AndroidJUnit4::class)
class StateFlowTestViewModelTest  {

    @get:Rule
    val instantTaskExecutorRule = InstantTaskExecutorRule()

    @Test
    fun test_setDataAndSave() = runTest {
        val viewModel = StateFlowTestViewModel()

        viewModel.setData(true)

        assertThat(viewModel.dataState.value)
            .isTrue() // doesn't fail

        // assertThat(viewModel.state.value.data)
        //     .isTrue() // fails

        assertThat(viewModel.state.first().data)
            .isTrue() // doesn't fail.
        // ^ calling viewModel.state.first() is also somehow required for below viewModel.save call
        // so it doesn't fail with ui out of sync error (bcz viewModel.state.value.data is still null &  viewModel.state.value.isSaveEnabled is false)

        viewModel.save()
        // also tried viewModel.save().join()

        assertThat(viewModel.errorState.value)
            .isNotNull() // doesn't fail

        assertThat(viewModel.state.value.error)
            .isNotNull() // fails bcz still null

        // also tried
        // assertThat(viewModel.state.first().error).isNotNull()
        // viewModel.state.test {
        //     assertThat(awaitItem().error)
        //         .isNotNull()
        // }
    }
}

[Albert Chang]

https://developer.android.com/kotlin/coroutines/test#setting-main-dispatcher

[Berkeli Alashov]

I will try that, thanks! I've seen that before but for some reason assumed it was being handled by runTest & Roboelectric

[Berkeli Alashov]

Things are mostly working now. Additionally I had to use state.first() instead of state.value in vm.save() to read latest set values right after calling vm.setData(true) in tests. Do you know why that is?