Document configuration for AGP to unit test Molecule

I’m trying out Molecule with AAC ViewModel. Here is how the VM code is written exposing stateFlow stream using launchMolecule()

Molecule version used : 0.5.0-SNAPSHOT

@HiltViewModel
class PhotosListViewModel @Inject constructor(
    private val unsplashRepository: UnsplashRepository,
    @MoleculeScope private val scope: CoroutineScope,
    @CompositionClock private val clock: RecompositionClock,
) : ViewModel() {

    private val events = Channel<Event>()
    val stateFlow = scope.launchMolecule(clock = clock) {
        present(events.receiveAsFlow())
    }

    init {
        processEvent(InitialPageEvent)
    }

    @Composable
    fun present(events: Flow<Event>): PhotosListUIState {
        var isLoading by remember { mutableStateOf(false) }
        var error by remember { mutableStateOf<String?>(null) }
        val images = remember { mutableStateListOf<UnsplashImage>() }

        LaunchedEffect(Unit) {
            events.collect { event ->
                when (event) {
                    InitialPageEvent -> {
                        isLoading = true
                        error = null
                        when (val result =
                            unsplashRepository.getPhotos(page = 1, perPage = ITEM_PER_PAGE)) {
                            is Error -> {
                                isLoading = false
                                error = result.message
                            }
                            is Success -> {
                                isLoading = false
                                images.addAll(result.data.images)
                            }
                        }

                    }
                }
            }
        }
        return PhotosListUIState(
            isLoading = isLoading,
            error = error,
            images = images,
        )
    }

    fun processEvent(event: Event) {
        scope.launch {
            events.send(event)
        }
    }
}

// UI State
data class PhotosListUIState(
    val isLoading: Boolean = false,
    val error: String? = null,
    val images: List<UnsplashImage> = emptyList(),
)

// Events
sealed interface Event
object InitialPageEvent : Event

App works fine on Android devices without any issue when VM is provided with correct coroutineScope and frameClock. However, when I write Unit test for VM it requires android.os.Trace to be mocked. Added both unit test and it’s error log below.

VM’s Unit Test

import app.cash.molecule.RecompositionClock
import app.cash.turbine.testIn
import dev.punitd.unplashapp.data.fake.FakeUnsplashRepository
import dev.punitd.unplashapp.model.images
import dev.punitd.unplashapp.model.pageLinks
import dev.punitd.unplashapp.util.CoroutineRule
import kotlinx.coroutines.ExperimentalCoroutinesApi
import kotlinx.coroutines.test.runTest
import org.junit.Assert.assertEquals
import org.junit.Rule
import org.junit.Test


@OptIn(ExperimentalCoroutinesApi::class)
class PhotosListViewModelTest {

    @get:Rule
    val coroutineRule = CoroutineRule()

    @Test
    fun successStateIfApiSucceeds() = runTest {
        val viewModel = PhotosListViewModel(
            unsplashRepository = FakeUnsplashRepository(coroutineRule.testDispatcher),
            scope = this, // TestScope
            clock = RecompositionClock.Immediate,
        )

        // InitialPageEvent is send inside VM's init{} block by default
        // That's why we're not sending any events here.

        val turbine = viewModel.stateFlow.testIn(this)
        assertEquals(PhotosListUIState(isLoading = false), turbine.awaitItem())
        assertEquals(PhotosListUIState(isLoading = true), turbine.awaitItem())
        assertEquals(
            PhotosListUIState(
                isLoading = false,
                error = null,
                images = images,
                pageLinks = pageLinks
            ),
            turbine.awaitItem()
        )
        turbine.cancel()
    }
}

Test Error log

Method beginSection in android.os.Trace not mocked. See http://g.co/androidstudio/not-mocked for details.
java.lang.RuntimeException: Method beginSection in android.os.Trace not mocked. See http://g.co/androidstudio/not-mocked for details.
	at android.os.Trace.beginSection(Trace.java)
	at androidx.compose.runtime.Trace.beginSection(ActualAndroid.android.kt:30)
	at androidx.compose.runtime.ComposerImpl.doCompose(Composer.kt:4357)
	at androidx.compose.runtime.ComposerImpl.composeContent$runtime_release(Composer.kt:3119)
	at androidx.compose.runtime.CompositionImpl.composeContent(Composition.kt:584)
	at androidx.compose.runtime.Recomposer.composeInitial$runtime_release(Recomposer.kt:811)
	at androidx.compose.runtime.CompositionImpl.setContent(Composition.kt:519)
	at app.cash.molecule.MoleculeKt.launchMolecule(molecule.kt:163)
	at app.cash.molecule.MoleculeKt.launchMolecule(molecule.kt:108)
	at dev.punitd.unplashapp.screen.photos.PhotosListViewModel.<init>(PhotosListViewModel.kt:32)

I’m sure i’m doing something stupid here because unit tests written in sample app of this repo works just fine without any such error.

Questions:

• Is there something I need to mock which i’m missing here?
• Are Coroutinescope and FrameClock passed to VM in test incorrect?

Repo to reproduce it : https://github.com/punitda/Tinysplash

Thanks!

[JakeWharton]

Upstream bug about exception message: https://issuetracker.google.com/issues/246752914

[JakeWharton]

Oh, you’re right. I’ll file a bug on AGP for that.