runTest with combine flow and backgroundScope update bug

[cprestera]

Trying to create a test case around a viewModel that has flows that trigger other flows while using combine() seems to be showing some weird behavior. I seemed to have narrowed it down to using the Combine() mixed with the backgroundScope in the runTest block, for some reason the combine() is detecting value changes only once and then not again.

Here is a generic example I made of the test:

@Test
    fun genericTest() = runTest(UnconfinedTestDispatcher(), timeout = 15.seconds) {

        val deferred = CompletableDeferred<Int>()

        backgroundScope.launch {
            viewModel.msf1.collectLatest { println("latest msf1: $it") }
        }

        backgroundScope.launch {
            viewModel.msf2.collectLatest { println("latest msf2: $it") }
        }

        backgroundScope.launch {
            viewModel.csf.collectLatest {
                println("latest csf: $it")
                if (it == 3) {
                    deferred.complete(it)
                }
            }
        }

        backgroundScope.launch {
            viewModel.sf.collectLatest {
                println("latest sf: $it")
                if (it == 4) {
                    deferred.complete(it)
                }
            }
        }

        viewModel.msf1.value = 1
        viewModel.msf2.value = 2
        viewModel.msf2.value = 3
        viewModel.msf1.value = 2
        viewModel.msf2.value = 4

        deferred.await()

        assertEquals(4, viewModel.sf.value)
        assertEquals(3, viewModel.csf.value)
    }

Here is the generic flows in my viewModel:

val msf1 = MutableStateFlow(0)
val msf2 = MutableStateFlow(1)
val csf = combine(msf1, msf2) { a, b -> a + b }.stateIn(
        scope = viewModelScope,
        started = SharingStarted.WhileSubscribed(stopTimeoutMillis = 5_000L),
        initialValue = 0
    )
val sf = msf2.mapLatest { it }.stateIn(
        scope = viewModelScope,
        started = SharingStarted.WhileSubscribed(stopTimeoutMillis = 5_000L),
        initialValue = 0
    )

Here is my test output failure:

latest msf1: 0
latest msf2: 1
latest csf: 0
latest csf: 1
latest sf: 0
latest sf: 1
latest msf1: 1
latest msf2: 2
latest sf: 2
latest msf2: 3
latest sf: 3
latest msf1: 2
latest msf2: 4
latest sf: 4

Expected :3
Actual   :1

I would expect:

The flows to update properly. I can verify these flows update correctly in app. Using the backgroundScope, mixed with the viewModelScope that my viewModel has seems to be causing the issue when mixed with combined. My other flow that does not use combine but does use the viewModelScope updates as expected. The combined flow will just never update and the test will timeout.

Not sure if I am missing something, but seems like a bug.

[dkhalanskyjb]

I couldn't reproduce the issue you're having. I get:

Expected :3
Actual   :6

Which does make sense, because the last value emitted by msf1 and msf2 are 2 and 4, respectively.

Full code that I used:

    object viewModel {
        val scheduler = TestCoroutineScheduler()
        val scope = TestScope(UnconfinedTestDispatcher(scheduler))
        val viewModelScope = scope.backgroundScope
        val msf1 = MutableStateFlow(0)
        val msf2 = MutableStateFlow(1)
        val csf = combine(msf1, msf2) { a, b -> a + b }.stateIn(
            scope = viewModelScope,
            started = SharingStarted.WhileSubscribed(stopTimeoutMillis = 5_000L),
            initialValue = 0
        )
        val sf = msf2.mapLatest { it }.stateIn(
            scope = viewModelScope,
            started = SharingStarted.WhileSubscribed(stopTimeoutMillis = 5_000L),
            initialValue = 0
        )
    }

    @Test
    fun genericTest() = viewModel.scope.runTest(timeout = 15.seconds) {
        val deferred = CompletableDeferred<Int>()

        backgroundScope.launch {
            viewModel.msf1.collectLatest { println("latest msf1: $it") }
        }

        backgroundScope.launch {
            viewModel.msf2.collectLatest { println("latest msf2: $it") }
        }

        backgroundScope.launch {
            viewModel.csf.collectLatest {
                println("latest csf: $it")
                if (it == 3) {
                    deferred.complete(it)
                }
            }
        }

        backgroundScope.launch {
            viewModel.sf.collectLatest {
                println("latest sf: $it")
                if (it == 4) {
                    deferred.complete(it)
                }
            }
        }

        viewModel.msf1.value = 1
        viewModel.msf2.value = 2
        viewModel.msf2.value = 3
        viewModel.msf1.value = 2
        viewModel.msf2.value = 4

        deferred.await()

        assertEquals(4, viewModel.sf.value)
        assertEquals(3, viewModel.csf.value)
    }

I've also tried other variations (have viewModelScope just be a normal CoroutineScope with a properly injected test dispatcher, both using UnconfinedTestDispatcher and StandardTestDispatcher), but only the println output changed, the reason for the assertion failure stayed the same.

[cprestera]

I think what you have works still because you are setting the viewModelScope to be the backgroundScope:
val scheduler = TestCoroutineScheduler() val scope = TestScope(UnconfinedTestDispatcher(scheduler)) val viewModelScope = scope.backgroundScope

The viewModel I am testing against uses the default scope defined in the viewmodel class. So my runTest block uses the UnconfinedTestDispatcher() with the backgroundScope for the coroutine calls and the flows I am trying to test against in my viewmodel use the default viewmodelScope.

This is the defined scope I am referring to for my viewModel:
public val ViewModel.viewModelScope: CoroutineScope get() = synchronized(VIEW_MODEL_SCOPE_LOCK) { getCloseable(VIEW_MODEL_SCOPE_KEY) ?: createViewModelScope().also { scope -> addCloseable(VIEW_MODEL_SCOPE_KEY, scope) } }

Can you confirm using the androidx.lifecycle ViewModel() class with the default scope if it still works?

[dkhalanskyjb]

I think what you have works still because you are setting the viewModelScope to be the backgroundScope

I also checked this with other options (including what should be equivalent to the default viewModelScope).

Could it be that you are missing a call to Dispatchers.setMain? If you do call it, what do you pass there? Also, what version of kotlinx.coroutines are you using?

[cprestera]

I do not call Dispatchers.setMain, within the ViewModel class the viewModelScope is passed to .stateIn().
I am using kotlinx.coroutines 1.9.0.

As mentioned before the combine() flow seems to not work as expected when using my setup. My guess is that the backgroundScope from runTest{} mixed with the combine flow which is scoped by the viewModelScope is causing the issue.

Any other recommendations?

[dkhalanskyjb]

I do not call Dispatchers.setMain

That's the root of your issues. Please see https://developer.android.com/kotlin/coroutines/test#setting-main-dispatcher and/or https://github.com/Kotlin/kotlinx.coroutines/blob/master/kotlinx-coroutines-test/README.md#dispatchersmain-delegation. The test framework doesn't know anything about tasks happening on your main dispatcher, so the coroutines that are internally spawned by combine effectively run in parallel to your normal test code. I don't know where your main dispatcher comes from if you are not setting it, so it could be that you need to poll its tasks manually, or just Thread.sleep until the actual main thread processes the tasks created by combine. The easiest fix, though, would certainly be making sure your code is properly interacting with the test framework.

[cprestera]

I see, I must have missed that in the documentation. I was able to get combine to trigger now. I used Dispatchers.setMain().
Thank you!! I will close this bug.