cancelAndIgnoreRemainingEvents() does not cancel a Molecule StateFlow

[Egorand]

This might be Molecule-specific, but I still expect cancelAndIgnoreRemainingEvents() to be able to cancel any StateFlow.

Here's a simple Molecule presenter:

@Composable fun MoleculePresenter(): Model {
  return Model("Hello!")
}

And a test for it:

class MoleculePresenterTest {
  @Test fun test() = runBlocking {
    makePresenter().test {
      assertEquals(Model("Hello!"), awaitItem())
      cancelAndIgnoreRemainingEvents()
    }
  }

  private fun CoroutineScope.makePresenter(): StateFlow<Model> {
    return launchMolecule(RecompositionClock.Immediate) {
      MoleculePresenter()
    }
  }
}

test() never completes when executed, neither with cancelAndIgnoreRemainingEvents() nor without it.

The repro project is here: https://github.com/Egorand/turbine-cancel-hangs. Simply run ./gradlew app:testDebugUnitTest to reproduce the issue.

(Crappy) workaround:

@Test(expected = CancellationException::class)
fun test() = runBlocking {
  makePresenter().test {
    assertEquals(Model("Hello!"), awaitItem())
    coroutineContext.cancel()
  }
}

[JakeWharton]

As far as Turbine is concerned, this is working as expected. The coroutine which is driving the StateFlow was launched separately from Turbine's own coroutine which collects from the StateFlow. When you cancel, Turbine can only cancel the one which is doing the collection.

You would see similar behavior with code like:

val flow = neverFlow().stateIn(this, Unit)
flow.test { cancelAndIgnoreRemainingEvents() }

If you launch any coroutines separately from Turbine they will also need to be canceled at the end of the test. You can do this by canceling all children of the runBlocking scope, or starting a child scope with an explicit Job that you can cancel, or, in this case, by switching to a moleculeFlow instead of launchMolecule.

[PaulWoitaschek]

In my pet project I overcame this by using runTest in combination with the background dispatcher:
https://github.com/PaulWoitaschek/Voice/blob/4625edbc891327bfa15aff3641b78c153d9444ed/folderPicker/src/test/kotlin/voice/folderPicker/selectType/SelectFolderTypeViewModelTest.kt#L38

[JakeWharton]

Nice! Can you contribute something to the README about that pattern?

[PaulWoitaschek]

About which pattern? Utilizing molecule + turbine to test composables?

[JakeWharton]

A bit more general in that the background scope can be used for launching things which produce a StateFlow which become the subject of a Turbine test.

Although, in general, testing a StateFlow with Turbine (at least with 0.9.0) is a bit sketchy. We no longer use the Unconfined dispatcher for collections so you are subject to races with which actual states you see reflected as events. Ideally you can just call .value after performing an action, but that relies on no threading to be in play. If there is threading at play, awaitItem() may conflate two updates into a single event which can surprise.

So I'm not actually sure how detailed we want to get here.

[Egorand]

Gonna close the issue cause it's not really a bug.