`cancelAndIgnoreRemainingEvents()` does not cancel a Molecule StateFlow

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]

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.

[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