I have a test where a flow should emit with delay,...

# coroutines

[Lukasz Kalnik]

I have a test where a flow should emit with delay, but it doesn't emit:

@Test
fun `test flow`() = runTest {
    val eventUpdateService = mockk { 
        every { subscribeToEvents() } returns flowOf(SomeEvent).onStart { delay(1000) } 
    }
    val presenter = Presenter(
        UnconfinedTestDispatcher(),
        eventUpdateService
    )
    presenter.attachView(view)
    advanceTimeBy(2000)
    verify { view.displayEvent() } // fails because the flow didn't emit
}

class Presenter(
    coroutineContext: CoroutineContext,
    val eventUpdateService: EventUpdateService    
) {

    val coroutineScope = CoroutineScope(coroutineContext)

    fun attachView(view: View) {
        coroutineScope.launch {
            eventUpdateService.subscribeToEvents().collect { event ->
                view.displayEvent()
            }
        }
    }
}

[Lukasz Kalnik]

Presenter for the test has injected an UnconfinedTestDispatcher for the CoroutineScope to collect the flow.

[Joffrey]

There is no code here that collects the flow here AFAICT. The flow is cold, nothing happens until some consumer collects it

[Lukasz Kalnik]

The consumer is inside the tested presenter, consuming should cause a side effect.
Sorry, I didn't post all relevant code.

[Joffrey]

presenter.subscribeToEvents() is mocked to return a flow that is never used or collected, what is inside the presenter is irrelevant

[Lukasz Kalnik]

You are obviously right, sorry. By trying to make my example minimal I actually made it wrong...

[Lukasz Kalnik]

Edited, should be correct now.

[Lukasz Kalnik]

Thanks for pointing this out.

[Joffrey]

My guess is that the advanceTimeBy call doesn't affect the Presenter because you're passing a new UnconfinedTestDispatcher (which is not linked to runTest) to the Presenter instance

[Lukasz Kalnik]

Yes, that's also my suspicion.

[Lukasz Kalnik]

Looks like for this case I need to use e.g. common TestScope instance for both running the test and initializing the presenter.

[Joffrey]

Yep. You should have access to the coroutineContext of the current test scope inside runTest, so you should pass this one directly to the presenter without creating a new dispatcher

[Lukasz Kalnik]

@Test
fun `test flow`() = runTest {
    val presenter = Presenter(coroutineContext, //...)
    // ...
}

This indeed works. Thank you!

[Nick Allen]

btw, creating a CoroutineScope from an external CoroutineContext seems a little odd. If the context has a Job then that means the caller owns the lifetime, If it doesn't have a Job then CoroutineScope factory function creates a Job and the Presenter is in charge of managing the lifetime. I generally pass in CoroutineScope and it's up to the caller to manage the lifetime or I pass in a specific context like CoroutineDispatcher that definitely does not contain a Job so the class can create it's own CoroutineScope and be in change of cancelling it.

[Lukasz Kalnik]

Yes, that's a very good point.
This example is obviously simplified. We don't use the CoroutineScope factory function itself. We have our own PresenterCoroutineScope which creates a Job and lets the Presenter internally manage its own lifecycle.
But you are right, in this case we should only pass a dispatcher and not the whole context, including the externally created Job.
Thank you for bringing this to my attention.

[Joffrey]

I kinda disagree (at least as a general statement). Depending on how generic a component is, it may be nice to allow users to specify other things like an exception handler or a coroutine name, not just a dispatcher. And yet you may want control over the lifecycle using cancel() which would be unadvisable on a user-provided scope

[Lukasz Kalnik]

In production code we actually always inject in the PresenterCoroutineScope a Dispatchers.Default.

[Joffrey]

Yep it might be correct for your use case. I was disagreeing with @Nick Allen's general statement

[Lukasz Kalnik]

I wonder how it works for tests though. Tests need to inject an exception handler I guess?

[Lukasz Kalnik]

Because runTest has to rethrow all exceptions that possibly happened in coroutines launched by Presenter during the test in order for the test to fail.