Hi team, I’ve got a testing question. It only very...

# squarelibraries

[Tim Malseed]

Hi team, I’ve got a testing question. It only very loosely relates to Turbine, I’m sure it’s a more general coroutines/testing problem. But I thought I’d ask here anyway.

It’s to do with testing the interactions between a Channel and a SharedFlow. See 🧵

[Tim Malseed]

Suppose we have this class:

class SystemUnderTest(
    externalEvents: SharedFlow<Int>
) {
    private val _internalEvents = Channel<String>()
    val internalEvents = _internalEvents.receiveAsFlow()

    val mappedEvents = externalEvents.map { number ->
        _internalEvents.send(number.toString())
    }
}

This is a representation of a real ViewModel that I’m dealing with. Ideally we’d refactor it, but let’s say we need test coverage first..

So, here’s a test that tries to assert that when an external event receives a new value, our internal channel is updated:

class MixedFlowsTest() {

    @Test
    fun testCase() = runTest {
        // Arrange
        val externalEvents = MutableSharedFlow<Int>()
        val systemUnderTest = SystemUnderTest(externalEvents)

        // Act
        systemUnderTest.internalEvents.test {
            externalEvents.emit(1)

            // Assert
            Assertions.assertEquals("1", awaitItem())
        }
    }
}

[Tim Malseed]

This doesn’t work.. presumably it’s because nothing is collecting mappedEvents, so _internal_events.send() never gets called

[Tim Malseed]

We can try to fix that like so:

class MixedFlowsTest() {

    @Test
    fun testCase() = runTest {
        // Arrange
        val externalEvents = MutableSharedFlow<Int>()
        val systemUnderTest = SystemUnderTest(externalEvents)

        val job = launch {
            systemUnderTest.mappedEvents.collect()
        }

        // Act
        systemUnderTest.internalEvents.test {
            externalEvents.emit(1)

            // Assert
            Assertions.assertEquals("1", awaitItem())
        }

        // Cleanup

        job.cancel()
    }
}

But still, the test fails with ‘no value produced’

[Tim Malseed]

I’m guessing it’s a timing issue around when the events are emitted vs. when they are collected. But I really don’t understand what’s going on

[Tim Malseed]

Moving externalEvents.emit(1) to its own launch{} block, before the .test{} seems to solve the problem. I think it’s to do with the code indefinitely suspending on that emit call. I don’t understand it tho