Hi! I have a problem with testing the Flow using <...

# coroutines

[Adam Grzybkowski]

Hi!
I have a problem with testing the Flow using FlowAssert from SqlDelight library. It might be related to FlowAssert, but tbh I feel like I know nothing right now :D
I created a repo with the problem here, you can find there two branches master and sqldelight
On master branch I've tried to reproduce the original problem I've encountered when testing SqlDelight but without the SqlDelight code.

The problem
Tests are stuck when TestCoroutineDipsatcher is injected and used in flowOn operator here.

fun getData(): Flow<Pair<String?, String?>> {
    val flow1 = channel1.asFlow()
        .flowOn(dispatcher)
    val flow2 = channel2.asFlow()
        .flowOn(dispatcher)
    return flow1.combine(flow2) { data1, data2 ->
            data1 to data2
        }
        .flowOn(dispatcher)
}

suspend fun addData1(value: String?) = withContext(dispatcher) {
    channel1.send(value)
}

@Test
fun `receives data1`() = runBlocking {
    exampleDataSource.getData()
        .test {
            assertEquals(null to null, expectItem())

            exampleDataSource.addData1("data1")
            assertEquals("data1" to null, expectItem())

            cancel()
        }
}

So far I've been able to make the tests pass by removing the flowOn operator or by changing the injected dispatcher to Dispatchers.Unconfined


Question
Why is this happening? Could anyone explain me why using flowOn operator is causing this?
I may be using this wrong coming from Rx word 😕


repository url https://github.com/AdamGrzybkowski/coroutineTestingProblem

[tseisel]

When using TestCoroutineDispatcher, you need to also use `runBlockingTest`: https://github.com/Kotlin/kotlinx.coroutines/blob/master/kotlinx-coroutines-test/README.md#providing-an-explicit-testcoroutinedispatcher
I don't think you can use TestCoroutineDispatcher with runBlocking.

[Adam Grzybkowski]

yup, you are probably right that I shouldn't use TestCoroutineDispatcher with runBlocking but adding runBlockingTest will not fix the tests.

And still for me the most weird behavior is that removing the flowOn operator from combine is fixing the tests.

[wasyl]

@Adam Grzybkowski One thing that fixes the tests is also switching from runBlocking to testCoroutineDispatcher.runBlockingTest. I also have no idea why, but it seems to work