Trying to figure out how to test channels + flow b...

# coroutines

[voben]

Trying to figure out how to test channels + flow but I keep running into a IllegalStateException: This job has not completed yet. I have a channel in my viewmodel which I expose as a flow

ViewModel

private val _channel = BroadcastChannel<Boolean>(1)
val myFlow = _channel.asFlow()

fun sendToChannel() {
    viewModelScope.launch {
        _channel.send(true)
    }
}

And in my activity, I collect the flow

lifecycleScope.launch {
     myFlow.collect { println(it) }
}

My Test

@Test
fun `myTest`() = runBlockingTest {
    
    viewModel.myFlow.collect { myVal ->
        assertTrue(myVal)
    }

    viewModel.sendToChannel()
}

How do I ensure the assertion is called. Or what’s a better way of testing this?

[octylFractal]

https://github.com/Kotlin/kotlinx.coroutines/issues/1204 possibly

[voben]

Weird, so there’s currently no way of testing a BroadcastChannel with Flow?

[streetsofboston]

The runBlockingTest provides a TestCoroutineScope that dispatches on a TestCoroutineDispatcher.
Your viewModelScope uses the Dispatchers.Main. This mismatch may be the problem, where sendToChannel really never winds up calling _channel.send().

[streetsofboston]

Use Dispatchers.setMain to make the Main dispatcher use the TestCoroutineScope's TestCoroutineDispatcher instead for your unit tests

[voben]

I’m currently already doing that with the gist I got from the Google IO app

@get:Rule
var coroutineRule = MainCoroutineRule()

@Test
fun myTest() = coroutineRule.runBlockingTest {}

https://gist.githubusercontent.com/manuelvicnt/049ce057fa6b5e5c785ec9fff7c22a7c/raw/b668143cbc1a8d52ef7267796b8a787165497702/CoroutinesTestRule.kt

[streetsofboston]

I looked at your myTest code.
collect suspends and you never get to call viewModel.sendToChannel().

[streetsofboston]

Wrap the viewModel.myFlow.collect {...} inside a launch ....

[voben]

If we wrap the collect call inside of launch it throws an UncompletedCoroutinesError: Test finished with active jobs . I assume this is because the flow is still open to collecting. Do I then need to call cancel inside the of launch? because that seems seems a little flaky to me but the test passes. Here’s what I have so far

@Test
fun `myTest`() = coroutineRule.runBlockingTest {

    launch {
        viewModel.myFlow.collect { myVal ->
            assertTrue(myVal)
            cancel()
        }
    }
    viewModel.sendToChannel()
}

[voben]

Was able to resolve the issue by using .take(n).toList() rather than collect. I think this might currently be the best way to test flow according to Manuel https://stackoverflow.com/a/58611767/5067435

[tseisel]

I wrote that (long) post a few days ago to introduce a Flow.test operator. This might help you : https://kotlinlang.slack.com/archives/C1CFAFJSK/p1575206282080900