Hi. I’m not sure if it is a bug or I treat the doc...

# coroutines

[azabost]

Hi. I’m not sure if it is a bug or I treat the docs too literally so please advise.
The doc for Flow.launchIn() says it is “a shorthand for `scope.launch { flow.collect() }`” but when I compare behaviors in unit tests it is a little different.

Having a simple flow:

private val integersFlow = flow {
        var i = 1
        while(true) {
            delay(100)
            emit(i++)
        }
    }

and using test scope and dispatcher:

private val observingScopeJob1 = SupervisorJob()
private val observingScope1 = CoroutineScope(observingScopeJob1)
private val testDispatcher = TestCoroutineDispatcher()

When I do the following:

@Test
    fun `should receive 10 integers - collect`() = runBlocking {
        var observedIntegers = 0

        observingScope1.launch(testDispatcher) {
            integersFlow.collect {
                observedIntegers++
            }
        }

        observedIntegers.shouldEqual(0)
        testDispatcher.advanceTimeBy(1000)
        observedIntegers.shouldEqual(10)
        Unit
    }

the test passes ✅

but when I convert it into RxJava-style like this:

@Test
    fun `should receive 10 integers - launchIn`() = runBlocking {
        var observedIntegers = 0

        integersFlow
            .onEach { observedIntegers++ }
            .flowOn(testDispatcher)
            .launchIn(observingScope1)

        observedIntegers.shouldEqual(0)
        testDispatcher.advanceTimeBy(1000)
        observedIntegers.shouldEqual(10)
        Unit
    }

the test fails ❎
unless I add an additional delay(100) before testDispatcher.advanceTimeBy(1000) which seems very weird to me. Like… why is that needed to delay the main thread in unit test to allow the collecting coroutine to be launched at all?

[louiscad]

Hi @azabost
Your two snippets are not equivalent because one uses flowOn while the other changes the dispatcher of the scope.
Two solutions to have equivalence between the two:
1. Don't pass testDispatcher to launch and use flowOn instead, like in the second snippet.
2. Remove flowOn and put the testDispatcher in the scope (e.g. using launchIn(observingScope1 + testDispatcher)).

[azabost]

I used the 2. approach and it works as expected. Thanks for the explanation 🙏