Hello :wave: I have a question regarding Unit Test and Flow. I'm following the Testing flows <Andro...

# coroutines

[Lucien Guimaraes]

Hello 👋 I have a question regarding Unit Test and Flow. I'm following the Testing flows Android documentation to test my work. But I'm stuck with the following error This job has not completed yet

Here is my implementation:

class MyUseCaseImpl(
    myRepository: MyRepository,
    myID: String,
    ioDispatcher: CoroutineDispatcher,
) : MyUseCase {

    override val state = MutableStateFlow<MyState>(MyState.Loading)

    init {
        CoroutineScope(ioDispatcher).launch {
            myRepository
                .requestStuff(myID)
                .map { MyState.Success(it) }
                .catch { state.emit(MyState.Error(it)) }
                .collect { state.emit(it) }
        }
    }
}

interface MyUseCase {

    sealed class MyState {
        object Loading: MyState()
        data class Error(val throwable: Throwable) : MyState()
        data class Success(val hikeTrack: HikeTrackResponse) : MyState()
    }

    val state: Flow<MyState>
}

And here is the UT failing:

...
private val coroutineDispatcher = TestCoroutineDispatcher()
...
@Test
    fun `Given ... `() = coroutineDispatcher.runBlockingTest {

    When calling myRepository.requestStuff(myID) `it returns` flowOf(stuffResponse)

    val myUseCase = MyUseCase(
        myRepository,
        myID,
        coroutineDispatcher,
    )

    myUseCase.state.take(2).toList() shouldBeEqualTo listOf(
        MyState.Loading,
        MyState.Success(stuffResponse),
    )
  }
}

From the documentation it says If the test needs to check multiple values, calling toList() causes the flow to wait for the source to emit all its values and then returns those values as a list. Note that this works only for finite data streams . For what I understand, the UT fails because the flow I'm returning to my repository is an infinite streams. But shouldn't flowOf(value) create a finite stream ?

Thanks !

[Alex Vasilkov]

That’s a known issue: https://github.com/Kotlin/kotlinx.coroutines/issues/1204,
runBlockingTest does not work well with dispatchers other than TestCoroutineDispatcher. It is not clear from your code what dispatcher you use though.

I’m using the following code to get correct dispatcher from current `TestCoroutineScope`:

@ExperimentalCoroutinesApi
val TestCoroutineScope.testDispatcher: CoroutineDispatcher
    get() = coroutineContext[ContinuationInterceptor] as CoroutineDispatcher

[Lucien Guimaraes]

sorry I forgot to mention I'm using `runBlockingTest`from TestCoroutineDispatcher (I updated the code)

[Lucien Guimaraes]

Thanks for the link, the issue is almost 2 years old 😅

[Lucien Guimaraes]

I tried your code, and I still have the same issue. Do you have any sample that I can test and run it successfully ?

[Alex Vasilkov]

Well, if you already use test dispatcher then you should be fine, in theory 🙂 I recently had similar issue in one of my tests, and the reason was that internally I was calling the code like this:

// ... crash the scope (and the app)
scope.launch { throw th }.join()

Where scope didn’t have any dispatcher by default. I fixed it by using scope.launch(dispatcher) ....

[Lucien Guimaraes]

I must miss something then, because as you can see in the init of the class MyUseCaseImpl, I'm doing the fetch on the repository by using CoroutineScope(ioDispatcher).launch { ... }

[Alex Vasilkov]

It’s not clear what is going on in myRepository.requestStuff(myID) , if it does something asynchronous (e.g. https://github.com/square/retrofit/issues/3330) then you have to test with runBlocking instead.

[Lucien Guimaraes]

myRepository.requestStuff(myID) is a mock interface, that as the following signature: fun requestStuff(myID: String): Flow<StuffResponse> There is no Retrofit information on this UT level

[Lucien Guimaraes]

by the way if I replace hikeDetailInteractor.state.take(2).toList() shouldBeEqualTo listOf(...) by hikeDetailInteractor.state.first() shouldBeEqualTo HikeTrackState.Success(hikeTrackResponse) the UT is successful. And my using coroutineDispatcher.runBlockingTest

[Alex Vasilkov]

Are you sure “Loading” state is actually emitted?

[Alex Vasilkov]

It seems like it is not, you’re probably missing onStart { emit (Loading) }

[Lucien Guimaraes]

By removing the init, and using the first() it will be the loading state

[Lucien Guimaraes]

It's a MutableStateFlow init with a given value (loading here)

[Alex Vasilkov]

Ah, indeed, missed that part. Can you share the code under myRepository.requestStuff(myID)?

[Alex Vasilkov]

You can also try to use Default dispatcher with runBlocking, does it still fail?

[Lucien Guimaraes]

Here is the impl of myrepository

class MyRepositoryImpl(
    private val myApi: MyApi,
) : MyRepository {

    override fun requestStuff(myID: String): Flow<StuffResponse> = flow {
        emit(myApi.getStuff(myID))
    }
}

interface MyRepository {
    fun requestStuff(myID: String): Flow<StuffResponse>
}

[Alex Vasilkov]

And what is behind myApi.getStuff? 🙂 BTW, it seems like you don't need to use flow in your repo, just use a suspend function.

[Lucien Guimaraes]

You can also try to use Default dispatcher with runBlocking, does it still fail?

Infinite loading in the UT

[Lucien Guimaraes]

And what is behind myApi.getStuff?

interface MyApi{

    @GET("test/{myID}/stuff")
    suspend fun getStuff(
        @Path("myID") myID: String,
    ): StuffResponse
}

[Lucien Guimaraes]

BTW, it seems like you don't need to use flow in your repo, just use a suspend function.

For now yes, but later I will have to transform it as data streams 🙂

[Alex Vasilkov]

Checking the code again now, I think the problem here is that state flow is immediately updated inside the constructor, it means that when you start collecting it you only get latest Success result, without initial Loading. Thus take(2) never receives second value and hangs forever.

[Alex Vasilkov]

Try to add delay before returning mocked response

[Lucien Guimaraes]

You mean something like `When calling myRepository.requestStuff(myID) it returns flow { delay(Long.MAX_VALUE)`
emit(stuffResponse) } ?

[Alex Vasilkov]

Almost, delay(Long.MAX_VALUE) won’t work as it is equivalent to awaitCancellation. Try delay(1L).

[Lucien Guimaraes]

It works 🎉

[Lucien Guimaraes]

Thank you very much @Alex Vasilkov

[Lucien Guimaraes]

The only downside of this trick, it that testing a method that throw an exception can't be done as easily. In this case instead of returning a flowOf(yourValue) you answers { throw YourException }

[Alex Vasilkov]

You can use similar mocked response I think:

flow {
  delay(1L)
  throw ...
}

[Lucien Guimaraes]

Oh yeah you right, I didn't even think about it

[Lucien Guimaraes]

Good catch 👍