Hi I have faced a problem with hot flows while unit testing

Question

Hi I have faced a problem with hot flows while unit testing I saw people had similar problems but not in the exact way so it would be nice if someone could explain the following behaviour 1 This test fails with `java lang IllegalStateException This job has not completed yet` ``` Test fun testHotFlow = coroutinesTestRule testDispatcher runBlockingTest val eventChannel = Channel lt String gt val events = eventChannel receiveAsFlow val event = events first assertEquals event null ``` 2 This test doesn t fail ``` Test fun testHotFlow = coroutinesTestRule testDispatcher runBlockingTest val eventChannel = Channel lt String gt val events = eventChannel receiveAsFlow launch eventChannel send First element val event = events first assertNotNull event ```

ephemient · Answer

did you mean ` firstOrNull `

Андрей Коровин · Answer

it s the same with first or firstOrNull

ephemient · Answer

your test fails either way but ` first ` is wrong even if it worked because it won t return null

ephemient · Answer

whereas ` firstOrNull ` is only broken because it needs the flow to either emit or terminate which is also wrong with ` first `

Андрей Коровин · Answer

gt it needs the flow to either emit or terminate But why does it fail with java lang IllegalStateException This job has not completed yet in the first example And why it doesn t fail after it emits something `eventChannel send First element ` Isn t the job still running

Андрей Коровин · Answer

Or is it because of how first firstOrNull works

ephemient · Answer

in the first case the test dispatcher knows that there are zero runnable children you cannot possibly proceed so the test ends instead of deadlocking forever

ephemient · Answer

in the second case first firstOrNull terminates as soon as it has an answer so everything can wrap up

ephemient · Answer

compare to if you used ` shareIn this ` then the second test wouldn t pass because that launches a coroutine that will keep running

Андрей Коровин · Answer

gt zero runnable children Meaning that nothing can send terminate the flow

Андрей Коровин · Answer

gt so everything can wrap up but isn t the flow still running Or it is the `first ` function that creates the job that has not completed yet

Андрей Коровин · Answer

gt a coroutine that will keep running ``` Test fun testHotFlow = coroutinesTestRule testDispatcher runBlockingTest val eventChannel = Channel lt String gt val events = eventChannel receiveAsFlow shareIn this SharingStarted Lazily launch eventChannel send First element val event = events first assertNotNull event ``` you are right it fails but with more meaningful message ```Test finished with active jobs coroutine 2 StandaloneCoroutine Active 244e30dd kotlinx coroutines test UncompletedCoroutinesError Test finished with active jobs coroutine 2 StandaloneCoroutine Active 244e30dd ```

ephemient · Answer

1 nothing is able to proceed 2 the flow is not still running receiveAsFlow does nothing when there are no collectors and first stops collecting as soon as it has an answer

ephemient · Answer

if you want to see first throw or firstOrNull return null simply close the channel or use a different kind of flow that ends as well

Андрей Коровин · Answer

the channel itself is a private field in my view model and I collect events flow in my fragment So I wanted to test that events flow is empty in one case and it s has a particular value in another

ephemient · Answer

if the channel never closes you don t know that is empty

Андрей Коровин · Answer

I can send the value to the channel by calling a function in my view model But if I don t call it the channel is empty and it results in the exception

Андрей Коровин · Answer

gt you don t know that is empty so it s wrong to assume that it s empty in the first place