I have a ViewModel that launched with `viewModelScope.launch {}` a `while(isActive)` loop where I do...

# android

[Luca Nicoletti]

I have a ViewModel that launched with viewModelScope.launch {} a while(isActive) loop where I do things.
I’m trying to test that viewModel, mocking everything it needs.
The test is run inside a runTest block.
The test reaches the assertEqual line, which passes, but then it doesn’t cancel the launch and that keeps running (found it out while debugging).
Am I doing something wrong?

[Stepan Churyukanov]

did you replace main dispatcher? I.e. with JUnit extension

class ReplaceMainDispatcherExtension(
    private val testDispatcher: TestDispatcher = UnconfinedTestDispatcher()
) : BeforeEachCallback, AfterEachCallback {

    override fun beforeEach(context: ExtensionContext) {
        Dispatchers.setMain(testDispatcher)
    }

    override fun afterEach(context: ExtensionContext?) {
        Dispatchers.resetMain()
    }
}

[Luca Nicoletti]

Yes that works, the test run jjust fine

[Luca Nicoletti]

Not missing main or anything

[Luca Nicoletti]

All other tests are passing, but this one which starts the loop, just won’t complete/cancel that when the test is over

[Stepan Churyukanov]

Did you try to run such test with the TestScope instead of view model scope? You can pass it outside to check

[Luca Nicoletti]

No, haven’t, DMed you

[zsmb]

These docs might help you as well: https://developer.android.com/kotlin/coroutines/test#inject-scope

[Luca Nicoletti]

Read them already, there’s nothing specific for my scenario

[Stepan Churyukanov]

What I have proposed in DM:

class MyViewModel: ViewModel() {

    fun runSomething(scope: CoroutinesScope = viewModelScope) {
        scope.launch {
            // your problematic code
        }
    }
}

class MyViewModelTest {
    val scope = TestScope(UnconfinedTestDispatcher())
    
    @Test 
    fun test() {
        MyViewModel().runSomething(scope)
        scope.cancel()
    }
}

it works

[zsmb]

It's not covered specifically, but the main idea is that the coroutine needs to be in a scope which is cancelled. If you use the coroutine testing library, TestScope.backgroundScope is something you can pass into the object you're testing, as coroutines in there will be cancelled at the end of runTest.

[Luca Nicoletti]

Is there a way to instruct viewModelScope.launch - without parameter - to be cancelled as well?

[Luca Nicoletti]

Something like a @Rule I can setup on my tests

[zsmb]

Alternatively, if you want to keep using the Jetpack viewModelScope directly (which generally makes testing more difficult, unfortunately), you can attempt to clear the ViewModel at the end of the test, which would also cancel that scope. Not sure off the top of my head how difficult that is to achieve though.

[zsmb]

If you're writing tests for these objects, I'd go for injecting a scope so that they can be easily replaced in tests.

[Luca Nicoletti]

I see, I recall on a previous project we used viewModelScope but provided dispatchers in the launch, using different ones for testing. Would that work as well?

[zsmb]

You could technically solve it using contexts. If you were to inject a Job into the ViewModel that you then pass in as an extra parameter every time you call viewModelScope.launch(), then each of those newly created coroutines would become children of the injected Job, and cancelling the parent would cancel them as well.

This would, however, mean that the coroutines are no longer tied to the ViewModel's lifecycle, so you'd need to also cancel the injected job whenever the ViewModel is cleared, for example like this https://developer.android.com/topic/libraries/architecture/viewmodel#clear-dependencies

Having to remember to pass it in every time you launch a coroutine though, my opinion is that this isn't a clean way to do it. Ideally a scope where you're launching should already contain a correct configuration for its coroutines (dispatcher, error handler, and especially parent job), with an extra context parameter only required in rare cases.

[Haroon Iftikhar]

I agree with @zsmb especially because that's leveraging one of the selling points of a coroutine

[Erfannj En]

Are you used to yield() inside the loop?

[Luca Nicoletti]

No, I don’t have any yield

[Erfannj En]

You should call it inside loop

[Luca Nicoletti]

for what?