I'm using `viewModelScope` to collect an endless Flow in my `viewModel`. In my unit test I want to "...

# android

[Lukasz Kalnik]

I'm using viewModelScope to collect an endless Flow in my viewModel. In my unit test I want to "destroy" the ViewModel in order for viewModelScope to cancel the Flow collection. I'm calling viewModel.onCleared() but it doesn't cancel the `viewModelScope`'s children jobs.
How can I make sure all coroutines started inside viewModelScope are canceled before my test ends, so runTest doesn't hang waiting for the coroutines to finish?

[Javier]

I would use turbine library

[Javier]

it has multiple APIs to cancel flows

[Lukasz Kalnik]

Turbine is useful when I assert that the flow emitted something

[Lukasz Kalnik]

Actually here the flow doesn't emit anything, it is just subscribed to

[Lukasz Kalnik]

class MyViewModel(coroutineContext: CoroutineContext = Dispatchers.Default) : ViewModel() {
    
    private val coroutineScope = viewModelScope + coroutineContext

    init {
        coroutineScope.launch { myFlow.collect { /* do something with collected items */ } }
    }
}

class MyViewModelTest {

    @Test
    fun testInitMyViewModel() = runTest {
        MyViewModel(coroutineContext) // coroutineContext from test injected here
        // test hangs and times out eventually because myFlow collection is not canceled
    }
}

[Javier]

but turbine allows cancel it too, why you can't use it?

[Lukasz Kalnik]

Turbine doesn't allow you to cancel collecting the flow inside of the ViewModel, only inside of your test

[Lukasz Kalnik]

So MyViewModel will still hang

[Lukasz Kalnik]

And myFlow is not even accessible inside the test, because it comes from a dependency

[Lukasz Kalnik]

Turbine is meant to assert that a public (visible in tests) flow emitted something which should be asserted upon in the test

[Lukasz Kalnik]

My test case is that I'm not interested in what the flow emitted and it's not part of the ViewModel's public API (the View doesn't subscribe on this flow)

[Lukasz Kalnik]

So it simply cannot be tested using Turbine

[Umar Ata]

I think you could assign launch to a job and cancel it in onCleared

[Lukasz Kalnik]

Yes, that's one approach

[Lukasz Kalnik]

However then I don't need to use viewModelScope

[Lukasz Kalnik]

I would like to avoid creating jobs for everything. That's the point of viewModelScope, to cancel all children jobs when the ViewModel is destroyed. The only problem is that apparently a ViewModel cannot be destroyed from a unit test.

[Lukasz Kalnik]

Unit testing support is still second class citizen on Android.

[Ian Lake]

Just creating a ViewModel doesn't tie it to any scope - you have to add it to a ViewModelStore via the ViewModelProvider APIs to actually scope it to something. It is ViewModelStore that has the clear() method that cancels and clears every ViewModel added to it

[Lukasz Kalnik]

Yes, in my Fragments I use the by viewModels(factoryProducer = {}) helper function to create a ViewModel tied to the Fragment's scope.
My question concerned however how to do it from a unit test.

Is there a possibility to also use a ViewModelStore inside a unit test to call clear() at the end of the test to cancel all children jobs of the viewModelScope?

[Ian Lake]

Yep, it is just a regular class you can instantiate in your test, ViewModelProvider is just a regular class you can use in your test, etc.

[Lukasz Kalnik]

Awesome, I will try that. Thank you for your help!