Im trying to test a ViewModel that lazily loads da...

# android

[Michael Friend]

Im trying to test a ViewModel that lazily loads data when the LiveData is accessed and first emits a Loading state, the loads the data, then emits a Data state with the result. The data is loaded in a coroutine on the viewModelScope and I’m using TestCoroutineScope and MockK in my test but for some reason my mock observer doesn’t get the loading emission unless I pause the dispatcher then resume it. Here’s a basic example of my issue

// The view model 
class LazyViewModel(val repo: Repo) : ViewModel() {
    val state: LiveData<State<String>> by lazy {
        loadData()
        _state
    }

    private val _state = MutableLiveData<State<String>>()
    private fun loadData() {
        viewModelScope.launch {
            _state.value = State.Loading
            _state.value = State.Data(repo.getData())
        }
    }
}
// the test
    @Test
    fun `get loading then data`() = coroutinesTestRule.testScope.runBlockingTest {
        val mockObserver: Observer<State<String>> = mockk(relaxUnitFun = true)

        val mockRepo: Repo = mockk {
            coEvery { getData() } returns "data"
        }
        val viewModel = LazyViewModel(mockRepo)
        // If I don't pause then resume the dispatcher _state.value = State.Loading seemingly gets skipped
        pauseDispatcher()
        viewModel.state.observeForever(mockObserver)
        resumeDispatcher()
        verifyOrder {
            mockObserver.onChanged(State.Loading)
            mockObserver.onChanged(State.Data("data"))
        }
    }

[Michael Friend]

This is the failure i get. I don’t understand how the Loading state just doesn’t get received but pausing and resuming the dispatcher fixes it

[Mgj]

I dont see where you've replaced your viewModelScope with a TestCoroutineScope...?

[Michael Friend]

Is that the issue? Do you have to inject a CoroutineScope to ViewModels instead of using viewModelScope directly? My coroutine test rule sets main and follows the example here

https://youtu.be/KMb0Fs8rCRs?t=801▾

[Mgj]

Im not sure if you have to but its what i've been doing and it seems to work 🤷

Instead of injecting it you could consider doing something like this:

open class MyViewModel {
    protected open val mScope = CoroutineScope(Dispatchers.Main)
}

and then in tests use:

class MyViewModelStub : MyViewModel {
	override val mScope = TestCoroutineScope()
}

[Michael Friend]

So that just doesn't use the viewModelScope extension provided by android? I feel like there has to be a way to utilize androids extension

[Mgj]

You can use viewModelScope if you want:

open class MyViewModel {
    protected open val mScope = viewModelScope
}

[Michael Friend]

Also something I've noticed is that if instead of calling loadData in the lazy block you call it manually from the test it works as expected

[Michael Friend]

I figured out a way to make it work, but i still don’t quite understand why it wasn’t working before. If you use the liveData coroutine builder to get the lazy load then the test works as expected.

val state: LiveData<State<String>> = liveData {
        // This HAS to be called first for the test to work
        emitSource(_state)
        loadData()
    }
    private val _state = MutableLiveData<State<String>>()
    
   private fun loadData() {
        viewModelScope.launch {
            _state.value = State.Loading
            _state.value = State.Data(repo.getData())
        }
    }

[Michael Friend]

I initially avoided using the liveData build because I wanted to be able to allow retrying loadData so i needed the backing MutableLiveData. using emitSource(_state) instead of emit(Loading) and emit(Data) lets me do that still