How to test code before a suspended coroutine function - unit-testing

Question

Consider the following example in a view model class:
override fun signInViewSelected(
email: String,
password: String
) {
viewModelScope.launch {
loadingViewState.value = LoadingState.Loading("Loading")
withContext(dispatcher.ioDispatcher) {
authManager.signIn(email, password) // suspend fun signIn(email: String, password: String): Boolean -> makes network call
}
loadingViewState.value = LoadingState.NotLoading
}
}
How could I test this method so that I can verify that I start out in loading state, call the authManager.signIn method, and then end up in the not loading state?
When I had this setup with completion handlers I was able to capture the argument passed to my mock authManager class, and then call that manually to advance the completion, but with coroutines I'm not familiar with how to do the equivalent behavior.
What I'd want is something like this, ideally:
#Test
fun `sign in loading state`() {
signInViewModel.signInViewSelected("email#email.com", "password")
val inProgressLoadingViewState = signInViewModel.loadingViewState.getOrAwaitValue()
assertLoadingStateIsLoading(inProgressLoadingViewState, progressMessage)
// delay mockAuthManager until now, have it execute at this point
val finishedLoadingViewState = signInViewModel.loadingViewState.getOrAwaitValue()
assertLoadingStateIsNotLoading(finishedLoadingViewState)
}
Any thoughts?

Answer 1

Assuming you're familiar with libraries like AssertJ and Mockito, I would go about it the following way:
First off, mock the observer of loadingViewState (assuming you have some LiveData in place) and authManager:
private lateinit var viewModel: SomeViewModel
private val loadingViewStateObserver = mock<Observer<LoadingViewState>>()
fun initViewModel() {
viewModel = SomeViewModel().apply {
loadingViewStateLiveData.observeForever(loadingViewStateObserver)
}
}
#Test
fun `sign in loading state`() {
runBlocking {
initViewModel()
viewModel.signInViewSelected("email", "password")
inOrder(loadingViewStateObserver, authManager) {
verify(loadingViewStateObserver).onChanged(LoadingState.NotLoading)
verify(authManager).signIn("email", "password")
verify(loadingViewStateObserver).onChanged(LoadingState.Loading("Loading"))
}
}
}