Get livedata latest value while testing view model - testing

Question

I'm trying to unit test my viewmodel:
private val loginRepository: LoginRepository = LoginRepository()
private val _loginSuccess = MutableLiveData<Resource<String>>()
val loginSuccess : LiveData<Resource<String>>
get() = _loginSuccess
fun login(credentials : RequestLogin){
_loginSuccess.value = Resource.loading()
viewModelScope.launch {
_loginSuccess.postValue(loginRepository.login(credentials))
}
With this:
#Test
fun login_success(){
val loginRequest = RequestLogin("username", "test")
val app:Application = ApplicationProvider.getApplicationContext()
PreferencesHelper.init(app)
val viewModel = LoginViewModel(app)
viewModel.loginSuccess.observeForever(dataObserver)
runBlocking {
viewModel.login(loginRequest)
assertEquals(viewModel.loginSuccess.getOrAwaitValue(), Resource.success("OK"))
}
viewModel.loginSuccess.removeObserver(dataObserver)
}
But everytime i'm getting just the first value of the liveData object Resource.loading() instead of the one obtained with the postValue method.
How can i ignore the result of the first liveData update and just get the final one?

Answer 1

runBlocking executes and waits for completion for the block you pass to it, in this case it is
viewModel.login(loginRequest)
assertEquals(viewModel.loginSuccess.getOrAwaitValue(), Resource.success("OK"))
But this code does not have any suspend calls, so runBlocking does not have any effect here. In particular it does not affect the viewModelScope.launch call.
There are a couple of ways to test this code. I would suggest using kotlinx-coroutines-test library. It provides TestCoroutineDispatcher which is very convenient in this case.
viewModelScope uses Dispatchers.Main dispatcher by default, so you need to replace it with TestCoroutineDispatcher. E.g. you can create a simple test rule:
class CoroutineTestRule(val dispatcher: TestCoroutineDispatcher = TestCoroutineDispatcher()) : TestWatcher() {
override fun starting(description: Description?) {
super.starting(description)
Dispatchers.setMain(dispatcher)
}
override fun finished(description: Description?) {
super.finished(description)
Dispatchers.resetMain()
}
}
Then apply it to your test:
#get:Rule
var coroutineTestRule: CoroutineTestRule = CoroutineTestRule()
And use it like this
#Test
fun login_success(){
...
viewModel.login(loginRequest)
coroutineTestRule.dispatcher.advanceUntilIdle()
assertEquals(viewModel.loginSuccess.getOrAwaitValue(), Resource.success("OK"))
...
}
And here is a bit of how it works:
CoroutineTestRule replaces Dispatcher.Main with the CoroutineTestRule.dispatcher
Your viewmodel launches a login job, using viewModelScope, which uses the same CoroutineTestRule.dispatcher
coroutineTestRule.dispatcher.advanceUntilIdle() makes the dispatcher to execute all outstanding tasks, so it will execute all coroutines, which are using this dispatcher and are ready to be executed.
There is also very convenient advanceTimeBy method on TestCoroutineDispatcher which allows you to fast-forward and skip e.g. delay calls.