TestViewModelTest.kt

@ExperimentalCoroutinesApi
class ViewModelTest {

    @get:Rule
    var coroutinesTestRule = TestCoroutineDispatcherRule()

    @get:Rule
    var rule: TestRule = InstantTaskExecutorRule()

    private val stateObserver: Observer<State> = mock()

    private lateinit var lifeCycleTestOwner: LifeCycleTestOwner
    private lateinit var testViewModel: TestViewModel

    @Before
    fun setUp() {
        lifeCycleTestOwner = LifeCycleTestOwner()
        lifeCycleTestOwner.onCreate()
        testViewModel = TestViewModel()
        testViewModel.pollStateData.observe(lifeCycleTestOwner, stateObserver)
    }

    @After
    fun tearDown() {
        lifeCycleTestOwner.onDestroy()
    }

    @Test
    fun `when polling starts the observer is notified the correct amount of times depending on delay`() {
        // Given
        lifeCycleTestOwner.onResume()
   
        // When
        runBlockingTest {
            testViewModel.fetchSomething()
            // Then
            Verify on stateObserver that pollStateObserver.onChanged(State.Loading) was called
            Verify on stateObserver that pollStateObserver.onChanged(State.Post) was called
            VerifyNoFurtherInteractions on pollStateObserver
        }
    }
}