PhotosViewModelTest.kt

@ExperimentalCoroutinesApi
@RunWith(MockitoJUnitRunner::class)
class PhotosViewModelTest {

    @get:Rule
    val testInstantTaskExecutorRule: TestRule = InstantTaskExecutorRule()
    @get:Rule
    val testCoroutineRule = TestCoroutineRule()
    private lateinit var viewModel: PhotosViewModel
    @Mock
    private lateinit var photosRepository: PhotosRepository
    @Mock
    private lateinit var photosResponseObserver: Observer<Result<ArrayList<Photo>>>

    @Before
    fun setUp() {
        viewModel = PhotosViewModel(photosRepository)
    }

    @Test
    fun `when fetching results ok then return a list successfully`() {
        val emptyList = arrayListOf<Photo>()
        testCoroutineRule.runBlockingTest {
            viewModel.getPhotos().observeForever(photosResponseObserver)
            whenever(photosRepository.getPhotosFromApi(anyInt(), anyInt())).thenAnswer {
                Result.Success(emptyList)
            }
            viewModel.loadData()
            assertNotNull(viewModel.getPhotos().value)
            assertEquals(Result.Success(emptyList), viewModel.getPhotos().value)
        }
    }

    @Test
    fun `when calling for results then return loading`() {
        testCoroutineRule.runBlockingTest {
            viewModel.getPhotos().observeForever(photosResponseObserver)
            viewModel.loadData()
            verify(photosResponseObserver).onChanged(Result.InProgress)
        }
    }

    @Test
    fun `when fetching results fails then return an error`() {
        val exception = mock(HttpException::class.java)
        testCoroutineRule.runBlockingTest {
            viewModel.getPhotos().observeForever(photosResponseObserver)
            whenever(photosRepository.getPhotosFromApi(anyInt(), anyInt())).thenAnswer {
                Result.Error(exception)
            }
            viewModel.loadData()
            assertNotNull(viewModel.getPhotos().value)
            assertEquals(Result.Error(exception), viewModel.getPhotos().value)
        }
    }

    @After
    fun tearDown() {
        viewModel.getPhotos().removeObserver(photosResponseObserver)
    }
}