Unit test ViewModel class

Question

I am trying to write unit test for my ViewModel class. In this case, I have an activity that when created, it subscribes to my repo (LiveData from viewModel) and retrieves a list of Github repositories from network or database.

What should I be testing here? I tried to write two test methods:

dontFetchWithoutObservers which fails with the following:

[![enter image description here][1]][1]

fetchWhenObserved which fails with the following: [![enter image description here][2]][2]

Here's my ViewModel class:

class MainViewModel @ViewModelInject constructor(mainRepository: MainRepository) : ViewModel() {

    val repo: LiveData<Resource<List<Repository>>> = mainRepository.getRepositories()

}

And my ViewModel test class:

@RunWith(JUnit4::class)
class MainViewModelTest {

    @Rule
    @JvmField
    val instantTaskExecutorRule = InstantTaskExecutorRule()

    private val mainRepository = mock(MainRepository::class.java)
    private lateinit var mainViewModel: MainViewModel

    @Before
    fun init() {
        mainViewModel = MainViewModel(mainRepository)
    }

    @Test
    fun dontFetchWithoutObservers() {
        verify(mainRepository, never()).getRepositories()
    }

    @Test
    fun fetchWhenObserved() {
        mainViewModel.repo.observeForever(mock())
        verify(mainRepository).getRepositories()
    }
}

Answer 1

you are not mocking the getRepositories method and in fact the error you get is: NPE. Provide a mock for getRepositories method.

`when`(mainRepository.getRepositories()).thenReturn(YourObject)

Also I would go and check the contents of getRepositories instead of just verifying that it is being invoked.