@RunWith(JUnit4::class)
class MainViewModelTest {

    @get:Rule
    val instantTaskExecutorRule = InstantTaskExecutorRule()

    @Mock
    lateinit var userService: UserService

    lateinit var mainViewModel: MainViewModel

    @Before
    fun setUp() {
        MockitoAnnotations.initMocks(this)
        this.mainViewModel = MainViewModel(this.userService)
    }
  //...
}