LoginFragmentTest.kt

@MediumTest
@RunWith(AndroidJUnit4::class)
class LoginFragmentTest {

    @Rule
    @JvmField
    val activityRule = IntentsTestRule(SingleFragmentActivity::class.java)

    @Rule
    @JvmField
    val taskExecutorRule = InstantTaskExecutorRule()

    @Rule
    @JvmField
    val mockitoRule: MockitoRule = MockitoJUnit.rule()

    @Mock
    private lateinit var viewModel: LoginViewModel

    @Mock
    private lateinit var emailTextChangedListener: TextWatcher

    @Mock
    private lateinit var passwordTextChangedListener: TextWatcher

    private val emailField = ObservableField<String>()
    private val passwordField = ObservableField<String>()

    private val isSignInButtonEnabled = ObservableBoolean()
    private val isProgressBarVisible = ObservableBoolean()

    private val navigateToForgotPasswordPage = SingleLiveEvent<Unit>()
    private val navigateToNextScreen = SingleLiveEvent<Unit>()
    private val showErrorToast = SingleLiveEvent<Int>()

    private lateinit var loginFragment: LoginFragment

    @Before
    fun setUp() {
        loginFragment = LoginFragment()

        `when`(viewModel.emailField).thenReturn(emailField)
        `when`(viewModel.passwordField).thenReturn(passwordField)

        `when`(viewModel.isSignInButtonEnabled).thenReturn(isSignInButtonEnabled)
        `when`(viewModel.isProgressBarVisible).thenReturn(isProgressBarVisible)

        `when`(viewModel.navigateToForgotPasswordPage).thenReturn(navigateToForgotPasswordPage)
        `when`(viewModel.navigateToNextScreen).thenReturn(navigateToNextScreen)
        `when`(viewModel.showErrorToast).thenReturn(showErrorToast)

        `when`(viewModel.emailTextChangedListener()).thenReturn(emailTextChangedListener)
        `when`(viewModel.passwordTextChangedListener()).thenReturn(passwordTextChangedListener)

        loginFragment.viewModelFactory = createViewModelFactory(viewModel)

        activityRule.activity.setFragment(loginFragment)
    }
    
    private fun <T : ViewModel> createViewModelFactory(viewModel: T): ViewModelProvider.Factory {
        return object : ViewModelProvider.Factory {
            override fun <T : ViewModel> create(viewModelClass: Class<T>): T {
                if (viewModelClass.isAssignableFrom(viewModel.javaClass)) {
                    @Suppress("UNCHECKED_CAST")
                    return viewModel as T
                }
                throw IllegalArgumentException("Unknown view model class " + viewModelClass)
            }
        }
    }
}