Android Mocking a Dagger2 injected dependency for a Espresso test

Question

I have a heavily dependency injected (dagger2) application. I would like to run an espresso test without having the test navigate through the whole application, and log into the application.

I would like to start on my teleActivity, and mock the login manager. However in any @test function, we have already hit the null pointer as we have called onCreate. If I override it before we launch the activity (show below) the activity is null.

To my understanding, the ability to switch our underlining dependencies is a large reason why we use Dagger2, else it would be just a very over engineered singleton. How do I override, mock, or switch the injection to a testing dagger module -- so I can create this simple espresso test.

Note I also wrote all this in the MVP design pattern if that makes a difference.

TeleActivity

@Inject
TelePresenter mTelePresenter;
@Inject
public LoginStateManager mLoginStateManager;

    @Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    ButterKnife.bind(this);
    DaggerInjectorTele.get().inject(this);
    mTelePresenter.setTeleDependencies(this);
    Intent intent = getIntent();

    String searchId = null;

    if (intent != null) {
        searchId = intent.getStringExtra(Constants.SEARCH_ID);
       }

    mTelePresenter.onCreateEvent(searchId,
            Helper.makeAuthorizationHeader(
            // CRASH Null pointer
            mLoginStateManager.getBaseLoginResponse().getAccessToken()));

}

Espresso

@LargeTest
@RunWith(AndroidJUnit4.class)
public class TeleTest {
    @Rule
    public ActivityTestRule<TeleActivity> mActivityTestRule = new ActivityTestRule(
            TeleActivity.class) {
        @Override
        protected void beforeActivityLaunched() {
            super.beforeActivityLaunched();
            TeleActivity teleActivity = (TeleActivity)getActivity();
             //teleActivity NULL!
            teleActivity.mLoginStateManager = mock(LoginStateManager.class);
            LoginResponse loginResponse = mock(LoginResponse.class);
            when(loginResponse.getAccessToken()).thenReturn("1234");
            // Nope here still null


when(teleActivity.mLoginStateManager.getBaseLoginResponse()).thenReturn(loginResponse);

        }
    };

Dagger Injector

  public class DaggerInjectorTele {
    private static TelePresenterComponent telePresenterComponent =
            DaggerTelePresenterComponent.builder().build();

    public static TelePresenterComponent get() {
        return telePresenterComponent;
    }
}

TelePresenterComponent

@Singleton
@Component(modules = {TelePresenterModule.class,
        LoginStateManagerModule.class})
public interface TelePresenterComponent {
    void inject(TeleActivity activity);
}

TelePresenterModule

@Module
public class TelePresenterModule {

    @Provides
    @Singleton
    public TelePresenter getTelePresenter() {
        return new TelePresenter();
    }
}

LoginStateManagerModule

@Module
public class LoginStateManagerModule {

    @Provides
    @Singleton
    public LoginStateManager getLoginStateManager(){
        return new LoginStateManager();
    }
}

Answer 1

First, your decision to use dependency injection (Dagger2) is a very good one and will indeed make your tests easier to write.

You have to override dependency injection configuration (module) and inject a mock. Here is a simple example of how it can be done.

First you need a mock:

LoginStateManager lsmMock = mock(LoginStateManager.class);

Now override the DI config to use this mock:

//Extend your TelePresenterModule, override provider method
public class TestTelePresenterModule extends TelePresenterModule{
    @Override
    public LoginStateManager getLoginStateManager() {
        //simply return the mock here
        return lsmMock;
    }
}

Now to the test:

@Test
//this is an espresso test
public void withAMock() {
    //build a new Dagger2 component using the test override
    TelePresenterComponent componentWithOverride = DaggerTelePresenterComponent.builder()
            //mind the Test in the class name, see a class above
            .telePresenterModule(new TestTelePresenterModule())
            .build();

    //now we initialize the dependency injector with this new config
    DaggerInjectorTele.set(componentWithOverride);

    mActivityRule.launchActivity(null);

    //verify that injected mock was interacted with
    verify(lsmMock).whatever();
}

Example from: https://github.com/yuriykulikov/DIComparison/blob/master/app/src/androidTest/java/com/example/yuriy/dependencyinjectioncomparison/Dagger2Test.java