Getting Dagger to inject mock objects when doing Espresso functional testing for Android

With Dagger 2 and Espresso 2 things have indeed improved. This is how a test case could look like now. Notice that ContributorsModel is provided by Dagger. The full demo available here: https://github.com/pmellaaho/RxApp

@RunWith(AndroidJUnit4.class)
public class MainActivityTest {

ContributorsModel mModel;

@Singleton
@Component(modules = MockNetworkModule.class)
public interface MockNetworkComponent extends RxApp.NetworkComponent {
}

@Rule
public ActivityTestRule<MainActivity> mActivityRule = new ActivityTestRule<>(
        MainActivity.class,
        true,     // initialTouchMode
        false);   // launchActivity.

@Before
public void setUp() {
    Instrumentation instrumentation = InstrumentationRegistry.getInstrumentation();
    RxApp app = (RxApp) instrumentation.getTargetContext()
            .getApplicationContext();

    MockNetworkComponent testComponent = DaggerMainActivityTest_MockNetworkComponent.builder()
            .mockNetworkModule(new MockNetworkModule())
            .build();
    app.setComponent(testComponent);
    mModel = testComponent.contributorsModel();
}

@Test
public void listWithTwoContributors() {

    // GIVEN
    List<Contributor> tmpList = new ArrayList<>();
    tmpList.add(new Contributor("Jesse", 600));
    tmpList.add(new Contributor("Jake", 200));

    Observable<List<Contributor>> testObservable = Observable.just(tmpList);

    Mockito.when(mModel.getContributors(anyString(), anyString()))
            .thenReturn(testObservable);

    // WHEN
    mActivityRule.launchActivity(new Intent());
    onView(withId(R.id.startBtn)).perform(click());

    // THEN
    onView(ViewMatchers.nthChildOf(withId(R.id.recyclerView), 0))
            .check(matches(hasDescendant(withText("Jesse"))));

    onView(ViewMatchers.nthChildOf(withId(R.id.recyclerView), 0))
            .check(matches(hasDescendant(withText("600"))));

    onView(ViewMatchers.nthChildOf(withId(R.id.recyclerView), 1))
            .check(matches(hasDescendant(withText("Jake"))));

    onView(ViewMatchers.nthChildOf(withId(R.id.recyclerView), 1))
            .check(matches(hasDescendant(withText("200"))));
}

Your approach doesn't work because it only happens once, and as Matt mentioned, when the activity's real injection code runs, it will wipe out any variables injected by your special object graph.

There are two ways to get this to work.

The quick way: make a public static variable in your activity so a test can assign an override module and have the actual activity code always include this module if it's not null (which will only happen in tests). It's similar to my answer here just for your activity base class instead of application.

The longer, probably better way: refactor your code so that all activity injection (and more importantly graph creation) happens in one class, something like ActivityInjectHelper. In your test package, create another class named ActivityInjectHelper with the exact same package path that implements the same methods, except also plusses your test modules. Because test classes are loaded first, your application will execute with the testing ActivityInjectHelper. Again it's similar to my answer here just for a different class.

UPDATE:

I see you've posted more code and it's close to working, but no cigar. For both activities and applications, the test module needs to be snuck in before onCreate() runs. When dealing with activity object graphs, anytime before the test's getActivity() is fine. When dealing with applications, it's a bit harder because onCreate() has already been called by the time setUp() runs. Luckily, doing it in the test's constructor works - the application hasn't been created at that point. I briefly mention this in my first link.