@RunWith(AndroidJUnit4.class)
public class MyActivityTest {

    @Rule public ActivityTestRule<MyActivity> activityTestRule = new ActivityTestRule<>(MyActivity.class, false, false);

    private MyApp myApp;

    @Before public void setup() {
        myApp = (MyApp) InstrumentationRegistry.getTargetContext().getApplicationContext();
    }

    @Test public void mockInjection() throws Exception {
        // Create test object graph & inject.
        DaggerMyActivityTest_TestAppComponent.builder()
                .build()
                .inject(myApp);

        activityTestRule.launchActivity(new Intent(myApp, MyActivity.class));
        MyActivity myActivity = activityTestRule.getActivity();

        // Check things got injected correctly.
        Assert.assertEquals("test-singleton-value", myActivity.singletonString);
        Assert.assertEquals("test-per-activity-value", myActivity.perActivityString);
    }

    @Singleton @Component(modules = { TestAppModule.class, AndroidInjectionModule.class })
    public interface TestAppComponent {

        void inject(MyApp myApp);
    }

    @Module
    public static class TestAppModule {
        @Provides @IntoMap @ActivityKey(MyActivity.class)
        AndroidInjector.Factory<? extends Activity> factory() {
            return new AndroidInjector.Factory<MyActivity>() {
                @Override
                public AndroidInjector<MyActivity> create(MyActivity instance) {
                    return new AndroidInjector<MyActivity>() {
                        @Override
                        public void inject(MyActivity instance) {
                            instance.singletonString = "test-singleton-value";
                            instance.perActivityString = "test-per-activity-value";
                        }
                    };
                }
            };
        }
    }
}