Dagger 2 on Android: inject same dependency in Activity and retained Fragment

Question

I have objects of classes F1 and F2 that I want to inject in a retained Fragment. I also have an object of class A that depends on Activit

Answer 1

Considering your retained fragment lives longer than your activity, I'd wager that the proper way to do this would be to make the FragmentScope contain the ActivityScope, and not vice versa.

Meaning your FragmentComponent would have

@FragmentScope
@Component(modules = {FragmentModule.class})
public interface FragmentComponent {
    void inject(MyFragment presenter);
}

And your Activity component would have

@ActivityScope
@Component(dependencies = {FragmentComponent.class}, modules = {MainActivityModule.class})
public interface MainActivityComponent extends FragmentComponent { //provision methods
    void inject(MainActivity activity);

    // make the A object available to dependent components
    A getA();
}

Which is possible if your Fragment injected classes don't rely on the Activity module as dependencies.

This can be done with something akin to

public class MainActivity extends AppCompatActivity {

    private MainActivityComponent mainActivityComponent;

    private MyFragment myFragment;

    @Override
    public void onCreate(Bundle saveInstanceState) {
         super.onCreate(saveInstanceState);
         setContentView(R.layout.activity_main);

         if(saveInstanceState == null) { // first run
             myFragment = new MyFragment(); //headless retained fragment
             getSupportFragmentManager()
                .beginTransaction()
                .add(myFragment, MyFragment.class.getName()) //TAG
                .commit();
         } else {
             myFragment = (MyFragment)(getSupportFragmentManager()
                               .findFragmentByTag(MyFragment.class.getName()));
         }
    }

    @Override
    public void onPostCreate() {
         mainActivityComponent = DaggerMainActivityComponent.builder()
              .fragmentComponent(myFragment.getComponent())
              .build();
    }
}

And

public class MyFragment extends Fragment {
    public MyFragment() {
         this.setRetainInstance(true);
    }

    private FragmentComponent fragmentComponent;

    @Override
    public void onCreate(Bundle saveInstanceState) {
        super.onCreate(saveInstanceState);
        this.fragmentComponent = DaggerFragmentComponent.create();
    }

    public FragmentComponent getFragmentComponent() {
        return fragmentComponent;
    }
}

EDIT:

public class MyFragment extends Fragment {
    public MyFragment() {
         this.setRetainInstance(true);
         this.fragmentComponent = DaggerFragmentComponent.create();
    }

    private FragmentComponent fragmentComponent;

    public FragmentComponent getFragmentComponent() {
        return fragmentComponent;
    }
}

public class MainActivity extends AppCompatActivity {

    private MainActivityComponent mainActivityComponent;

    private MyFragment myFragment;

    @Inject
    A mA;

    @Override
    public void onCreate(Bundle saveInstanceState) {
         super.onCreate(saveInstanceState);
         setContentView(R.layout.activity_main);

         if(saveInstanceState == null) { // first run
             myFragment = new MyFragment(); //headless retained fragment
             getSupportFragmentManager()
                .beginTransaction()
                .add(myFragment, MyFragment.class.getName()) //TAG
                .commit();
         } else {
             myFragment = (MyFragment)(getSupportFragmentManager().findFragmentByTag(MyFragment.class.getName()));
         }
         mainActivityComponent = DaggerMainActivityComponent.builder()
              .fragmentComponent(myFragment.getComponent())
              .build();
         mainActivityComponent.inject(this);
         mA.onCreate();
    }
}