How to callback an Activity in onAttach with Otto?

Question

I am working on an app which uses the NavigationDrawer. Different fragments are placed into the content view of the MainActivity whenever a menu item in the drawer is selected. To inform the MainActivity that a Fragment successfully attached the following callback is executed:

public class CustomFragment extends Fragment {

    @Override
    public void onAttach(Activity activity) {
        super.onAttach(activity);
        ((MainActivity) activity).onSectionAttached();
    }

}

Since I am started using Otto with Dagger in the project I am curious how I can substitute the callback with a .post() event such as:

mBus.post(new CustomFragmentAttachedEvent);

The problem is that mBus is null in onAttach(). It gets initialized in onCreate().

@Override
public void onCreate(final Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    ((MyApp) getActivity().getApplication()).getObjectGraph().inject(this);
}

Here is an example of such a Fragment class.

References:

• Complete Android Fragment & Activity Lifecycle

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You can easily try out the example yourself: Create a new project from the NavigationDrawer template available in Android Studio, add Dagger and Otto and try to substitute the mentioned callback.

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Working solution:

• Here is the example Fragment in the final working version.

Answer 1

You can create a provider for your Otto bus with Dagger as follows:

@Module(injects = {
    YourFragment.class, 
    MainActivity.class
},complete = true)    
public class EventBusModule {

    @Provides
    @Singleton
    Bus provideBus() {
        return new Bus();
    }
}

Then you register the EventBusModule when you create your ObjectGraph. You can create your graph in the Application's onCreate():

public class MyApplication extends Application{

    public void onCreate() {
        Object[] modules = new Object[]{new EventBusModule()};
        Injector.init(modules);
    }

}

You would need to create an Injector that has some static methods and a reference to the ObjectGraph so you can manipulate it without having a reference to the Application. Something like this:

public final class Injector {

    private static ObjectGraph objectGraph = null;

    public static void init(final Object... modules) {

        if (objectGraph == null) {
            objectGraph = ObjectGraph.create(modules);
        }
        else {
            objectGraph = objectGraph.plus(modules);
        }

        // Inject statics
        objectGraph.injectStatics();
    }

    public static final void inject(final Object target) {
        objectGraph.inject(target);
    }

    public static <T> T resolve(Class<T> type) {
        return objectGraph.get(type);
    }
}

Then you just use @Inject in your Fragment to let Dagger give you a Bus instance and inject the Fragment:

public class MyFragment extends Fragment{

    @Inject Bus mBus;

    public void onAttach(){
        Injector.inject(this);
    } 

}