Dagger `ObjectGraph` utility to hide the mess of injecting the different Android classes. Note this is inspired by https://github.com/pyricau/CleanAndroidCode/blob/master/cleanandroidcode/src/main/java/info/piwai/cleanandroidcode/base/GraphRetriever.java

ObjectGraph.java

import javax.annotation.Nonnull;

import android.app.Activity;
import android.app.Service;
import android.support.v4.app.Fragment;
import android.content.Context;

/**
 * <p>Retrieves the {@link dagger.ObjectGraph} and injects dependencies.</p>
 * @author Dandré Allison
 */
public final class ObjectGraph {

    /**
     * <p>An {@link android.app.Application} that wants to inject dependencies from an 
     * {@link dagger.ObjectGraph object graph} must implement {@link ObjectGraphApplication}.</p>
     */
    public interface ObjectGraphApplication {
        /**
         * <p>Injects dependencies into the given Object.</p>
         */
        @Nonnull void inject(Object dependent);
    }
    
    /**
     * <p>Injects the dependencies for the given {@link Activity}.</p>
     * @param activity The given activity
     */
    public static void inject(@Nonnull Activity activity) {
        ((ObjectGraphApplication) activity.getApplication()).inject(activity);
    }

    /**
     * <p>Injects the dependencies for the given {@link Fragment}.</p>
     * @param fragment The given fragment
     */
    public static void inject(@Nonnull Fragment fragment) {
        final Activity activity = fragment.getActivity();
        if (activity == null)
            throw new IllegalStateException("Attempting to get Activity before it has been attached to "
                    + fragment.getClass().getName());
        ((ObjectGraphApplication) activity.getApplication()).inject(fragment);
    }

    /**
     * <p>Injects the dependencies for the given {@link Service}.</p>
     * @param service The given service
     */
    public static void inject(@Nonnull Service service) {
        ((ObjectGraphApplication) service.getApplication()).inject(service);
    }
    
    /**
     * <p>Injects the dependencies for the given {@link Object} from the given {@link Context}.</p>
     * @param context The given context
     * @param object The given object
     */
    public static void inject(@Nonnull Context context, @Nonnull Object object) {
        ((ObjectGraphApplication) context.getApplicationContext()).inject(object);
    } 

/* Private Constructor */
    /** Blocks instantiation of the {@link ObjectGraph} class. */
    private ObjectGraph() { }
}

I mean that will work only if the context is able to provide you an ObjectGraphApplication, which could not always be the case. That's what I meant with asking for ObjectGraphApplication, which is what the method needs, rather than a way to get there.

@nimeacuerdo it is simply a convenience method that removes the "how" from the "what". When you're in a class that has a reference to the Context and needs it dependencies injected, you will be calling (ObjectGraphApplication) context.getApplicationContext() to get to the ObjectGraphApplication, so I extracted it out into the method. The Context passed doesn't have to be an ObjectGraphApplication in the way the method is implemented, merely a Context that can get to the ObjectGraphApplication.

I have simplified setting up the basic injection pattern in Android using a library I made called Hilt.

Now all you have to do is extend a couple classes, and set up your own modules and place them in the getActivityModules() methods in your subclass of HiltActivity and HiltApplication.