Using otto : minimal

Otto.java

//Create a file BusProvider to provide singleton BusProivder
public class BusProvider {
    private static final Bus bus = new Bus();

    public static Bus getInstance()  {
        return bus;
    }

}

//Create another file which extends application
public class ExampleApplication extends Application {

    @Override
    public void onCreate() {
        super.onCreate();
        //Persistent BusProvider, so GC doesn't remove this.
        new BusProivder();
    }
}

Now time to create Events in events/ , create a class
LoadReviewsEvent.java
public class LoadReviewsEvent {
  public final String data;
  public LoadReviewsEvent(String data) {
    this.data = data;
    }
}

Now in your activitys onPause and onResome methods

@Override
public void onResume()  {
  super();
  BusProvider.getInstance().register(this);
}

@Override
public void onPause()  {
  BusProvider.getInstance().unregister(this);
  super();
}

Subscribe for events
@Subscribe
public void handleReviewEvent(LoadReviewsEvent loadReviews) {
  //Load reviews here.
  //use loadReviews.data
}

someAsyncTask() {
  //when we download data , publish and event using otto
  BusProvider.getInstance().publish(new LoadReviewsEvent("This is our data"));
}