public class ObserverCallBack
extends Object
implements EventListener

Use this class to implement call back methods for the Region observers onAppear, onVanish and onChange.
by overriding the contained empty methods appeared, vanished and changed

 example:
 aRegion.onAppear(anImage,
   new ObserverCallBack() {
     appeared(ObserveEvent e) {
       // do something
     }
   }
 );
 

when the image appears, your above call back appeared() will be called see ObserveEvent about the features available in the callback function