Daily Bytes

import java.util.Vector;

import java.util.Iterator;



public abstract aspect ObserverPattern perthis(subjectConstructed(Subject))

{

        public interface Observer { }

        public interface Subject { }

        

        Vector observers = new Vector();

        

        public void addObserver(Observer o)

        {

                observers.add(o);

        }

        

        protected pointcut subjectConstructed(Subject s) : 

                execution(Subject+.new(..)) && this(s);

        

        abstract protected pointcut subjectChanged(Subject s);

                

        after(Subject s) : subjectChanged(s)

        {

                Iterator itr = observers.iterator();

                while (itr.hasNext())

                        updateObserver((Observer)itr.next(), s);

        }

        

        public abstract void updateObserver(Observer o, Subject s);

}

public aspect PointObserverPattern extends ObserverPattern

{

        declare parents: Screen implements Observer;

        declare parents: Point implements Subject;

        

        protected pointcut subjectChanged(Subject s) :

                execution(void Point.set*(..)) && this(s);



        public void updateObserver(Observer o, Subject s) 

        {

                ((Screen)o).updateDisplay();

        }

}

public class Point

{

        private float x;

        private float y;

        

        Point (float x, float y) 

        {

                this.x = x;

                this.y = y;

        }

        

        public void setX(float x){

                this.x = x;

        }



        public void setY(float y){

                this.y = y;

        }

}

public class Screen

{

        public void updateDisplay(){

                System.out.println("Display has been updated");

        }

}

public class Application

{

        public static void main(String[] args) {

                Point p = new Point(0,0);

                Screen s = new Screen();

                

                PointObserverPattern.aspectOf(p).addObserver(s);                



                p.setX(2);

                p.setY(2);

                

                System.exit(0);

        }

}