import java.awt.*;
import java.util.*;

/**
 * A portal is a both scene's view and its controller. It must:
 *    1) display the scene
 *    2) translate user input into transformation operations
 */

public abstract class Portal extends Canvas
   implements Observer
{
   protected Scene _scn;

   /**
    * The constructor.
    */

   public Portal(Scene scn)
   {
      if (scn == null) throw new NullPointerException("null scene");

      _scn = scn;

      _scn.addObserver(this);
   }

   /**
    * The observable's update callback.
    */

   public void update(Observable obs, Object obj)
   {
      if (obs == _scn) repaint();
   }

   /**
    * Automatically handle double-buffering.
    */

   private Image _im = null;
   private Graphics _gr = null;
   private Dimension _dim = null;

   public void update(Graphics gr)
   {
      Dimension dim = size();

      if (_im == null ||
          _gr == null ||
          _dim == null ||
          _dim.height != dim.height ||
          _dim.width != dim.width)
      {
         _dim = dim;
         _im = createImage(_dim.width, _dim.height);
         _gr = _im.getGraphics();
      }

      _gr.setColor(getBackground());

      _gr.fillRect(0, 0, _dim.width, _dim.height);

      _gr.setColor(getForeground());

      paintScene(_gr);

      gr.drawImage(_im, 0, 0, this);
   }

   public void paint(Graphics gr)
   {
      update(gr);
   }

   /**
    * Define this.
    */

   protected abstract void paintScene(Graphics gr);
}