Java AWT: Lightweight UI Framework

1. Native windows can be heavyweight, so it's undesirable to have too many of them.
2. Native windows are opaque, so they can't be used to implement transparent regions.
3. Native windows are handled differently across platforms, so the AWT has to struggle to maintain a consistent view across these varied platforms.

Lightweight UI Framework

The advantages of creating lightweight components are the following:

• The Lightweight component can now have transparent areas by simply not rendering to those areas in its paint() method (although, until we get full shape support from Java2D, the bounding box will remain rectangular).
• The Lightweight component is "lighter" in that it requires no native data-structures or peer classes.
• There is no native code required to process lightweight components, hence handling of lightweights is 100% implemented in common java code, which leads to complete consistency across platforms.

We are using this framework in an upcoming version of the toolkit (beyond 1.1) to implement pure-java versions of the base UI controls (Button, List, etc.) which implement a common look-and-feel across the platforms (and don't use the native peers). For more information on our future work, see the AWT: The Next Generation overview.

Mixing Lightweight & Heavyweight Components

Putting Lightweight components in Existing Panels

Double Buffering

public class DoubleBufferPanel extends Panel {    
  Image offscreen;

  /**
   * null out the offscreen buffer as part of invalidation
   */
  public void invalidate() {
      super.invalidate();
      offscreen = null;
  }

  /**
   * override update to *not* erase the background before painting
   */
  public void update(Graphics g) {
      paint(g);
  }

  /**
   * paint children into an offscreen buffer, then blast entire image
   * at once.
   */
  public void paint(Graphics g) {
      if(offscreen == null) {
         offscreen = createImage(getSize().width, getSize().height);
      }
      Graphics og = offscreen.getGraphics();
      og.setClip(0,0,getSize().width, getSize().height);
      super.paint(og);
      g.drawImage(offscreen, 0, 0, null);
      og.dispose();
  }
}

Sample Code

import java.lang.*;
import java.util.*;
import java.awt.*;
import java.awt.event.*;

/**
 * RoundButton - a class that produces a lightweight button.
 */
public class RoundButton extends Component {

  String label;                      // The Button's text
  protected boolean pressed = false; // true if the button is detented.
  
  /**
   * Constructs a RoundButton with the specified label.
   * @param label the label of the button
   */
  public RoundButton(String label) {
      this.label = label;
      enableEvents(AWTEvent.MOUSE_EVENT_MASK);
  }
  
  /**
   * paints the RoundButton
   */
  public void paint(Graphics g) {
      int s = Math.min(getSize().width - 1, getSize().height - 1);
      
      // paint the interior of the button
      if(pressed) {
          g.setColor(getBackground().darker().darker());
      } else {
          g.setColor(getBackground());
      }
      g.fillArc(0, 0, s, s, 0, 360);
      
      // draw the perimeter of the button
      g.setColor(getBackground().darker().darker().darker());
      g.drawArc(0, 0, s, s, 0, 360);
      
      // draw the label centered in the button
      Font f = getFont();
      if(f != null) {
          FontMetrics fm = getFontMetrics(getFont());
          g.setColor(getForeground());
          g.drawString(label,
                       s/2 - fm.stringWidth(label)/2,
                       s/2 + fm.getMaxDescent());
      }
  }
  
  /**
   * The preferred size of the button. 
   */
  public Dimension getPreferredSize() {
      Font f = getFont();
      if(f != null) {
          FontMetrics fm = getFontMetrics(getFont());
          int max = Math.max(fm.stringWidth(label) + 40, fm.getHeight() + 40);
          return new Dimension(max, max);
      } else {
          return new Dimension(100, 100);
      }
  }
  
  /**
   * The minimum size of the button. 
   */
  public Dimension getMinimumSize() {
      return new Dimension(100, 100);
  }
   
   /**
    * Paints the button and distribute an action event to all listeners.
    */
   public void processMouseEvent(MouseEvent e) {
       Graphics g;
       switch(e.getID()) {
          case MouseEvent.MOUSE_PRESSED:
            // render myself inverted....
            pressed = true;

            // Repaint might flicker a bit. To avoid this, you can use
            // double buffering (see the Gauge example).
            repaint(); 
            break;
          case MouseEvent.MOUSE_RELEASED:
            // render myself normal again
            if(pressed == true) {
                pressed = false;
                // Repaint might flicker a bit. To avoid this, you can use
                // double buffering (see the Gauge example).
                repaint();
            }
            break;
          case MouseEvent.MOUSE_ENTERED:
            break;
          case MouseEvent.MOUSE_EXITED:
            if(pressed == true) {
                // Cancel! Don't send action event.
                pressed = false;

                // Repaint might flicker a bit. To avoid this, you can use
                // double buffering (see the DoubleBufferPanel example above).
                repaint();

                // Note: for a more complete button implementation,
                // you wouldn't want to cancel at this point, but
                // rather detect when the mouse re-entered, and
                // re-highlight the button. There are a few state
                // issues that that you need to handle, which we leave
                // this an an excercise for the reader (I always
                // wanted to say that!)
            }
            break;
       }
       super.processMouseEvent(e);
   }   
}