// base class for double buffered applets. Drawing is done to imageBuffer,
//   an offscreen buffer that's BLTed to the frame as need be. This gives
//   you less flashies and some measure of image persistence across exposes.
// (it'd be nice if we could do this for arbitrary components, but that
//  would require changing the root Component class)

// Draw model:
//   When drawing, don't use this.getGraphics() or paint()'s argument-
//     use imageBuffer.getGraphics()
//   override paint() as expected to paint, but make sure to
//     call super.paint() to actually render your changes
//   call this.repaint() to actually cause the drawing to show up
//     (or wait for something else to call this.update())
//   use this.setBackground() as you would normally
//   call clearImageBuffer() to reset the entire drawing to background color

import java.applet.*;
import java.awt.*;

public class DoubleBufferApplet extends Applet {
  protected Image imageBuffer;
  protected Dimension imageBufferSize;
  
  // paint - just call update. Subclass can override this, be sure to call
  // super.paint() as the last step.
  public void paint(Graphics g) {
    this.update(g);
  }

  // clear the image buffer to whatever the background colour is.
  public void clearImageBuffer() {
    Graphics ig = imageBuffer.getGraphics();
    Color oldColor = ig.getColor();
    ig.setColor(this.getBackground());	  // clear the buffer
    ig.fillRect(0, 0, imageBufferSize.width, imageBufferSize.height);
    ig.setColor(oldColor);
  }

  // update - render the buffer onto the screen
  // also handles lazy creation of the offscreen buffer
  public void update (Graphics g) {
    Dimension appletSize = this.size();		  // read the current size

    // check that the buffer is valid - if not, build one
    if (imageBuffer == null ||
	appletSize.width != imageBufferSize.width ||
	appletSize.height != imageBufferSize.height) {
      // System.out.println("Building a buffer of size " + appletSize);
      imageBuffer = createImage(appletSize.width, appletSize.height);
      imageBufferSize = appletSize;

      this.clearImageBuffer();
    }

    g.drawImage(imageBuffer, 0, 0, this);
  }
}