/*
   This applet demonstrates how an applet can respond when
   the user drags the mouse.  The user can draw on the applet
   by dragging the mouse.  Right-click on the applet to
   clear it.  (This applet does not preperly deal with
   changes in size.)
*/


import java.awt.*;
import java.awt.event.*;
import java.applet.Applet;

public class TrivialSketch extends Applet implements MouseListener, MouseMotionListener {

   Image OSC;     // an offscreen image, to hold a copy of the sketch for repainting
   Graphics OSG;  // graphics context for drawing to OSC
   
   public void init() {
         // Initialize the applet.
      setBackground(Color.white);
      addMouseListener(this);         // Register the applet to receive
      addMouseMotionListener(this);   //   mouse events from itself.
   }
   
   public void update(Graphics g)  {
         // override this to avoid erasing the applet before drawwing
      paint(g);
   }
   
   public void paint(Graphics g) {
         // copy the off-screen image to the applet, creating it first
         // if it does not already exist
      if (OSC == null)
         makeOSC();
      g.drawImage(OSC,0,0,this);
   }
   
   void makeOSC() {
          // create the off-screen image and graphics context
      OSC = createImage(getSize().width,getSize().height);
      OSG = OSC.getGraphics();
      clearImage();
   }
   
   void clearImage() {
          // make the off-screen image into a white rectagle with a red border
      OSG.setColor(Color.white);
      OSG.fillRect(0,0,getSize().width,getSize().height);
      OSG.setColor(Color.red);
      OSG.drawRect(0,0,getSize().width - 1,getSize().height - 1);
      OSG.setColor(Color.black);
   }
   
   void drawLine(int x1, int y1, int x2, int y2) {
         // Draw a line from (x1,y1) to (x2,y2) both on the off-screen
         // image and on the applet itself.  This keeps the two
         // images in proper sync.
      OSG.drawLine(x1,y1,x2,y2);
      Graphics g = getGraphics();  // Graphics context for drawing to the applet.
      g.drawLine(x1,y1,x2,y2);
      g.dispose();
   }
   
   boolean dragging = false;  // This is true when a drag is in progress.
   
   int prev_x;  // Previous position of cursor during a drag.
   int prev_y;
   
   public void mousePressed(MouseEvent evt) {
          // If user right-clicks the applet, clear the image;
          // otherwise start drawing by setting dragging = true
      if (OSC == null)
         makeOSC();
      if (evt.isMetaDown()) {
         clearImage();
         repaint();
      }
      else {
         dragging = true;
         prev_x = evt.getX();
         prev_y = evt.getY();
      }
   }
   
   public void mouseDragged(MouseEvent evt) {
           // Draw a line from previous mouse position to current
           // position, but only if a drag is in progress.
      if (dragging) {
         int new_x = evt.getX();
         int new_y = evt.getY();
         drawLine(prev_x, prev_y, new_x, new_y);
         prev_x = new_x;
         prev_y = new_y;
      }
   }
   
   public void mouseReleased(MouseEvent evt) {
           // Finish off the drag, if one is in progress.
      if (dragging) {
         int new_x = evt.getX();
         int new_y = evt.getY();
         drawLine(prev_x, prev_y, new_x, new_y);
         dragging = false;
      }
   }
   
   public void mouseClicked(MouseEvent evt) { }  // required by listener interface
   public void mouseEntered(MouseEvent evt) { }
   public void mouseExited(MouseEvent evt) { }
   public void mouseMoved(MouseEvent evt) { }
   
}  // end of class TrivialSketch