// This program is free software; you can redistribute it and/or modify // it under the terms of the GNU General Public License as published by // the Free Software Foundation; either version 2, or (at your option) // any later version.

// // This program is distributed in the hope that it will be useful, // but WITHOUT ANY WARRANTY; without even the implied warranty of // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the // GNU General Public License for more details.

// // To obtain a copy of the GNU General Public License write to the Free // Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.

package rob.applets ; import java.applet.*; import java.awt.*; import java.awt.event.*; /** * An applet that displays a simple animation using double-buffering * and clipping. To add the applet to a web page put the class file in the * same directory as the page and use the tag: * 


 *
 * 
 * 
 * 
 *
 *
**/ public class BouncingTextApplet extends Applet implements Runnable, ActionListener { private int x = 150, y = 100 ; // Initial position of text private int dx = 5, dy = 3; // Trajectory of text private int adx = dx, ady = dy; // Absolute values of dx and dy private Dimension size; // The size of the applet private Image buffer; // The off-screen image for double-buffering private Graphics bufferGraphics; // A Graphics object for the buffer private Thread runner; // Thread that performs the animation private boolean stop; // A flag asking animation thread to stop private String text ; // The text to display private int ascent, height, width ; // Font details /** Set up an off-screen Image for double-buffering and some buttons */ public void init() { // Set the font and the background setFont(new Font("SansSerif", 14, Font.PLAIN)) ; setBackground(Color.white) ; // A box to hold the buttons so that they don't look horrible Panel box = new Panel(new FlowLayout(FlowLayout.CENTER, 15, 5)) ; Button b = new Button("Start") ; b.addActionListener(this) ; box.add(b) ; b = new Button("Stop") ; b.addActionListener(this) ; box.add(b) ; // Set the layout so that we can put the button box across the bottom setLayout(new BorderLayout()) ; add(box, BorderLayout.SOUTH) ; // We need this information to implement double buffering so that // we can move the text without flicker size = this.size(); buffer = this.createImage(size.width, size.height); bufferGraphics = buffer.getGraphics(); // Read the "text" parameter - it should look like: // text = getParameter("text") ; if (text == null || text.equals("")) text = "Hello World" ; // Get the size of the text so that we know how to draw it. FontMetrics fm = Toolkit.getDefaultToolkit().getFontMetrics(getFont()) ; ascent = fm.getAscent() ; height = fm.getHeight() ; width = fm.stringWidth(text) ; } /** * Listen for button clicks and act on them. The command will be either * "Start" or "Stop". The methods of those names are called in response. * * @param e The ActionEvent generated by the button click - it may * be used to find the "Action Command", in this case the text on the * button. */ public void actionPerformed(ActionEvent e) { if ("Start".equals(e.getActionCommand())) start() ; else stop() ; } /** Draw the text at its current position, using double-buffering */ public void paint(Graphics g) { // Draw into the off-screen buffer. // Set the clip rectangle of bufferGraphics to be the same as that of g. bufferGraphics.setClip(g.getClip()); bufferGraphics.setColor(this.getBackground()); // set background colour bufferGraphics.fillRect(0, 0, size.width, size.height); // clear the buffer bufferGraphics.setColor(Color.red); // set foreground colour bufferGraphics.drawString(text, x, y) ; // draw the text // Then copy the off-screen buffer onto the screen g.drawImage(buffer, 0, 0, this); } /** Don't clear the screen; just call paint() immediately * It is important to override this method like this for double-buffering */ public void update(Graphics g) { paint(g); } /** * The body of the animation thread; the text needs to be moved in a separate * thread of execution otherwise the browser will not be able to control the * applet. This is done with the following steps: *
    *
  1. Indicating that we implement the Runnable interface - * that requires that we contain a method with signature * public void run() *
  2. Implement a method public void run() containing the code * we want executed. There should be some way to signal the thread to * stop so that when the browser tells the applet to stop (by calling its * stop method) we can stop the text.

    * Note One of the bugs in Netscape's implementation of the JVM * is that calling interrupt on a thread, catching the * InterruptedException and then calling * interrupt again simply does not work. Thus you must use * your own flag for signalling. *

  3. When the applet is started (by the browser calling our * start method) create a new thread with * Thread runner = new Thread(this) and start it with * runner.start(). *

* * This method shifts the text a little, calls repaint with * the region that has changed, waits for 1/20th of a second and repeats. */ public void run() { while(!stop) { // Bounce the text if we've hit an edge. if ((x + dx < 2) || (x + width + dx > size.width)) dx = -dx; if ((y + dy < height) || (y + 50 + dy > size.height)) dy = -dy; // Move the text. x += dx; y += dy; // Ask the browser to call our paint() method to redraw the circle // at its new position. Tell repaint what portion of the applet needs // be redrawn. // repaint only what has changed repaint(x-adx-2, y-ady-ascent, width+4+2*adx, height+2*ady); // Now pause 1/20th of a second before drawing the text again. try { Thread.sleep(50); } catch (InterruptedException e) { ; } } runner = null; } /** Start the animation thread */ public void start() { if (runner == null) { stop = false; runner = new Thread(this); runner.start(); } } /** Stop the animation thread */ public void stop() { stop = true; } }