Learning Java

Using a MediaTracker

java.awt.MediaTracker is a utility class that simplifies life if we have to wait for one or more images to be loaded before they’re displayed. A MediaTracker monitors the preparation of an image or a group of images and lets us check on them periodically, or wait until they are completed. MediaTracker uses the ImageObserver interface internally to receive image updates.

The following applet, TrackImage, uses a MediaTracker to wait while an image is prepared. It shows a "Loading . . .” message while it’s waiting. (If you are retrieving the image from a local disk or very fast network, this message might go by quickly, so pay attention.)

//file: TrackImage.java import java.awt.*; public class TrackImage extends javax.swing.JApplet implements Runnable { final int MAIN_IMAGE = 0; Image image; MediaTracker tracker; boolean loaded = false; Thread thread = null; String message = "Loading..."; public void init( ) { image = getImage(getClass( ).getResource(getParameter("image"))); tracker = new MediaTracker(this); tracker.addImage(image, MAIN_IMAGE); } public void start( ) { if (!tracker.checkID(MAIN_IMAGE)) { thread = new Thread(this); thread.start( ); } } public void stop( ) { thread.interrupt( ); thread = null; } public void run( ) { repaint( ); try { tracker.waitForID(MAIN_IMAGE); } catch(InterruptedException e) {} if (tracker.isErrorID(MAIN_IMAGE)) message = "Error"; else loaded = true; repaint( ); } public void paint(Graphics g) { if (loaded) g.drawImage(image, ...