ImageObserver Interface

The various flags associated with the ImageObserver are used for the infoflags argument to imageUpdate(). The flags indicate what kind of information is available and how to interpret the other arguments to imageUpdate(). Two or more flags are often combined (by an OR operation) to show that several kinds of information are available.

public static final int WIDTH

When the WIDTH flag is set, the width argument to imageUpdate() correctly indicates the image's width. Subsequent calls to getWidth() for the Image return the valid image width. If you call getWidth() before this flag is set, expect it to return -1.

public static final int HEIGHT

When the HEIGHT flag is set, the height argument to imageUpdate() correctly indicates the image's height. Subsequent calls to getHeight() for the Image return the valid image height. If you call getHeight() before this flag is set, expect it to return -1.

public static final int PROPERTIES

When the PROPERTIES flag is set, the image's properties are available. Subsequent calls to getProperty() return valid image properties.

public static final int SOMEBITS

When the SOMEBITS flag of infoflags (from imageUpdate()) is set, the image has started loading and at least some of its content are available for display. When this flag is set, the x, y, width, and height arguments to imageUpdate() indicate the bounding rectangle for the portion of the image that has been delivered so far.

public static final int FRAMEBITS

When the FRAMEBITS flag of infoflags is set, a complete frame of a multiframe image has been loaded and can be drawn. The remaining parameters to imageUpdate() should be ignored (x, y, width, height).

public static final int ALLBITS

When the ALLBITS flag of infoflags is set, the image has been completely loaded and can be drawn. The remaining parameters to imageUpdate() should be ignored (x, y, width, height).

public static final int ERROR

When the ERROR flag is set, the production of the image has stopped prior to completion because of a severe problem. ABORT may or may not be set when ERROR is set. Attempts to reload the image will fail. You might get an ERROR because the URL of the Image is invalid (file not found) or the image file itself is invalid (invalid size/content).

public static final int ABORT

When the ABORT flag is set, the production of the image has aborted prior to completion. If ERROR is not set, a subsequent attempt to draw the image may succeed. For example, an image would abort without an error if a network error occurred (e.g., a timeout on the HTTP connection).

public boolean imageUpdate (Image image, int infoflags, int x, int y, int width, int height)

The imageUpdate() method is the sole method in the ImageObserver interface. It is called whenever information about an image becomes available. To register an image observer for an image, pass an object that implements the ImageObserver interface to getWidth(), getHeight(), getProperty(), prepareImage(), or drawImage().

The image parameter to imageUpdate() is the image being rendered on the observer. The infoflags parameter is a set of ImageObserver flags ORed together to signify the current information available about image. The meaning of the x, y, width, and height parameters depends on the current infoflags settings.

Implementations of imageUpdate() should return true if additional information about the image is desired; returning false means that you don't want any additional information, and consequently, imageUpdate() should not be called in the future for this image. The default imageUpdate() method returns true if neither ABORT nor ALLBITS are set in the infoflags--that is, the method imageUpdate() is interested in further information if no errors have occurred and the image is not complete. If either flag is set, imageUpdate() returns false.

You should not call imageUpdate() directly--unless you are developing an ImageConsumer, in which case you may find it worthwhile to override the default imageUpdate() method, which all components inherit from the Component class.

Overriding imageUpdate

Instead of bothering with the MediaTracker class, you can override the imageUpdate() method and use it to notify you when an image is completely loaded. Example 12.1 demonstrates the use of imageUpdate(), along with a way to force your images to load immediately. Here's how it works: the init() method calls getImage() to request image loading at some time in the future. Instead of waiting for drawImage() to trigger the loading process, init() forces loading to start by calling prepareImage(), which also registers an image observer. prepareImage() is a method of the Component class discussed in Chapter 5, Components.

The paint() method doesn't attempt to draw the image until the variable loaded is set to true. The imageUpdate() method checks the infoflags argument to see whether ALLBITS is set; when it is set, imageUpdate() sets loaded to true, and schedules a call to paint(). Thus, paint() doesn't call drawImage() until the method imageUpdate() has discovered that the image is fully loaded.

import java.applet.*;
import java.awt.*;
import java.awt.image.ImageObserver;
public class imageUpdateOver extends Applet {
    Image image;
    boolean loaded = false;
    public void init () {
        image = getImage (getDocumentBase(), "rosey.jpg");
        prepareImage (image, -1, -1, this);
    }
    public void paint (Graphics g) {
        if (loaded)
            g.drawImage (image, 0, 0, this);
    }
    public void update (Graphics g) {
        paint (g);
    }
    public synchronized boolean imageUpdate (Image image, int infoFlags,
                        int x, int y, int width, int height) {
        if ((infoFlags & ImageObserver.ALLBITS) != 0) {
            loaded = true;
            repaint();
            return false;
        } else {
            return true;
        }
    }
}

Note that the call to prepareImage() is absolutely crucial. It is needed both to start image loading and to register the image observer. If prepareImage() were omitted, imageUpdate() would never be called, loaded would not be set, and paint() would never attempt to draw the image. As an alternative, you could use the MediaTracker class to force loading to start and monitor the loading process; that approach might give you some additional flexibility.