src/java.desktop/share/classes/sun/awt/image/ImageWatched.java

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
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package sun.awt.image;

import java.lang.ref.WeakReference;
import java.awt.Image;
import java.awt.image.ImageObserver;

public abstract class ImageWatched {
    public static Link endlink = new Link();

    public Link watcherList;

    public ImageWatched() {
        watcherList = endlink;
    }

    /*
     * This class defines a node on a linked list of ImageObservers.
     * The base class defines the dummy implementation used for the
     * last link on all chains and a subsequent subclass then
     * defines the standard implementation that manages a weak
     * reference to a real ImageObserver.
     */
    public static class Link {
        /*
         * Check if iw is the referent of this Link or any
         * subsequent Link objects on this chain.
         */
        public boolean isWatcher(ImageObserver iw) {
            return false;  // No "iw" down here.
        }

        /*
         * Remove this Link from the chain if its referent
         * is the indicated target or if it has been nulled
         * out by the garbage collector.
         * Return the new remainder of the chain.
         * The argument may be null which will trigger
         * the chain to remove only the dead (null) links.
         * This method is only ever called inside a
         * synchronized block so Link.next modifications
         * will be safe.
         */
        public Link removeWatcher(ImageObserver iw) {
            return this;  // Leave me as the end link.
        }

        /*
         * Deliver the indicated image update information
         * to the referent of this Link and return a boolean
         * indicating whether or not some referent became
         * null or has indicated a lack of interest in
         * further updates to its imageUpdate() method.
         * This method is not called inside a synchronized
         * block so Link.next modifications are not safe.
         */
        public boolean newInfo(Image img, int info,
                               int x, int y, int w, int h)
        {
            return false;  // No disinterested parties down here.
        }
    }

    /*
     * Standard Link implementation to manage a Weak Reference
     * to an ImageObserver.
     */
    public static class WeakLink extends Link {
        private final WeakReference<ImageObserver> myref;
        private Link next;

        public WeakLink(ImageObserver obs, Link next) {
            myref = new WeakReference<ImageObserver>(obs);
            this.next = next;
        }

        public boolean isWatcher(ImageObserver iw) {
            return (myref.get() == iw || next.isWatcher(iw));
        }

        public Link removeWatcher(ImageObserver iw) {
            ImageObserver myiw = myref.get();
            if (myiw == null) {
                // Remove me from the chain, but continue recursion.
                return next.removeWatcher(iw);
            }
            // At this point myiw is not null so we know this test will
            // never succeed if this is a pruning pass (iw == null).
            if (myiw == iw) {
                // Remove me from the chain and end the recursion here.
                return next;
            }
            // I am alive, but not the one to be removed, recurse
            // and update my next link and leave me in the chain.
            next = next.removeWatcher(iw);
            return this;
        }

        public boolean newInfo(Image img, int info,
                               int x, int y, int w, int h)
        {
            // Note tail recursion because items are added LIFO.
            boolean ret = next.newInfo(img, info, x, y, w, h);
            ImageObserver myiw = myref.get();
            if (myiw == null) {
                // My referent is null so we must prune in a second pass.
                ret = true;
            } else if (myiw.imageUpdate(img, info, x, y, w, h) == false) {
                // My referent has lost interest so clear it and ask
                // for a pruning pass to remove it later.
                myref.clear();
                ret = true;
            }
            return ret;
        }
    }

    public synchronized void addWatcher(ImageObserver iw) {
        if (iw != null && !isWatcher(iw)) {
            watcherList = new WeakLink(iw, watcherList);
        }
        watcherList = watcherList.removeWatcher(null);
    }

    public synchronized boolean isWatcher(ImageObserver iw) {
        return watcherList.isWatcher(iw);
    }

    public void removeWatcher(ImageObserver iw) {
        synchronized (this) {
            watcherList = watcherList.removeWatcher(iw);
        }
        if (watcherList == endlink) {
            notifyWatcherListEmpty();
        }
    }

    public boolean isWatcherListEmpty() {
        synchronized (this) {
            watcherList = watcherList.removeWatcher(null);
        }
        return (watcherList == endlink);
    }

    public void newInfo(Image img, int info, int x, int y, int w, int h) {
        if (watcherList.newInfo(img, info, x, y, w, h)) {
            // Some Link returned true so we now need to prune dead links.
            removeWatcher(null);
        }
    }

    protected abstract void notifyWatcherListEmpty();
}