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This is a backport of OpenJFX 8 to run on Java 7.
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/*
 * Copyright (c) 2011, 2013, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code 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
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package javafx.collections;

import javafx.beans.WeakListener;

import java.lang.ref.WeakReference;

/**
 * A {@code WeakMapChangeListener} can be used, if an {@link javafx.collections.ObservableMap}
 * should only maintain a weak reference to the listener. This helps to avoid
 * memory leaks, that can occur if observers are not unregistered from observed
 * objects after use.
 * 
 * {@code WeakMapChangeListener} are created by passing in the original
 * {@link javafx.collections.MapChangeListener}. The {@code WeakMapChangeListener} should then be
 * registered to listen for changes of the observed object.
 * 
 * Note: You have to keep a reference to the {@code MapChangeListener}, that
 * was passed in as long as it is in use, otherwise it will be garbage collected
 * to soon.
 *
 * @see MapChangeListener
 * @see ObservableMap
 * @see javafx.beans.WeakListener
 *
 * @param  the key element type
 * @param  the value element type
 *
 * @since JavaFX 2.1
 */
public final class WeakMapChangeListener implements MapChangeListener, WeakListener {

    private final WeakReference> ref;

    /**
     * The constructor of {@code WeakMapChangeListener}.
     *
     * @param listener
     *            The original listener that should be notified
     */
    public WeakMapChangeListener(MapChangeListener listener) {
        if (listener == null) {
            throw new NullPointerException("Listener must be specified.");
        }
        this.ref = new WeakReference>(listener);
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public boolean wasGarbageCollected() {
        return (ref.get() == null);
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public void onChanged(Change change) {
        final MapChangeListener listener = ref.get();
        if (listener != null) {
            listener.onChanged(change);
        } else {
            // The weakly reference listener has been garbage collected,
            // so this WeakListener will now unhook itself from the
            // source bean
            change.getMap().removeListener(this);
        }
    }
}