Spec-Zone .ru
спецификации, руководства, описания, API
001/*
002 * Copyright (c) 2011, 2013, Oracle and/or its affiliates. All rights reserved.
003 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
004 *
005 * This code is free software; you can redistribute it and/or modify it
006 * under the terms of the GNU General Public License version 2 only, as
007 * published by the Free Software Foundation.  Oracle designates this
008 * particular file as subject to the "Classpath" exception as provided
009 * by Oracle in the LICENSE file that accompanied this code.
010 *
011 * This code is distributed in the hope that it will be useful, but WITHOUT
012 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
013 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
014 * version 2 for more details (a copy is included in the LICENSE file that
015 * accompanied this code).
016 *
017 * You should have received a copy of the GNU General Public License version
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019 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
020 *
021 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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023 * questions.
024 */
025
026package javafx.collections;
027
028import javafx.beans.WeakListener;
029
030import java.lang.ref.WeakReference;
031
032/**
033 * A {@code WeakListChangeListener} can be used, if an {@link ObservableList}
034 * should only maintain a weak reference to the listener. This helps to avoid
035 * memory leaks, that can occur if observers are not unregistered from observed
036 * objects after use.
037 * <p>
038 * {@code WeakListChangeListener} are created by passing in the original
039 * {@link ListChangeListener}. The {@code WeakListChangeListener} should then be
040 * registered to listen for changes of the observed object.
041 * <p>
042 * Note: You have to keep a reference to the {@code ListChangeListener}, that
043 * was passed in as long as it is in use, otherwise it will be garbage collected
044 * to soon.
045 *
046 * @see ListChangeListener
047 * @see ObservableList
048 * @see javafx.beans.WeakListener
049 *
050 * @param <E>
051 *            The type of the observed value
052 *
053 */
054public final class WeakListChangeListener<E> implements ListChangeListener<E>, WeakListener {
055
056    private final WeakReference<ListChangeListener<E>> ref;
057
058    /**
059     * The constructor of {@code WeakListChangeListener}.
060     *
061     * @param listener
062     *            The original listener that should be notified
063     */
064    public WeakListChangeListener(ListChangeListener<E> listener) {
065        if (listener == null) {
066            throw new NullPointerException("Listener must be specified.");
067        }
068        this.ref = new WeakReference<ListChangeListener<E>>(listener);
069    }
070
071    /**
072     * {@inheritDoc}
073     */
074    @Override
075    public boolean wasGarbageCollected() {
076        return (ref.get() == null);
077    }
078
079    /**
080     * {@inheritDoc}
081     */
082    @Override
083    public void onChanged(Change<? extends E> change) {
084        final ListChangeListener<E> listener = ref.get();
085        if (listener != null) {
086            listener.onChanged(change);
087        } else {
088            // The weakly reference listener has been garbage collected,
089            // so this WeakListener will now unhook itself from the
090            // source bean
091            change.getList().removeListener(this);
092        }
093    }
094}