Source code

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
018 * 2 along with this work; if not, write to the Free Software Foundation,
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
022 * or visit www.oracle.com if you need additional information or have any
023 * questions.
024 */
025
026package javafx.beans.value;
027
028import javafx.beans.WeakListener;
029
030import java.lang.ref.WeakReference;
031
032/**
033 * A {@code WeakChangeListener} can be used, if an {@link ObservableValue}
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 WeakChangeListener} are created by passing in the original
039 * {@link ChangeListener}. The {@code WeakChangeListener} 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 ChangeListener}, that
043 * was passed in as long as it is in use, otherwise it will be garbage collected
044 * to soon.
045 *
046 * @see ChangeListener
047 * @see ObservableValue
048 *
049 * @param <T>
050 *            The type of the observed value
051 * 
052 * 
053 */
054public final class WeakChangeListener<T> implements ChangeListener<T>, WeakListener {
055
056    private final WeakReference<ChangeListener<T>> ref;
057
058    /**
059     * The constructor of {@code WeakChangeListener}.
060     * 
061     * @param listener
062     *            The original listener that should be notified
063     */
064    public WeakChangeListener(ChangeListener<T> listener) {
065        if (listener == null) {
066            throw new NullPointerException("Listener must be specified.");
067        }
068        this.ref = new WeakReference<ChangeListener<T>>(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 changed(ObservableValue<? extends T> observable, T oldValue,
084            T newValue) {
085        ChangeListener<T> listener = ref.get();
086        if (listener != null) {
087            listener.changed(observable, oldValue, newValue);
088        } else {
089            // The weakly reference listener has been garbage collected,
090            // so this WeakListener will now unhook itself from the
091            // source bean
092            observable.removeListener(this);
093        }
094    }
095
096}