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 * Copyright (c) 2011, 2022, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * 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.
 *
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 */

package javafx.collections;

import javafx.beans.NamedArg;
import javafx.beans.WeakListener;

import java.lang.ref.WeakReference;

/**
 * A {@code WeakSetChangeListener} can be used, if an {@link javafx.collections.ObservableSet}
 * 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.
 * <p>
 * {@code WeakSetChangeListener} are created by passing in the original
 * {@link SetChangeListener}. The {@code WeakSetChangeListener} should then be
 * registered to listen for changes of the observed object.
 * <p>
 * Note: You have to keep a reference to the {@code SetChangeListener}, that
 * was passed in as long as it is in use, otherwise it will be garbage collected
 * to soon.
 *
 * @see SetChangeListener
 * @see ObservableSet
 * @see javafx.beans.WeakListener
 *
 * @param <E>
 *            The type of the observed value
 *
 * @since JavaFX 2.1
 */
public final class WeakSetChangeListener<E> implements SetChangeListener<E>, WeakListener {

    private final WeakReference<SetChangeListener<E>> ref;

    /**
     * The constructor of {@code WeakSetChangeListener}.
     *
     * @param listener
     *            The original listener that should be notified
     */
    public WeakSetChangeListener(@NamedArg("listener") SetChangeListener<E> 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<? extends E> change) {
        final SetChangeListener<E> 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.getSet().removeListener(this);
        }
    }
}
