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The JGoodies Binding library connects object properties to Swing user interface components. And it helps you represent the state and behavior of a presentation independently of the GUI components used in the interface.
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/*
 * Copyright (c) 2002-2015 JGoodies Software GmbH. All Rights Reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 *  o Redistributions of source code must retain the above copyright notice,
 *    this list of conditions and the following disclaimer.
 *
 *  o Redistributions in binary form must reproduce the above copyright notice,
 *    this list of conditions and the following disclaimer in the documentation
 *    and/or other materials provided with the distribution.
 *
 *  o Neither the name of JGoodies Software GmbH nor the names of
 *    its contributors may be used to endorse or promote products derived
 *    from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS;
 * OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
 * WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE
 * OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE,
 * EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

package com.jgoodies.binding.value;

import static com.jgoodies.common.base.Preconditions.checkNotNull;

import java.beans.PropertyChangeEvent;
import java.beans.PropertyChangeListener;



/**
 * A ValueModel that allows to accept or reject proposed value changes.
 * Useful to request information from the user or to perform operations
 * before a value is changed.
 *
 * Wraps a given subject ValueModel and always returns the subject value
 * as this model's value. Observes subject value changes and forwards
 * them to listeners of this model. If a value is set to this model,
 * the abstract method {@code #proposedChange} is invoked. In this method
 * implementors define how to accept or reject value changes.

 *
 * Implementors may veto against a proposed change based on the application
 * state or by asking the user, and may also perform additional operations
 * during the check, for example to save editor contents. Here's an example:
 * 
 * public class CheckPendingEditValueModel extends AbstractVetoableValueModel {
 *
 *     public CheckPendingEditValueModel(ValueModel subject) {
 *         super(subject);
 *     }
 *
 *     public boolean proposedChange(Object oldValue, Object proposedNewValue) {
 *         if (equals(oldValue, proposedNewValue))
 *             return true;
 *         int option = JOptionPane.showConfirmDialog(
 *             Application.getDefaultParentFrame(),
 *             "Do you want to save the editor contents.");
 *         if (option == JOptionPane.YES_OPTION)
 *             model.save();
 *         return option != JOptionPane.CANCEL_OPTION;
 *     }
 * }
 * 
 *
 * @author Karsten Lentzsch
 * @version $Revision: 1.12 $
 *
 * @since 1.1
 */
public abstract class AbstractVetoableValueModel extends AbstractValueModel {

    /**
     * Holds the wrapped subject ValueModel that is used to read values from
     * and commit accepted changes to.
     */
    private final ValueModel subject;


    // Instance Creation ******************************************************

    /**
     * Constructs an AbstractVetoableValueModel for the given ValueModel.
     *
     * @param subject   the underlying (or wrapped) ValueModel
     *
     * @throws NullPointerException if the subject is {@code null}
     */
    protected AbstractVetoableValueModel(ValueModel subject) {
        this.subject = checkNotNull(subject, "The subject must not be null.");
        subject.addValueChangeListener(new SubjectValueChangeHandler());
    }


    // Abstract Behavior ******************************************************

    /**
     * Checks and answers whether the proposed value change shall be
     * accepted or rejected. Implementors may perform additional
     * operations, for example to save a pending editor content.
     *
     * @param oldValue          the value before the change
     * @param proposedNewValue  the new value if the change is accepted
     * @return true to accept the proposed value, false to veto against it.
     */
    public abstract boolean proposedChange(Object oldValue, Object proposedNewValue);


    // ValueModel Implementation **********************************************

    /**
     * Returns this model's current subject value.
     *
     * @return this model's current subject value.
     */
    @Override
	public final Object getValue() {
        return subject.getValue();
    }


    /**
     * Sets the given value as new subject value if and only if
     * 1) the new value differs from the old value and
     * 2) the proposed change is accepted as checked by
     * {@code proposedChange(oldValue, newValue)}.
     *
     * @param newValue   the value to set
     */
    @Override
	public final void setValue(Object newValue) {
        Object oldValue = getValue();
        if (oldValue == newValue) {
            return;
        }
        if (proposedChange(oldValue, newValue)) {
            subject.setValue(newValue);
        }
    }


    // Event Handling *****************************************************

    /**
     * Forwards value changes in the subject to listeners of this model.
     */
    private final class SubjectValueChangeHandler implements PropertyChangeListener {

        @Override
		public void propertyChange(PropertyChangeEvent evt) {
            fireValueChange(evt.getOldValue(), evt.getNewValue(), true);
        }
    }

}