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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 java.beans.PropertyChangeListener;

/**
 * Describes models with a generic access to a single value that allow
 * to observe value changes. The value can be accessed using the
 * {@code #getValue()}/{@code #setValue(Object)} methods.
 * Observers can register instances of {@code PropertyChangeListener}
 * to be notified if the value changes.
 *
 * If the value is read-only or write-only, an implementor may choose
 * to reject an operation using an {@code UnsupportedOperationException}
 * or may do nothing or perform an appropriate action, or may return an
 * appropriate value.

 *
 * The listeners registered with this ValueModel using #addValueChangeListener
 * will be invoked only with PropertyChangeEvents that have the name set to
 * "value".
 * In other words, the listeners won't get notified when a PropertyChangeEvent
 * is fired that has a null object as the name to indicate an arbitrary set
 * of the event source's properties have changed. This is the case
 * if you use the PropertyChangeSupport, either directly or indirectly,
 * to fire property changes with the property name "value" specified.
 * This constraint ensures that all ValueModel implementors behave
 * like the AbstractValueModel subclasses.
 * In the rare case, where you want to notify a PropertyChangeListener
 * even with PropertyChangeEvents that have no property name set,
 * you can register the listener with #addPropertyChangeListener,
 * not #addValueChangeListener.

 *
 * AbstractValueModel minimizes the effort required to implement this interface.
 * It uses the PropertyChangeSupport to fire PropertyChangeEvents, and it adds
 * PropertyChangeListeners for the specific property name "value". This ensures
 * that the constraint mentioned above is met.

 *
 * Implementors are encouraged to provide non-null values for the
 * PropertyChangeEvent's old and new values. However, both may be null.
 *
 * @author  Karsten Lentzsch
 * @version $Revision: 1.9 $
 *
 * @see AbstractValueModel
 * @see ValueHolder
 * @see com.jgoodies.binding.beans.PropertyAdapter
 */
public interface ValueModel {

    // Property Names *********************************************************

    /**
     * The name of the bound property value.
     */
    String PROPERTY_VALUE = "value";


    // Properties *************************************************************

    /**
     * Returns this model's value. In case of a write-only value,
     * implementors may choose to either reject this operation or
     * or return {@code null} or any other appropriate value.
     *
     * @return this model's value
     */
    Object getValue();


    /**
     * Sets a new value and notifies any registered value listeners
     * if the value has changed. In case of a read-only value
     * implementors may choose to either reject this operation
     * or to do nothing.
     *
     * @param newValue  the value to be set
     */
    void setValue(Object newValue);


    /**
     * Registers the given PropertyChangeListener with this
     * ValueModel. The listener will be notified if the value has changed.
     * The PropertyChangeEvents delivered to the listener must have the name
     * set to "value". The latter ensures that all ValueModel implementors
     * behave like the AbstractValueModel subclasses.
     *
     * To comply with the above specification implementors can use
     * the PropertyChangeSupport's #addPropertyChangeListener method
     * that accepts a property name, so that listeners will be invoked only
     * if that specific property has changed.
     *
     * @param listener  the listener to be added
     *
     * @see AbstractValueModel#addValueChangeListener(PropertyChangeListener)
     */
    void addValueChangeListener(PropertyChangeListener listener);


    /**
     * Deregisters the given PropertyChangeListener from this ValueModel.
     *
     * @param listener  the listener to be removed
     */
    void removeValueChangeListener(PropertyChangeListener listener);


}