/*******************************************************************************
 * Copyright (c) 2000, 2015 IBM Corporation and others.
 *
 * This program and the accompanying materials
 * are made available under the terms of the Eclipse Public License 2.0
 * which accompanies this distribution, and is available at
 * https://www.eclipse.org/legal/epl-2.0/
 *
 * SPDX-License-Identifier: EPL-2.0
 *
 * Contributors:
 *     IBM Corporation - initial API and implementation
 *******************************************************************************/
package org.eclipse.jface.util;

import java.util.EventObject;
import org.eclipse.core.runtime.Assert;

/**
 * An event object describing a change to a named property.
 * <p>
 * This concrete class was designed to be instantiated, but may
 * also be subclassed if required.
 * </p>
 * <p>
 * The JFace frameworks contain classes that report property
 * change events for internal state changes that may be of interest
 * to external parties. A special listener interface
 * (<code>IPropertyChangeListener</code>) is defined for this purpose,
 * and a typical class allow listeners to be registered via
 * an <code>addPropertyChangeListener</code> method.
 * </p>
 *
 * @see IPropertyChangeListener
 */
public class PropertyChangeEvent extends EventObject {

	/**
	 * Generated serial version UID for this class.
	 * @since 3.1
	 */
	private static final long serialVersionUID = 3256726173533811256L;

	/**
	 * The name of the changed property.
	 */
	private String propertyName;

	/**
	 * The old value of the changed property, or <code>null</code> if
	 * not known or not relevant.
	 */
	private Object oldValue;

	/**
	 * The new value of the changed property, or <code>null</code> if
	 * not known or not relevant.
	 */
	private Object newValue;

	/**
	 * Creates a new property change event.
	 *
	 * @param source the object whose property has changed
	 * @param property the property that has changed (must not be <code>null</code>)
	 * @param oldValue the old value of the property, or <code>null</code> if none
	 * @param newValue the new value of the property, or <code>null</code> if none
	 */
	public PropertyChangeEvent(Object source, String property, Object oldValue,
			Object newValue) {
		super(source);
		Assert.isNotNull(property);
		this.propertyName = property;
		this.oldValue = oldValue;
		this.newValue = newValue;
	}

	/**
	 * Returns the new value of the property.
	 *
	 * @return the new value, or <code>null</code> if not known
	 *  or not relevant (for instance if the property was removed).
	 */
	public Object getNewValue() {
		return newValue;
	}

	/**
	 * Returns the old value of the property.
	 *
	 * @return the old value, or <code>null</code> if not known
	 *  or not relevant (for instance if the property was just
	 *  added and there was no old value).
	 */
	public Object getOldValue() {
		return oldValue;
	}

	/**
	 * Returns the name of the property that changed.
	 * <p>
	 * Warning: there is no guarantee that the property name returned
	 * is a constant string.  Callers must compare property names using
	 * equals, not ==.
	 * </p>
	 *
	 * @return the name of the property that changed
	 */
	public String getProperty() {
		return propertyName;
	}
}
