/******************************************************************************* * Copyright (c) 2005 IBM Corporation and others. * All rights reserved. This program and the accompanying materials * are made available under the terms of the Eclipse Public License v1.0 * which accompanies this distribution, and is available at * http://www.eclipse.org/legal/epl-v10.html * * Contributors: *     IBM Corporation - initial API and implementation ******************************************************************************/package org.eclipse.core.commands.common;import org.eclipse.core.runtime.ListenerList;/** * <p> * A manager to which listeners can be attached. This handles the management of * a list of listeners -- optimizing memory and performance. All the methods on * this class are guaranteed to be thread-safe. * </p> * <p> * Clients may instantiate or extend. * </p> * <p> * <strong>EXPERIMENTAL</strong>. This class or interface has been added as * part of a work in progress. There is a guarantee neither that this API will * work nor that it will remain the same. Please do not use this API without * consulting with the Platform/UI team. * </p> *  * @since 3.2 */public abstract class EventManager {	/**	 * An empty array that can be returned from a call to	 * {@link #getListeners()} when {@link #listenerList} is <code>null</code>.	 */	private static final Object[] EMPTY_ARRAY = new Object[0];	/**	 * A collection of objects listening to changes to this manager. This	 * collection is <code>null</code> if there are no listeners.	 */	private transient ListenerList listenerList = null;	/**	 * Adds a listener to this manager that will be notified when this manager's	 * state changes.	 * 	 * @param listener	 *            The listener to be added; must not be <code>null</code>.	 */	protected synchronized final void addListenerObject(final Object listener) {		if (listenerList == null) {			listenerList = new ListenerList(ListenerList.IDENTITY);		}		listenerList.add(listener);	}	/**	 * Clears all of the listeners from the listener list.	 */	protected synchronized final void clearListeners() {		if (listenerList != null) {			// TODO Bug 117519			// listenerList.clear();			final Object[] listeners = listenerList.getListeners();			for (int i = 0; i < listeners.length; i++) {				listenerList.remove(listeners[i]);			}		}	}	/**	 * Returns the listeners attached to this event manager.	 * 	 * @return The listeners currently attached; may be empty, but never	 *         <code>null</code>	 */	protected final Object[] getListeners() {		final ListenerList list = listenerList;		if (list == null) {			return EMPTY_ARRAY;		}		return list.getListeners();	}	/**	 * Whether one or more listeners are attached to the manager.	 * 	 * @return <code>true</code> if listeners are attached to the manager;	 *         <code>false</code> otherwise.	 */	protected final boolean isListenerAttached() {		return listenerList != null;	}	/**	 * Removes a listener from this manager.	 * 	 * @param listener	 *            The listener to be removed; must not be <code>null</code>.	 */	protected synchronized final void removeListenerObject(final Object listener) {		if (listenerList != null) {			listenerList.remove(listener);			if (listenerList.isEmpty()) {				listenerList = null;			}		}	}}