plugins/org.eclipse.jem.proxy/proxy/org/eclipse/jem/internal/proxy/core/ListenerList.java

/*******************************************************************************
 * Copyright (c) 2003, 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.jem.internal.proxy.core;

/**
 * This class is used to maintain a list of listeners, and
 * is used in the implementations of several classes within jem
 * which allow you to register listeners of various kinds.
 * It is a fairly lightweight object, occupying minimal space when
 * no listeners are registered.
 * <p>
 * Note that the <code>add</code> method checks for and eliminates 
 * duplicates based on identity (not equality).  Likewise, the
 * <code>remove</code> method compares based on identity.
 * </p>
 * <p>
 * Use the <code>getListeners</code> method when notifying listeners.
 * Note that no garbage is created if no listeners are registered.
 * The recommended code sequence for notifying all registered listeners
 * of say, <code>FooListener.eventHappened</code>, is:
 * <pre>
 * Object[] listeners = myListenerList.getListeners();
 * for (int i = 0; i < listeners.length; ++i) {
 *    ((FooListener) listeners[i]).eventHappened(event);
 * }
 * </pre>
 * </p>
 */

public class ListenerList {

	/**
	 * The initial capacity of the list. Always >= 1.
	 */
	private int capacity;

	/**
	 * The current number of listeners.
	 * Maintains invariant: 0 <= size <= listeners.length.
	 */
	private int size;

	/**
	 * The list of listeners.  Initially <code>null</code> but initialized
	 * to an array of size capacity the first time a listener is added.
	 * Maintains invariant: listeners != null IFF size != 0
	 */
	private Object[] listeners = null;

	/**
	 * The empty array singleton instance, returned by getListeners()
	 * when size == 0.
	 */
	private static final Object[] EmptyArray = new Object[0];

	/**
	 * Creates a listener list with an initial capacity of 1.
	 */
	public ListenerList() {
		this(1);
	}

	/**
	 * Creates a listener list with the given initial capacity.
	 *
	 * @param capacity the number of listeners which this list can initially accept 
	 *    without growing its internal representation; must be at least 1
	 */
	public ListenerList(int capacity) {
		if (!(capacity >= 1))
			throw new IllegalArgumentException();
		this.capacity = capacity;
	}

	/**
	 * Adds the given listener to this list. Has no effect if an identical listener
	 * is already registered.
	 *
	 * @param listener the listener
	 */
	public void add(Object listener) {
		if (listener == null)
			throw new IllegalArgumentException();
		if (size == 0) {
			listeners = new Object[capacity];
		} else {
			// check for duplicates using identity
			for (int i = 0; i < size; ++i) {
				if (listeners[i] == listener) {
					return;
				}
			}
			// grow array if necessary
			if (size == listeners.length) {
				System.arraycopy(
					listeners,
					0,
					listeners = new Object[size * 2 + 1],
					0,
					size);
			}
		}
		listeners[size++] = listener;
	}

	/**
	 * Removes all listeners from this list.
	 */
	public void clear() {
		size = 0;
		listeners = null;
	}

	/**
	 * Returns an array containing all the registered listeners,
	 * in the order in which they were added.
	 * <p>
	 * The resulting array is unaffected by subsequent adds or removes.
	 * If there are no listeners registered, the result is an empty array
	 * singleton instance (no garbage is created).
	 * Use this method when notifying listeners, so that any modifications
	 * to the listener list during the notification will have no effect on the
	 * notification itself.
	 * </p>
	 *
	 * @return the list of registered listeners
	 */
	public Object[] getListeners() {
		if (size == 0)
			return EmptyArray;
		Object[] result = new Object[size];
		System.arraycopy(listeners, 0, result, 0, size);
		return result;
	}

	/**
	 * Returns whether this listener list is empty.
	 *
	 * @return <code>true</code> if there are no registered listeners, and
	 *   <code>false</code> otherwise
	 */
	public boolean isEmpty() {
		return size == 0;
	}

	/**
	 * Removes the given listener from this list. Has no effect if an identical
	 * listener was not already registered.
	 *
	 * @param listener the listener
	 */
	public void remove(Object listener) {
		if (listener == null)
			throw new IllegalArgumentException();
		for (int i = 0; i < size; ++i) {
			if (listeners[i] == listener) {
				if (size == 1) {
					listeners = null;
					size = 0;
				} else {
					System.arraycopy(
						listeners,
						i + 1,
						listeners,
						i,
						--size - i);
					listeners[size] = null;
				}
				return;
			}
		}
	}

	/**
	 * Returns the number of registered listeners.
	 *
	 * @return the number of registered listeners
	 */
	public int size() {
		return size;
	}

}