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Whack is a Java library that easily allows the creation of external components that follow the XEP-0114: Jabber Component Protocol.

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/**
 * Copyright 2004-2009 Jive Software.
 *
 * All rights reserved. Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.jivesoftware.util;

import java.util.List;
import java.util.Map;
import java.util.concurrent.CopyOnWriteArrayList;

/**
 * Dispatches property events. Each event has a {@link EventType type}
 * and optional parameters, as follows:
 *
 * 
 * 
 * 
 * 
 * 
 * 
 * Event Type Extra Params
{@link EventType#property_set property_set} A param named value that
 *      has the value of the property set.
{@link EventType#property_deleted property_deleted} None
{@link EventType#xml_property_set xml_property_set} A param named value that
 *      has the value of the property set.
{@link EventType#xml_property_deleted xml_property_deleted} None
 *
 * @author Matt Tucker
 */
public class PropertyEventDispatcher {

    private static List listeners =
            new CopyOnWriteArrayList();

    private PropertyEventDispatcher() {
        // Not instantiable.
    }

    /**
     * Registers a listener to receive events.
     *
     * @param listener the listener.
     */
    public static void addListener(PropertyEventListener listener) {
        if (listener == null) {
            throw new NullPointerException();
        }
        listeners.add(listener);
    }

    /**
     * Unregisters a listener to receive events.
     *
     * @param listener the listener.
     */
    public static void removeListener(PropertyEventListener listener) {
        listeners.remove(listener);
    }

    /**
     * Dispatches an event to all listeners.
     *
     * @param property the property.
     * @param eventType the event type.
     * @param params event parameters.
     */
    public static void dispatchEvent(String property, EventType eventType, Map params) {
        for (PropertyEventListener listener : listeners) {
            try {
                switch (eventType) {
                    case property_set: {
                        listener.propertySet(property, params);
                        break;
                    }
                    case property_deleted: {
                        listener.propertyDeleted(property, params);
                        break;
                    }
                    case xml_property_set: {
                        listener.xmlPropertySet(property, params);
                        break;
                    }
                    case xml_property_deleted: {
                        listener.xmlPropertyDeleted(property, params);
                        break;
                    }
                    default:
                        break;
                }
            }
            catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    /**
     * Represents valid event types.
     */
    public enum EventType {

        /**
         * A property was set.
         */
        property_set,

        /**
         * A property was deleted.
         */
        property_deleted,

        /**
         * An XML property was set.
         */
        xml_property_set,

        /**
         * An XML property was deleted.
         */
        xml_property_deleted
    }
}

Event Type	Extra Params
{@link EventType#property_set property_set}	A param named `value` that * has the value of the property set.
{@link EventType#property_deleted property_deleted}	None
{@link EventType#xml_property_set xml_property_set}	A param named `value` that * has the value of the property set.
{@link EventType#xml_property_deleted xml_property_deleted}	None