001/*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements.  See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License.  You may obtain a copy of the License at
008 *
009 *     http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017package org.apache.commons.configuration2.event;
018
019/**
020 * <p>
021 * An interface for configuration implementations which support registration of
022 * event listeners.
023 * </p>
024 * <p>
025 * Through the methods provided by this interface it is possible to register and
026 * remove listeners for different events supported by this library. The event
027 * type to be handled by a listener must be provided; the specified event listener
028 * must be compatible with this event type. By using generic type parameters, the
029 * compiler can check this.
030 * </p>
031 *
032 * @version $Id: EventSource.java 1678629 2015-05-10 20:19:08Z oheger $
033 * @since 2.0
034 */
035public interface EventSource
036{
037    /**
038     * Adds an event listener for the specified event type. This listener is
039     * notified about events of this type and all its sub types.
040     *
041     * @param eventType the event type (must not be <b>null</b>)
042     * @param listener the listener to be registered (must not be <b>null</b>)
043     * @param <T> the type of events processed by this listener
044     * @throws IllegalArgumentException if a required parameter is <b>null</b>
045     */
046    <T extends Event> void addEventListener(EventType<T> eventType,
047            EventListener<? super T> listener);
048
049    /**
050     * Removes the event listener registration for the given event type and
051     * listener. An event listener instance may be registered multiple times for
052     * different event types. Therefore, when removing a listener the event type
053     * of the registration in question has to be specified. The return value
054     * indicates whether a registration was removed. A value of <b>false</b>
055     * means that no such combination of event type and listener was found.
056     *
057     * @param eventType the event type
058     * @param listener the event listener to be removed
059     * @param <T> the type of events processed by this listener
060     * @return a flag whether a listener registration was removed
061     */
062    <T extends Event> boolean removeEventListener(EventType<T> eventType,
063            EventListener<? super T> listener);
064}