/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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
 *
 *     https://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.apache.commons.configuration2.event;

/**
 * <p>
 * An interface for configuration implementations which support registration of event listeners.
 * </p>
 * <p>
 * Through the methods provided by this interface it is possible to register and remove listeners for different events
 * supported by this library. The event type to be handled by a listener must be provided; the specified event listener
 * must be compatible with this event type. By using generic type parameters, the compiler can check this.
 * </p>
 *
 * @since 2.0
 */
public interface EventSource {

    /**
     * Adds an event listener for the specified event type. This listener is notified about events of this type and all its
     * sub types.
     *
     * @param eventType the event type (must not be <strong>null</strong>)
     * @param listener the listener to be registered (must not be <strong>null</strong>)
     * @param <T> the type of events processed by this listener
     * @throws IllegalArgumentException if a required parameter is <strong>null</strong>
     */
    <T extends Event> void addEventListener(EventType<T> eventType, EventListener<? super T> listener);

    /**
     * Removes the event listener registration for the given event type and listener. An event listener instance may be
     * registered multiple times for different event types. Therefore, when removing a listener the event type of the
     * registration in question has to be specified. The return value indicates whether a registration was removed. A value
     * of <strong>false</strong> means that no such combination of event type and listener was found.
     *
     * @param eventType the event type
     * @param listener the event listener to be removed
     * @param <T> the type of events processed by this listener
     * @return a flag whether a listener registration was removed
     */
    <T extends Event> boolean removeEventListener(EventType<T> eventType, EventListener<? super T> listener);
}
