/*
 * 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
 *
 *      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.
 */

/* $Id$ */

package org.apache.fop.layoutmgr;

import java.util.List;

/**
 * This class is used to observe Knuth element lists generated within the layout managers. This
 * is mainly used for the purpose of automated testing. This implementation here does nothing.
 * Please see the subclass within the test code.
 */
public final class ElementListObserver {

    private ElementListObserver() {
    }

    private static List<Observer> activeObservers;

    /**
     * Adds a new Observer to the list.
     * @param observer the observer implementation
     */
    public static void addObserver(Observer observer) {
        if (!isObservationActive()) {
            activeObservers = new java.util.ArrayList<>();
        }
        activeObservers.add(observer);
    }

    /**
     * Removes an Observer from the list. This call simply returns if the observer was not on
     * the list and does nothing.
     * @param observer the observer to remove
     */
    public static void removeObserver(Observer observer) {
        if (isObservationActive()) {
            activeObservers.remove(observer);
        }
    }

    /**
     * Notifies all registered observers about the element list.
     * @param elementList the Knuth element list
     * @param category the category for the element list (example: main, static-content, table-cell)
     * @param id ID for the element list (may be null)
     */
    public static void observe(List elementList, String category, String id) {
        if (isObservationActive()) {
            if (category == null) {
                throw new NullPointerException("category must not be null");
            }
            for (Object activeObserver : activeObservers) {
                ((Observer) activeObserver).observe(elementList, category, id);
            }
        }
    }

    /** @return true if observation is active, i.e. Observers are registered. */
    public static boolean isObservationActive() {
        return activeObservers != null;
    }

    /**
     * Implement this interface to receive notifications on element lists.
     */
    public interface Observer {

        /**
         * Notifies the observer about the element list.
         * @param elementList the Knuth element list
         * @param category the category for the element list (example: main, static-content or
         * table-cell)
         * @param id ID for the element list (may be null)
         */
        void observe(List elementList, String category, String id);

    }

}
