/*
 * 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: ElementListObserver.java 1761021 2016-09-16 11:40:57Z ssteiner $ */

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.
/**
 * 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 activeObservers;

    
Adds a new Observer to the list.
Params:
observer – the observer implementation/**
     * 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.
Params:
observer – the observer to remove/**
     * 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.
Params:
elementList – the Knuth element list
category – the category for the element list (example: main, static-content, table-cell)
id – ID for the element list (may be null)/**
     * 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);
            }
        }
    }

    
Returns:
true if observation is active, i.e. Observers are registered./** @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.
/**
     * Implement this interface to receive notifications on element lists.
     */
    public interface Observer {

        
Notifies the observer about the element list.
Params:
elementList – the Knuth element list
category – the category for the element list (example: main, static-content or
table-cell)
id – ID for the element list (may be null)/**
         * 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);

    }

}