content: /*
   * 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.
   */
  package org.apache.distributedlog.namespace;
  
  import java.util.concurrent.CopyOnWriteArraySet;
  import org.apache.distributedlog.callback.NamespaceListener;
  
  
  /**
   * Namespace Watcher watching namespace changes.
   */
  public abstract class NamespaceWatcher {
  
      protected final CopyOnWriteArraySet<NamespaceListener> listeners =
              new CopyOnWriteArraySet<NamespaceListener>();
  
      /**
       * Register listener for namespace changes.
       *
       * @param listener
       *          listener to add
       */
      public void registerListener(NamespaceListener listener) {
          if (listeners.add(listener)) {
              watchNamespaceChanges();
          }
      }
  
      /**
       * Unregister listener from the namespace watcher.
       *
       * @param listener
       *          listener to remove from namespace watcher
       */
      public void unregisterListener(NamespaceListener listener) {
          listeners.remove(listener);
      }
  
      /**
       * Watch the namespace changes. It would be triggered each time
       * a namespace listener is added. The implementation should handle
       * this.
       */
      protected abstract void watchNamespaceChanges();
  
  }
  