/**
 * Copyright 2010 Google Inc.
 *
 * Licensed 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.waveprotocol.wave.model.adt;

import org.waveprotocol.wave.model.wave.SourcesEvents;


/**
 * Extends {@link Singleton} to provide events whenever the value changes.
 *
 * @author anorth@google.com (Alex North)
 * @param <V> type of the value
 * @param <I> type of a value initializer
 */
public interface ObservableSingleton<V, I> extends Singleton<V, I>,
    SourcesEvents<ObservableSingleton.Listener<? super V>> {
  interface Listener<V> {
    /**
     * Called when the value changes.
     *
     * @param oldValue the previous value (may be null)
     * @param newValue the new value (may be null)
     */
    void onValueChanged(V oldValue, V newValue);
  }
}