/**
 * 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.util.CopyOnWriteSet;
import org.waveprotocol.wave.model.util.ValueUtils;

/**
 * Simple implementation of a singleton.
 *
 * @author anorth@google.com (Alex North)
 */
public final class ObservableSingletonImpl<V, I> implements ObservableSingleton<V, I> {
  /**
   * Factory which creates value objects from abstract initial state.
   */
  interface Factory<V, I> {
    V create(I initialState);
  }

  private final Factory<V, I> factory;
  private final CopyOnWriteSet<Listener<? super V>> listeners = CopyOnWriteSet.create();

  private V value = null;

  public ObservableSingletonImpl(Factory<V, I> factory) {
    this.factory = factory;
  }

  @Override
  public boolean hasValue() {
    return value != null;
  }

  @Override
  public V get() {
    return value;
  }

  @Override
  public V set(I initialState) {
    V oldValue = value;
    value = factory.create(initialState);
    maybeTriggerOnValueChanged(oldValue, value);
    return value;
  }

  @Override
  public void clear() {
    V oldValue = value;
    value = null;
    maybeTriggerOnValueChanged(oldValue, value);
  }

  @Override
  public void addListener(Listener<? super V> listener) {
    listeners.add(listener);
  }

  @Override
  public void removeListener(Listener<? super V> listener) {
    listeners.remove(listener);
  }

  private void maybeTriggerOnValueChanged(V oldValue, V newValue) {
    if (ValueUtils.notEqual(oldValue, newValue)) {
      for (Listener<? super V> l : listeners) {
        l.onValueChanged(oldValue, newValue);
      }
    }
  }
}