/**
 * 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.client.common.util;

import com.google.common.base.Preconditions;

import org.waveprotocol.wave.model.util.CollectionUtils;

import java.util.Queue;

/**
 * Something that holds a value, revealed by potentially asynchronous access.
 *
 * @param <T> type of held value
 */
public interface AsyncHolder<T> {

  /** Accessor of a value in this holder. */
  public interface Accessor<T> {
    void use(T x);
  }

  /**
   * Reveals the value held by this holder to a consumer. The value may be
   * revealed synchronously or asynchronously.
   *
   * @param accessor procedure to apply
   */
  void call(Accessor<T> accessor);

  public abstract class Impl<T> implements AsyncHolder<T>, Accessor<T> {
    private Queue<Accessor<T>> waiting;
    private T value;

    @Override
    public final void call(Accessor<T> accessor) {
      if (value == null) {
        addWaiter(accessor);
      } else {
        accessor.use(value);
      }
    }

    private void addWaiter(Accessor<T> accessor) {
      assert value == null;
      if (waiting == null) {
        waiting = CollectionUtils.createQueue();
        waiting.add(accessor);
        create(this);
      } else {
        waiting.add(accessor);
      }
    }

    protected abstract void create(Accessor<T> whenReady);

    @Override
    public void use(T x) {
      Preconditions.checkState(value == null && waiting != null);
      value = x;
      for (Accessor<T> waiter : waiting) {
        waiter.use(value);
      }
      waiting = null;
    }
  }
}