QueueProcessor.java

/**
 * Copyright 2009 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.scheduler;

import org.waveprotocol.wave.client.scheduler.Scheduler.IncrementalTask;
import org.waveprotocol.wave.model.util.CollectionUtils;

import java.util.Queue;

/**
 * Constructs a queue of items to be processed one by one.
 *
 */
public abstract class QueueProcessor<T> implements IncrementalTask {
  private final Queue<T> queue;
  private final TimerService timer;

  /**
   * Creates a serial queue processor which serializes processing of the queued
   * items.
   *
   * @param timerService
   */
  public QueueProcessor(TimerService timerService) {
    this.timer = timerService;
    this.queue = CollectionUtils.createQueue();
  }

  /**
   * Processes an item.
   */
  protected abstract void process(T item);

  /**
   * Adds an item to be processed.
   */
  public final void add(T item) {
    if (queue.isEmpty()) {
      timer.schedule(this);
    }
    queue.add(item);
  }

  /**
   * Processes an item.
   */
  @Override
  public final boolean execute() {
    process(queue.poll());
    return !queue.isEmpty();
  }

  /**
   * Stops this processor if it is scheduled.
   * <p>
   * It is undefined whether unprocessed items will be processed if this
   * processor becomes scheduled again, or if those items will be discarded.
   */
  public final void cancel() {
    timer.cancel(this);
  }
}