/**
    * Copyright 2010 The Kuali Foundation Licensed under the
    * Educational Community 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.osedu.org/licenses/ECL-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.kuali.student.common.ui.client.mvc;
  
  /**
   * @author Kuali Student Team
   */
  import java.util.LinkedList;
  
  public class WorkQueue {
      public static abstract class WorkItem {
          private boolean canceled = false;
  
          public abstract void exec(final Callback<Boolean> onCompleteCallback);
  
          public boolean isCanceled() {
              return this.canceled;
          }
  
          protected void setCanceled(final boolean canceled) {
              this.canceled = canceled;
          }
      }
  
      private boolean running = false;
      private final LinkedList<WorkItem> queue = new LinkedList<WorkItem>();
  
      private final Callback<Boolean> onCompleteCallback = new Callback<Boolean>() {
          @Override
          public void exec(final Boolean value) {
              processNext();
          }
      };
  
      public void clear() {
          for (final WorkItem item : queue) {
              item.setCanceled(true);
          }
          queue.clear();
      }
  
      public boolean isRunning() {
          return this.running;
      }
  
      private void processNext() {
          WorkItem item = null;
          while (!queue.isEmpty() && (item = queue.removeFirst()) != null) {
              if (item.isCanceled()) {
                  item = null;
              } else {
                  break;
              }
          }
          if (item == null) {
              running = false;
          } else {
              item.exec(onCompleteCallback);
          }
      }
  
      public void submit(final WorkItem item) {
          queue.add(item);
          if (!running) {
              running = true;
              processNext();
          }
      }
  }