content: /*
   * This library is part of OpenCms -
   * the Open Source Content Management System
   *
   * Copyright (c) Alkacon Software GmbH & Co. KG (https://www.alkacon.com)
   *
   * This library is free software; you can redistribute it and/or
   * modify it under the terms of the GNU Lesser General Public
   * License as published by the Free Software Foundation; either
   * version 2.1 of the License, or (at your option) any later version.
   *
   * This library is distributed in the hope that it will be useful,
   * but WITHOUT ANY WARRANTY; without even the implied warranty of
   * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
   * Lesser General Public License for more details.
   *
   * For further information about Alkacon Software, please see the
   * company website: https://www.alkacon.com
   *
   * For further information about OpenCms, please see the
   * project website: https://www.opencms.org
   *
   * You should have received a copy of the GNU Lesser General Public
   * License along with this library; if not, write to the Free Software
   * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
   */
  
  package org.opencms.gwt.client.validation;
  
  import java.util.LinkedList;
  
  /**
   * This is a helper class for scheduling form validations.<p>
   *
   * Since validations can be asynchronous, it would be possible for a validation to start while another one
   * is still waiting for a response from the server if it were ran directly. This might result in an inconsistent state
   * of the form fields being validated. To prevent this, validation controllers use this class to schedule validations,
   * and call this class again after they're finished to execute the next validation.<p>
   *
   * The result of this is that a validation will only start after all unfinished validations which have been scheduled
   * before it have finished running.<p>
   *
   * @since 8.0.0
   */
  public class CmsValidationScheduler {
  
      /** The singleton instance of this class. */
      private static CmsValidationScheduler instance = new CmsValidationScheduler();
  
      /** The queue of validations which still need to be run. */
      private LinkedList<CmsValidationController> m_actions = new LinkedList<CmsValidationController>();
  
      /** A flag which indicates whether there is no validation currently running. */
      private boolean m_idle = true;
  
      /**
       * Hidden default constructor.<p>
       */
      protected CmsValidationScheduler() {
  
          // do nothing
      }
  
      /**
       * Returns the singleton instance of the validation scheduler.<p>
       *
       * @return the validation scheduler
       */
      public static CmsValidationScheduler get() {
  
          return instance;
      }
  
      /**
       * This method should be called by the validation controller when it has finished running.<p>
       *
       * It will execute the next validation if there is one.<p>
       */
      public void executeNext() {
  
          if (!m_idle) {
              if (m_actions.isEmpty()) {
                  m_idle = true;
              } else {
                  CmsValidationController action = m_actions.removeFirst();
                  action.internalStartValidation();
              }
          } else {
              assert false; // this shouldn't happen
          }
      }
  
      /**
       * This schedules a new validation to be run after all currently scheduled or running validations have finished.<p>
       *
       * If there are no validations running, the validation will be started immediately.<p>
       *
       * @param action the validation to be scheduled
       */
      public void schedule(CmsValidationController action) {
  
          if (m_idle) {
              m_idle = false;
              action.internalStartValidation();
          } else {
              m_actions.add(action);
          }
      }
  }
  