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.util;
  
  import com.google.gwt.core.client.Scheduler;
  import com.google.gwt.core.client.Scheduler.ScheduledCommand;
  import com.google.gwt.event.dom.client.ScrollEvent;
  import com.google.gwt.event.dom.client.ScrollHandler;
  import com.google.gwt.user.client.ui.ScrollPanel;
  import com.google.gwt.user.client.ui.Widget;
  
  /**
   * Scroll handler which executes an action when the user has scrolled to the bottom.<p>
   *
   * @since 8.0.0
   */
  public class CmsScrollToBottomHandler implements ScrollHandler {
  
      /**
       * If the lower edge of the content being scrolled is at most this many pixels below the lower
       * edge of the scrolling viewport, the action is triggered.
       */
      public static final int DEFAULT_SCROLL_THRESHOLD = 30;
  
      /**
       * An internal flag that, if false, prevents the scroll action from being executed.<p>
       */
      protected boolean m_enabled = true;
  
      /**
       * The action which is triggered when the user scrolls to the bottom.<p>
       */
      private Runnable m_callback;
  
      /**
       * If the lower edge of the content being scrolled is at most this many pixels below the lower
       * edge of the scrolling viewport, the action is triggered.
       */
      private int m_scrollThreshold = DEFAULT_SCROLL_THRESHOLD;
  
      /**
       * Constructs a new scroll handler.<p>
       *
       * @param callback the action which should be executed when the user scrolls to the bottom.
       */
      public CmsScrollToBottomHandler(Runnable callback) {
  
          m_callback = callback;
      }
  
      /**
       * Constructs a new scroll handler with a custom scroll threshold.
       *
       * The scroll threshold is the distance from the bottom edge of the scrolled content
       * such that when the distance from the bottom edge of the scroll viewport to the bottom
       * edge of the scrolled content becomes lower than the distance, the scroll action is triggered.
       *
       * @param callback the action which should be executed when the user scrolls to the bottom.
       * @param scrollThreshold the scroll threshold
       */
      public CmsScrollToBottomHandler(Runnable callback, int scrollThreshold) {
  
          m_callback = callback;
          m_scrollThreshold = scrollThreshold;
      }
  
      /**
       * @see com.google.gwt.event.dom.client.ScrollHandler#onScroll(com.google.gwt.event.dom.client.ScrollEvent)
       */
      public void onScroll(ScrollEvent event) {
  
          if (!m_enabled) {
              return;
          }
          final ScrollPanel scrollPanel = (ScrollPanel)event.getSource();
          final int scrollPos = scrollPanel.getVerticalScrollPosition();
          Widget child = scrollPanel.getWidget();
          int childHeight = child.getOffsetHeight();
          int ownHeight = scrollPanel.getOffsetHeight();
          boolean isBottom = (scrollPos + ownHeight) >= (childHeight - m_scrollThreshold);
          if (isBottom) {
              m_callback.run();
              m_enabled = false;
              scrollPanel.setVerticalScrollPosition(scrollPos);
              Scheduler.get().scheduleDeferred(new ScheduledCommand() {
  
                  /**
                   * @see com.google.gwt.core.client.Scheduler.ScheduledCommand#execute()
                   */
                  public void execute() {
  
                      scrollPanel.setVerticalScrollPosition(scrollPos);
                      m_enabled = true;
                  }
              });
          }
      }
  }
  