001/*
002 * This library is part of OpenCms -
003 * the Open Source Content Management System
004 *
005 * Copyright (c) Alkacon Software GmbH & Co. KG (http://www.alkacon.com)
006 *
007 * This library is free software; you can redistribute it and/or
008 * modify it under the terms of the GNU Lesser General Public
009 * License as published by the Free Software Foundation; either
010 * version 2.1 of the License, or (at your option) any later version.
011 *
012 * This library is distributed in the hope that it will be useful,
013 * but WITHOUT ANY WARRANTY; without even the implied warranty of
014 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
015 * Lesser General Public License for more details.
016 *
017 * For further information about Alkacon Software, please see the
018 * company website: http://www.alkacon.com
019 *
020 * For further information about OpenCms, please see the
021 * project website: http://www.opencms.org
022 *
023 * You should have received a copy of the GNU Lesser General Public
024 * License along with this library; if not, write to the Free Software
025 * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
026 */
027
028package org.opencms.gwt.client.util;
029
030import com.google.gwt.core.client.Scheduler;
031import com.google.gwt.core.client.Scheduler.ScheduledCommand;
032import com.google.gwt.event.dom.client.ScrollEvent;
033import com.google.gwt.event.dom.client.ScrollHandler;
034import com.google.gwt.user.client.ui.ScrollPanel;
035import com.google.gwt.user.client.ui.Widget;
036
037/**
038 * Scroll handler which executes an action when the user has scrolled to the bottom.<p>
039 *
040 * @since 8.0.0
041 */
042public class CmsScrollToBottomHandler implements ScrollHandler {
043
044    /**
045     * If the lower edge of the content being scrolled is at most this many pixels below the lower
046     * edge of the scrolling viewport, the action is triggered.
047     */
048    public static final int DEFAULT_SCROLL_THRESHOLD = 30;
049
050    /**
051     * An internal flag that, if false, prevents the scroll action from being executed.<p>
052     */
053    protected boolean m_enabled = true;
054
055    /**
056     * The action which is triggered when the user scrolls to the bottom.<p>
057     */
058    private Runnable m_callback;
059
060    /**
061     * If the lower edge of the content being scrolled is at most this many pixels below the lower
062     * edge of the scrolling viewport, the action is triggered.
063     */
064    private int m_scrollThreshold = DEFAULT_SCROLL_THRESHOLD;
065
066    /**
067     * Constructs a new scroll handler.<p>
068     *
069     * @param callback the action which should be executed when the user scrolls to the bottom.
070     */
071    public CmsScrollToBottomHandler(Runnable callback) {
072
073        m_callback = callback;
074    }
075
076    /**
077     * Constructs a new scroll handler with a custom scroll threshold.
078     *
079     * The scroll threshold is the distance from the bottom edge of the scrolled content
080     * such that when the distance from the bottom edge of the scroll viewport to the bottom
081     * edge of the scrolled content becomes lower than the distance, the scroll action is triggered.
082     *
083     * @param callback the action which should be executed when the user scrolls to the bottom.
084     * @param scrollThreshold the scroll threshold
085     */
086    public CmsScrollToBottomHandler(Runnable callback, int scrollThreshold) {
087
088        m_callback = callback;
089        m_scrollThreshold = scrollThreshold;
090    }
091
092    /**
093     * @see com.google.gwt.event.dom.client.ScrollHandler#onScroll(com.google.gwt.event.dom.client.ScrollEvent)
094     */
095    public void onScroll(ScrollEvent event) {
096
097        if (!m_enabled) {
098            return;
099        }
100        final ScrollPanel scrollPanel = (ScrollPanel)event.getSource();
101        final int scrollPos = scrollPanel.getVerticalScrollPosition();
102        Widget child = scrollPanel.getWidget();
103        int childHeight = child.getOffsetHeight();
104        int ownHeight = scrollPanel.getOffsetHeight();
105        boolean isBottom = (scrollPos + ownHeight) >= (childHeight - m_scrollThreshold);
106        if (isBottom) {
107            m_callback.run();
108            m_enabled = false;
109            scrollPanel.setVerticalScrollPosition(scrollPos);
110            Scheduler.get().scheduleDeferred(new ScheduledCommand() {
111
112                /**
113                 * @see com.google.gwt.core.client.Scheduler.ScheduledCommand#execute()
114                 */
115                public void execute() {
116
117                    scrollPanel.setVerticalScrollPosition(scrollPos);
118                    m_enabled = true;
119                }
120            });
121        }
122    }
123}