ScrollPaneWheelScroller.java

/*
 * @(#)ScrollPaneWheelScroller.java	1.5 03/01/23
 *
 * Copyright  1990-2008 Sun Microsystems, Inc. All Rights Reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER
 * 
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License version
 * 2 only, as published by the Free Software Foundation. 
 * 
 * This program 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
 * General Public License version 2 for more details (a copy is
 * included at /legal/license.txt). 
 * 
 * You should have received a copy of the GNU General Public License
 * version 2 along with this work; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA
 * 02110-1301 USA 
 * 
 * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa
 * Clara, CA 95054 or visit www.sun.com if you need additional
 * information or have any questions. 
 */

package sun.awt;

import java.awt.Toolkit;
import java.awt.ScrollPane;
import java.awt.Insets;
import java.awt.Adjustable;
import java.awt.event.MouseWheelEvent;

/*
 * ScrollPaneWheelScroller is a helper class for implmenenting mouse wheel
 * scrolling on a java.awt.ScrollPane.  It contains only static methods.
 * No objects of this class may be instantiated, thus it is declared abstract.
 */
public abstract class ScrollPaneWheelScroller {
    private ScrollPaneWheelScroller() {}
    
    /*
     * Called from ScrollPane.processMouseWheelEvent()
     */
    public static void handleWheelScrolling(ScrollPane sp, MouseWheelEvent e) {
        int increment = 0;

        if (sp != null && e.getScrollAmount() != 0) {
            Adjustable adj = getAdjustableToScroll(sp); 
            if (adj != null) {
                increment = getIncrementFromAdjustable(adj, e);
                scrollAdjustable(adj, increment);
            }
        }
    }

    /*
     * Given a ScrollPane, determine which Scrollbar should be scrolled by the
     * mouse wheel, if any.
     */
    public static Adjustable getAdjustableToScroll(ScrollPane sp) {
        int policy = sp.getScrollbarDisplayPolicy();

        // if policy is display always or never, use vert
        if (policy == ScrollPane.SCROLLBARS_ALWAYS ||
            policy == ScrollPane.SCROLLBARS_NEVER) {
            return sp.getVAdjustable();

        }
        else {

            Insets ins = sp.getInsets();
            int vertScrollWidth = sp.getVScrollbarWidth();

            // Check if scrollbar is showing by examining insets of the
            // ScrollPane
            if (ins.right >= vertScrollWidth) {
                return sp.getVAdjustable();
            }
            else {
                int horizScrollHeight = sp.getHScrollbarHeight();
                if (ins.bottom >= horizScrollHeight) {
                    return sp.getHAdjustable();
                }
                else {
                    return null;
                }
            }
        }
    }

    /*
     * Given the info in a MouseWheelEvent and an Adjustable to scroll, return
     * the amount by which the Adjustable should be adjusted.  This value may
     * be positive or negative.
     */
    public static int getIncrementFromAdjustable(Adjustable adj,
                                                 MouseWheelEvent e) {
    // ASSUME: adj != null
        int increment = 0;

        if (e.getScrollType() == MouseWheelEvent.WHEEL_UNIT_SCROLL) {
            increment = e.getUnitsToScroll() * adj.getUnitIncrement();
        }
        else if (e.getScrollType() == MouseWheelEvent.WHEEL_BLOCK_SCROLL) {
            increment = adj.getBlockIncrement() * e.getWheelRotation();
        }
        return increment;
    }

    /*
     * Scroll the given Adjustable by the given amount.  Checks the Adjustable's
     * bounds and sets the new value to the Adjustable.
     */
    public static void scrollAdjustable(Adjustable adj, int amount) {
    // ASSUME adj != null
    // ASSUME amount != 0

        int current = adj.getValue();
        int upperLimit = adj.getMaximum() - adj.getVisibleAmount();

        if (amount > 0 && current < upperLimit) { // still some room to scroll
                                                  // down
            if (current + amount < upperLimit) {
                adj.setValue(current + amount);
                return;
            }
            else {
                adj.setValue(upperLimit);
                return;
            }
        }
        else if (amount < 0 && current > adj.getMinimum()) { // still some room 
                                                             // to scroll up
            if (current + amount > adj.getMinimum()) {
                adj.setValue(current + amount);
                return;
            }
            else {
                adj.setValue(adj.getMinimum());
                return;
            }
        }
    }
}