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From: salzman%gaucho@RAND.ORG (Isaac)
Newsgroups: comp.windows.x
Subject: Re: Need help with XView scrollbars
Message-ID: <9008090722.AA02805@gaucho.rand.org>
Date: 9 Aug 90 07:22:34 GMT
References: <8936@uhccux.uhcc.Hawaii.Edu>
Sender: daemon@athena.mit.edu (Mr Background)
Organization: The Internet
Lines: 119


>bu.edu!rpi!zaphod.mps.ohio-state.edu!samsung!munnari.oz.au!uhccux!hale.ifa.haw
>aii.edu@eddie.mit.edu (Tony Denault) writes:
>

>I need help with dealing with scrollbars in XView....

>What I need to know is:

>How can I be notified when the user interacts with the scrollbar?
>I need to read it's new postion. Update my internal variable and
>call my canvas redraw procedure. I don't want XView to scroll my
>canvas display. 

XView 2.0 (in the OpenWindows 2.0 distribution) includes a good example of
how to get a handle on the scrollbar events of a canvas (it's also in the
updated version of the O'Reilly XView Programming Manual). since it's such
a small file, i'll include it below.

i've written a canvas based text list package where i take complete control
of the scrollbar. it can get tricky at times, and there are some
undocumented side effects. for instance, funny things can happen if you
make the SCROLLBAR_OBJECT_LENGTH smaller than SCROLLBAR_VIEW_LENGTH (like
setting the SCROLLBAR_VIEW_START to something other than what you think it
should be, and not calling your notify_interpose_event_func to let you
know)....

if you (or anyone else) are interested, i'll be glad to mail you a copy of
my canvas text list package. if nothing else it serves as an example of how
to deal with controlling the scrollbar (and how to write a canvas based
package in general)....

--
* Isaac J. Salzman                                            ----     
* The RAND Corporation - Information Sciences Dept.          /o o/  /  
* 1700 Main St., PO Box 2138, Santa Monica, CA 90406-2138    | v |  |  
* AT&T      : +1 213-393-0411 x6421 or x7923 (ISL lab)      _|   |_/   
* Internet  : salzman@rand.org                             / |   |
* UUCP      : !uunet!rand.org!salzman                      | |   |     

----------------------------------------------------------------------------

[$OPENWINHOME/share/src/sun/xview/examples/scrollbar/scrollto.c]

/* scroll_to.c -- demonstrate how to monitor the scrolling
 * requests invoked by the user.  Requests can be monitored,
 * ignored or changed programmatically.  This program creates
 * a canvas window by default or a textsw with the -textsw
 * command line option.  Both contain a scrollbar.
 */
#include <stdio.h>
#include <xview/xview.h>
#include <xview/textsw.h>
#include <xview/canvas.h>
#include <xview/scrollbar.h>

main(argc, argv)
int argc;
char *argv[];
{
    Frame           frame;
    Textsw          textsw;
    Canvas          canvas;
    Scrollbar       sbar;
    Notify_value    monitor_scroll();

    (void) xv_init(XV_INIT_ARGC_PTR_ARGV, &argc, argv, NULL);

    frame = xv_create(NULL, FRAME, NULL);

    if (argc > 1 && !strcmp(argv[1], "-textsw")) {
        textsw = xv_create(frame, TEXTSW,
            TEXTSW_FILE_CONTENTS, "/etc/termcap",
            NULL);
        sbar = xv_get(textsw, TEXTSW_SCROLLBAR);
    } else {
        canvas = xv_create(frame, CANVAS,
            CANVAS_WIDTH, 1000,
            CANVAS_HEIGHT, 1000,
            CANVAS_AUTO_SHRINK, FALSE,
            CANVAS_AUTO_EXPAND, FALSE,
            NULL);
        sbar = xv_create(canvas, SCROLLBAR,
            SCROLLBAR_DIRECTION, SCROLLBAR_VERTICAL,
            SCROLLBAR_PIXELS_PER_UNIT, 10,
            NULL);
    }
    notify_interpose_event_func(xv_get(sbar, SCROLLBAR_NOTIFY_CLIENT),
        monitor_scroll, NOTIFY_SAFE);

    xv_main_loop(frame);
}

/*
 * To change the behavior of the scrolling of the canvas, do not pass
 * on the event via notify_next_event_func() when the event type is
 * SCROLLBAR_REQUEST.
 */
Notify_value
monitor_scroll(client, event, sbar, type)
Notify_client     client;
Event            *event;
Scrollbar         sbar;
Notify_event_type type;
{
    int     view_start, last_view_start, pixels_per, is_neg = 0, total;

    if (event_id(event) == SCROLLBAR_REQUEST) {
        view_start = (int)xv_get(sbar, SCROLLBAR_VIEW_START);
        last_view_start = (int)xv_get(sbar, SCROLLBAR_LAST_VIEW_START);
        pixels_per = (int)xv_get(sbar, SCROLLBAR_PIXELS_PER_UNIT);
        if ((total = view_start - last_view_start) < 0)
            total = -total, is_neg = 1;
        printf("scrolled from %d to %d: %d pixels (%d units) %s\n",
            last_view_start, view_start, pixels_per * total, total,
            is_neg? "up" : "down");
    }
    return notify_next_event_func(client, event, sbar, type);
}