Q166473: PRB: CScrollView Scroll Range Limited to 32K

Article: Q166473
Product(s): Microsoft C Compiler
Version(s): 4.0,4.1,4.2,5.0,6.0
Operating System(s): 
Keyword(s): kbProgramming kbusage kbDocView kbMFC kbScrollBar KbUIDesign kbVC kbVC400 kbVC410 kbVC4
Last Modified: 04-AUG-2001

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The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft Visual C++, 32-bit Editions, versions 4.0, 4.1, 4.2, 5.0, 6.0 
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SYMPTOMS
========

If the scroll range exceeds 32K when you are using the Scroll Thumb to scroll,
CScrollView does not handle scrolling correctly. Unusual behavior occurs when
you try to scroll beyond 32K.

CAUSE
=====

This behavior is by design. The code to handle ranges greater than 32K was not
added to CScrollView because it could fail if a WM_xSCROLL message is sent to
the view to programatically scroll the contents. In Visual C++ version 6.0 on
Windows NT, the limit has been extended to 2 gigabytes.

RESOLUTION
==========

You can work around the problem easily by adding support to handle scroll ranges
greater than 32K to a CScrollView derived class. Override the function as
follows:

     BOOL CMyScrollView::OnScroll(UINT nScrollCode, UINT nPos, BOOL
                                  bDoScroll)
     {
        SCROLLINFO info;
        info.cbSize = sizeof(SCROLLINFO);
        info.fMask = SIF_TRACKPOS;

        if (LOBYTE(nScrollCode) == SB_THUMBTRACK)
        {
           GetScrollInfo(SB_HORZ, &info);
           nPos = info.nTrackPos;
        }

        if (HIBYTE(nScrollCode) == SB_THUMBTRACK)
        {
           GetScrollInfo(SB_VERT, &info);
           nPos = info.nTrackPos;
        }

        return CScrollView::OnScroll(nScrollCode, nPos, bDoScroll);
     }

In the code sample above, note that you can not depend on the value of nPos that
is sent to the handler. It will be invalid for positions greater than 32K.
Instead, use GetScrollInfo() to assign the correct position to nPos.

For additional information on this problem, click the article number below to
view the article in the Microsoft Knowledge Base:

  Q152252 HOWTO: How To Get 32-bit Scroll Position During Scroll Messages

NOTE: If you are trying to programatically scroll the contents in CscrollView,
use the SetScrollInfo to correctly set the scroll bar information and then send
or post the WM_xSCROLL message.

NOTE: Windows 95, Windows 98, Windows Millennium Edition (Me) and Win32s only
support logical and device coordinates up to 32K. Because CScrollView uses
logical units in SetScrollSizes(), you should specify a scroll range < 32K on
when using CSrollView derived classes on these operating systems.

(c) Microsoft Corporation 1997, All Rights Reserved. Contributions by Sridhar S.
Madhugiri, Microsoft Corporation


Additional query words: CScrollView scrolling

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Keywords          : kbProgramming kbusage kbDocView kbMFC kbScrollBar KbUIDesign kbVC kbVC400 kbVC410 kbVC420 kbVC500 kbVC600 kbVS600 kbprb kbGrpDSMFCATL 
Technology        : kbAudDeveloper kbMFC
Version           : :4.0,4.1,4.2,5.0,6.0
Issue type        : kbprb

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