Q133034: FIX: CSpinButtonCtrl Causes Assertion in viewscrl.cpp line 698

Article: Q133034
Product(s): Microsoft C Compiler
Version(s): winnt:
Operating System(s): 
Keyword(s): kbcode kbnokeyword kbMFC kbVC210fix kbVC220fix kbGrpDSMFCATL kbNoUpdate
Last Modified: 06-MAY-2001

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The information in this article applies to:

- The Microsoft Foundation Classes (MFC), used with:
   - Microsoft Visual C++, versions 2.1, 2.2, on platform(s):
      - the operating system: Microsoft Windows NT, on platform(s):
         - the operating system: Microsoft Windows 
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SYMPTOMS
========

When a CSpinButtonCtrl is created as a child of a view derived from CScrollView,
user interaction (clicking with the mouse or pressing an arrow key) with the
Up-Down Control's arrow buttons causes an assertion in the viewscrl.cpp file on
line 698.

CAUSE
=====

A WM_VSCROLL message (or a WM_HSCROLL message if the style is UDS_HORZ) is sent
as a notification when the user interacts with the Up-Down Control's arrow
buttons, either by clicking them or by pressing an arrow key.

The problem is that CScrollView has implemented handlers for messages sent by its
own scrollbars, so even when CSpinButtonCtrl is the source of the message, the
first opportunity to handle the message occurs in the parent's (CScrollView)
implementation of the handler. CScrollView's handler makes sure the message was
sent by its own scrollbars, or the assertion occurs.

Here is the code for default implementation of CScrollView::OnVScroll from
viewscrl.cpp:

     void CScrollView::OnVScroll(UINT nSBCode, UINT nPos,
                                  CScrollBar* pScrollBar)
     {
       ASSERT(pScrollBar == GetScrollBarCtrl(SB_VERT));
   
       UNUSED pScrollBar;  // unused in release builds
   
       OnScroll(MAKEWORD(-1, nSBCode), nPos);
     }

Because the message originated from the Up-Down Control and not the parent's
scrollbar, the assertion in the first line fails.

RESOLUTION
==========

Add a handler for the WM_VSCROLL or WM_HSCROLL message in the Up-Down control's
parent class. Test to see if the message originated from the Up- Down control or
the scrollbar of the parent. If it originated from the Up- Down control, perform
operations intended for the control, otherwise, pass the call to the parent's
default handler as in this example:

  // Here CMyFormView is derived from CFormView, which itself is
  // derived from CScrollView

  void CMyFormView::OnVScroll(UINT nSBCode, UINT nPos,
                               CScrollBar* pScrollBar)
  {
           // IDC_SPINCONTROL is the ID of the Up-Down Control
    if ((pScrollBar != NULL) &&
              (pScrollBar->GetDlgCtrlID() == IDC_SPINCONTROL))
    {
      // Do whatever you want to do when Up-Down control buttons are
      // pressed

        TRACE("spin button\r\n"); // for debugging..
        return;
    }

    CFormView::OnVScroll(nSBCode, nPos, pScrollBar);
  }

STATUS
======

Microsoft has confirmed this to be a bug in the Microsoft products listed at the
beginning of this article. This problem was fixed in Microsoft Visual C++,
32-bit Edition, version 4.0.

Additional query words: 2.10 2.20 3.10 3.20 spin control CFormView

======================================================================
Keywords          : kbcode kbnokeyword kbMFC kbVC210fix kbVC220fix kbGrpDSMFCATL kbNoUpdate 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:
Issue type        : kbbug
Solution Type     : kbfix

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