FIX: CSpinButtonCtrl Causes Assertion in viewscrl.cpp line 698 (133034)

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.

// 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);
}