FIX:CScrollView::OnHScroll/OnVScroll Do Not Call OnChildNotify

WINDOWS        | WINDOWS NT

The information in this article applies to:

• The Microsoft Foundation Classes (MFC), included with:

      - Microsoft Visual C++ for Windows, versions 1.5, 1.51, 1.52
      - Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1, 2.2
  

SYMPTOMS

Scroll messages generated by scroll related controls in a CScrollView- derived object fail to invoke the control's OnChildNotify function.

CAUSE

CScrollView::OnVScroll and CScrollView::OnHScroll do not call SendChildNotifyLastMsg, which is responsible for calling OnChildNotify.

RESOLUTION

Override OnVScroll and/or OnHScroll for your CScrollView-derived class, and implement this code:

   void CMyView::OnVScroll(UINT nSBCode, UINT nPos, CScrollBar*
   pScrollBar)
   {
       if (pScrollBar != NULL && pScrollBar->SendChildNotifyLastMsg())
           return;     // eat it

       // ignore scroll bar msgs from other controls
       if (pScrollBar != GetScrollBarCtrl(SB_VERT))
           return;

       OnScroll(MAKEWORD(-1, nSBCode), nPos);
   }

   void CMyView::OnHScroll(UINT nSBCode, UINT nPos, CScrollBar*
   pScrollBar)
   {
       // same as above
       ...
   }

STATUS

Microsoft has confirmed this to be a bug in the Microsoft products listed at the beginning of this article. This problem is corrected in Microsoft Visual C++ version 4.0.

MORE INFORMATION

OnChildNotify is called by the control's parent window when the parent window receives a notification message that applies to this control. Notification messages include control notification messages such as WM_COMMAND sent by controls, some specific Windows messages such as WM_HSCROLL, WM_VSCROLL, and WM_CTLCOLOR, and a few others.

NOTE: For more information, see Tech note #21.

OnChildNotify allows a control an initial opportunity to view these messages before the control reaches its parent.

The code reflects corrections for a bug reported in the following article in the Microsoft Knowledge Base:

   ARTICLE-ID: Q133034
   TITLE     : BUG: CSpinButtonCtrl Causes Assertion in Viewscrl.cpp
               line 698

REFERENCES

For additional information, please see the following article in the Microsoft Knowledge Base:

   ARTICLE-ID: Q133034
   TITLE     : BUG: CSpinButtonCtrl Causes Assertion in viewscrl.cpp
               line 698.

Books Online - CWnd::OnChildNotify