How To Get 32-bit Scroll Position During Scroll Messages

The information in this article applies to:

• The Microsoft Foundation Classes (MFC), included with: Microsoft Visual C++, 32-bit Edition, versions 2.0, 2.1, 2.2, 4.0, 4.1

SUMMARY

The current scroll bar position accompanying the SB_THUMBTRACK and SB_THUMBPOSITION type of scroll messages is only 16-bits wide. Functions such as GetScrollPos, SetScrollPos, GetScrollRange, and SetScrollRange can handle 32-bit value scroll-box positions. This article discusses how the GetScrollInfo call can be used to retrieve 32-bit scroll position during scroll messages.

MORE INFORMATION

One of the arguments to the MFC scroll message handlers, OnHScroll and OnVScroll, is the scroll box position. This argument is meaningful only in the case of scroll messages with scroll bar code SB_THUMBPOSITION and SB_THUMBTRACK. This value, even though of type UINT, is really of type short int. In other words, the scroll thumb position is only 16-bits wide. This behavior occurs because Windows sends the thumb position in the High Word of WPARAM accompanying the scroll message.

To obtain 32-bit scroll positions, the OnxScroll handler should make a call to GetScrollInfo with the mask set to SIF_TRACKPOS. The immediate position of the scroll thumb is returned in the nTrackPos member of the SCROLLINFO structure passed in to GetScrollInfo.

This functionality was embedded in MFC 4.0 classes and was handled by the function CWnd::OnWndMsg. However, this gave rise to other problems. As a consequence, the functionality was withdrawn in MFC 4.1 and it was left to the user to extract 32-bit values for the scroll box. For more information on this subject, please see the following article in the Microsoft Knowledge Base:

   ARTICLE ID: Q147684
   TITLE     : BUG: Sending WM_xSCROLL Messages Causes Invalid ASSERT

Sample Code

   void CMyWnd::OnHScroll(UINT nSBCode, UINT nPos, CScrollBar* pScrollBar)
   {
       if(nSBCode == SB_THUMBTRACK || nSBCode == SB_THUMBPOSITION)
       {
       // First determine if the user scrolled a scroll bar control
       // on the window or scrolled the window itself
           HWND hWndScroll;
           if ( pScrollBar == NULL )
               hWndScroll = m_hWnd;
           else
               hWndScroll = pScrollBar->m_hWnd;

           SCROLLINFO info;
           info.cbSize = sizeof(SCROLLINFO);
           info.fMask = SIF_TRACKPOS;
           ::GetScrollInfo(hWndScroll, SB_HORZ, &info);

           nPos = info.nTrackPos;
       }

      //......
      //......
   }