HOWTO: How To Get 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 As a consequence, the functionality was withdrawn in MFC 4.1 and it was left to the user to extract 32-bit values for the scroll box.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: This method of extracting 32-bit scroll box positions works only in those scroll messages that are sent to a window when the user grabs the thumb and drags it. It is not possible to obtain 32-bit scroll box position in cases of programmatically sent SB_THUMBTRACK and SB_THUMBPOSITION scroll messages. First, the 32-bit information cannot be packed into the WPARAM of the messages because only the High Word contains the scroll position. Second, the nTrackPos member of the SCROLLINFO structure maintained for scroll bars cannot be set programmatically. Any calls to SetScrollInfo to set the immediate scroll positions are ignored by the operating system.

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;
       }

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