December 1999

Code for this article: Dec99CQA.exe (274KB) Paul DiLascia is the author of Windows ++: Writing Reusable Code in C++ (Addison-Wesley, 1992) and a freelance consultant and writer-at-large. He can be reached at askpd@pobox.com or http://pobox.com/~askpd.

Q How can I disable the scrolling of a virtual list control when the scroll box (thumb) is being dragged? I tried to handle WM_VSCROLL in my CListCtrl-derived class

 void CMyListCtrl::OnVScroll(UINT nSBCode, UINT nPos, 
                             CScrollBar* pScrollBar)
 {
   if (nSBCode == SB_THUMBTRACK) {
     return;
   }
   CListCtrl::OnVScroll(nSBCode, nPos, pScrollBar);
 }

 // in OnLVScroll handler
 if (nSBCode==SB_THUMBTRACK) {
   if (m_bIgnoreThumb)
     return 0; // ignore
 } else if nSBCode==SB_THUMBPOSITION) {
   m_bIgnoreThumb=FALSE; // don't ignore
   SendMessage(WM_VSCROLL, MAKEWPARAM(SB_THUMBTRACK, 
                                      nPos));
   m_bIgnoreThumb=TRUE; // keep ignoring
   return 0; // handled
 }
 // default: pass to MFC
 CScrollView::OnLVScroll(...);

Assuming m_bIgnoreThumb is initialized to TRUE, this code ignores all SB_THUMBTRACK notifications, but when SB_THUMBPOSITION comes along, it temporarily turns off its ignore switch and sends SB_THUMBTRACK to itself. In other words, this code tricks MFC into handling SB_THUMBPOSITION by converting it to SB_THUMBTRACK. Pretty sneaky, eh?       Figure 1 ListView Scroll       Since that was too easy, I decided to add another feature to this deferred scrolling business: a scroll tip that pops up during deferred scrolling to at least give the user some feedback (see Figure 1). Microsoft® Excel and Word have the same feature, so you can easily scroll to a particular spreadsheet cell or page. The text looks like a tooltip, but I implemented it as a completely separate standalone class, CPopupText, because I'm too stupid to figure out how tooltips work (I can never get it right on the first try—I always have to futz around). Since the scroll tip doesn't time out the way a tooltip does, it remains displayed until the user lets go of the mouse. Figure 2 shows the code, which is entirely straightforward, so I won't bore you with details.       To make life as easy as possible for you, I encapsulated the scrolling action together with the popup scroll tip in a class, CDeferScrollHook (see Figure 3). All you have to do is instantiate one of these gizmos in your scroll view class and call Install. Presto—instant retro scrolling behavior. CDeferScrollHook is derived from my ubiquitous CSubclassWnd class, which lets you subclass (in the Windows sense) any CWnd. It intercepts WM_VSCROLL messages to perform the required magic shown previously.       The vanilla CDeferScrollHook has no scroll tips. To add them, you have to derive your own class and implement the virtual function OnGetScrollTipText.

 BOOL CMyDeferScrollHook::
   OnGetScrollTipText(CString& s, UINT nPos)
 {
   s.Format(_T("Item #%d"), nPos);
   return TRUE; // handled
 }

 BOOL CMyViewDeferScrollHook::
   OnGetScrollTipText(CString& s, 
                      UINT nPos)
 {
   s.Format(_T("Line %d"),
            nPos/m_cyLine);
   return TRUE; // handled
 }

 BOOL CListCtrlDialog::OnInitDialog()
 {
   // subclass list contrl
   m_lcData.SubclassDlgItem(IDC_LIST1, this);
   
      // (add items)
 .
 .
 .
   // install hook
   m_deferScrollHook.Install(&lc);
   return TRUE;
 }

Figure 4 Deferred Scrolling

As you can see in Figure 4, LVScroll has a checkbox that lets the user turn deferred scrolling on or off. The on/off feature is implemented trivially in the command handler for the checkbox.

 void CListCtrlDialog::OnToggleDefer()
 {
   m_butnDefer.GetCheck() ? 
     m_deferScrollHook.Install(&m_lcData)
   : m_deferScrollHook.Unhook();
 }

If you decide to implement deferred scrolling for your app, I encourage you to likewise make it a user-selectable feature. Whether to do real-time or deferred scrolling is purely a performance matter, and hardware evolution continues to follow the Moore curve. What runs slowly today might zip tomorrow. If you make deferred scrolling an option, your customers will love you when they upgrade. You don't have to tell them you got the code from MSJ. Q I'm developing a specialized tree control that does custom painting using NM_CUSTOMDRAW. The painting works fine, but my control needs to initialize some fonts and other structures. At first I did the initialization in my OnCreate handler, which worked fine, but now someone at work wants to use my control in a dialog. They call SubclassDlgItem to subclass my control, and my OnCreate handler is never called because the window is already created. How can I initialize my control so it will work for either creating the control or subclassing it? Right now I'm using a m_bInitialized flag, which I check in every function. It works, but it seems inefficient. Marcel Didier Paris, France A I love questions like this that have short, one-line answers. You're right, the set-a-flag thing is kludgy. So put your initialization code in the little-known virtual function CWnd::PreSubclassWindow. MFC calls this function when the window is created or when it's subclassed. Problem solved. Next, please.

Have a question about programming in C or C++? Send it to Paul DiLascia at askpd@pobox.com

From the December 1999 issue of Microsoft Systems Journal.