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MFC: Two listboxes with one scrollbar

Posted on 2011-02-15
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Last Modified: 2012-08-14
Dear experts,

Could you please tell me how I could make two listboxes use one common scrollbar to move content of both vertically for my MFC application?
Thanks.
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Question by:tantormedia
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DeepuAbrahamK earned 250 total points
ID: 34896946
You may try overriding the WindowProc for the ListCtrl and capture VSCROLL and post the message to the other list box.
LRESULT CMyListCtrl::WindowProc(UINT message, WPARAM wParam, LPARAM lParam)
{
if(message==WM_VSCROLL) {
::PostMessage(m_hwndList2, message, wParam, lParam);
}
return CListCtrl:::WindowProc(message, wParam, lParam);
}

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Author Comment

by:tantormedia
ID: 34896999
Thank you for your answer. Could you please tell me how I can get m_hwndList2?
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Expert Comment

by:DeepuAbrahamK
ID: 34897080
You could create an object of your list control

Have a look at this:
http://www.functionx.com/visualc/controls/listbox.htm
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Author Comment

by:tantormedia
ID: 34897113
This I understand. My question was about the window handle that is passed to PostMessage().
Also, my ListBox class should be generic, so I guess, I would have to pass the handle to the second Listbox to its constructor...
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Expert Comment

by:DeepuAbrahamK
ID: 34897198
HWND m_hwndList2 = NULL;
m_hwndList2 = m_MyList2.m_hWnd;
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Expert Comment

by:sarabande
ID: 34897259
tantormedia, deepuabrahamk speeks of a list control while you were talking of list boxes.

nevertheless, you can do the parallel scrolling with listboxes as well:

  - derive a own listbox class CMyListBox from CListBox
  - exchange CListBox in dialog header by CMyListBox
  - add a function CMyListBox::PreTranslateMessage  to CMyListBox like

BOOL CMyListBox::WindowProc(MSG* pMsg)
{
    if(pMsg->message==WM_VSCROLL) {
         CMyDialog* pDlg = (CMyDialog*)GetParent();

         pDlg->m_listbox2.PostMessage(pMsg->message, pMsg->wParam, pMsg->lParam);
    }
    return FALSE;   // not handled cause the scrolling must be done by CListBox
}

using a list control in report style would allow you to have two text columns (and line number column if you want). so you automatically have one scrollbar for both text files.

Sara

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Author Comment

by:tantormedia
ID: 34897341
Thank you for your answers.
Sara, your code for WindowProc() is very similar to DeepuAbrahamK's. Is there any real difference?

Thanks.
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Assisted Solution

by:sarabande
sarabande earned 250 total points
ID: 34897446
i made a mistake. the function is

BOOL CMyListBox::PreTranslateMessage(MSG* pMsg)

not WindowProc.

you also must find out whether the 'this' is m_listbox1 or m_listbox2 in order to post the message to the other control. also you need to add code to prevent from infinite messages for example by saving the last message.

BOOL CMyListBox::PreTranslateMessage(MSG* pMsg)
{
    static UINT lastListbox = 0;
    static MSG lastMsg = { 0 };

   
    if(pMsg->message==WM_VSCROLL) {
         CMyDialog* pDlg = (CMyDialog*)GetParent();
         if (pDlg->GetDlgItem(IDC_LIST1) == this)
         {
             if (memcmp(pMsg, &lastMsg) != 0)
                 pDlg->m_listbox2.PostMessage(pMsg->message, pMsg->wParam, pMsg->lParam);
          }
         else /* if (pDlg->GetDlgItem(IDC_LIST2) == this) */
         {
             if (memcmp(pMsg, &lastMsg) != 0)
                 pDlg->m_listbox1.PostMessage(pMsg->message, pMsg->wParam, pMsg->lParam);
          }
          memcpy(&lastMsg, pMsg, sizeof(MSG));
    }
    return FALSE;   // not handled cause the scrolling must be done by CListBox
}


Sara
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LVL 33

Expert Comment

by:sarabande
ID: 34897537
the PreTranslateMessage and WindowProc both will be called from message loop so that you can hook with them into message processing.

for your purpose the one is as good as the other. but you should use the CWnd member functions rather than pure WINAPI as deepuabrahamk posted. The CWnd is baseclass of CListBox and has the window handle as a member.

Sara
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Author Comment

by:tantormedia
ID: 34897642
Strange, but it doesn't work. WM_VSCROLL is not caught.

Sara, could you please answer also my question there:
http://www.experts-exchange.com/Programming/Languages/CPP/Q_26820685.html#a34897334

Thanks.
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LVL 33

Expert Comment

by:sarabande
ID: 34898046
then try the WindowProc.

i mean to remember that PreTranslateMessage only was called for dialog windows.

unfortunately there are also two different techniques for scrollbar. one is the built-in scroll bar of a normal control window. another is a separate scroll bar window attached to a control. i mean listbox has the first and list control the second.

Sara
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Author Closing Comment

by:tantormedia
ID: 34898233
Thanks.
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