Article: Q117563
Product(s): Microsoft C Compiler
Version(s): winnt:2.0,2.1,4.0
Operating System(s):
Keyword(s): kbDlg kbHook kbKeyIn kbMFC kbVC100 kbVC150 kbVC200 kbVC400 kbArchitecture
Last Modified: 07-MAY-2001
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The information in this article applies to:
- The Microsoft Foundation Classes (MFC), used with:
- Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5, 1.51, 1.52
- Microsoft Visual C++, 32-bit Editions, versions 2.0, 2.1, 4.0
- Microsoft Visual C++ 32-bit Edition, versions 2.0, 2.1, 4.0
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SUMMARY
=======
Certain Windows messages are difficult to trap in a dialog box because they are
either processed by the Windows internal dialog procedure or sent to the control
instead of the dialog box. There are several ways to do this, but they usually
involve subclassing all the controls in the dialog box or using a Windows hook
function. The method described in this article uses the predefined overridable
MFC hook function ProcessMessageFilter() to capture these messages. The idea is
to override the MFC preinstalled hook function, ProcessMessageFilter(), to trap
the messages before they get to the dialog box.
MORE INFORMATION
================
The following steps are the required to implement the hook function. The
SCRIBBLE sample was used as the base for the steps. In Visual C++ 32-bit
Edition, version 4.0, use SCRIBBLE sample STEP3.
In this example, the steps outlined below trap WM_KEYDOWN for a specific dialog
box:
1. Add a member variable to your CWinApp-derived class to hold the handle to the
dialog box for which you want to trap the messages. You need to know the
handle because ProcessMessageFilter is receiving messages for the entire
application, while you only want a small selection of those messages:
SCRIBBLE.H
<Inside CScribbleApp class>
public:
HWND m_hwndDialog;
SCRIBBLE.CPP
<Inside CScribbleApp::InitInstance>
m_hwndDialog = NULL;
NOTE: Because this message filter applies to the entire application, there may
be a performance hit when you use this method.
2. Override CWinApp::ProcessMessageFilter in your CWinApp-derived class.
CWinApp::ProcessMessageFilter is a virtual function of CWinApp, so all you
need to do is put a declaration in the header file and an implementation in
the source file.
SCRIBBLE.H
public:
virtual BOOL ProcessMessageFilter(int code, LPMSG lpMsg);
SCRIBBLE.CPP
BOOL CScribbleApp::ProcessMessageFilter(int code, LPMSG lpMsg)
{
// Check to make sure CPenWidthsDlg is up
if (m_hwndDialog != NULL)
{
if ((lpMsg->hwnd == m_hwndDialog) ||
::IsChild(m_hwndDialog, lpMsg->hwnd))
// Use ::IsChild to get messages that may be going
// to the dialog's controls. In the case of
// WM_KEYDOWN this is required.
{
if (lpMsg->message == WM_KEYDOWN)
TRACE("Got WM_KEYDOWN\n");
}
}
// Default processing of the message.
return CWinApp::ProcessMessageFilter(code, lpMsg);
}
For more information on overriding ProcessMessageFilter, please see the MFC
online Help.
3. Add initialization code for the dialog handle member variable in our
OnInitDialog for the desired dialog box. You also have to add exit code to
reset the member variable after the dialog box closes:
PENDLG.H
protected:
virtual BOOL OnInitDialog();
afx_msg void OnDestroy();
PENDLG.CPP
<Add to Message Map>
ON_WM_DESTROY()
BOOL CPenWidthsDlg::OnInitDialog()
{
CDialog::OnInitDialog();
// Use AfxGetApp() to get pointer to CWinApp-derived
// object; then cast the pointer to our type and assign
((CScribbleApp *)AfxGetApp())->m_hwndDialog = m_hWnd;
return TRUE; // Return TRUE
// unless you set the focus to a control.
}
void CPenWidthsDlg::OnDestroy()
{
CDialog::OnDestroy();
// Use AfxGetApp() to get pointer to CWinApp-derived
// object; then cast the pointer to our type and assign
((CScribbleApp *)AfxGetApp())->m_hwndDialog = NULL;
}
Once these steps are completed, you can trap any message intended for the
CPenWidthsDlg or any of its children (such as edit controls, combo boxes, and
radio buttons)
If this method is unacceptable because of the application-wide nature of the
message filter, please see the following article in the Microsoft Knowledge
Base:
Q72219 Context-Sensitive Help in a Dialog Box Through F1
This article explains how to set a hook function for a specific dialog and how to
remove the hook after the dialog box has terminated. That method requires more
Windows programming knowledge.
Additional query words: kbinf 1.00 1.50 2.00 2.10 2.50 3.00 3.10 4.00 key keystroke
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Keywords : kbDlg kbHook kbKeyIn kbMFC kbVC100 kbVC150 kbVC200 kbVC400 kbArchitecture
Technology : kbAudDeveloper kbMFC
Version : winnt:2.0,2.1,4.0
Issue type : kbhowto
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