HOWTO: How to Trap WM_KEYDOWN Messages in a CDialog

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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;
   
         }