Q97693: INFO: Dynamic Processing of WM_COMMAND Messages

Article: Q97693
Product(s): Microsoft C Compiler
Version(s): winnt:1.0,2.0,2.1,4.0
Operating System(s): 
Keyword(s): kbDocView kbMFC kbVC100 kbVC150 kbVC200 kbVC400 kbGrpDSMFCATL 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 1.0, 2.0, 2.1, 4.0 
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SUMMARY
=======

The message-map mechanism provided by the Microsoft Foundation Classes (MFC) can
process WM_COMMAND messages for a constant ID. However, in some cases, an
application needs to process WM_COMMAND messages for an ID that is not known
until run time. This can occur when an application modifies menus or dynamically
creates controls at run time. To process these messages, your application must
override the CCmdTarget::OnCmdMsg() function.

MORE INFORMATION
================

The Microsoft Foundation Classes provide both the ON_COMMAND() and
ON_UPDATE_COMMAND_UI() message-map macros to process WM_COMMAND messages. The
ON_COMMAND() macro dispatches a WM_COMMAND message generated when the user
selects a user interface object (for example, a control or a menu item). The
ON_UPDATE_COMMAND_UI() macro updates the status of the user interface object
that generates the WM_COMMAND message. Each of these macros requires a constant
for the command ID. If the command ID is not known until run time, the message
must be processed without using a message map.

The sample code below demonstrates overriding the OnCmdMsg() member function to
dynamically process messages from a dynamically-created button.

The sample code processes messages for a class derived from CDocument. The
command ID is stored in the m_MyButtonID member variable. The code calls the
DoMyButton() member function when the user chooses the button, and calls the
DoUpdateMyButton() member function when the Microsoft Foundation Classes library
requests the status of the button.

Sample Code
-----------

     BOOL CMyDocument::OnCmdMsg(UINT nID, int nCode, void * pExtra,
                                AFX_CMDHANDLERINFO* pHandlerInfo)
     {
        // If pHandlerInfo is NULL, then handle the message
        if (pHandlerInfo == NULL)
           {
           // Filter the commands sent to a button the
           // ID of which is stored in m_MyButtonID.

           if (nID == m_MyButtonID)
              {
              if (nCode == CN_COMMAND)
                 DoMyButton();      // Handles WM_COMMAND messages
                                    // sent by the button.
              else if (nCode == CN_UPDATE_COMMAND_UI)
                 DoUpdateMyButton((CCmdUI*) pExtra);
                                    // Update buttons UI.
              return(TRUE);
              }
           }

        // Call the base class version of OnCommand
        // so the message-map can handle the message.
        return CDocument::OnCmdMsg(nID, nCode, pExtra, pHandlerInfo);
     }

REFERENCES
==========

MFC Technical Note 6: Message Maps.
MFC Technical Note 21: Command and Message Routing.

Additional query words:

======================================================================
Keywords          : kbDocView kbMFC kbVC100 kbVC150 kbVC200 kbVC400 kbGrpDSMFCATL kbArchitecture 
Technology        : kbAudDeveloper kbMFC
Version           : winnt:1.0,2.0,2.1,4.0
Issue type        : kbinfo

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