INFO: ProcessMessageFilter gets called only for messages posted (166212)
The information in this article applies to:
- The Microsoft Foundation Classes (MFC), when used with:
- Microsoft Visual C++ for Windows, 16-bit edition 1.52
- Microsoft Visual C++, 32-bit Editions 2.0
- Microsoft Visual C++, 32-bit Editions 2.1
- Microsoft Visual C++, 32-bit Editions 2.2
- Microsoft Visual C++, 32-bit Editions 4.0
- Microsoft Visual C++, 32-bit Editions 4.0a
- Microsoft Visual C++, 32-bit Editions 4.1
- Microsoft Visual C++, 32-bit Enterprise Edition 4.2
- Microsoft Visual C++, 32-bit Professional Edition 4.2
- Microsoft Visual C++, 32-bit Enterprise Edition 4.2b
- Microsoft Visual C++, 32-bit Professional Edition 4.2b
- Microsoft Visual C++, 32-bit Enterprise Edition 5.0
- Microsoft Visual C++, 32-bit Professional Edition 5.0
- Microsoft Visual C++, 32-bit Enterprise Edition 6.0
- Microsoft Visual C++, 32-bit Professional Edition 6.0
- Microsoft Visual C++, 32-bit Learning Edition 6.0
This article was previously published under Q166212 SUMMARY
ProcessMessageFilter is only called for messages that are posted to the
message queue and is not called for messages that are send directly to the
window procedure. Therefore, this function cannot be used to monitor
messages that are sent (by "Windows" or by the user) to a dialog box,
message box, menu, or a scroll bar.
ProcessMessageFilter is called from _AfxMsgFilterHook, which is the MFC
hook procedure for WH_MSGFILTER hook. This hook procedure will be called by
the system after a message generated by an input event (in a dialog box,
message box, menu, or scroll bar) is retrieved from the message queue.
Since messages sent directly to the window procedure (by SendMessage) do
not go through the message queue, this hook procedure is not called by
Windows. This in turn blocks the call to ProcessMessageFilter.
MORE INFORMATION
The WH_MSGFILTER hook is a task-specific hook that enables an application
to monitor messages passed to a menu, scroll bar, message box, or dialog
box created by the application that installed the hook procedure. In a
typical MFC application the WH_MSGFILTER is set in the global function
AfxWinInit(), and in _AfxThreadEntry for multithreaded applications, with
_AfxMsgFilterHook as the callback or the hook procedure.
The callback function for this hook is called after these messages are
retrieved from the queue, just before dispatching them, performing special
processing as appropriate. Since messages sent directly to the window
procedure (by SendMessage) do not go through the message queue, the hook
procedure (_AfxMsgFilterHook) is not called by Windows.
ProcessMessageFilter of CWinThread (in MFC 3.x and up) and CWinApp (in MFC
2.x) is called from the _AfxMsgFilterHook hook procedure. Since the hook
procedure will not be called for messages sent directly to the window
procedure, ProcessMessageFilter will not get called and none of the special
code inside will execute.
In general, all the keyboard and mouse messages, along with the WM_PAINT
and WM_TIMER messages, are posted to the message queue.
ProcessMessageFilter will not be called for messages like WM_SETFOCUS,
WM_KILLFOCUS, WM_SETCURSOR, WM_COMMAND, WM_CTLCOLOR, WM_ACTIVATE, etc.,
since these are sent directly to the window procedure.
To monitor messages that are sent to a menu, message box, dialog box or a
scroll bar, set a WH_CALLWNDPROC hook. This hook is called whenever a
message is sent directly to a window procedure. Note that there will be a
tradeoff with performance since this hook procedure is called for every
message that are sent through SendMessage.
REFERENCES
Additional information on SetWindowsHookEx, MessageProc, WH_MSGFILTER can
be found in the Windows API documentation.
| Modification Type: | Major | Last Reviewed: | 12/2/2003 |
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| Keywords: | kbcode kbHook kbinfo KB166212 |
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