There are a substantial number of Windows programmers who insist, often very
vehemently, that a programmer should avoid overriding
have their reasons for saying so and I believe they are correct. But in this
article my intention is not to contemplate on whether
good for you or whether you should avoid it like the plague. I have found that
PreTranslateMessage can come in quite handy in dialog-based applications for
handling keyboard messages. In addition to using
PreTranslateMessage I also show
you how you can override
ProcessMessageFilter for handling accelerator keys in a
dialog based application.
Using PreTranslateMessage to handle dialog keystrokes
Very often you hear questions from novice programmers asking how they can
trap keystrokes in a dialog based application. Presumably they tried to handle
WM_KEYDOWN/WM_KEYUP unsuccessfully. The whole problem is that in a dialog based
application the focus is always on one of the child controls and not on the main
dialog window. So what do you need to do? You need to override
PreTranslateMessage. I'll show you a simple example.
Suppose that you have a dialog based app with a lot of edit boxes on the
dialog. It's basically a data entry program and thus you feel it would make it
easier for the end-user if pressing the ENTER key would take the focus to the
next edit box, just as if he had pressed TAB. The solution is so very easy and
PreTranslateMessage as I'll demonstrate below.
BOOL CPreTransTestDlg::PreTranslateMessage(MSG* pMsg)
All I have done is to check whether the message is a
WM_KEYDOWN, and if it is
so, then I check to see if the
VK_RETURN. If I find it so, I change the
VK_TAB and then the base class implementation is called. Easy huh?
Using ProcessMessageFilter to handle dialog-based accelerator keys
Let's say you have a menu in your dialog based app. And you have an
accelerator key for some particular task. You'll soon be disappointed to find
that the hotkey does not work. The problem is that the modal dialog app's
message loop does not call
TranslateAccelerator. I do not know why this is so.
Presumable the Microsoft team decided that people shouldn't use dialog based
apps to write complicated applications, with hotkeys and menus.
But as usual they have suggested a workaround too. Here's is how you go about
implementing it. I'd like to state again, that even though this is a Microsoft
recommended technique there will be a good majority of MFC gurus, like Joseph
Newcomer for example, who would tell you that you shouldn't be doing this. But
then sometimes you have to sacrifice elegance for getting things done quickly
and with minimum effort.
- Add a member variable to your
CWinApp derived class.
Use the resource editor to create a new Accelerator, by default it will be
named IDR_ACCELERATOR1. And add a new accelerator key that is a short cut
for some menu item.
Put the following line in your
InitInstance just before the line where the
CDialog derived object is declared
ProcessMessageFilter and modify the function so that it looks
BOOL CPreTransTestApp::ProcessMessageFilter(int code, LPMSG lpMsg)
if (::TranslateAccelerator(m_pMainWnd->m_hWnd, m_haccel, lpMsg))
return CWinApp::ProcessMessageFilter(code, lpMsg);
All we did was to call
TranslateAccelerator and if it succeeds then we don't
need to call the base class
ProcessMessageFilter, as the message has been
handled. So we return
The author wishes to state here that the two methods mentioned above are
generally used methods and the author is not in any way endorsing these methods.
Users should read more on the usage of
ProcessMessageFilter before they use it in their programs.