Advanced MFC Programming

Chapter 5. Common Controls
From this structure, we know which window is going to receive the message (from member hwnd),
what kind of message it is (from member message). Also, we can obtain the message parameters from
members wParam and lParam. If the message is not the one we want to intercept, we can just forward the
message to its original destination by calling the base class version of this function.
Function CWnd::PreTranslateMessage(…)
Sample 5.9\CCtl demonstrates how to trap RETURN keystrokes for combo box. It is based on sample
5.8\CCtl. First, function PreTranslateMessage(…) is overridden. This function can be added by using
Class Wizard through following steps: 1) Open Class Wizard, click “Message Maps” tab, select
“CCCtlDlg” from “Class name” window. 2) Highlight “CCCtlDlg” in window “Object IDs”. 3) Locate and
highlight “PreTranslateMessage” in window “Messages”. 4) Press “Add function” button.
The default member function looks like the following:
BOOL CCCtlDlg::PreTranslateMessage(MSG *pMsg)
{
return CDialog::PreTranslateMessage(pMsg);
}
If we do not want to process the message, we need to call function CDialog::
PreTranslateMessage(…) to let the dialog box process it as usual. Otherwise we need to return a TRUE
value to give the operating system an impression that the message has been processed properly.
In the overridden function, first we need to check if the message is WM_KEYDOWN and the key being
pressed is RETURN:
BOOL CCCtlDlg::PreTranslateMessage(MSG *pMsg)
{
CString str;
HWND hwndDlg;
HWND hwndFocus;
HWND hwndEdit;
CEdit *ptrEdit;
int nVirtKey;
char szClassName[256];
……
if(pMsg->message == WM_KEYDOWN)
{
nVirtKey=(int)pMsg->wParam;
if(nVirtKey == VK_RETURN)
{
}
}
Message WM_KEYDOWN is a standard Windows message for non-system key strokes, and VK_RETURN is
a standard virtue key code defined for RETUN key (For a list of virtual key codes, see appendix A). Some
local variables are declared at the beginning. They will be used throughout this function.
Accessing the Edit Box of a Combo Box
We need to find out which combo box has the current focus in order to decide if we should process this
message. If the item that has the current focus is either IDC_COMBO_DROPDOWN or IDC_COMBO_SIMPLE, we will
update the corresponding list items.
In Windows operating system, windows are managed through using handles. Like menu and bitmap
resources, a window handle is also a number which could be used to identify a window. Each window’s
handle has a different value. As a programmer, we do not need to know the exact value of the handle,
however, we can use handle to access or identify a window.
In MFC, there is a function CWnd::GetFocus(), which can be used to obtain a pointer to the child
window that has the current focus. From this pointer, we can obtain that window’s handle. Then we can
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