Advanced MFC Programming

Chapter 5. Common Controls
(CBN_CLOSEUP for IDC_COMBO_DROPDOWN and IDC_COMBO_DROPLIST, CBN_SELCHANGE for IDC_COMBO_SIMPLE).
3) Click button “Add function” and confirm the member function name.
These functions will be called when the user makes a new selection from the list box of a combo box.
We can retrieve the index of the current selection of a combo box by calling function CComboBox::
GetCurSel(), and further retrieve the text of that item by calling function CComboBox::GetLBText(…). At
last we can call function CWnd::SetWindowText(…) to display the updated content of the selected item in
one of the static text controls. The implementations of three message handlers are almost the same. The
following is one of them:
void CCCtlDlg::OnCloseupComboDropdown()
{
CString szStr;
int nSel;
}
nSel=m_cbDropDown.GetCurSel();
if(nSel != CB_ERR)
{
m_cbDropDown.GetLBText(nSel, szStr);
}
GetDlgItem(IDC_STATIC_DROPDOWN)->SetWindowText(szStr);
First index of the current selection is retrieved and stored in variable nSel. Then we check if the
returned value is CB_ERR. This is possible if there is nothing being currently selected. If the returned value
is a valid index, we call function CComboBox::GetLBText(…) to retrieve the text string and store it in
CString type variable szStr. Finally function CWnd::GetDlgItem(…) is called to obtain the pointer to the
static text window, and CWnd::SetWindowText(…) is called to update its contents.
5.9 Trapping RETURN key strokes for the Combo Box
Problem & Workaround
One feature we may want to add to the combo boxes is to let the user dynamically add new items
through using their edit boxes. We can let the user input a string into the edit box of a drop-down or simple
combo box, then hit the RETURN key to add the input to list item. However, in a dialog box, the RETURN
key (also the ESC key) is used to exit the application by default. Even if we add message handler for
combo box to trap RETURN keystrokes, it still can not receive this message because after the message
reaches the dialog box, the application will exit. The message has no chance to be further routed to the
child windows of a dialog box.
If we want to process RETURN keystroke events, we need to intercept the message before it is
processed by the dialog box. In MFC, there is a function CWnd::PreTranslateMessage(…) that can be
overridden for this purpose. This function will be called just before a message is about to be processed by
the destination window. Since CDialog is derived from CWnd, we can trap any message sent to the dialog
box if we override the above function. This function has the following format:
BOOL CWnd::PreTranslateMessage(MSG *pMsg);
Its only parameter is a pointer to MSG type object:
typedef struct tagMSG {
HWND hwnd;
UINT message;
WPARAM wParam;
LPARAM lParam;
DWORD time;
POINT pt;
} MSG;
// msg
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