Is there an event that fires from a C++ program when a control is about to lose focus? - c++

Question

I am trying to fix a validation bug in a MFC CEdit control. Currently, validation is performed in an OnChange event handler. But this does not work because it validates data before the user is finished entering it.
So, instead, I am trying to validate inside an OnKillFocus event handler. If validation fails, then I use GotoDlgCtrl() to return focus to the edit box that contained the invalid data. And when I call GotoDlgCtrl(), the kill focus event fires again, and I'm in an infinite loop.
So, I'd like to handle an event that fires just before the control loses focus, so that if I determine that the data is invalid, I can stop focus from leaving and instead get the user to enter correct data.
I know I've seen a Validating event someplace, but that was probably in the .Net world. But it offers the functionality I'm looking for.

Answer 1

Right-click the dialog resource and invoke Class Wizard:
Next, go to the Virtual Functions tab, locate PreTranslateMessage and add it:
Then, you can do something like this:
BOOL CTestDlgDlg::PreTranslateMessage(MSG* pMsg)
{
if (pMsg->message == WM_CHAR)
{
CWnd *pControl = GetDlgItem(IDC_EDIT1);
if (pControl->GetSafeHwnd() == pMsg->hwnd)
{
if (pMsg->wParam == _TINT('!'))
{
AfxMessageBox(_T("Not allowed ! character"));
return TRUE;
}
}
}
return CDialogEx::PreTranslateMessage(pMsg);
}
Normally the control is a member variable of type CEdit so you could compare against m_edit.GetSafeHwnd() instead.
Results:
Update
I realise you stated:
But this does not work because it validates data before the user is finished entering it.
You could use WM_KEYUP instead:
BOOL CTestDlgDlg::PreTranslateMessage(MSG* pMsg)
{
if (pMsg->message == WM_KEYUP)
{
CWnd *pControl = GetDlgItem(IDC_EDIT1);
if (pControl->GetSafeHwnd() == pMsg->hwnd)
{
CString str;
GetDlgItemText(IDC_EDIT1, str);
if (str.Find(_T("!")) >= 0)
{
AfxMessageBox(_T("Not allowed ! character"));
return TRUE;
}
}
}
return CDialogEx::PreTranslateMessage(pMsg);
}
That is give you a chance to validate after the display has been updated.
An alternative it to customize your DoDataExchange handler. In there you can validate as required. Then in your code you simple test the return value of UpdataData(TRUE) for FALSE.