December 04, 2000, 12:00 AM — In a previous column, I talked about UpdateData(), which moves data
between member variables and controls. Timothy Purves, Director of
Systems Architecture at the Henry Ford Health System, wrote me about
UpdateData(). The code given for the OK button handler was:
void CSetWinner::OnOK()
{
UpdateData(TRUE);
if (m_winning_value < 1 || m_winning_value > 40)
{
AfxMessageBox("Value out of range");
}
else
CDialog::OnOK();
}
He noted that "This code should check the return from UpdateData...,
otherwise if you have a mixture of hand validate and DDX_Validate you
might get into a condition where you call OnOK with bad values."
UpdateData(TRUE) returns a BOOL value. The value is meaningful if the
parameter is TRUE (updating the member values from the controls).
(Note: I think that this emphasizes why this function should have been
broken into two functions. The return value is not meaningful if you
FALSE to update the controls from the member values).
If you used ClassWizard to set up a minimum and maximum value for a
member variable, there will be a line in your AFX_DATA_MAP(CSetWinner)
that looks like:
DDV_MinMaxInt(pDX, m_winning_value, 1, 40);
DDV stands for Dialog Data Validation. When UpdateData(TRUE) is
called, this validation is performed. If the value is not valid, then
a message is displayed to the user and they are directed back to the
appropriate control. UpdateData() will return FALSE if any validation
fails. If you use both DDV validation and manual validation, the user
will get two error messages - one from DDV and one you generate. To
avoid this, you should test the result of UpdateData(). The code will
then look like:
void CSetWinner::OnOK()
{
if (!UpdateData(TRUE))
return;
if (m_winning_value < 1 || m_winning_value > 40)
{
AfxMessageBox("Value out of range");
}
else
CDialog::OnOK();
}
The test is then redundant since the value has already been checked by
DDV. However you might want to put additional validation for other
fields in this routine. If you use DDV, you'll get a set of error
messages that may not resemble the error messages you want to manually
code.
You can create your own DDV routines. If you've got a field whose
validation is going to be used several times (in a single program or a
set of programs), then that's a good idea. If the field is only used
in a single dialog box, then it's probably overkill to create a
separate validation routine.
You can use the standard DDV routines in a more dynamic manner. The
ClassWizard only allows fixed values for the minimum and maximum
values. If you move the DDV line from inside the DATA_MAP, ClassWizard
will not worry about it. You'll probably need to move the DDX line as
well, since the DDV line for a control needs to immediately follow the
DDX line for that control.
