Advanced MFC Programming

Chapter 5. Common Controls
}
ptrEdit->SetSel(0, -1);
nSize=ptrCombo->GetCount();
bHit=FALSE;
for(i=0; iGetLBText(i, szStrLB);
if(szStrLB == szStr)
{
bHit=TRUE;
break;
}
}
}
if(bHit == FALSE)
{
ptrCombo->AddString(szStr);
if(wParam == IDC_COMBO_DROPDOWN)
{
GetDlgItem(IDC_STATIC_DROPDOWN)->SetWindowText(szStr);
}
if(wParam == IDC_COMBO_SIMPLE)
{
GetDlgItem(IDC_STATIC_SIMPLE)->SetWindowText(szStr);
}
}
return (LRESULT)TRUE;
In this message handler, we first obtain a pointer to the combo box using the ID passed through WPARAP
message parameter. Then we use above-mentioned method to get the pointer to the edit box (a child
window of combo box), and assign it to variable ptrEdit. Then we use this pointer to call function CWnd::
GetWindowText(…) to retrieve the text contained in the edit box window. If the edit box is not empty (this is
checked by calling function CString::IsEmpty()), we select all the text in the edit box by calling function
CEdit::SetSel(…), which has the following format:
void CEdit::SetSel(int nStartChar, int nEndChar, BOOL bNoScroll = FALSE);
The first two parameters of this function allow us to specify a range indicating which characters are to
be selected. If we pass 0 to nStartChar and -1 to nEndChar, all the characters in the edit box will be
selected. Then we use a loop to check if the text contained in the edit box is identical to any item string in
the list box. In case there is no hit, we will add this string to the list box by calling function
CComboBox::AddString(…). Finally, a TRUE value is returned before this function exits.
Using this method, we can also trap other keystrokes such as DELETE, ESC to the combo box. This
will make the application easier to use.
5.10 Implementing Subclass for the Edit Box of a Combo Box
Under certain conditions we may want to put restrictions on the contents of the list items. For example,
sometimes we may want the combo box to hold only numerical characters (‘0’-‘9’), and sometimes we may
expect it to hold only alphabetical characters (‘a’-‘z’, ‘A’-‘Z’). In these cases, we may want to customize
the properties of the edit box so that only a special set of characters can be accepted. If we are creating an
edit box resource in dialog template, this can be easily achieved by setting its customizable styles. But for
the edit box of a combo box, we can not customize its styles before it is created, so the edit box contained
in a combo box will have only the default styles.
To customize the behavior of the edit box in a combo box, we need to use “subclass” technique. We
can design our own class to intercept and process the messages sent to the edit box. Sample 5.10\CCtl
demonstrates how to customize the edit box that belongs to a combo box. It is based on sample 5.9\CCtl,
with two combo boxes customized as follows: combo box IDC_COMBO_SIMPLE allows only numerical
characters to be input into the edit box; combo box IDC_COMBO_DROPDOWN accepts only alphabetic
characters.
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