Advanced MFC Programming

Chapter 5. Common Controls
In this function, first the number of selected items is retrieved by calling function
CListBox::GetSelCount(), and the retrieved value is saved to variable nSelSize. If the size is not zero,
we allocate an integer type array with size of nSelSize. Then by calling function
CListBox::GetSelItems(…), we fill this buffer with the indices of selected items. Next, a loop is used to
retrieve the text of each item. The procedure of retrieving selected text for list box IDC_LIST_DIR is the
same.
5.8 Combo Box
Combo box is another type of common control that allows the user to select one object from a list.
While a list box allows multiple selections, a combo box allows only single selection at any time. A combo
box is made up of two other controls: an edit box and a list box. There are three types of combo boxes: 1)
Simple combo box: the list box is placed below the edit box, and is displayed all the time; the edit box
displays the currently selected item in the list box. 2) Drop down combo box: the list box is hidden most of
the time; when the user clicks the drop-down arrow button located at the right corner of the edit box, the list
box is shown and can be used to select an item. In both 1) and 2), the edit box can be used to input a string.
3) Drop list combo box: it is the same with drop down combo box, except that its edit box cannot be used to
input string.
Using a combo box is more or less the same with that of a list box. We must first create combo box
resources in the dialog template, set appropriate styles, then in the dialog’s initialization stage (in function
CDialog::OnInitDialog()), initialize the combo box. We can add message handlers to trap mouse or
keyboard related events for combo box. Two most important messages for combo boxes are CBN_CLOSEUP
and CBN_SELCHANGE. The first message indicates that the user has clicked the drop-down arrow button,
made a selection from the list box, and the drop down list is about to be closed. The second message
indicates that the user has selected a new item.
In MFC, combo box is supported by class CComboBox. Like CListBox, class CComboBox has a function
CComboBox::AddString(…) which can be used to initialize the contents of its list box. Besides this, we can
also initialize the contents of a list box when designing dialog template. In the property sheet whose caption
is “Combo Box Properties”, by clicking “Data” tab, we will have a multiple-line edit box that can be used
to input initial data for combo box. We can use CTRL+RETURN keys to begin a new line (Figure 5-8).
Class CComboBox has two functions that allow us to change the contents contained in the list box
dynamically: CComboBox::InsertString(…) and CComboBox:: DeleteString(…).
When designing drop-down combo box, we must set its vertical size, otherwise it will be set to the
default value zero. In this case, there will be no space for the list box to be dropped down when the user
clicks drop down button. To set this size, we can click the drop-down button in the dialog template. After
doing this, a resizable tracker will appear. The initial size of a combo box can be adjusted by dragging the
tracker’s border (Figure 5-9).
Click Data tab
Initialize list box
contents
Figure 5-8. Initialize combo box contents
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