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NOTE
VCL Components
form. Like the ListBox and ComboBox components discussed earlier, the RadioGroup component
has an ItemIndex property that you can read at runtime to determine which item in the
group is selected. Oh, by the way—the answer to the quiz is Redtailed Hawk (American
Kestrel would also have been an acceptable answer, but it was not presented in the list).
You can also use a GroupBox component to hold radio buttons. The
GroupBox component is less convenient to use than the RadioGroup
component, but it has more flexibility. You can place any type of
control in a group box. Once placed in the group box, the controls and
the group box itself can be moved as a unit at design time.
The CheckBox component is used to allow users to turn an option on or off or to indicate to
a user that an option is currently on or off. A check box can have up to three states, depending
on its style: on, off, or grayed. If the check box’s AllowGrayed property is false, it can only
be checked or unchecked. When the AllowGrayed property is true, the check box can be any
one of the three states. The grayed, or indeterminate, state is handled programmatically. In
other words, it’s up to you to decide what the grayed state means for your application. If the
AllowGrayed property is false (the default), you can use the Checked property to determine
whether the check box is checked or unchecked. If the AllowGrayed property is true, you must
use the State property to determine (or set) the check box state. State will return either
cbChecked, cbUnchecked, or cbGrayed.
TIP
To make a check box read-only but not grayed, place it on a panel and
change the panel’s Enabled property to false.
The Label Component
The Label component is used to display text on a form. Sometimes the label text is
determined at design time and never changed. In other cases, the label is dynamic and is
changed at runtime as the program dictates. Use the label’s Caption property to set the label
text at runtime. The Label component has no specialized methods or events beyond what is
available with other components. Table 8.8 lists the properties specific to the Label
component.
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