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VCL Components
The GroupIndex Property
Speed buttons can be grouped to make them behave like radio buttons (radio buttons are
discussed later in the chapter in the section “Radio Buttons and Check Boxes”). When one
button in the group is pressed, it stays down, and the button that was previously pressed pops
up again. To group speed buttons, simply assign the same value to the GroupIndex property
for all buttons in a group. (The default value of 0 indicates that the button is not part of any
group.) To illustrate, do the following exercise:
1. Create a blank form and place five speed buttons on the form. (I won’t bother with
adding glyphs to the buttons in this simple exercise, but you certainly may if you
want.)
2. Select all the buttons and change the value of the GroupIndex property to 1. The
GroupIndex for all buttons will be changed to 1.
3. Optional: Change the Down property of one of the buttons to true.
4. Click the Run button to compile and run the program.
When you run the program, click on several of the buttons. You will notice that only one
button can be in the down state at one time. As you can see when you assign a nonzero value
to GroupIndex, the speed buttons change their behavior. A speed button with a GroupIndex
of 0 pops back up when you click it, whereas a speed button that is part of a group stays down
when clicked.
The AllowAllUp Property
By default, one button in the group must be down at all times. You can change that behavior
by setting the AllowAllUp property to true. Doing this for one button automatically changes
the AllowAllUp property for all other buttons in the group to true as well. Now you can have
any one button in the group selected or no buttons at all.
TIP
Sometimes you want a speed button to act as a toggle button. A toggle
button is used to turn an option on or off and is not part of a button
group. To make an individual speed button a toggle button, assign a
nonzero value to its GroupIndex property and set its AllowAllUp property
to true. Be sure to set the GroupIndex property to a value not used by any
other components on the form. When the user clicks the button, it stays
down. When the button is clicked again, it pops back up.
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