The Be Book - System Overview - The Interface Kit
BControl
is an abstract class for views that draw control devices on the
screen. Objects that inherit from
BControl
emulate, in software, real-world control devices—like the switches
and levers on a machine, the check lists and blank lines on a form to fill
out, or the dials and knobs on a home appliance.
Controls translate the messages that report generic mouse and keyboard
events into other messages with more specific instructions for the
application. A BControl
object can be customized by setting the message
it posts when invoked and the target object that should handle the
message.
Controls also register a current value, stored as an int32 integer that's
typically set to B_CONTROL_ON or B_CONTROL_OFF. The value is changed only
by calling
SetValue(),
a virtual function that derived classes can implement to be notified of the change.
The Interface Kit currently includes six classes derived from
BControl
—
BButton,
BPictureButton,
BRadioButton,
BCheckBox,
BColorControl,
and BTextControl.
In addition, it has two classes—
BListView and
BMenuItem
—that implement control
devices but are not derived from this class.
BListView
and its subclass,
BOutlineListView,
share an interface with the
BList
class (of the Support Kit) and
BMenuItem
is designed to work with the other classes in the menu system. Like
BControl,
BListView and
BMenuItem
inherit from the Application Kit's
BInvoker class.
As BListView and
BMenuItem demonstrate, it's possible to implement a
control device that's not a
BControl.
However, it's simpler to take
advantage of the code that's already provided by the BControl class. That
way you can keep a simple programming interface and avoid reimplementing
functions that
BControl has defined for you. If your application defines
its own control devices—dials, sliders, selection lists, and the
like—they should be derived from BControl.
