4.6.3 Callbacks

All choices can have callbacks associated with them. These callbacks are activated when the application user makes a selection, and different sorts of gesture can have different sorts of callback associated with them.

The following callbacks are available: :selection-callback , :retract-callback (called when a deselection is made), :extend-callback , and :action-callback (called when a double-click occurs). What makes one choice different from another is that they permit different combinations of these callbacks. This is a consequence of the differing interactions. For example, you cannot have an :extend-callback in a radio button panel, because you cannot extend selection in one.

Callbacks pass data to the function they call. There are default arguments for each type of callback. Using the :callback-type keyword allows you to change these defaults. It can take any of the following arguments.

:interface

causes the interface to be passed as an argument to the callback function.

:data

causes the value of the selected data to be passed to the callback function.

:item

causes the selected item to be passed as an argument to the callback function.

The following combinations of two of the above are also allowed - :interface-data , :interface-item , :data-interface and :item-interface . In each of these cases two arguments are passed to the callback function.

A final option is to pass no arguments, using the :none keyword The following example uses a push button and a callback function to display the arguments it receives.

(defun show-callback-args (arg1 arg2)

  (display-message "The arguments were ~S and ~S" arg1 arg2))

(setq example-button

      (make-instance 'push-button

                     :text "Push Me"

                     :callback 'show-callback-args

                     :data "Here is some data"

                     :callback-type :data-interface))

Try changing the :callback-type to other types.

If you do not use the :callback-type argument and you do not know what the default is, you can define your callback function with lambda list (&rest args) to account for all the arguments that might be passed.

Specifying a callback that is invalid for a particular choice causes a compilation error.