4.6 General properties of choices

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))

(contain example-button)

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 include a&restargs statement in your callback function to account for all the arguments that might be passed.

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

CAPI User Guide, Liquid Common Lisp Version 5.0 - 2 OCT 1997

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