Every widget has an XtNdestroyCallbacks callback list resource.
Widgets can define additional callback lists as they see fit.
For example, the Pushbutton widget has a callback list to notify clients when the button has been activated. Except where otherwise noted, it is the intent that all Intrinsics functions may be called at any time, including from within callback procedures, action routines, and event handlers. Using Callback Procedure and Callback List Definitions Callback procedure pointers for use in callback lists are of type XtCallbackProc. For example, when Scrollbar executes its XtNthumbChanged callback list, it passes the new position of the thumb. The client can use the more general data retrieval methods, if necessary.