User level events are mapped into actions. Actions call application-level callback functions. Xt replay system Actions define widget methods. A sequence of XtCallActionsProc's allows replay of an Xt application. Simulation method must allow queuing, and must allow processing.
Some actions require event information.
Gadgets don't have actions. Preference for non-proprietary language. Require extensible language. An application-independant command language. An extensible language.