Comparison of screendumps. Drawbacks of X approaches X events require x, y co-ordinates. Moving or resizing windows may invalidate this. Xt defines objects called widgets. 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.