
DIRECTORY
EditNotify USING [Change];
UndoEvent: CEDAR DEFINITIONS ~ BEGIN
Event: TYPE ~ REF EventRep;
EventRep: TYPE ~ RECORD [subevents: SubEvent]; -- actual type of Tioga.EventRep
SubEvent: TYPE = REF SubEventBody;
SubEventBody: TYPE = RECORD [
next: SubEvent,
undoProc: UndoProc,
undoRef: REF Change
];

Change: TYPE ~ EditNotify.Change;

UndoProc: TYPE = PROC [undoRef: REF Change, currentEvent: Event];

Create: PROC RETURNS [Event];

Note: PROC [event: Event, undoProc: UndoProc, undoRef: REF Change];

Undo: PROC [undoEvent: Event, currentEvent: Event ¬ NIL];

Reset: PROC [event: Event];

Empty: PROC [event: Event] RETURNS [BOOL];

END.
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UndoEvent.mesa
Copyright Ó 1985, 1986, 1991, 1992 by Xerox Corporation.  All rights reserved.
written by Bill Paxton, June 1981
last edit by Bill Paxton, April 5, 1982 12:39 pm
Doug Wyatt, February 26, 1992 12:20 pm PST
An undo event is a series of actions that are to be undone as a unit.
The event record contains sufficient information to undo the actions.


undo list contains changes.
procedure supplied by client to undo the effects of a particular subevent
adds <undoProc, undoRef> to list of subevents
otherwise, calls undoProc[undoRef,currentEvent] for each subevent 
in reverse order that subevents originally happened
currentEvent arg is to record the effects of Undo so it too will be undoable
resets the list of subevents to NIL
Undo automatically does a Reset when it is finished
returns true if subevents list is empty
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