[gme-users] Constraint problem

Larry Howard larry.howard at vanderbilt.edu
Thu Mar 2 10:32:06 CST 2006

I'm having a problem defining a constraint that concerns a change in number
of children in a container.  My intention is to check this constraint on an
event, like "onlostchild", and to prevent the action by setting the
constraint's Priority=1.  My problem is to find an approach that does not
subsequently trigger a violation when the user chooses the 'Check All'
action of the Constraint Manager.
It is my current belief that there is no way to define such a constraint.
The reason is that the focus of this constraint is "change", whereas the
focus of the Constraint Manager, in general, is "current state".
Constraints concerning "change" typically involve knowing "state before" and
"state after".  Have I failed to consider something?
One thing that occurs to me is that the Constraint Manager might consider
not checking constraints on "Check All" that are checked on events with
Priority=1.  The intuition is that, since such event-based constraints can
only result in aborting the action, there is simply no reason to check them
again on "Check All".  Such a feature would allow the desired constraint to
be defined quite simply with a "false" predicate.
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