[Xotcl] Improvement proposal for instfilters and guards

[Gustaf Neumann]

Dear Eckhard,

Eckhard Lehmann schrieb:
> One thing comes to mind since I play around with Tclhttpd and a 
> persistency framework: Tclhttpd maintains sessions as safe slave 
> interpreters. These interpreters must know about persistency objects 
> in the server, so it is necessary that
> - whenever a persistency object is created, a command alias for it 
> must be created in certain sessions (interpreters)
> - whenever a session is created, certain persistency objects must be 
> "informed" (-> a command alias for them must be created in the new 
> interpreter).
> - whenever a session is destroyed, it's alias must be removed from all 
> persistency objects that contain it.
i would not recommend to use method combinators
or filters for this kind of problem: This problem looks to me
like an perfect example for metaclasses.

Meta-classes allow to create special kinds of classes.
Sometimes you would like to have PersistentClasses,
sometimes RemoteClasses, or LoggedClasses,
etc. One can certainly use object or class mixins
as well to mix the feature "persistency" into one or
several classes. The mixin approach has advantages
for finer-grained control, but i doubt that you will
need it for e.g. session management.

So, for your problem, define a meta-class for classes
with persistant objects and overwrite
on the class level the methods "create" and "instdestroy".  The same
can be done with the Session class as well. I have not looked at
the session model of TclHttpd, but from your specification, the
class structure can look like the code below. The script
produces the following output:

creating object p1 of class ::Person
... create an alias for p1
creating session s1
... inform instances of ::Person
creating object p1 of class ::Person
... create an alias for p1
creating object e1 of class ::Employee
... create an alias for e1
creating session s2
... inform instances of ::Employee ::Person
deleting session ::s2
deleting session ::s1
object ::e1 destroy
... delete alias for ::e1
object ::p1 destroy
... delete alias for ::p1

Hope, this helps.
-gustaf neumann




##############################################
package require XOTcl
namespace import -force ::xotcl::*

Class PersistentClass -superclass Class
PersistentClass instproc create {name args} {
  puts stderr "creating object $name of class [self]"
  puts stderr "... create an alias for $name"
  next
}
PersistentClass instproc instdestroy {name} {
  puts stderr "object $name destroy"
  puts stderr "... delete alias for $name"
  next
}

Class Session
Session proc create {name args} {
  puts stderr "creating session $name"
  puts stderr "... inform instances of [PersistentClass info instances]"
  next
}
Session proc instdestroy {name} {
  puts stderr "deleting session $name"
  next
}


PersistentClass Person
Person p1

Session s1

Person p1
PersistentClass Employee -superclass Person
Employee e1

Session s2

s2 destroy
s1 destroy