Location-related events (was Re: [Zope3-dev] Using parent references rather than context wrappers to represent containment)

[Phillip J. Eby]

At 10:58 AM 8/13/03 -0400, Jim Fulton wrote:
>I worked up a preproposal on location-related events at:
>
>   http://dev.zope.org/Zope3/LocationRelatedEvents
>
>I've conviced, based on this preliminary analysis, that containers cannot be
>responsible for generating location-change events.

Per our previous discussion, I disagree with "location implies traversal", 
since a component may wish to use another component as its context, and yet 
that context has no references to the component.  So, if it's desired to 
find all of the items that have a particular item as a parent, some kind of 
index would be needed.

However, it's not clear to me *why* that's needed.  That is, I'm not sure 
what the use case is for propagating an event to all descendants of a 
container.  It seems to me that one would simply tell the container that 
the object is moving, and the object that it's moving, and leave it to the 
container to tell its parents (if applicable) and the object to notify its 
children (if desirable), and maybe you'd notify the system event service, 
so any other interested parties could know about it.

As far as the event structure...  in ZPatterns I found it useful to 
structure the event differently according to who was receiving it.  I don't 
know if this will do what you want, but anyway, what I did was that each 
recipient saw the event from its own point of view.  Let's say that you had 
this structure:

     A
    / \
   B   C

And you moved an object O from folder B to folder C.  A received an event 
that said "O moved from B to C", while B received an event that said "O has 
been removed", and C received an event that said "O has just arrived".  The 
purpose of this is that if B and C have local invariants to preserve 
regarding their contents, they are seeing the right kind of event, from 
their local perspective.  Meanwhile, A could maintain any location-based 
things, but would not think an object had been created or destroyed.

Perhaps this is the sort of event structure you are looking for?