[Zope3-dev] instance- and type-based event subscriptions

[Jim Fulton]

Sometimes, we want to subscribe to events *only* for specific
objects or for specific types of objects.  For example, we currently
have notification interfaces, IAddNotifiable and IRemoveNotifiable
that we call when objects are added or removed. These are used to
implement class-specific notification policies.

Similarly, we have a dependency framework, for, essentially, defining
integrity constraints mong objects.  We prevent objects from being deleted
if they have dependents. This mechanism is fairly crude.  We might, in some
cases, perform some action on a dependent object when the object it depends
on is removed. In any case, we want to be able to subscribe to events on the
affected object.

I'm thinking about providing the following mechanisms:

1. Type-based subscriptions through adapters

    For object events, get partial multi adapters from
    the affected object and event to ISubscriber.  Subscriptions
    can thus be registered for any of an object's interfaces, or
    even for it's class.

    This illustrates a need to provide an option to provide duplicate
    partial adapters.

2, Instance-based subscriptions

    For object events, we'll adapt the object to ISelfSubscriber:

      class IInstanceEventChannel(IEventChannel):
          """Objects that support instance-based subscriptions
          """

    and call it's notify method.

    Similarly, to subscibe to events for an instance, we'll
    get an IInstanceEventChannel adapter and call it's subscribe method.

    We'll provide an adapter from IAnnotatable to ISubscriptions.

Note that both of these policies can be implemented with global subscribers
to object events. The policy need not be implemented in the event service
itself.

Jim

-- 
Jim Fulton           mailto:jim at zope.com       Python Powered!
CTO                  (540) 361-1714            http://www.python.org
Zope Corporation     http://www.zope.com       http://www.zope.org