[Grok-dev] view predicates in Grok?

Paul Wilson paulalexwilson at gmail.com
Fri Jan 1 21:22:48 EST 2010

2010/1/1 Martijn Faassen <faassen at startifact.com>:
> * investigate how BFG's implementation works, and whether it can reused
> and/or extracted somehow. (if extracted we'll need to figure out where
> we can maintain the extracted bits, i.e. what are the possible rights
> issues around importing this into svn.zope.org?).

Its interesting, at a high level:.

It looks like their scanning procedure finds views and groups them by
(context, request, name). Since a collection of views may share such a
type-tuple, there is a one-to-many relationship between them:

(ContextA, RequestA, 'some_name') ------> [ViewA, ViewB, ViewC]

and the various views differ only by their predicates only. The list
of views is modeled as a 'MultiView' class, which is what actually
gets registered. (My initial thought was that subscription adapters
must be at work - but apparently not.)

During the scanning phase, the type tuple of a view is found, and
looked up in the component registry, If no such MultiView exists for
it, one is created and the view is added to it. This wouldn't fit with
Grok btw, because the grokker only registers a configuration *action*
- a callable - and doesn't play with the registry until after all
views have been found, as far as I'm aware. The view grokker would
need to keep track of what's going on here.... This shouldn't be too
difficult because at this stage we don't care about type-relationships
between the tuple-types.

So, when a request comes in, the component registry resolves to a
multiview (rather than directly to a view). This indirection allows
the system to interrogate the predicates to find a matching view. The
order in which the views are evaluated depends on a pre-computed score
that is determined by the relative importance of the predicates that
it defines. I'm a bit vague on this at the moment though.

So a multi-view class would need to be defined, the view grokker
revised, and some strategy for communicating predicates (for which the
obvious choice would be martian directives).


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