[pmwiki-devel] objectizing pagelist?

[Patrick R. Michaud]

On Wed, Nov 07, 2007 at 05:26:28PM -0500, Stirling Westrup wrote:
> I've been thinking of writing a recipe to implement the YouSendIt clone I
> mentioned on the list a few months ago. In my mind, what would be ideal would
> be to have an (:attachlist...:) and a (:filelist ...:) directive that would
> use much of the same code as pagelist, and have most of the same features,
> including the fmt=... option.

The big problem with using (:pagelist:) for things like attachlist
or filelist is that pagelist is really built to work with lists of
pages (and all of the associated page properties), whereas 
attachlist/filelist would need to deal with lists of attachments
or files (and a different set of properties).  While this may sound 
like a minor difference, in terms of implementation I've found it 
to be very significant -- especially since pagelist is one of the
most intensive operations we have.

I think the biggest difference between the two is that attachments
don't have "page variables" in the same way that pages do, thus
it's very difficult to use things like {$Name} or {$Group} for
display or sorting criteria.  We'd probably need to come up with
a set of "attachment variables", and they probably should not
use the {$var} page variable syntax.  It might make sense for them
to possibly use the {$$var} template syntax, but then we'll also
be looking for things like {=$$var} and {<$$var} and that starts
to get really confusing.

The other big difference is that attachments don't have the same
access control mechanism that pages do, so much of what pagelist
is trying to do is irrelevant to an attachment context.  Even
things like pagelist caching don't seem to make much sense in
an attachment context, since we're caching completely different
types of data.

So, I really think that attachlist/filelist are fundamentally
different beasts from pagelist, and any attempt to get one routine
to handle both would be more superficial than helpful.

One possible exception to this would be if the whole of PmWiki
became very heavily object oriented, where both pages and
attachments are internally represented as objects that share 
some common interfaces.  That might make it easier to develop a
common base for pagelist/filelist, and there could be some other
advantages.  However, it's certainly not likely to happen anytime
soon, and I'm not sure that the supposed advantage would actually
be all that useful in the overall scheme of things.

Please correct me if I'm wrong, but I think the main reason
that people are so interested in trying to re-use (:pagelist:)
code for attachments is to gain the flexibility we currently
have with pagelist templates.  But that's really more a function
of the templating code than of (:pagelist:), and the templating
functions in PmWiki have been factored out into re-usable 
subroutines.  So, if we're just looking for a templatable
attachlist, we just need a driver that builds a list of attachments
according to whatever filtering/ordering criteria we want,
and then have it perform template substitutions with the
appropriate variables set.  And we need to figure out what
those variables are going to be.  :-)

Hope this helps explain why I haven't tried to unify
pagelist/attachlist in the codebase.  Ultimately I find there
are just too many differences in the underlying data components
to make a single routine have that much benefit.

Pm