[Zope-dev] RE: [Zope] ZPatterns Question

[RC Compaan]

> At 01:57 PM 8/8/00 +0200, RC Compaan wrote:
> >I've added a propertysheet called "properties" to my ZClass and i notice
> >there is Persistent Sheetprovider under the default rack already.  The
> >Sheetprovider has properties Sheet_Names and Sheet_Namespaces.  I guess
> >Sheet_Names should refer to the sheetname i created for my
> ZClass??? but how
> >do Sheet_Namespaces come into play.
>
> Actually, neither relates.  Property sheets created on ZClasses
> always have
> their data stored in attributes of the object itself, so if you want to
> control those property sheets you would not use a sheet provider at all.
> Sheet providers are only used to provide property sheets whose data is
> stored external to the object.
>

Oh ok, this divide clears it up a bit.  I somehow imagined that an object's
properties are always stored inside the object and that Providers are the
managers of those properties.

> The Specialist does not talk to the sheet provider.  The Specialist asks
> its racks for an object.  The Rack which "retrieves" the object tells the
> object that it belongs to the rack and should ask the rack for anything it
> needs.  When the object needs a property sheet or attribute or
> whatever, it
> asks the rack to give it a list of relevant providers for that
> attribute/sheet/whatever.  It then walks the list asking those
> providers if
> they can perform the function it desires.  If none of the providers are
> successful, it performs a default behavior (such as raising an
> AttributeError to indicate the attribute does not exist).  This general
> pattern is followed for almost anything that can be delegated from a skin
> to a provider.

I considered the scenario where attributes are stored external to the object
according to the filing clerk analogy on the wiki:

The Specialist (filing clerk) asks its racks (filing cabinet) for an object
(dental records).  The rack (filing cabinet) tells the object (the dental
records) that it belongs to the rack AND that the object should ask the rack
for anything it needs.

Why would the object "need" anything.  Is it not the specialist with the
"need"?

If object data is stored external to the dataskin object does the dataskin
only implement behaviour?  I find it difficult to understand because I
associate Dataskin with Storage.  If object data is stored externally is it
stored in Persistent Property Sheets?  How does one define properties for
Persistent Property Sheets?

Roché