Re: Options for making AT schemas more adaptive

Martin Aspeli wrote:
> Hi guys,
> 
> Whit made a very important observation the "story for re-using content 
> types" thread that (a) logic should be in adapters outside the content 
> type and (b) without that, the content type is just the setup code, the 
> CMF boilerplate and a schema. You probably don't mind creating a new FTI 
> and deriving from BaseObject and doing the installTypes() in Install.py 
> if you're trying to re-use a content type and you still want a *new* 
> content type. It's trickier, however, having to duplicate or override 
> the schema, especially if that schema is involved. And, if all you 
> wanted was to change some behaviour in that schema, creating a whole new 
> content type is a pain.
> 
> Whit noted:
> 
> """
> I would say the major step to make here is making BaseObject.Schema use 
> interfaces to return the schema.
> second step is to remove the schema as a tree of persistent object 
> inside the content (unless instance local
> configuration of your schema is part of your usecase). where it come 
> from, the interface doesn't care.
> """
> 
> I'll extrapolate two real use cases from that:
> 
>  - I want to write an adapter that provides a somewhat different schema 
> (or part of a schema) for an existing content object without registering 
> a brand new content object.
> 
>  - I want to inject something into every schema, for example some 
> site-wide required metadata. Take for example PloneOntology, which 
> requires you to choose some keywords when you save a content object. 
> Currently, that either requires a separate object tab, which sucks 
> because people don't necessarily click on it, or a monkey-patch to 
> BaseSchema (ouch), or a custom field/partial schema that every content 
> type needs to explicitly include.
> 
> Both of these directly relate to things I really want to enable with 
> Plone 3.0: (1) generic UI mechanisms for things like PloneOntology or a 
> tagging system or anything else that needs to apply site-wide or generic 
> metadata and (2) improved re-use of existing content types (my main 
> gripe is all the people who want PoiIssues to have a RichWidget instead 
> of a TextWidget with intelligenttext).
> 
> I'd like to discuss some general design ideas for how we can achieve 
> this without having to re-write all of Archetypes, but obviously in 
> keeping with the evoluationary path of making AT more re-usable and less 
> monolithic.
> 
> Two potential caveats:
> 
>  - there may be several components installed that want to, say, inject 
> metadata into every schema for different purposes (e.g. you have a 
> company-specific metadata provider + a tagging solution installed)
> 
>  - only when a product is actually installed should these things apply
> 
> I'm going to think about this in more detail too, of course, but some 
> specific design outlines would go a long way towards letting me write a 
> PLIP for this.

the first step in this process, i'd say, would be to define 
ISchemaConsumer and ISchemaProvider interfaces.  BaseObject, and 
therefore all derived types, would implement ISchemaConsumer.  the 
BaseObject Schema() method would call ISchemaProvider(self) to retrieve 
an adapter that would actually return the schema object.

the default implementation of this adapter would do exactly what it does 
now, simply accessing self.schema.  we'll want to deprecate direct 
schema attribute access... not quite sure how to achieve that; where do 
we put the deprecation message?

anyway, now we've got more flexibility.  adapters from more specific 
interfaces to ISchemaProvider can be implemented that will override the 
schema retrieval behaviour.  ATSENG support can then be gained via a 
marker interface, for instance, instead of the mix-in currently 
required.  also, the default adapter could be easily overridden if you 
want to change global schema behaviour.  we can also require that 
adapters to ISchemaProvider fire events to allow for subscribers to 
impact the process.

-r



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