Re: [Sax-devel] SAX-ext proposal #4: feature/property discovery

[David B. <da...@pa...>]

> How would this work for filters?  They'd have to pass the request on
> to any ancestor filters, ...

It seems to me that touches on a general issue with the notion of
exposing properties (features too) of complex objects:  delegation
models don't necessarily work appropriately, except for the very
simplest types of global properties.

Expecting delegation to work in all cases is the object-oriented
analogue of saying that since it's a black car, it must accordingly
have black controls labeled in black on a black background ... :)


> It would also be dangerous for applications, because a filter might
> want to change its parent dynamically during runtime, and the features
> or properties supported by the parent might be different.

The simple solution to that is to say that filters shouldn't generally
delegate _everything_ to their "parent" reader/filter.  Components
of an XMLFilter chain might have different values for any of those
features/properties.


> This is doable, but I'm not sure that the dubious benefits outweigh
> the costs, especially for the filter writers.

Well, I wouldn't encourage that particular implementation for filters,
that's for sure!

That said -- even though the Xerces team has this functionality
in their parser, I think good documentation will be just as good
a solution to the problem in most cases.

- Dave