Pier/Magritte Dynamic Descriptions

> Recently I was trying to puzzle out how to provide some descriptions on some
> objects that changed based on other descriptions.
>
> A concrete example of this is: a Flickr photo, with two descriptions:
> - photo ID
> - which JPG size to use
>
> The second description depends on the first -- one can't retrieve the
> available sizes to choose from without first getting the photo ID.
>
> So I did a bit of Googling around, and came across an On Smalltalk [1]
> article, where it mentioned something about this:
>
>> Generally speaking you'll keep these descriptions on the class side of
>> your domain object. On occasion, you'll want some property of the
>> description to be dynamic and depend on the actual value of an instance
>> side value, so you'll move the description to the instance side.
>> Strictly speaking, it doesn't matter where you put them, sometimes
>> you'll use them to build forms when you don't even have a domain object,
>> but this isn't the normal case.
>
> Then I did some further looking, and found some tutorial [2] that actually
> gives some information on doing this (without Pier).  It seems like in
> general (based on this tutorial) you implement #descriptionEdit, and then
> grab that when handling the editing function in whatever UI you choose.
>
> That said, I've been struggling to figure out how I can do this with a
> PRWidget in Pier.  There seems to be some way of doing it with a
> PRStructure, as PRFile provides editing capabilities on a text-based file by
> overriding #editDescription.  But I'm not sure how it would work with a
> PRWidget, or if it is even possible.
>
> Basically, I would envision it as follows in the UI, continuing with the
> Flickr photo example:
> - Create a new photo widget
> - Edit the photo widget -- the size property either does not show or is not
> populated
> - Input a photo ID
> - Either submit the form, or have something realize it has what it needs to
> populate the next field and transmit the data without a page refresh
> - Select the photo size, and submit
>
> Is there any way of accomplishing this with the existing support for
> Magritte in Pier?  Or is this something that could be (or should be) hacked
> into PRWidget?  Or am I going about this in a completely wrong way?
>
> Any advice would be much appreciated.
>
> Thanks,
> Joel
>
> [1] http://onsmalltalk.com/using-magritte-with-seaside
> [2]
> http://www.hpi.uni-potsdam.de/hirschfeld/seaside/tutorial?chapter=10#part4
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