Linking documents, XLink, XML-SVG compliance

["Ian Smith" <daraknor gmail com>]

I have been trying to figure out if Dia can be used for a specific
purpose. The purpose seems simple, create a bunch of icons for
subsystems with values attached to the icon. Link together systems so
connections between say, a web browser and a web server, can be
documented on the client and server respectively. All of these things
have been requested before, and it took my crawling through many emails
to find somewhat vague or speculative answers.

Storing values can be done using ext_attribute tags, which I did not
find documented anywhere and only briefly mentioned in email. I would
never have found this at all without looking at the CVS, only one shape
uses this feature (I grepped the shapes) and found it implemented with
the original patch and settings (Circuits/hresistor.shape). This was a
great find. I mention this because the rest of my requests may have
been implemented but not documented.

Linking diagrams together, and providing external references. There are
several ways to accomplish this, most have been discussed on the
mailing list previously:
1. Create a shape that serves to connect documents. It would store a
filename in a field, and some context or property item would trigger
dia to open the document. It would be nice if we could use the link to
provide information on how to open the document (focused on the
corresponding link in the other diagram as an SVG anchor for example).

2. Create a shape or property to trigger external program execution,
like the suggestion for $BROWSER $URL, but leave an option to use $PATH
$DIA $FILE as well ($PATH for supporting windows users of dia, perhaps
$DIA would include a full path instead). This would obviously negate
the ability to use anchors unless Dia would support that functionality
from the commandline.

3. (Recommended) Implement more of the SVG standard, which includes
links via xlink. http://www.w3.org/TR/2000/CR-SVG-20001102/linking.html
It would be preferrential for several reasons, one of them being SVG
View Specification like: MyDrawing.svg#svgView(viewBox(0,200,1000,1000))
. XLink has a great many attributes and several modes, but I can see a
very clear usage for links to "resources", embedding data from other
XML documents, etc. http://www.w3.org/TR/2000/CR-xlink-20000703/ The
previously mentioned hack with ext_attribute could be replaced by a
fully XML compliant and addressable mechanism. One XML file (or
database with an XML wrapper) could contain all of my values, fetched
from xlinks in my dia document. If dia follows this route, Firefox 1.5+
will be able to follow href links automatically after using its new SVG
support.

I need some way to have hierarchical SVG diagrams, Dia could easily be
that tool even with a bad code. I am concerned that my code would be
too bad, implemented as C, to be included. I made some Python
extensions as a test, using menu callback from LarsClausen TWiki (which
is very much out of date) and some of the included examples. I looked
through the existing shapes with custom properties, all of them are in
C that I looked at. Is there a way to write Python Shapes? Even a stub
would be a great help.

All of this may be working right now, but I wouldn't know about it
without documentation. I saw the request for ext_attribute tags about 6
times looking through the mailing list, aside from 3 people outside the
dev team I don't think anyone is aware that it is still included and
working in 0.94. 

If linking doesn't work out, I'm going to need to use raw SVG to make
these diagrams :/ Seems counterproductive considering this seems like
such a common request, and hopefully won't be much work to include an
additional part of the SVG 1.0 spec.

Ian Smith

P.S. it would be nice if the shape XML Namespace link worked - so we could do DTD validation.