Re: Named connection points?

[Hans Breuer <hans breuer org>]

At 16.02.2012 16:07, Dan Katz wrote:
> Hi -
>
> I'm doing a bunch of work on yet another Python diagram->code
> generator plugin, and several times now I've found myself wishing that
> connection points (and handles, to a lesser extent) had names that
> were assignable in the Custom Shape Module shape files and accessible
> through the Python API.
For the use case at hand this would be names on the type level, not 
changeable per object. But I don't see why you could not use named 
constants for the connection point index instead.
The connection points are parsed in the order in the shape (see 
objects/custom/shape_info.c) and put into the connection point array in 
that order.
In any case (either with index or names) you have to ensure that your 
script and your shapes are matching.

> I actually got so far as beginning to hack
> the C code to add this capability when I found that Dia actually used
> to have named connection points and that they were removed in 2009
> (https://mail.gnome.org/archives/commits-list/2009-August/msg00148.html).
>   I guess I have two questions:  could we bring back named connection
> points,
Not without a convincing use case. As my removal comment says they were 
unused and just wasting memory ( 
http://git.gnome.org/browse/dia/commit/?id=a8193caff ) And it even was per 
object, not per type.

> and if not, then how do people usually deal with the issues
> I'm having.
Just rely on the stable numbering of connection points and handles. If it 
would be broken, than even the standard loading and saving of diagrams 
would break.

> A bit of context:  I'm writing a Python plug-in for Dia which
> translates a data-flow diagram to code in a home-grown legacy
> job-control language.  The plugin is exposed as an export filter (like
> codegen.py and siblings) [...]
Where are the siblings of codegen.py?

> The annoyance comes when I want to differentiate among the connection
> points in the back-end pass of the Python code generator.  I'd like to
> be able to name the connection points as part of the custom object
> definition (e.g. 'minuend', 'subtrahend', 'difference') and then  then
> get them by name in the Python API.  In my imagination, the change in
> the shape file would look something like this (a small syntactic
> extension to what's there today):
>
>     <connection>
>         <point x="2" y="3" name="minuend"/>
>         <point x="2" y="5" name="subtrahend"/>
>         <point x="6" y="4" name="difference"/>
>     <connections>
>
> (where the name attribute is optional, of course, defaulting to the
> empty string) and would then allow code like
>
>     foo = [c for c in differencer.connections if c.name() == 'minuend']
You could spare the loop and the string compare by just using named 
constants ...

> which is rather cleaner than what I'm doing now which involves things
> like getting the list of connections and sorting the connections by
> y-coordinate to find the 'top' one and knowing that 'top'='minuend',
> or sorting by x-coordinate and knowing that the right-most connection
> point is the output.  (In fact, is that sort of logic even reliable in
> the face of actions like "Flip horizontal"?I haven't checked that…)
>
No, it is not. But the connection point index will stay stable.

HTH,
        Hans

-------- Hans "at" Breuer "dot" Org -----------
Tell me what you need, and I'll tell you how to
get along without it.                -- Dilbert