Problems with Properties and Notify Signal Parts

[Richard Cole <rcole itee uq edu au>]

Dear Gtkers,

I just spent a day on a problem related to property names and notify 
signal parts. Since I solved the problem I thought I'd email it here 
incase someone else has the same problem. Maybe they'll find this email.

Let me preface my comments by saying that although I'm quite new to Gtk 
I've been quite impressed by the 2.0 code and the very serious attempt 
to produce high quality software (clean and regular interfaces, not too 
many bugs).

So onto the description of my problem.

I created a widget derived from GtkDrawingArea and added some properties 
to it as described in: The Official Gnome 2 Developers Guide by Matthias 
Warkus (nice book).

So then, and please excuse my Ruby here as I haven't the heart to type 
it out in C.

 ...
 view = LatticeView.new(diagram, attr_names, obj_names)
 view.connect_signal("notify::font_size") {
   |view, value|
   puts("Font Size updated, view=" + view.to_s + ", value=" + value.to_s)
 }
 view.set_property("font_size", 10)
 ...

But no dice! The callback attached to notify::font_size was not being 
called. Due to a trace statement in my lattice_view_property_set 
function I could see that the property was being set. So what is wrong?

Well you see somewhere in glib the property "font_size" is helpfully 
changed to "font-size". (The underscore turned into a dash). I guess so 
that if one's code says:

 view.set_property("font-size", 10)

Then rather than a warning about a non-existent property one will get 
what one intended: a change to the font size [1].

However this helpfull modification to property names isn't being 
performed for signal details. At least not in: glib-2.6.5 as packaged by 
debian. So what happens is the property name is being changed to 
font-size and the signal being emitted is "notify::font-size", but the 
modification to the signal name from "notify::font_size" to 
"notify::font-size" is not being performed.

My guess is that this situation should be fixed so that underscores are 
changed to hyphens in each of: property names, signal names, and signal 
details. At the moment it seems to be missing on signal details. But 
that is a bit of guess.

I think it would also be helpfull to put a note about signal names and 
this kind of hidden conversion that goes on behind the scenes into the 
description part of

 file:///usr/share/doc/libgtk2.0-doc/gobject/gobject-Signals.html

i.e. from whereever that file is being generated.


regards,

Richard.


[1] Going from "font_size" to "font-size" seems a bit like lisp inspired 
madness, but it turns out to be quite usefull in the following 
situation: some widget writer has defined a property called font-desc 
and you want to set it in Ruby (which has nicely automatically wrapped 
property names) then you type:

view.font_desc = Pango::FontDescription.new_from_string("Sans 12")

and font_desc gets turned into font-desc and matches the property name. 
If there was no conversion from '_' to '-' then you'd be stuck because 
font-desc isn't a ruby identifier, it is 3 of them.


PS: Contrary to some statements floating around the web, one does not 
need to call g_object_notify(gobject, "font_size"), or even 
g_object_notify(gobject, "font-size") from within the 
lattice_view_property_set function since the notify signal is 
automatically produced by the apparatus in glib. In fact looking at the 
code in gobject.c (2.6.5) it looks like you might get the font_size 
notification signal twice if you interpose a g_object_notify(gobject, 
"other_property") between you font_size notification and the end of the 
property_set function. But, well, maybe such a thing is mostly harmless.