Problems with Properties and Notify Signal Parts

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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.



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