Re: Hacking on Gnome2::Canvas

[muppet <scott asofyet org>]

On Thursday, June 5, 2003, at 12:46 AM, Dov Grobgeld wrote:

> I started hacking on implementing the Gnome2::Canvas tonight and
> I made enough progress so that the following snippet of code works:

eeek!  i started working on this, too.  i got functional bindings in 
about two hours on monday, and spent several more hours porting 
portions of the canvas demo app.  i was about to commit it to cvs but 
don't like the fact that it's currently done as part of Gnome2 --- 
since the canvas itself can be used without the gnome libraries, i'd 
like to have the bindings be usable without gnome as well.



>     my $item = Gnome2::Canvas::Item->new($root,
>                                      "GnomeCanvasEllipse",
>                                      x1 => 50,
>                                      y1 => 70,
>                                      x2 => 200,
>                                      y2 => 180,
>                                      fill_color=>"red",
>                                      outline_color=>"black"
>                                     );
>     $item->signal_connect(event=>sub {print "I got an event!\n"; });
>
> (Note that I choose to use a string argument for providing the type of
> the canvas item as opposed to using the 
> Gnome2::Canvas::Ellipse->get_type()
> which in my opinion is ugglier. The cost is another string lookup
> within Glib. If needed it should be pretty easy to support both 
> syntaxes,
> and perhaps then we can predeclare GNOME_CANVAS_ELLIPSE_TYPE etc const
> subs for speed. (The string is the GLib registered type, btw)).

since these types are actual, registered GTypes, they should get actual 
perl package names and be referred to as such, e.g., you'd pass 
'Gnome2::Canvas::Ellipse' to Gnome2::Canvas::Item->new.

it is a design decision of the bindings that the type id concept shall 
be completely replaced by perl package names.


> But, I eventully got stuck on type mapping for the GnomeCanvasLine.
> One of its properties is "points" which take an argument of type
> GnomeCanvasPoints. The latter should be mapped from a perl list,
> but how do I do that? Where do I define the type conversion from
> a perl ARRAY-ref to the GnomeCanvasPoint structure?

there's not a clean way to do this, unfortunately, in the current 
object representation.  i looked at this for quite some time the other 
day, and decided eventually that something like

$points = Gnome2::Canvas::Points->new (@coords);
@coords = $points->coords;

was the best solution under the current circumstances.  the problem is 
that the code which will extract the C object from the perl wrapper is 
called deep within the library (gperl_value_from_sv, actually) during 
the call to g_object_set, so normal binding tricks don't work.

i've been talking offlist with pcg about possible changes to the object 
representation; if done right, that could provide a way to solve this 
perlishly.


> I get the very informative ;-) error message:
>
>     internal nastiness: boxed wrapper contains NULL pointer at 
> ./test-canvas line 37

basically, the autogeneration code registered standard code to handle 
the registered boxed type.  you passed an array ref, which is 
essentially an RV pointing to an AV, to the function which extracts the 
boxed type from a wrapper which is an RV pointing to an IV, whose value 
is the pointer to a C structure describing the C object.  since the RV 
didn't point to an IV, the value was 0, and that's how you got a NULL 
inside a boxed wrapper.  it actually should've complained when it 
discovered that the wrapper wasn't derived from the proper class....


> And finally, may I please have cvs write access btw so that I can put
> back my work there?

talk to me off-list so we can combine our code and decide where to put 
it.