Re: size_request/size_allocate question

[muppet <scott asofyet org>]

On Oct 14, 2005, at 5:52 AM, Jaap Karssenberg wrote:

> I try to subclass Gtk2::HBox and I want to handle size allocation  
> myself. I figured overloading size_request() and size_allocation()  
> would  do the trick, but these methods don't get called when the  
> widget is shown :( I've included my code below, does anyone have a  
> suggestion what I do wrong ?

That's not how you override signals.  This is another case of the  
GObject-as-a-perl-object abstraction leaking some of the GObject  
details.

size-request and size-allocate are widget signals.  The methods  
size_request() and size_allocate() emit the corresponding signals,  
and the real work happens in the class closure for the signal handler.

What you have done is provide a new implementation of the methods  
that will be seen by perl when it searches for the method by $widget- 
>size_request() by looking at the actual type of $widget and  
searching @ISA.  The problem is that for any *interesting* invocation  
of this method, it will be called from the C function  
gtk_widget_size_request(), deep within gtk+ --- that is, control  
never gets to the perl method lookup!  You have to ensure that these  
things are invoked by C code, which means that normal perl OO stuff  
doesn't work quite right.

The way to do this, then, is to override the "class closure" for the  
signal.

See also: http://gtk2-perl.sourceforge.net/doc/ 
subclassing_widgets_in_perl.html (which explains the above situation)  
and http://gtk2-perl.sourceforge.net/doc/examples/ 
histogramplot.pl.html (which has a size-request override).


Also, as you'd written it, the constructor was completely broken.   
SUPER::new would find Gtk2::HBox::new(), which unconditionally calls  
gtk_hbox_new(), giving you an actual GtkHBox blessed as a  
Gtk2::HBox.  Your own methods would not be found, and your new  
subclass would not be created (the GType is wrong).  The correct way  
to instantiate GObject subclasses is to use Glib::Object::new();  
Glib::Object::Subclass will alias this for you, so you don't need to  
provide a constructor.

Changes to your code:
- rename size_request() to _size_request()
- rename size_allocate() to _size_allocate()
- change SUPER::size_request and SUPER::size_allocate to  
signal_chain_from_overridden.
- add a signal map to the Subclass pragma to tell your new GType to  
invoke your class closures
- remove new()
- if necessary, do instance initialization (defaults, etc) in  
INIT_INSTANCE().


> package Gtk2::Ex::ButtonPathBar;
>
> our $VERSION = '0.01';
>
> use strict;
> use Gtk2;

use Glib::Object::Subclass
    Glib::HBox::,
    signals => {
        # provide overrides for widget signals
        size_allocate => \&_size_allocate,
        size_request => \&_size_request,
    },
    ;

# use a different name to avoid clashes and the resulting infinite loops
sub _size_request {
    my $self = shift;
    print "Got a size request\n";
    # SUPER doesn't work for signals.
    $self->signal_chain_from_overridden (@_);
}

#again, a different name
sub _size_allocate {
>     my ($self, $allocation) = @_;
>     my ($w, $h) = ($allocation->width, $allocation->height);
>     print "got allocation for $w, $h\n";
    $self->signal_chain_from_overridden ($allocation);
}

> 1;
> # End Code listing


--
elysse (pregnant): are your hands cold?
me: uh, i suppose so.
elysse: will you put them on me?