Re: event callback order

James Muir said:
Based on this output I'd say that the driver program is handling the
event before the object that it instantiates, but I'd really like the
MapItem to have first dibs. Any insights and help appreciated.

That's the way that run-last signals work.  GnomeCanvasItem::event happens to
be a run-last signal:

  $ perl -MData::Dumper -MGnome2::Canvas -e 'print "".Glib::Type->list_signals
  [ run-last ]

This is part of how GObject signal emission works; it's described in fairly
useful detail in the tutorial portion of the GObject manual:

(That used to be Matthieu Lacage's GObject tutorial, which was merged this
year into the GObject docs.)

There's a more terse description in the reference portion of the manual :

Events wind up being run-last signals because they use a return value (the "is
the event handled?" flag).

On the other hand, if you're creating your own signals, you get to choose. 
Here's a primitive illustration of how those things work:

  package Thing;
  use strict;
  use Glib qw(:constants);
  use Glib::Object::Subclass
      signals => {
          poked => {
              flags => 'run-first',
              class_closure => sub { print "poked: class closure\n" },
          prodded => {
              flags => 'run-last',
              class_closure => sub { print "prodded: class closure\n" },

  sub poke { $_[0]->signal_emit ('poked') }
  sub prod { $_[0]->signal_emit ('prodded') }

  package main;

  my $thing = new Thing;

  $thing->signal_connect (poked => sub { print "poked: user callback 1\n"; });
  $thing->signal_connect (poked => sub { print "poked: user callback 2\n"; });

  $thing->signal_connect (prodded => sub { print "prodded: user callback 1\n";
  $thing->signal_connect (prodded => sub { print "prodded: user callback 2\n";


The output is:

  $ perl
  poked: class closure
  poked: user callback 1
  poked: user callback 2
  prodded: user callback 1
  prodded: user callback 2
  prodded: class closure

User callbacks are executed in the order they were connected, and the signal
flags determine when the class closure runs.

muppet <scott at asofyet dot org>

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