First a little bit of setup. I am creating a simple image viewer, designed to be easily controlled via external programs. It waits on STDIN, and each time it receives a line of input, it attempts to use that as a filename. The viewer window will not size itself past set limits in either dimension, so if the image is too big it produces a set of scrollbars. This all works fine.

My problem comes when I attempt to modify the position of the scrollbars. I would like to center the scrollbars, so the middle portion of the image is viewable. Here's a bad ASCII diagram in case this is unclear:

....................
:                  :
:   ------------   :
:   | viewable |   :
:   | portion  |   :
:   ------------   :
:                  :
....................

[download]

The dotted lines are the image extents, and the viewable portion is what can be seen in the ScrolledWindow. The scrollbars should be adjusted so the viewable portion is cenetered on the image extents.

It seems like it should be a simple case of creating the appropriate Gtk2::Adjustment objects, and calling the set_hadjustment and set_vadjustment methods on my Gtk2::ScrolledWindow object. This does not change my scrollbars, however. If I check the values on the newly set objects, they contain what I put in them, yet the scrollbars are still unchanged. I thought, "perhaps the signal is not getting emitted properly," so then I tried adding Glib::Object->signal_emit($adjustment, "changed"); This produces a segfault.

At this point, I'm not sure what's going on. Am I setting the adjustments on the wrong object? I've been trying on my ScrolledWindow object, but there's also a Gtk2::Viewport object inside that. Should I be setting the adjustments on the Viewport object? That seems to have the same effect (i.e. none). I'm just running out of ideas. One thing to note is someone on IRC reported that the code works fine. Perhaps there's simply something wrong with my Gtk2-Perl install, or my Gtk2 library in general?

Here's the code:

#!/usr/bin/perl
use strict;
use warnings;
use Gtk2 '-init';
use Gtk2::Helper;

my @landscape = qw(800 600);
my @portrait  = qw(600 800);

# Copyright (C) 2004  Carey Tilden <revdiablo@wd39.com>

# This code is dual licensed. You may choose from one of the following
+:
#  - http://creativecommons.org/licenses/by/1.0/
#  - http://d.revinc.org/pages/license

my ($mw, $sw) = create_widgets();
my $vp;
Gtk2::Helper->add_watch(fileno(STDIN), 'in', \&load_image);
Gtk2->main;

sub load_image {
    exit if eof STDIN;

    my $file = <STDIN>;
    chomp $file;

    $sw->remove($vp) if defined $vp;

    my $img = Gtk2::Image->new_from_file($file);

    my $pb = $img->get_pixbuf;
    my ($x, $y) = ($pb->get_width, $pb->get_height);

    $vp = Gtk2::Viewport->new(undef, undef);
    $vp->add($img);

    $sw->add($vp);

    if ($x > $y) {
        $mw->resize($x > $landscape[0] ? $landscape[0] : $x + 2,
                    $y > $landscape[1] ? $landscape[1] : $y + 2);
    } else {
        $mw->resize($x > $portrait[0] ? $portrait[0] : $x + 2,
                    $y > $portrait[1] ? $portrait[1] : $y + 2);
    }

    # all these values are the same as default, except the first. thes
+e
    # values are just for testing. the eventual plan is to adjust them
    # so the viewport is centered, but even with static values it
    # doesn't work.
    my $ha = Gtk2::Adjustment->new(50, 0, 1, 0.1, 0.9, 1);
    my $va = Gtk2::Adjustment->new(50, 0, 1, 0.1, 0.9, 1);

    $sw->set_hadjustment($ha);
    $sw->set_vadjustment($va);

    $mw->set_title("$file ${x}x${y}");
    $mw->show_all();

    return 1;
}

sub create_widgets {
    my $sw = Gtk2::ScrolledWindow->new(undef, undef);
    $sw->set_policy('automatic','automatic');

    my $mw = Gtk2::Window->new;
    $mw->add($sw);

    return ($mw, $sw);
}

[download]

Since this thing was designed to be controlled by an external program, using it might seem a little funky. Filenames come in via STDIN, and once STDIN is closed, the program exits. Here's an easy way to test it:

    perl -le "$|++; print and sleep 5 for qw(file1.jpg file2.jpg)" | perl imageviewer.pl

In reply to setting scrollbar position of a Gtk2 ScrolledWindow by revdiablo

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