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Ever get stuck in a console based app, where you need a nested while(1) loop to sit in a holding pattern, while you watch variables, or filehandles? It is handy to have a Event loop in these cases. You can start the loop anytime in your script, and bust out of it later, to continue linear program flow.

The Gtk2 GUI system, is built upon a non-gui base called Glib. For unix-like systems, you first need the c-libs installed (Cairo, Glib, Pango, Atk, Gtk2) available at gtk+ Most linux distributions come with these preinstalled, so you can run mozilla, gimp, etc. Once you have the c libs installed, you get the Perl interfaces to them, at Perl/Gtk2 . There are win32 ports at Gtk2-win32

You can use the Glib::MainLoop in console based programs to watch filehandles, run multiple timers( which can do just about anything ), and probably other things which your creativity can come up with. This is the console based version of the MainLoop, which Tk and Gtk2 programmers are familiar with.

After you launch the program, start another to write to the test.log file, and see the io_watch monitor the filehandle. Like

echo 'yadda yadda' >>test.log

#!/usr/bin/perl
use warnings;
use strict;
use Glib;

# Glib provides a few ready-made event  sources, 
#  Glib::Timeout, Glib::Idle, and Glib::IO->add_watch
#  read perldoc Glib::MainLoop

my $main_loop = Glib::MainLoop->new;

my $count = 1;
my $timer  = Glib::Timeout->add (1000, \&timer_callback, undef, 1 );

#my $timer  = Glib::Timeout->add ($interval, $callback, $data=undef, $
+pri-
#      ority=G_PRIORITY_DEFAULT)

#    * $interval (integer) number of milliseconds
#    * $callback (subroutine)
#    * $data (scalar)
#    * $priority (integer)

#    Run $callback every $interval milliseconds until $callback return
+s
#    false.  Returns a source id which may be used with
#   "Glib::Source->remove".  Note that a mainloop must be active for
#    the timeout to execute.

sub timer_callback{
     $count++;
     print "$count\n";
    return 1;
}

my $count1 = 1;
my $timer1  = Glib::Timeout->add (100, \&timer1_callback, undef, 1 );

sub timer1_callback{
     $count1++;
     print "\t$count1\n";
     return 1;
}


###  filehandle watch
open (FH, "+> test.log") or warn "$!\n";

Glib::IO->add_watch (fileno 'FH', ['in'], \&watch_callback, 'FH', 1 );

# integer = Glib::IO->add_watch ($fd, $condition, $callback, $data=und
+ef, $priority=G_PRIORITY_DEFAULT)
#           * $fd (Glib::IO) file number, e.g. fileno($filehandle)
#           * $condition (Glib::IOCondition)
#           * $callback (subroutine)
#           * $data (scalar)
#           * $priority (integer)


$main_loop->run;
####################################################################
sub watch_callback {
    my ($fd, $condition, $fh) = @_;
    my @lines = <FH>;
    print @lines;
    #always return TRUE to continue the callback 
    return 1;
}
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In reply to Roll your own Event-loop by zentara

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