#!/usr/bin/perl -w

$usage = "usage: $0 pdf-file";
@ARGV || die $usage;


## User variables.

$verbose = 1;			# Set to non-zero for feedback.
$delay = 0.10;			# delay time, in seconds, between polling
				# to see if pdf has changed.

######################################################################
$remote = "wxpdf_$$";		# create a unique remote name for xpdf.
$pdf = shift;			# get name of pdf from the command line.
$pid = $$;			# Get the current process i.d.
$cmd =  "xpdf -remote $remote $pdf"; # cmd to start xpdf
$reload = "xpdf -remote $remote -reload"; # cmd to reload pdf


## FORK code takne from Programming Perl, p167.
 FORK: {
     if ($pid = fork) {
	 ## This is the parent.
	 $verbose && print "I am the parent; child has pid $pid.";

	 # get last mod time of pdf; this is needed by atchange below.
	 $old = (stat($pdf))[9]; 
	 
	 while (1) {
	     select(undef, undef, undef, $delay); # wait some delay.


	     if (&atchange($pdf)) {
		 $verbose && print "reload\n";
		 system($reload);
	     }
	 }
     }
     elsif (defined $pid) {
	 ## This is the child.
	 ## $pid will be zero here if defined.
	 system("$cmd");	# start viewer.

	 ## If we get this far in the program, it means that the viewer
	 ## has been closed, so the child can kill the parent to stop
	 ## it keeping in watch.
	 $parent_pid = getppid();
	 $verbose && print "Child ending... parent $parent_pid\n";
	 if ($parent_pid > 1) {
	     kill 9, $parent_pid;
	 }
	 
	 exit;
     } else {
	 # weird fork error
	 die "Can't fork: $!\n";
     }
}



## This taken from the atchange program, see below.
sub atchange {		# if $file has changed, do $cmd{$file}
	my($file) = @_;
	my($new);

	$new = (stat($file))[9];
	return 0 if ($old == $new);
	while (1) {			# wait until it stops changing
		$old = $new;
		sleep 1;
		$new = (stat($file))[9];
		if ($old == $new) {
			return 1;
		}
	}
}

__END__

=head1 NAME

wxpdf - watch a pdf file

=head1 SYNOPSIS

wxpdf <file.pdf>

=head1 DESCRIPTION

Run the xpdf viewer on file.pdf, but provide a watchfile facility.  If
the pdf file changes, then xpdf automatically reloads the pdf.
(file.pdf is checked every `delay' seconds, where delay is set at the
top of the script.)

If xpdf viewer is killed by the user, this program finishes watching
the file.


Some of the code for watching for file changes comes from the
excellent utility atchange, http://www.lecb.ncifcrf.gov/~toms/atchange.html


=head1 OPTIONS

=head1 ENVIRONMENT

=head1 EXAMPLES

wxpdf main.pdf


=head1 FILES

=head1 AUTHOR

Stephen Eglen <stephen@gnu.org>
GPL.

=head1 SEE ALSO

=head1 DIAGNOSTICS

=head1 BUGS

=cut