Waiting for Events to Happen (Mastering Perl/Tk)

13.21.1. File Events

There is a special method in Perl/Tk called fileevent, which watches and notifies you when a file is readable or writable. For complete details see Chapter 15, "Anatomy of the MainLoop", Chapter 19, "Interprocess Communicationwith Pipes and Sockets", and Chapter 22, "Perl/Tk and the Web".

Here is an example that shows how fileevent can be used (this code is meant to be executed on a Unix system because we use the Unix tail command):[31]

[31] Thanks to Phivu Nguyen for sharing this code.

use Tk;
open (FH, "tail -f -n 25 text_file|") || die "Could not open file!\n";
my $mw = MainWindow->new;
my $text = $mw->Scrolled("Text", 
                         -width => 80, 
                         -height => 25)->pack(-expand => 1);
$mw->fileevent(FH, 'readable', [\&insert_text]);
MainLoop;

sub insert_text{
  my $curline;
  if ($curline = <FH>)   {
    $text->insert('end', $curline);
    $text->yview('moveto', 100);
  } else {
    $mw->fileevent(FH, 'readable', "");
  }
}

This short program sits around and waits until a file is readable and then inserts the newly read information into a Listbox. You can also use 'writable'.

$mw->fileevent(FH, 'writable', callback);

If you omit the callback portion, the callback will be returned. Replace the callback with an empty string ("") and the callback is removed.

13.21. Waiting for Events to Happen

13.21.1. File Events