Experienced Monks!

I have a command-line app that parses commands and prints the output to the terminal. In search of new features, I'm migrating the terminal library from readline to tickit.

To simplify this changeover, my plan is to use open and select to direct the default output filehandle to a $variable, and then periodically dump the contents of $variable to a tickit widget.

I'm close, but my naive implementation loses every other line of output. Seems like I need to away to ensure that writes to $command_output are held up while printing and then deleting the contained text. Some kind of first-in-last-out buffer with synchronization.

Will be grateful for any hints on an easy way to accomplish this. Here is sample code demonstrating my conundrum: 

#!/usr/bin/env perl
use v5.36;
=comment

vbox - root
    scrollbox 
        vbox 
           static
           static
           ...
    entry

=cut
use Tickit::Async;
use Tickit::Widgets qw(ScrollBox Static VBox Entry);
use IO::Async::Loop;
use IO::Async::Timer::Periodic;
use IO::Async::Timer::Countdown;
use IO::Async::Loop::Select;
STDOUT->autoflush;
my $text = {};

$text->{loop} = IO::Async::Loop->new;

my $root =         Tickit::Widget::VBox->new; 
my $vbox =         Tickit::Widget::VBox->new; # contains multiple item
+s to scroll through
my $scrollbox = Tickit::Widget::ScrollBox->new->set_child( $vbox );
for (1..100){
my $a = 100 - $_;
$vbox->add( Tickit::Widget::Static->new( text   => "a hundred bottles 
+minus $_ is $a \n" ))
}

my $tickit = Tickit::Async->new( root => $root);
my $term = $tickit->term;
my $lines = $term->lines;

 
$root->add($scrollbox, force_size => $lines - 1); # , expand => 1);
my $entry =     Tickit::Widget::Entry->new( 
    text      => "enter command > ",
    on_enter => sub {
          my ( $self, $line ) = @_;
        print_to_terminal($line);
        $scrollbox->scroll_to(1e5);
        $line =~ s/^.+?>\s*//;
        $self->set_text('');
        my $prompt = 'enter command > ';
        $self->set_text($prompt);
        $self->set_position(99);
    }
    );
my $prompt = 'enter command > ';
$entry->set_text($prompt);
$entry->set_position(99);
$root->add($entry);
my $i;
redirect_stdout();
timer(0.5,0.5, sub{ ++$i; say(join'',$i,'-','X'x40)});
#timer(2,1, sub{ print( join '',1..40)  });

$tickit->run;

sub prompt {
    my $prompt = 'enter command > ';
    $entry->set_text($prompt);
    $entry->set_position(99);
}
our ($command_output, $output_fh, $old_output_fh);
sub redirect_stdout {
    open($output_fh, '>', \$command_output) or die; 
    $output_fh->autoflush;
    $old_output_fh = select $output_fh;
    my $j;
    timer(1, 1, sub{
        return unless $command_output;
        print_to_terminal($command_output); 
        $command_output = "" 
    });
}
sub restore_stdout {
    select $old_output_fh;
    close $output_fh;
}
sub print_to_terminal ($txt) {
    $vbox->add( Tickit::Widget::Static->new( text => $txt ));
    $scrollbox->scroll_to(1e5);
}
sub timer ($delay, $interval, $coderef ) {
    my $timer;
    if ($interval == 0){
        $timer = IO::Async::Timer::Countdown->new(
            delay => $delay,
            on_expire => $coderef, 
        );
    }
    else {
        $timer = IO::Async::Timer::Periodic->new(
            interval => $interval,
            on_tick => $coderef,
        );
    }
    $timer->start;
    $text->{loop}->add($timer);
    $timer
}

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In reply to Race when redirecting output. by gnosti

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