$app-add_show_handler (>
    sub {
        $app->draw_rect( [ 0, 0, $app->w, $app->h ], 0x000000FF );
        $app->draw_rect( $ball->{rect}, 0xFF0000FF );
        $app->draw_rect( $player1->{paddle}, 0xFF0000FF );
        $app->draw_rect( $player2->{paddle}, 0xFF0000FF );
        $app->update;
      }
    );