Tk, LWP, and blocking?

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Hi.

I'm trying to write a web client that uses LWP and Tk. Unfortunately, calls to get() keep blocking and freezing-up the program for often-significant amounts of time while LWP sends the request and receives response. Is there some way to get around this besides using Thread.pm or threads.pm? I tried update(), but that didn't seem to work. What I really need is way to call get() (or request()) and get access to the response object without the windows freezing and becoming unresponse for a while. Here's some example code:

#!/usr/bin/perl -w
use strict;
use Tk;
use Tk::Pane;
use LWP;

use vars qw($URL);
use vars qw($DOC_VIEWER);
use vars qw($DOC);

my $lwp = new LWP::UserAgent;

my $browser = new MainWindow;

my $url_entry_frame = $browser->Frame->pack(-side => 'top', -padx => 3
+, -pady => 4);
$url_entry_frame->Entry(
    -textvariable => \$URL, -width => 100
)->pack(-side => 'left', -padx => 1);
$url_entry_frame->Button(
    -text => 'GO', -command => sub { sendRequest() }
)->pack(-side => 'left', -padx => 1);

$DOC_VIEWER = $browser->Scrolled('Pane', Name => 'doc_viewer',
    -scrollbars => 'se',
    -sticky     => 'we',
    -gridded    => 'y'
)->pack(-expand => 1, -fill => 'both');

my $status = $browser->Label->pack(-side => 'bottom');


MainLoop;



sub sendRequest {
    $DOC->destroy if ($DOC);

    $status->update;
    $status->configure(-text => 'Requesting...');
    my $response = $lwp->get($URL);
    $status->configure(-text => 'Done.');

    if ($response->is_success)
    {
        $DOC = $DOC_VIEWER->Label(-text => $response->content)->pack;
    }
    else
    {
        die $response->error_as_HTML;
    }
}
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Comment on Tk, LWP, and blocking? Download Code

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Re: Tk, LWP, and blocking? by saintmike (Vicar) on Mar 05, 2004 at 08:35 UTC
You're looking at a fundamental problem with GUIs and synchronous I/O: While an I/O operation (a HTTP get in your case) is running, the GUI doesn't get refreshed, it looks "frozen". There's a couple of different ways to fix this. You could use multiple processes or threads (one GUI-refresher, one HTTP-getter) or you could use an asynchronous approach like POE. You've timed your question perfectly, because there's a new article out on how to do the latter: Using GUIs (Gtk in this case, but it works for Tk, too) and POE for I/O, all in one process, all smoothly: The April issue of the German Linux-Magazin features a GUI-driven stock ticker, fetching stock quotes via HTTP. The British "Linux-Magazine" (also available in the US at Barnes & Noble) will print the article in its May issue (update: now available here). Whichever language you prefer, the listing (at the bottom of the online article) should illustrate the general concept.	[reply]
Re: Tk, LWP, and blocking? by mawe (Hermit) on Mar 05, 2004 at 08:27 UTC
Hi! This seems to be the same problem discussed in Tk and socket question. Take a look at the replies.	[reply]
Re: Re: Tk, LWP, and blocking? by Anonymous Monk on Mar 06, 2004 at 07:58 UTC
Strange... Two of us posted pretty much the same question on the same day.	[reply]
•Re: Tk, LWP, and blocking? by merlyn (Sage) on Mar 05, 2004 at 15:56 UTC
You'll probably get more leverage from biting the bullet and learning about POE, which can play nicely with Tk just fine. Then, you can include prebuilt solutions such as POE::Component::Client::HTTP, which can fetch pages asynchronously, allowing the rest of your app to continue functioning as the page is being fetched. -- Randal L. Schwartz, Perl hacker Be sure to read my standard disclaimer if this is a reply.	[reply]
Re: Tk, LWP, and blocking? by zentara (Cardinal) on Mar 05, 2004 at 14:34 UTC
use Tk::fileevent I'm not really a human, but I play one on earth. flash japh	[reply]
Re: Tk, LWP, and blocking? by smackdab (Pilgrim) on Mar 05, 2004 at 23:36 UTC
One more idea, not sure you would want to modify LWP (or subclass it if possible...) You would probably need to make LWP use non-blocking sockets and also use select(). You would then have to do your own timeout loop and callback into your program to that you could call Tk::DoOneEven(ALL_EVENTS) to ensure the GUI keeps ticking Probably not too hard if you know socket communication, but likely not simple ;-)	[reply]
Re: Tk, LWP, and blocking? by Anonymous Monk on Mar 06, 2004 at 08:15 UTC
Thanks everyone. I've decided to go with Mike and Randal's suggestion to learn and use POE. I've already started on it, and I must say, I'm quite impressed with it.	[reply]