Re: Problems with Tk Events

[Thomas Bayen <tbayen bayen de>]

Philippe de Rochambeau schrieb:
> Hello,

You are wrong here. This is the GTK List, not the Tk list. Try the 
usenet group comp.lang.perl.tk. There are _very_ helpful people - like 
here, but they know about Tk :-)


But if you are alreasdy here. I'll give it a try. My Tk-Experience is 
some time ago...


> I am having problems with the Event Object in Perl/Tk 804.025. When I 
> run the example shown on page 365 of Mastering Perl/Tk with the old 
> syntax shown on page 364
>
> Courier0000,0000,FFFE#!/usr/bin/perl -w
>
> use Tk;
> use strict;
>
> print "Tk version = ", $Tk::VERSION, "\n";
> my $mw = MainWindow->new;
> my $b = $mw->Button(-text=>'click b1 then b2', -command => \&callback);
> $b->bind('<ButtonRelease-2>' => \&callback);
> $b->pack;
>
> MainLoop;
>
> sub callback {
> my ($widget) = @_;
> my $e = $widget->XEvent;
> print "\n";
> print "widget event = $e\n";
> print "\$e->W = ", $e->W, "\n";
> }
>
> I get the following warning although, as you can see, $widget was 
> previously defined.
>
> Courier0000,0000,FFFE$ perl callbacktest.pl
> Tk version = 804.025
> Tk::Error: Can't call method "XEvent" on an undefined value at 
> callbacktest.pl line 16.

Yes - you defined $widget as a "my" variable. But this is not the 
message. The message is: $window contains the "undef" value. This means 
that your subroutine was called without parameter.

How can this be? Where is it called? It is called via the callback 
mechanism. I don't really know wether this mechanism automatically adds 
the widget value but if you want to be shure you should read

  perldoc Tk::callbacks

I myself use mostly the closure syntax for callbacks. With this syntax I 
have a better feeling about what happens inside the callback and which 
arguments are used (but its personal taste):

  my $b = $mw->Button(
    -text=>'click b1 then b2',
    -command => sub{callback($my,$own,$parameters,$first,$then,@_)},
  );



The example you used was made for keypress events, not for button-1 events.

In the book "Mastering Perl/Tk" on page 365 is explained that the 
callbacks doesn't get arguments if you click button 1. No arguments = no 
value = undef value = error message!



> Furthermore, when I run it with the new syntax:
>
> Courier0000,0000,FFFE#!/usr/bin/perl -w
>
> use Tk ':variables';
> [...]
> MainLoop;
> [...]
> Bareword "MainLoop" not allowed while "strict subs" in use at 
> callbacktest.pl line 12.
> Execution of callbacktest.pl aborted due to compilation errors.

If you only export ":variables", the MainLoop subroutine will not be 
exported. This has nothing to do with the above problem. Perhaps this helps:

use Tk;
use Tk ':variables';


> Any help with these problems would be much appreciated.
>
> Cheers,
>
> Philippe

Thomas