Button

The button widget is one of the most common widgets used in Perl/Tk. Its basic purpose is to execute some prescribed action, and as such it is most commonly associated with something that requires no further user input - for example, a button to exit the application, or to submit some information. A button widget is created with the syntax

my $label = $mw->Button( [ option => value ] );

Two of the more common options are

• -text => text
  sets the string printed on the button as text
• -command => callback
  indicates a pointer to some code that will be called when the button is pressed

A basic example of a button is

#!perl
# file button.pl
use Tk;
use strict;
use warnings;
my $mw = MainWindow->new;
$mw->title('Button');
my $hello = $mw->Button(-text => 'Greetings!',
                        -command => \&print_hello);
my $exit = $mw->Button(-text => 'Exit',
                       -command => [$mw => 'destroy']);
$hello->pack(-side=>'left');
$exit->pack(-side => 'right');
MainLoop;
sub print_hello {
  print "Hello, earthling ....\n";
}

which appears in the figure below.

Figure 3.1: Example of a button widget

The first button, $hello, invokes the print_hello subroutine when pressed, which simply prints out a message to the terminal used to launch the script. The second button, $exit, is used to destroy the main window and to begin executing any code after the MainLoop statement. We will see some more examples of the use of buttons in the sections that follow.

It is also possible to use an image on a button, rather than a text label. GIF and PPM/PGM formats are supported, as well as the JPEG format through the Tk::JPEG module. To do this, first create an image pointer as

my $img = $mw->Photo(-file => 'filename');

and then create the button as

my $button = $mw->Button(-image => $img);

For example, the script

#!perl
# file button1.pl
use Tk;
use strict;
use warnings;
my $mw = MainWindow->new;
$mw->title('Button');
my $img = $mw->Photo(-file => 'dino.gif');
my $hello = $mw->Button(-image => $img,
                        -command => \&print_hello);
my $exit = $mw->Button(-text => 'Exit',
                       -command => [$mw => 'destroy']);
$hello->pack(-side=>'left');
$exit->pack(-side => 'right');
MainLoop;
sub print_hello {
  print "Hello, earthling ....\n";
}

results in the following:

Figure 3.2: Example of a button widget using a gif

Alternatively, you can specify a bitmap to use, as in

my $button = $mw->Button( -bitmap => bitmap);

Several bitmaps available in the standard Tk distribution - error, gray12, gray25, gray50, gray75, hourglass, info, questhead, question, and warning. As an example,

#!perl
# file button2.pl
use Tk;
use strict;
use warnings;
my $mw = MainWindow->new;
$mw->title('Button');
my $hello = $mw->Button(-bitmap => 'questhead',
                        -command => \&print_hello);
my $exit = $mw->Button(-text => 'Exit',
                       -command => [$mw => 'destroy']);
$hello->pack(-side=>'left');
$exit->pack(-side => 'right');
MainLoop;
sub print_hello {
  print "Hello, earthling ....\n";
}

results in the following

Figure 3.3: Example of a button widget using a bitmap