Hello World Example (Mastering Perl/Tk)

1.4. Hello World Example

Every programming language goes through the Hello World example, which is a complete program that prints a string (typically "Hello World") and exits. "Hello World" may get its share of ridicule, but it's a remarkably effective tool that shows readers how to write and execute a working program while they're still in the first chapter of the book. In our Hello World example, we'll have the title of our window say "Hello World" and create a Button that will dismiss the application:

#!/usr/bin/perl
use Tk;
my $mw = MainWindow->new;
$mw->title("Hello World");
$mw->Button(-text => "Done", -command => sub { exit })->pack;
MainLoop;

Despite being only six lines long, there is quite a bit going on in our little program. The first line, as any Perl programmer knows, invokes Perl.[8] The second line tells Perl to use the Tk module.

[8] On Unix, that is. In Win32 you have to type perl hello.pl to invoke the program or twiddle with Explorer to call up the perl executable when .pl files are double-clicked.

The third line:

my $mw = MainWindow->new;

is how we create a window. The window will have the same basic window manager decorations as all your other windows.

The title of our window is changed using the title method. If we hadn't used this method, the text across the top of the window would be the same as the name of the file containing the code, excluding any extension. For instance, if the code were stored in a file named hello_world, the string "Hello_world" would appear across the title bar of the application (Tk automatically capitalizes the first character for you). Using the title method is not required, but it makes the application look more polished.

Any string we put as an argument becomes the title. If we wanted the title to be "Hey! Look at my great program!," this would be the place. This is akin to using the -title option when starting any standard X Windows application. We cover more methods for a MainWindow object later in Chapter 11, " Frame, MainWindow,and Toplevel Widgets".

The next line creates a Button widget, sets basic properties, and packs the widget. (See Chapter 4, " Button, Checkbutton, and Radiobutton Widgets" for all available configuration options for Button.)

The Button is set to display the text "Done" and to perform the Perl command exit when pushed. Finally, the last item of concern is the MainLoop command. This starts the event loop in motion, and from then on the application will do only what we have told it to do: if the user clicks on the Button, the application will exit. Anything else the user does—minimizing, resizing, changing to other applications—will be processed by the window manager and ignored by our application. See Figure 1-2 for a picture of the Hello World window.

Figure 1-2. Hello World window

1.4.1. exit Versus destroy

In most of the examples in this book, you will see sub { exit; } ( or its equivalent, \&exit) used to quit the Perl/Tk application. This works fine as long as you have done a use Tk; in the same file. Perl/Tk defines its own exit routine, which does some cleanup and various other things that are important to Tk. The program is then unconditionally terminated, and control returns to the operating system.

Another way to quit the Tk portion of the application is to call $mw->destroy, which destroys the MainWindow and returns to the code listed after MainLoop. This allows your program to do post-GUI processing before exiting.