Tk Module - GUI process in PERL

Tk Module - GUI process in PERL

• Perl/Tk (also known as pTk) is a collection of modules and code that attempts to wed the easily configured Tk 8 widget toolkit to the powerful lexigraphic, dynamic memory, I/O, and objectoriented capabilities of Perl 5 In other words, it is an interpreted scripting language for making widgets and programs with Graphical User Interfaces (GUI).

• Tk, the extension(or module) that makes GUI programming in perl possible, is taken from Tcl/Tk Tcl(Tool Command Language) and Tk(ToolKit) was created by Professor John Ousterhout of the University of California, Berkeley Tcl is a scripting language that runs on Windows, UNIX and Macintosh platforms Tk is a standard addon to Tcl that provides
  commands to quickly and easily create user interfaces Later on Tk was used by a lot of other scripting languages like Perl, Python, Ruby etc.

Hello World:

Let us begin, as all other tutorials begin, with the "Hello World" program Create a file called "Hellopl" and enter the following into it.

#!/usr/local/bin/perl

use Tk;

# Main Window
my $mw = new MainWindow;
my $label = $mw -> Label(-text=>"Hello World") -> pack();
my $button = $mw -> Button(-text => "Quit",
-command => sub { exit })
-> pack();
MainLoop;

The first line

#!/ usr/local/bin/perl is not needed in windows In Linux, it tells the name of the script language processor In our case it is perl Don't understand what that means? Don't worry your gray cells over it Just put it at the top of the file

The second line use Tk; tells the interpreter that our program will use the Tk module This line is an absolute must in all GUI programs you make using perl When the interpreter encounters this line, it will load the Tk components that we will be using to create our program.

The third line This is a comment Any line that starts with a '#' char is a comment Comments are not of any use in the program It is used by programmer to talk to themselves A programmer cannot be expected to remember every thing a script does So he uses a comment to write it down Next time he edits the script, he can read the comment and understand what the program is for It is good practice to make as much comments as possible

The fourth line:                                                                                                                                           my $mw = new MainWindow;, will create a window into which the GUI elements will be placed The variable $mw is a object of type 'MainWindow' We will have to use this element when we want to place any widget inside it

The fifth line,   

$mw -> Label(-text=>"Hello World") -> pack(); makes a label and writes "Hello world" in it You can change the text to any thing you like Note the structure of the command $

label This variable assigned to that particular widget Ever widget must have a UNIQUE variable This name will be used when ever that widget must be accessed $mw -> $mw is the MainWindow's object We will be placing our label widget inside this window  Label(-text=>"Hello World") ' Label' is the name of the widget A widget is a user interface object in X graphical user interfaces Confused? Lets just say that it is the name of the object that appears on screen There are many other widgets too If you want to display a button, you use the button widget For text, you use the text widget For entry, you guessed it, the entry widget If you want, you can see more about widgets text=>"Hello World" The option for this widget This option says that this widget must be given the text "Hello World" Options change according to the widgets a button widget will not have all the options of the label widget and  vise versa But there will be many common ones Please note that operator used here is '=>' as opposed to the one used earlier '>' in $mw -> One uses the minus() sign while the other uses the equals(=) sign Do not confuse between these two You can keep writing other options can also be written here For example, let us make a label for showing the text "Hello World" The other lines are same as the Hello World program.

$mw -> Label(-text=>"Hello World",-font=>"courierfont",-relief=>"raised") ->
pack();

In this example, a lot more options are used The font option is used to tell which font must be used to make the text and the relief option tells whether the text should appear raised, sunken, flat etc To know all the options for a particular widget, read the manual that comes with Perl It lists every widget and every option they have If you are going to program in Perl, you will find your self peeking into the manual every few minutes The most important and most commonly used options are listed here All options must separated by a comma But as you have noted, this line is a little difficult to read As the number of options increase, the more difficult to read it So a more readable version is

$mw -> Label(-text=>"Hello World",-font=>"courierfont",-relief=>"raised")-> pack();

Next comes the -> pack(); This will pack the widget '$label' into the window '$mw' 'pack' is a geometry manager Another geometry manager is 'grid' Personally, I like grid better Once again, putting all this in one line is an eye sore so you can put this part in the next line

          my $label = $mw -> Label(-text=>"Hello World")-> pack();

In this case, pack has no options within it But that is not always the case

          my $label = $mw -> Label(-text=>"Hello World")-> pack(-side=>"left",-anchor=>'w');

You don't have to pack the widget in the same line of creating it but it is convenient in small programs You can pack the widget later using the widget's variable For example

my $label = $mw -> Label(-text=>"Hello World");

#We created the widget

$label -> pack(-side=>"left", -anchor=>'w');

#We pack it in another line

So we have the final syntax of how to create and display a widget

my $WidgetVariable = $Window -> WidgetType(?Option 1=>Value 1, ?Option 2=>Value2 ??)

-> pack();

The next three lines

my $button = $mw -> Button(-text => "Quit",-command => sub { exit })-> pack();

will create and display a button Here the widget variable is '$button' When we look at the options, we will find two options ' text' and 'command' The given text is Quit so the button will have the text "Quit" on it The command option determines what should happen when the user click on the button You can specify a function to execute when the user clicks on the button In this case the program will exit when this button is pressed One can also call functions that you

have created from here

#!/usr/local/bin/perl
use Tk;
# Main Window
my $mw = new MainWindow;
my $label = $mw -> Label(-text=>"Hello World") -> pack();
my $button = $mw -> Button(-text => "Quit",
-command =>\&exitProgam)
-> pack();
MainLoop;
sub exitProgam {
$mw->messageBox(-message=>"Goodbye");
exit;
}

The next line MainLoop:

is the Main Loop or the Event Loop Its job is to invoke callbacks in response to events

such as button presses or timer expirations If this line is missing, the program will run and exit with out waiting for the user to do any thing This is another one of those 'absolute musts' of Perl/Tk programming