perl/Tk - Writing Tk applications in perl5.


perl/Tk - Writing Tk applications in perl5.


This manual page is for beginners. It assumes you know some perl, and have got perl+Tk running. Please run the 'widget' demo before reading this text; it will teach you the various widget types supported by Tk.

Some background

Tk GUI programming is event-driven. (This may already be familiar to you.) In event-driven programs, the main GUI loop is outside of the user program and inside the GUI library. This loop will watch all events of interest, and activate the correct handler procedures to handle these events. Some of these handler procedures may be user-supplied; others will be part of the library.

For a programmer, this means that you're not watching what is happening; instead, you are requested by the toolkit to perform actions whenever necessary. So, you're not watching for 'raise window / close window / redraw window' requests, but you tell the toolkit which routine will handle such cases, and the toolkit will call the procedures when required.

First requirements

Any perl program that uses Tk needs to include use Tk. A program should also use use strict and the -w switch to ensure the program is working without common errors.

Any Tk application starts by creating the Tk main window. You then create items inside the main window, or create new windows, before starting the mainloop. (You can also create more items and windows while you're running.) The items will be shown on the display after you pack them; more info on this later. Then you do a Tk mainloop; this will start the GUI and handle all events. That's your application. A trivial one-window example is show below:

        #! /usr/bin/perl5 -w
        use strict;
        use Tk;
        my $main = MainWindow->new;
        $main->Label(-text => 'Hello, world!')->pack;
        $main->Button(-text => 'Quit',
                      -command => [$main => 'destroy']

Please run this example. It shows you two items types also shown in the widget demo; it also shows you how items are created and packed. Finally, note the typical Tk style using -option => value pairs.

Item creation

Tk windows and widgets are hierarchical, i.e. one includes one or more others. You create the first Tk window using MainWindow->new. This returns a window handle, assigned to $main in the example above. Keep track of the main handle.

You can use any Tk handle to create sub-items within the window or widget. This is done by calling the Tk constructor method on the variable. In the example above, the Label method called from $main creates a label widget inside the main window. In the constructor call, you can specify various options; you can later add or change options for any widget using the configure method, which takes the same parameters as the constructor. The one exception to the hierarchical structure is the Toplevel constructor, which creates a new outermost window.

After you create any widget, you must render it by calling pack. (This is not entirely true; more info later). If you do not need to refer to the widget after construction and packing, call pack off the constructor results, as shown for the label and button in the example above. Note that the result of the compound call is the result of pack, which is a valid Tk handle.

Windows and widgets are deleted by calling destroy on them; this will delete and un-draw the widget and all its children.

Standard Tk types


Variables and callback routines

Most graphical interfaces are used to set up a set of values and conditions, and then perform the appropriate action. The Tk toolkit is different from your average text-based prompting or menu driven system in that you do not collect settings yourself, and decide on an action based on an input code; instead, you leave these values to your toolkit and only get them when the action is performed.

So, where a traditional text-based system would look like this: (yes, this is obviously dumb code)

        #! /usr/bin/perl5 -w
        use strict;
        print "Please type a font name\n";
        my $font = <>; chomp $font;
        # Validate font
        print "Please type a file name\n";
        my $filename = <>; chomp $filename;
        # Validate filename
        print "Type <1> to fax, <2> to print\n";
        my $option = <>; chomp $option;
        if ($option eq 1) {
            print "Faxing $filename in font $font\n";
        } elsif ($option eq 2) {
            print "Now sending $filename to printer in font $font\n";

The (slightly larger) example below shows how to do this is Tk. Note the use of callbacks. Note, also, that Tk handles the values, and the subroutine uses get to get at the values. If a user changes his mind and wants to change the font again, the application never notices; it's all handled by Tk.

        #! /usr/bin/perl5 -w
        use strict;
        use Tk;
        my $main = MainWindow->new;
        $main->Label(-text => 'Print file')->pack;
        my $font = $main->Entry(-width => 10);
        my $filename = $main->Entry(-width => 10);
        $main->Button(-text => 'Fax',
                      -command => sub{do_fax($filename, $font)}
        $main->Button(-text => 'Print',
                      -command => sub{do_print($filename, $font)}
        sub do_fax {
            my ($file, $font) = @_;
            my $file_val = $file->get;
            my $font_val = $font->get;
            print "Now faxing $file_val in $font_val\n";
        sub do_print {
            my ($file, $font) = @_;
            my $file_val = $file->get;
            my $font_val = $font->get;
            print "Sending file $file_val to printer in $font_val\n";

The packer. Grouping and frames.

In the examples above, you must have noticed the pack calls. This is one of the more complicated parts of Tk. The basic idea is that any window or widget should be subject to a Tk widget placement manager; the packer is one of the placement managers.

The actions of the packer are rather simple: when applied to a widget, the packer positions that widget on the indicated position within the remaining space in its parent. By default, the position is on top; this means the next items will be put below. You can also specify the left, right, or bottom positions. Specify position using -side => 'right'.

Additional packing parameters specify the behavior of the widget when there is some space left in the frame or when the window size is increased. If widgets should maintain a fixed size, specify nothing; this is the default. For widgets that you want to fill up the current horizontal space, specify -fill => 'x', y, or both; for widgets that should grow, specify -expand => 1. These parameters are not shown in the example below; see the widget demo.

If you want to group some items within a window that have a different packing order than others, you can include them in a Frame. This is a do-nothing window type that is meant for packing (and to play games with borders and colors).

The example below shows the use of pack and frames:

        #! /usr/bin/perl5 -w
        use strict;
        use Tk;
        # Take top, the bottom -> now implicit top is in the middle
        my $main = MainWindow->new;
        $main->Label(-text => 'At the top (default)')->pack;
        $main->Label(-text => 'At the bottom')->pack(-side => 'bottom');
        $main->Label(-text => 'The middle remains')->pack;
        # Since left and right are taken, bottom will not work...
        my $top1 = $main->Toplevel;
        $top1->Label(-text => 'Left')->pack(-side => 'left');
        $top1->Label(-text => 'Right')->pack(-side => 'right');
        $top1->Label(-text => '?Bottom?')->pack(-side => 'bottom');
        # But when you use frames, things work quite alright
        my $top2 = $main->Toplevel;
        my $frame = $top2->Frame;
        $frame->Label(-text => 'Left2')->pack(-side => 'left');
        $frame->Label(-text => 'Right2')->pack(-side => 'right');
        $top2->Label(-text => 'Bottom2')->pack(-side => 'bottom');

More than one window

Most real applications require more than one window. As you read before, you can create more outermost windows by using Toplevel. Each window is independent; destroying a toplevel window does not affect the others as long as they are not a child of the closed toplevel. Exiting the main window will end the application. The example below shows a trivial three-window application:

        #! /usr/bin/perl5 -w
        use strict;
        use Tk;
        my $main = MainWindow->new;
        fill_window($main, 'Main');
        my $top1 = $main->Toplevel;
        fill_window($top1, 'First top-level');
        my $top2 = $main->Toplevel;
        fill_window($top2, 'Second top-level');
        sub fill_window {
            my ($window, $header) = @_;
            $window->Label(-text => $header)->pack;
            $window->Button(-text => 'close',
                            -command => [$window => 'destroy']
                            )->pack(-side => 'left');
            $window->Button(-text => 'exit',
                            -command => [$main => 'destroy']
                            )->pack(-side => 'right');

More callbacks

So far, all callback routines shown called a user procedure. You can also have a callback routine call another Tk routine. This is the way that scroll bars are implemented: scroll-bars can call a Tk item or a user procedure, whenever their position has changed. The Tk item that has a scrollbar attached calls the scrollbar when its size or offset has changed. In this way, the items are linked. You can still ask a scrollbar's position, or set it by hand - but the defaults will be taken care of.

The example below shows a listbox with a scroll bar. Moving the scrollbar moves the listbox. Scanning a listbox (dragging an item with the left mouse button) moves the scrollbar.

        #! /usr/bin/perl5 -w
        use strict;
        use Tk;
        my $main = MainWindow->new;
        my $box = $main->Listbox(-relief => 'sunken',
                                 -width => -1, # Shrink to fit
                                 -height => 5,
                                 -setgrid => 1);
        my @items = qw(One Two Three Four Five Six Seven
                       Eight Nine Ten Eleven Twelve);
        foreach (@items) {
           $box->insert('end', $_);
        my $scroll = $main->Scrollbar(-command => ['yview', $box]);
        $box->configure(-yscrollcommand => ['set', $scroll]);
        $box->pack(-side => 'left', -fill => 'both', -expand => 1);
        $scroll->pack(-side => 'right', -fill => 'y');

Canvases and tags

One of the most powerful window types in Tk is the Canvas window. In a canvas window, you can draw simple graphics and include other widgets. The canvas area may be larger than the visible window, and may then be scrolled. Any item you draw on the canvas has its own id, and may optionally have one or more tags. You may refer to any item by its id, and may refer to any group of items by a common tag; you can move, delete, or change groups of items using these tags, and you can bind actions to tags. For a properly designed (often structured) canvas, you can specify powerful actions quite simply.

In the example below, actions are bound to circles (single click) and blue items (double-click); obviously, this can be extended to any tag or group of tags.

        #! /usr/bin/perl5 -w
        use strict;
        use Tk;
        # Create main window and canvas
        my $main = MainWindow->new;
        my $canvas = $main->Canvas;
        $canvas->pack(-expand => 1, -fill => 'both');
        # Create various items
        create_item($canvas, 1, 1, 'circle', 'blue', 'Jane');
        create_item($canvas, 4, 4, 'circle', 'red', 'Peter');
        create_item($canvas, 4, 1, 'square', 'blue', 'James');
        create_item($canvas, 1, 4, 'square', 'red', 'Patricia');
        # Single-clicking with left on a 'circle' item invokes a procedure
        $canvas->bind('circle', '<1>' => sub {handle_circle($canvas)});
        # Double-clicking with left on a 'blue' item invokes a procedure
        $canvas->bind('blue', '<Double-1>' => sub {handle_blue($canvas)});
        # Create an item; use parameters as tags (this is not a default!)
        sub create_item {
            my ($can, $x, $y, $form, $color, $name) = @_;
            my $x2 = $x + 1;
            my $y2 = $y + 1;
            my $kind;
            $kind = 'oval' if ($form eq 'circle');
            $kind = 'rectangle' if ($form eq 'square');
            $can->create(($kind, "$x" . 'c', "$y" . 'c',
                          "$x2" . 'c', "$y2" . 'c'),
                         -tags => [$form, $color, $name],
                         -fill => $color);
        # This gets the real name (not current, blue/red, square/circle)
        # Note: you'll want to return a list in realistic situations...
        sub get_name {
            my ($can) = @_;
            my $item = $can->find('withtag', 'current');
            my @taglist = $can->gettags($item);
            my $name;
            foreach (@taglist) {
                next if ($_ eq 'current');
                next if ($_ eq 'red' or $_ eq 'blue');
                next if ($_ eq 'square' or $_ eq 'circle');
                $name = $_;
            return $name;
        sub handle_circle {
            my ($can) = @_;
            my $name = get_name($can);
            print "Action on circle $name...\n";
        sub handle_blue {
            my ($can) = @_;
            my $name = get_name($can);
            print "Action on blue item $name...\n";

 perl/Tk - Writing Tk applications in perl5.