Perl/Tk - Writing Tk applications in Perl 5

This document is for beginners. It assumes you know some Perl, and have it and Tk running. If you are not currently reading this document courtesy of the widget demonstration program, please be sure to run widget, as it will show you the various widget types supported by Tk and how to use them. widget should be installed in your default path, so type widget at a command prompt.

Here are links to other novice tutorials:

http://www.lehigh.edu/~sol0/ptk/tpj1.html http://www.lehigh.edu/~sol0/ptk/perlmonth01/pm1.html

Mastering Perl/Tk is the definitive book on Perl/Tk:

http://www.oreilly.com/catalog/mastperltk

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 - initiated by calling MainLoop - watches all events of interest and activates 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. These procedures are known as callbacks, and some of them you write yourself.

Perl programs that use Tk need to include CWuse Tk. A program should also use CWuse strict and the -w switch to ensure the program is working without common errors.

Any Perl/Tk application starts by creating the Tk MainWindow. You then create items inside the MainWindow, and/or create new windows called Toplevels that also contain child items, before starting the MainLoop, which is the last logical statment in your program. You can also create more items and windows while you're running, using callbacks. Items are only shown on the display after they have been arranged by a geometry manager like pack; more information on this later. MainLoop starts the GUI and handle all events. That's all there is to it!. A trivial one-window example is shown below:

    #!/usr/bin/perl -w  
    use Tk;
    use strict;

    my $mw = MainWindow->new;
    $mw->Label(-text => 'Hello, world!')->pack;
    $mw->Button(
        -text    => 'Quit',
        -command => sub { exit },
    )->pack;
    MainLoop;

Please run this example. It shows you two widget types, a Label and a Button, and how they are packed. When clicked, the <Button> widget invokes the callback specified by the -command option. Finally, note the typical Tk style using CW-option => CWvalue pairs.

Tk windows and widgets are hierarchical, i.e. one window includes one or more other windows. You create the first Tk window using CWMainWindow->new. This returns a window handle, assigned to CW$mw in the example above. Keep track of the main handle, commonly called a widget reference.

You can use any Tk handle to create child widgets within the window (or widget). This is done by calling the Tk constructor method on the variable. In the example above, the CWLabel method called from CW$mw creates a Label widget inside the MainWindow. 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 (other than the MainWindow or Toplevels, you must render it by calling pack. (This is not entirely true; more 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 utton 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.

Here is an itemize of the standard Tk widget set.

Button

Canvas

Checkbutton

Entry

Frame

Label

Labelframe

Listbox

Menu

Menubutton

Message

Panedwindow

Radiobutton

Scale

Scrollbar

Spinbox

Text

Toplevel

Perl/Tk provides an equal number of new widgets, above and beyond this core set.

Adjuster

Ballon

BrowseEntry

ColorEditor

DirTree

ErrorDialog

FileSelect

HList

LabEntry

LabFrame

NoteBook

Optionmenu

Pane

ProgressBar

ROText

Table

TextUndo

Tiler

TList

Tree

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:

    #!/usr/bin/perl -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 in Tk. Note the use of callbacks. Note, also, that Tk handles the values, and the subroutine uses the method 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/perl -w
    use Tk;
    use strict;

    my $mw = MainWindow->new;

    $mw->Label(-text => 'File Name')->pack;
    my $filename = $mw->Entry(-width => 20);
    $filename->pack;

    $mw->Label(-text => 'Font Name')->pack;
    my $font = $mw->Entry(-width => 10);
    $font->pack;

    $mw->Button(
        -text => 'Fax',
        -command => sub{do_fax($filename, $font)}
    )->pack;

    $mw->Button(
        -text => 'Print',
        -command => sub{do_print($filename, $font)}
    )->pack;

    MainLoop;

    sub do_fax {
        my ($file, $font) = @_;
        my $file_val = $file->get;
        my $font_val = $font->get;
        print "Now faxing $file_val in font $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 $font_val\n";
    }

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 geometry manager; the packer is one of the placement managers, and Bgrid> is another.

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 and/or vertical 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 demonstration.

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 or filling (and to play games with borders and colors).

The example below shows the use of pack and Frames:

    #!/usr/bin/perl -w
    use Tk;
    use strict;

    # Take top and the bottom - now implicit top is in the middle
    my $mw = MainWindow->new;
    $mw->title( 'The MainWindow' );
    $mw->Label(-text => 'At the top (default)')->pack;
    $mw->Label(-text => 'At the bottom')->pack(-side => 'bottom');
    $mw->Label(-text => 'The middle remains')->pack;

    # Since left and right are taken, bottom will not work...
    my $top1 = $mw->Toplevel;
    $top1->title( 'Toplevel 1' );
    $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 = $mw->Toplevel;
    $top2->title( 'Toplevel 2' );
    my $frame = $top2->Frame;
    $frame->pack;
    $frame->Label(-text => 'Left2')->pack(-side => 'left');
    $frame->Label(-text => 'Right2')->pack(-side => 'right');
    $top2->Label(-text => 'Bottom2')->pack(-side => 'bottom');

    MainLoop;

Most real applications require more than one window. As you just saw, you can create more outermost windows by using a Toplevel widget. 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. However, exiting the MainWindow will destroy all remaining Toplevel widgets and end the application. The example below shows a trivial three-window application:

    #!/usr/bin/perl -w
    use Tk;
    use strict;

    my $mw = MainWindow->new;
    fill_window($mw, 'Main');
    my $top1 = $mw->Toplevel;
    fill_window($top1, 'First top-level');
    my $top2 = $mw->Toplevel;
    fill_window($top2, 'Second top-level');
    MainLoop;

    sub fill_window {
        my ($window, $header) = @_;
        $window->Label(-text => $header)->pack;
        $window->Button(
            -text    => 'close',
            -command => [$window => 'destroy']
        )->pack(-side => 'left');
        $window->Button(
            -text    => 'exit',
            -command => [$mw => 'destroy']
        )->pack(-side => 'right');
    }

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/perl -w
     use Tk;
     use strict;

     my $mw = MainWindow->new;
     my $box = $mw->Listbox(
         -relief => 'sunken',
         -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 = $mw->Scrollbar(-command => ['yview', $box]);
    $box->configure(-yscrollcommand => ['set', $scroll]);
    $box->pack(-side => 'left', -fill => 'both', -expand => 1);
    $scroll->pack(-side => 'right', -fill => 'y');

    MainLoop;

One of the most powerful widgets 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/perl -w
    use Tk;
    use strict;

    # Create B<MainWindow> and canvas
    my $mw = MainWindow->new;
    my $canvas = $mw->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)});
    MainLoop;

    # 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 = $_;
            last;
        }
        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";
    }