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Author image Gisle Aas


Tkx::Tutorial - How to use Tkx


Tk is a toolkit that allows you to create applications with graphical interfaces for Windows, Mac OS X and X11. The Tk toolkit is native to the Tcl programming language, but its ease of use and cross-platform availability has made it the GUI toolkit of choice for many other dynamic languages as well.

Tkx is a Perl module that makes the Tk toolkit available to Perl programs. By loading the Tkx module Perl programs can create windows and fill them with text, images, buttons and other controls that make up the user interface of the application.

Hello World

Let's start with the mandatory exercise of creating an application that greats the world. Here we make the application window contain a single button which will shut down the application if clicked. The code to make this happen is:

    use Tkx;
        -text => "Hello, world",
        -command => sub { Tkx::destroy("."); },

Save this to a file called hello.pl and then run perl hello.pl to start up the application. A window with the text "Hello, world" should appear on your screen.

After the Tkx module has been loaded by the use Tkx statement the application will show an empty window called ".". We create a button with the name ".b" and tell the window to display the button with the call to Tkx::pack(). After the layout of the window has been set up we need to pass control back to Tk so that it can draw the window and invoke our callback if the button is clicked. This is achieved by the Tkx::MainLoop() call at the end. Clicking the button will invoke the subroutine registered with the -command option of the button. In this case the callback simply destroys the window, which in turn will terminate the application.

For reference this is how the same program would look in Tcl:

    package require Tk
    button .b \
        -text "Hello, world" \
        -command { destroy . }
    pack .b

This program can be executed by the tclsh binary that comes with Tcl/Tk. As you can see the code is mostly identical, but with a slightly different syntax. The only real difference is that the call to MainLoop() is implicit in Tcl and does not have to be spelled out.

Tkx does not come with documentation that explain all the widgets (like "button" above) that are available for use. Instead you will need to read the mostly excellent documentation that comes with Tcl/Tk and then figure out how this translates to Tkx yourself. As you can see this translation is straight forward. You basically only have add the prefix "Tkx::" to all the functions and use the Perl way of passing arguments. Tcl is a very simple language, so it should not take long to understand enough of it to be able to read its documentation with ease.

A great place to look up the Tk documentation is http://aspn.activestate.com/ASPN/docs/ActiveTcl/at.pkg_index.html. This documents core Tk as well as all the useful add-on packages that are part of ActiveTcl. The ActiveTcl HTML documentation can also be downloaded from http://downloads.activestate.com/ActiveTcl/html/ and installed locally. The official Tcl/Tk docs are found at http://www.tcl.tk/doc/.

Hello World with objects

The windows and controls that make up a Tk interface are called widgets. The widgets are identified by path names of the form .foo.bar.baz. These names are hierarchical in the same way as file system names are, but "." is used instead of "/" to separate levels. The name .foo.bar.baz is the name of a widget that is child of widget .foo.bar which in turn is a child of .foo. At the top of this hierarchy we have a widget called ., which is the main window of the application.

The Tkx module provide a class called Tkx::widget, that can be used to hide the details of Tk path names from Tkx applications. This provide a more "perlish" way to create and manipulate Tk widgets.

Our "Hello, world" program can be rewritten like this using the Tkx::widget class:

    use Tkx;
    my $mw = Tkx::widget->new(".");
    my $b = $mw->new_button(
        -text => "Hello, world",
        -command => sub { $mw->g_destroy; },

By loading the Tkx module we make the Tkx::widget class available and create the main window (the widget called .). Next we instantiate a new Tkx::widget object wrapping the main window. It is customary to name this object $mw.

To create a new button child widget we call $mw->new_button method. Constructor methods are always prefixed with new_. The rest of the method name is the name of the Tk widget to create; i.e. "button" in this case. Arguments are passed as before.

Calling a "g_" method will invoke the corresponding Tk command with the widget path as argument. In the code above we destroy the main window by calling $mw->g_destroy and we pack the button in the main window by invoking $b->g_pack.

In the end we need to invoke the MainLoop as before.

For trivial programs like the one above using Tkx::widget wrappers does not appear to be a win, but as the application grows and the Tk path names get longer it surely is.

Hello World expanded

In order to introduce a few more Tkx features, we now present a slightly expanded version of the previous Hello World program. This time we added line numbers to the program so that it is easier to reference back to its statements:

     1  use strict;
     2  use Tkx;
     4  my $mw = Tkx::widget->new(".");
     5  $mw->g_wm_title("Hello, world");
     6  $mw->g_wm_minsize(300, 200);
     8  my $b;
     9  $b = $mw->new_button(
    10      -text => "Hello, world",
    11      -command => sub {
    12          $b->m_configure(
    13              -text => "Goodbye, cruel world",
    14          );
    15          Tkx::after(1500, sub { $mw->g_destroy });
    16      },
    17  );
    18  $b->g_pack(
    19      -padx => 10,
    20      -pady => 10,
    21  );
    23  Tkx::tk___messageBox(
    24     -parent => $mw,
    25     -icon => "info",
    26     -title => "Tip of the Day",
    27     -message => "Please be nice!",
    28  );
    30  Tkx::MainLoop()

The first thing we added is the use strict statement, simply because that's a good idea in general.

In line 5 and 6 we set up some window manager attributes of the main application window. What is noteworthy here is that we use underscore in the g_ method names where Tcl would use space between words. Same rules apply to the function names in the Tkx:: namespace directly. We could alternatively have modified the window attributes with:

    Tkx::wm_title($mw, "Hello, world");
    Tkx::wm_minsize($mw, 300, 200);

which in Tcl would have been:

    wm title . "Hello, world"
    wm minsize . 300 200

The rule is: A single underscore on the Perl side turns into space on the Tcl side.

In line 11 to 16 we have expanded the button callback to change the text of button and then wait 1.5 seconds before we shut down the application. In addition to the "g_" methods described in the previous section, Tkx::widget also provide "m_" methods which are forwarded as Tcl subcommands of the current widget. The most commonly used subcommand is "configure" that is used to change the attributes of a widget as we do in line 12. Since we now reference $b from the callback, we had to declare the variable upfront in line 8 instead of declaring it together with the assignment as we did previously. In line 15 we schedule for the window to be destroyed after a delay of 1500 ms. The delay is needed so that the new button text is actually show before the window goes away.

Btw, the "m_" method prefix is actually optional, so you might prefer just to leave it out.

Padding around buttons is usually a good idea as we now do in line 18.

In line 23 we invoke the messageBox command to pop up a useful reminder to our user. But what's up with the "tk___" prefix? In the Tcl docs you will find that the name of this command is actually "tk_messageBox". Remember the previous rule that an underscore in Tkx:: names turn into a space on the Tcl side? If you try to call Tkx::tk_messageBox() you will get an error telling you:

    bad option "messageBox": must be appname, caret, scaling,
    useinputmethods, or windowingsystem

What happens is that Tkx invoked the "tk messageBox" command, but the Tcl "tk" command only take the subcommands listed in the error message above and refuse to do anything about "messageBox". In order to invoke Tcl commands with underscore their name, you need to triple the underscore on the Perl side, which gives us Tkx::tk___messageBox(). Double underscores in names have yet another meaning that we will tell you about in the next section.

Setting up a menu line

Most real GUI application will need a menu line at the top of the application window or screen. The following runnable program shows how a minimal menu can be set up with Tkx:

     1  #!/usr/bin/perl -w
     3  use strict;
     4  use Tkx;
     6  our $VERSION = "1.00";
     8  (my $progname = $0) =~ s,.*[\\/],,;
     9  my $IS_AQUA = Tkx::tk_windowingsystem() eq "aqua";
    11  Tkx::package_require("style");
    12  Tkx::style__use("as", -priority => 70);
    14  my $mw = Tkx::widget->new(".");
    15  $mw->configure(-menu => mk_menu($mw));
    17  Tkx::MainLoop();
    18  exit;
    20  sub mk_menu {
    21      my $mw = shift;
    22      my $menu = $mw->new_menu;
    24      my $file = $menu->new_menu(
    25          -tearoff => 0,
    26      );
    27      $menu->add_cascade(
    28          -label => "File",
    29          -underline => 0,
    30          -menu => $file,
    31      );
    32      $file->add_command(
    33          -label => "New",
    34          -underline => 0,
    35          -accelerator => "Ctrl+N",
    36          -command => \&new,
    37      );
    38      $mw->g_bind("<Control-n>", \&new);
    39      $file->add_command(
    40          -label   => "Exit",
    41          -underline => 1,
    42          -command => [\&Tkx::destroy, $mw],
    43      ) unless $IS_AQUA;
    45      my $help = $menu->new_menu(
    46          -name => "help",
    47          -tearoff => 0,
    48      );
    49      $menu->add_cascade(
    50          -label => "Help",
    51          -underline => 0,
    52          -menu => $help,
    53      );
    54      $help->add_command(
    55          -label => "\u$progname Manual",
    56          -command => \&show_manual,
    57      );
    59      my $about_menu = $help;
    60      if ($IS_AQUA) {
    61          # On Mac OS we want about box to appear in the application
    62          # menu.  Anything added to a menu with the name "apple" will
    63          # appear in this menu.
    64          $about_menu = $menu->new_menu(
    65              -name => "apple",
    66          );
    67          $menu->add_cascade(
    68              -menu => $about_menu,
    69          );
    70      }
    71      $about_menu->add_command(
    72          -label => "About \u$progname",
    73          -command => \&about,
    74      );
    76      return $menu;
    77  }
    80  sub about {
    81      Tkx::tk___messageBox(
    82          -parent => $mw,
    83          -title => "About \u$progname",
    84          -type => "ok",
    85          -icon => "info",
    86          -message => "$progname v$VERSION\n" .
    87                      "Copyright 2005 ActiveState. " .
    88                      "All rights reserved.",
    89      );
    90  }

We start out as all proper Perl programs should by enabling warnings and stricture at line 1 and 3. Then we load Tkx which will create our main application window at line 4.

In line 9 we initialize $IS_AQUA constant. Aqua is the native interface of Mac OS X. We need this constant because the menu layout on Aqua is not the same as for the other windowing systems. Note that Tk on Mac OS X can be compiled against either Aqua or X11. When our application runs under X11 we want to use the standard Unix menu layout, so it would not be correct to just make our code conditional on what operating system it runs under ($^O eq 'darwin' for Mac OS X).

In line 11 and 12 we override the default look&feel style of Tk to a more modern variant. Tcl packages can be loaded with the Tkx::package_require() function and we can access the Tcl command style::use as Tkx::style__use in Perl, i.e. we need to turn the double colon into a double underscore. More about Tcl packages and namespaces in the next section.

In line 14 we obtain a Tkx::widget reference to the main window as before and then we simply set up the application menu by setting up the -menu option of the main window in line 15.

In a real application there would be additional code between line 15 and 17 to set up the rest of the application window, but for this demonstration we just leave the window empty.

In line 17 we ask Tk to start processing events, by invoking Tkx::MainLoop(). This function will return when the application window has been destroyed and when that happens we are done at line 18.

The application menu itself is set up and returned by the mk_menu() function in line 20 to 77. This code should be easy enough to follow. Note how we make File | New and Help | Foo Manual both reference functions that are not yet written. The application will still run, but when you try to invoke these menu entries you get an "Application Error Dialog" from Tk. It is handy to be able to leave stubs like this around during the development. Just remember to add the new and show_manual functions before the application ships. :-)

The -underline options are provided to make it possible to select menu entries with the keyboard. The corresponding character of the -label will be underlined and you will be able to select this entry by pressing the key when the menu is active.

It is also possible to set up direct keyboard shortcuts like we done for the File | New function at line 32. Note that the -accelerator option only adds the text to the menu item, so we need to use an explicit call to set up this binding as done in line 38.

For Aqua we don't want to add the "File | Exit" entry to the menu because the OS itself always provide a Quit action in the application menu. Proper Aqua applications will also need to add the "About" function on the application menu instead of the "Help" menu as is common on other platforms.

The menu names "apple" and "help" provided in line 46 and 65 has special significance to Tk. Menu items added to the "apple" menu will show up in the application menu. The name is historic from the time when this stuff would actually show in the menu with the picture of the Apple logo. In Mac OS X these entries show up at the top of the menu just right of the apple. If not provided Tk will provide its own "About" entry that will tell you about what version of Tcl/Tk you are using. A menu called "help" menu will be flushed right on Unix even though this style seems to be out of fashion in modern Unix applications.

The Tkx distribution contain a script called menu which is a runnable version of the program shown here. You might want to use this as a starting point for your own Tkx applications.

Using Tcl packages

When the Perl application starts up and load Tkx, the only functions available in the Tkx:: namespace are those commands provided by core Tcl/Tk. These commands are described in the "Tcl" and "Tk" sections at http://aspn.activestate.com/ASPN/docs/ActiveTcl/at.pkg_index.html.

Additional commands can be loaded from Tcl packages. Once loaded new commands show up in the Tkx:: namespace. This example loads the "Tktable" package in order to make the table command to create table widgets available:

    use Tkx;

    my $mw = Tkx::widget->new(".");
    my $t = $mw->new_table(
        -rows => 5,
        -cols => 3,

Packages are loaded by calling the Tkx::package_require() function taking the package name as argument. An optional version number can be provided as the second argument if you want to make sure a certain version or newer is loaded.

One source of confusion here is what the proper spelling of the package name to provide to Tkx::package_require() is. The Tcl/Tk documentation will call the package in the example above TkTable (with two upper case "T"s) and not really mention the exact spelling of the package name (only one upper case "T"). In some cases the "synopsis" section describing the package will spell out the package name, but in cases like this we have found no better way than to peek into the pkgIndex.tcl files in the Tcl lib/ area if loading the package fails. The package documented as "BWidgets" should be loaded as "BWidget" (without the "s") and the package documented as "IWidgets" should be loaded as "Iwidgets" (with a lower case "w"). Somebody ought to be spanked.

Most modern Tcl packages will not create names at the top level like TkTable did above. Instead they will create functions in a Tcl namespace with a name matching the package name. In the menu example of the previous section we loaded the "style" package which created a command called "use" in the "style" namespace. This command can be referenced as "::style::use" or "style::use" from Tcl. From Perl this maps to a function called Tkx::style__use, i.e. we replace the double colon with double underscore and ignore the colon in the front. Read Tkx for details about how sequences of "_" in Tkx:: names are mapped to Tcl names.

Subclassing Tkx::widget

In Tkx applications it is often convenient to use your own subclass of Tkx::widget where you can introduce shortcuts and adapters for the raw Tcl commands. This shows an example class, that should be saved to the file MyWidget.pm:

     1  package MyWidget;
     3  use strict;
     4  use base qw(Tkx::widget);
     5  use Carp qw(croak);
     7  sub messageBox {
     8      my $self = shift;
     9      return Tkx::tk___messageBox(-parent => $self, @_);
    10  }
    12  sub getOpenFile {
    13      my $self = shift;
    14      return Tkx::tk___getOpenFile(-parent => $self, @_);
    15  }
    17  sub bell {
    18      my $self = shift;
    19      Tkx::bell(-displayof => $self, @_);
    20  }
    22  sub children {
    23      my $self = shift;
    24      croak("children must be called in list context")
    25          unless wantarray;
    26      return map { $self->_nclass->new($_) }
    27             Tkx::SplitList($self->g_winfo_children);
    28  }
    30  sub pack {
    31      my $self = shift;
    32      $self->g_pack(@_);
    33      return $self;
    34  }
    36  sub _nclass {
    37      return __PACKAGE__;
    38  }
    40  1;

In the main program you would use it like this:

    use Tkx;
    use MyWidget;
    my $mw = MyWidget->new(".");

    for my $kid ($mw->children) {

The MyWidget class above provide shortcuts for the "messageBox" and "getOpenFile" in order to hide the triple underscore ugliness and propegate the -parent attribute. Similar reasoning exists for the "bell".

The children method is provided since calling $w->g_winfo_children just return a Tcl-list of widget path names. What we usually want to get back is a list of widget objects, so that we can call further methods on them.

The pack method is provided so that we can initialize and pack a widget in the same statement and avoid repeated typing of "g_" method prefix:

    my $b = $mw->new_button(...)->pack;

The _nclass method need to be overridden so that any new widget kids created also ends up as MyWidget objects. This method is called internally by methods like $mw->new_button(...) to determine which kind of object will get a chance to wrap the newly created widget path.

Having you own application specific widget class provide a place to add methods discovered by refactoring of repeated code in your application.


This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

Copyright 2005 ActiveState. All rights reserved.



The bundled sample programs; tkx-ed, tkx-prove.