FAQ part2 of 5 
Author Message FAQ part2 of 5

Subject: FAQ part2 of 5

Archive-name: perl-faq/ptk-faq/part2
Summary: Frequently Asked Questions.

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Perl/Tk FAQ part 1 of 5 - Programming      


 9. How do I get widget X to do Y ?

 There are a number of tasks that can be accomplished with perl/Tk widgets,
 configurations, and bindings (a few that can't and a few that require specific
 tricks). Beginners are encouraged to work through the examples in
 UserGuide.pod. Some examples from UserGuide.pod are addressed
 in this document among those that follow.

 Basically a widget can be "created" by simply calling the sub of the same

     my $main = new MainWindow;

 will set aside the necessary system memory etc. for a new MainWindow
 widget (it does not appear until after the MainLoop; call). The object
 "created" is then callable via the variable $main. So, for example, if you
 wanted a Button in your MainWindow, then this:


 would be a very basic example of a widget command. If you wanted to later
 call this button widget you would need a "widget tag or ID" to "get a handle
 on it". Instead of the above call try something like:

     my $button = $main->Button();

 The variable $button is how you refer to the Button widget in subsequent
 calls, such as when we call the pack routine:

     $button -> pack;

 A complete script that incorporates these ideas to make a very plain button
 would look like:

     #!/usr/bin/perl -w
     use Tk;
     use strict;
     my $main = new MainWindow;
     my $button = $main -> Button();
     $button -> pack;

 But who wants such a plain looking button? You can provide a number of
 different widget configurations via calls to the configure routine as in:

     #!/usr/bin/perl -w
     use Tk;
     use strict;
     my $main = new MainWindow;
     my $button = $main->Button();
     $button -> configure(-text => 'Press me!');
     $button -> pack;

 The Perl motto is "there is more than one way to do it." - perl/Tk remains
 quite true to this motto as well. Note that the above script could have been
 written quite succinctly without the use of either the $main or $button
 variables as:

     #!/usr/bin/perl -w
     use Tk;
     use strict;
     new MainWindow -> Button(-text => 'Press me!') -> pack;

 But if you want your widgets to actually do things then you must set up
 callback procedures as discussed later...

 As of Tk-b9(.01) the - sign in front of options (like -text in the above
 example) will be optional (hence ->Button(text => 'Press me!')).


 9.1. How do I get a Button to call a Perl subroutine?

 You may specify the -command option in the call to create & pack the
 button as in:

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

 Where sub do_print { } is a subroutine that handles two arguments
 and is declared elsewhere in the script. A full script example of the use of
 the above code is presented in the second example(s) in UserGuide.pod

 (Full source code for this and other examples from UserGuide.pod may be
 found at http://www.*-*-*.com/ ~pvhp/ptk/pod/. To load code from the
 web save as a local file say, edit the first line to point to your perl
 interpreter, then change permission: %chmod u+x, then execute
 the script:

 The above method is called the "anonymous subroutine (closure)" method.
 As discussed in Callback.pod one might have re-written that statement
 to use the "reference to a sub" method thusly:

     $main->Button(-text => 'Print',
                    -command => [ \&do_print , $filename, $font ]

 Note the backslash in front of \&do_print. This causes perl to generate a
 reference to sub do_print rather than call it. (thanks Jim Stern :-)


 9.2. How do I arrange the layout of my widgets?

 To control the layout and appearance of widgets in a window one makes use
 of a geometry manager, as well as -padding, -fill, -expand, and -anchor
 options of individual widgets.

 A geometry manager is any Tk procedure for controlling the arrangement of
 widgets in your application window. The pre{*filter*} geometry manager
 used in both Tcl/Tk and perl/Tk is pack also known informally as the
 "packer" (other geometry managers are the "placer" and the canvas
 widget itself but are much less popular. There is also Nick Ing-Simmon's
 Table widget [discussed in a later question] and BLT_Table [which made it's
 way into perl/Tk thanks to Guy Decoux - but is also discussed in a later
 question]. So far tixForm is for Tcl/Tk only, but a perl/Tk version of Tix is
 in the works. You can invoke pack at the time of widget creation via calls


 where widget can be any of the perl/Tk widget primitives. Widget option
 lists are usually passed as an associative array (hash) in parentheses thusly:

     $widget(-option0 => value0,-option1 => value1)->pack;

 pack is often used in conjunction with the frame container widget to
 arrange your widgets much like a hiearchically arranged set of window panes
 (ultimately in a rectangular "tiling" fashion of sorts). An example of this
 would be:

     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');

 Note that pack itself is given parameters in this example. The default
 behavior for pack is equivalent to specifying -side => 'top' which
 can be overridden as in the above example.

 (Full source code for this and other examples from UserGuide.pod may be
 found at http://www.*-*-*.com/ ~pvhp/ptk/pod/. To load code from the
 web save as a local file say, edit the first line to point to your perl
 interpreter, change permission using: chmod u+x, then type the
 name of your script:

 One of the more helpful options to pass to pack when trying to get a given
 widget layout "just right" is through padding: either -padx or -pady. The
 details of the use of pad depend on which specific widget you are trying to
 pack. In fact you can often add the -pad in the call to create the widget
 rather than in the call to pack.

 There is also the -anchor configuration option for widgets. A good
 introduction to the 9 possible -anchor (and -overanchor) values is
 given by the popup demo in your Tk-b#/ directory.

 When setting a widget within a frame next to another widget one may wish
 to make use of the -fill => 'style' (where style = none | x | y |
 both) options of either pack or the widget itself. A typical situation where
 this is used is in setting up the Scrollbar next to a Canvas or Text

 Another aspect to consider when laying out your widgets is their behavior
 under resize operations (grabbing a part of the window frame and making it
 bigger or smaller - details depend on your window manager). This may be
 controlled by the -expand option of either pack or the widget itself.


 9.3. How do I get a Popup to popup?

 For things like a simple "are you sure?" dialog box you might want to take a
 look at which is discussed in a later question [15.1].

 If you don't wish to require Tk::Dialog, you need something more
 complicated, or you simply want to create your own independent window
 with widgets; you must first setup a Toplevel in perl/Tk. The fourth
 example in UserGuide.pod gives a simple example of how to call Toplevel.
 Quoting from that script:

     my $main = new MainWindow;
     fill_window($main, 'Main');
     my $top1 = $main->Toplevel;

 Where sub fill_window is declared after the call to MainLoop;.
 When running that script take careful note of which window pops up first,
 which window has grabbed the active attention of your input device(s), and
 which widget within the active window has the keyboard/mouse focus when
 all three windows are open.

 The use of Toplevels brings up the issue of grab - or which independent
 window is presently "active" and which are activatable. To make a
 Toplevel window active call grab thusly:


 where $Top_widget identifies the desired Toplevel (it would be either
 $top1 or $top2 in the sample script referred to above). grab_option
 could be -global - but this is discouraged as a sign of "desparate
 programming style". To give a Toplevel "local grab" you may simply say:


 That is, without an argument.

 The use of Toplevels may also bring up the issue of focus - or which
 window - even which widget within a window - is presently "hot". You may
 call focus on an entire Toplevel:


 However, focus is most often used with individual widgets rather than a
 whole Toplevel.

 To de-iconify a widget there is in fact a Popup function that may be called



 9.4. How do I bind keyboard keys?

 There are many default key bindings built in to the widgets of perl/Tk.
 Making proper use of them often involves setting up the right callback. (You
 may wish to consult the examples in BindTable.pod for help with this

 The basic idea is:

     $widget -> bind('<keyname>' => action);

 Where $widget is the tag or ID of the widget for which the bindings are to
 hold (note for global bindings you have to bind to <All>, for semi-global
 bindings you need to bind to all the relevant widgets in your application),
 '<keyname>' can be things like:

     <Key> or <KeyPress> or <Any-KeyPress>
     <Button> or <ButtonPress>
     <Button-1> or <B1>

 To get an idea of what the code is for the key that you are interested in try
 running the xlib_demo that comes in your Tk-b# directory. Hold your
 mouse pointer over the window that appears and simply type the key that
 you are interested in. The code should appear in the window. If you do not
 have perl/Tk up and running yet try "xmodmap -pk" or look directly at the
 /usr/include/X11/keysymdef.h file where keysym names are
 given with an XK_ pre-pended. Do not try things like the Tcl/Tk %k
 symbols - they will be mis-interpreted as non-existant perl hashes. Instead
 look at the Xevent function.

 script for determining keyboard key bindings on a MainWindow:

     #!/usr/local/bin/perl -w

     use Tk;

     $top = MainWindow->new();

     $frame = $top->Frame( -height => '6c', -width => '6c',
                             -background => 'black', -cursor => 'gobbler' );

     $top->bind( '<Any-KeyPress>' => sub

         my $e = $c->XEvent;
         my( $x, $y, $W, $K, $A ) = ( $e->x, $e->y, $e->K, $e->W, $e->A );

         print "A key was pressed:\n";
         print "  x = $x\n";
         print "  y = $y\n";
         print "  W = $K\n";
         print "  K = $W\n";
         print "  A = $A\n";
     } );


 To bind the action of one widget to that of another try taking a look at the
 .pm file for the widget of interest - is there a binding function already
 defined? If so you may use it. An example would be the use of "Up" &
 "Down" Buttons for a Listbox: one could bind the Buttons to call
 Tk::Listbox::UpDown, however, Guy Decoux describes a much more
 clever way to use the <Up> and <Down> already defined in
 (this does not work with Tk-b9.01):

     use Tk;
     $top = MainWindow->new;
     $lb = $top->Listbox(-height => 10);
     for($i=0; $i < 120; $i++) {
       $lb->insert('end', $i);
     $f = $top->Frame;
     $up = $f->Button(
            -text => "Up",
            -command => [ $lb->bind(ref $lb, '<Up>'), $lb]
     $down = $f->Button(
              -text => "Down",
              -command =>sub {&{$lb->bind(ref $lb, '<Down>')}($lb)}
     $up->pack(-side => 'left');


 9.5. How do I add bindings?

 On Fri, 15 Sep 95 10:30:56 BST Nick Ing-Simmons

 Re: Multiple binds to a single widget?

 On Thu, 14 Sep 1995 14:57:54 -0400

 >In the tcl/tk doc I have, they say that prepending the script
 >with '+' appends the new binding to the current one.
 >How do I do that in perlTk?

 You cannot do that that way (yet?) - one issue is what it would mean to
 prepend '+' to a perl/Tk callback :

     # did not look right to me

 Other issue is that I would need to manage a list-of-callbacks in glue code.

 Bind your new command to a new tag:


 And add Extra to the widgets bindtags:



 9.6. How do I bind the action of a slider (sic) to ... ?

 Technically speaking they are called Scrollbars (not sliders) and one
 must configure the action of the desired widget to call the Scrollbars
 (i.e. bind is not involved here)

 A common task using Scrollbars is to configure things like Canvas,
 Listbox, or a Text widgets to be updated (change appearance) when the
 slider of the acompanying Scrollbar is moved by the user.

 As an example consider the code that sets up a twelve element Listbox
 and an accompanying vertical Scrollbar:

     my $main = new MainWindow;
     my $box = $main->Listbox(-relief => 'sunken',
                              -width => -1, # Shrink to fit
                              -height => 5,
                              -setgrid => 'yes');

                    Eight Nine Ten Eleven Twelve);

        $box->insert('end', $_);
     my $scroll = $main->Scrollbar(-command => ['yview', $box]);

 So far so good. But merely setting them up does not mean that the
 Listbox even knows that the Scrollbar is lying next to it. Note that
 the scalar variable $scroll is how we refer to the Scrollbar, thus,
 hooking the $box up to handle $scroll events is a matter of configur

     $box->configure(-yscrollcommand => ['set', $scroll]);

 A complete script that makes use of this code (and adds the necessary calls
 to pack and MainLoop;) is given as the fifth example in UserGuide.pod
 (and may be found at http://www.*-*-*.com/ ~pvhp/ptk/pod/.)


 9.7. How do I configure a Scrollbar to scroll multiple widgets?

 Note that the widget type that you wish to scroll can be important as a scroll
 "unit" on a Text or Listbox may be a character (several pixels -
 depending on font) whereas it would be an X "units" on a Canvas (could be
 pixel - but you may also specify other units).

 A concrete answer for scrolling 3 Listboxes comes courtesy of Frederick

 From a working example of multi-xscrolling:

 sub multiscrollx
 {  # multiscrollx

  my $w;


 }  # multiscrollx

 # %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   $sh->configure( -command => [ \&multiscrollx, $sh,
   $ruletable->configure(  -xscrollcommand => [ 'set', $sh]);
   $ruleheader->configure( -xscrollcommand => [ 'set', $sh]);
   $scratchrule->configure(-xscrollcommand => [ 'set', $sh]);

 In this case,
   $sh is a horizontal Scrollbar,
   $ruletable and $scratchrule are Tables
   $ruleheader is an Entry

 However, this approach is good for any widget with
 X-scrolling capability, I think.  
 So the Y counterpart should be:

 sub multiscrolly
 {  # multiscrolly

  my $w;


 }  # multiscrolly

 # %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

   $sv->configure( -command => [ \&multiscrolly, $sv,
   $l1->configure( -yscrollcommand => [ 'set', $sv]);
   $l2->configure( -yscrollcommand => [ 'set', $sv]);
   $l3->configure( -yscrollcommand => [ 'set', $sv]);

 Hope that helps.


 9.8. How do I display a bitmap?

 You can display X bitmaps on your widgets with the -bitmap configuration
 option. Typically -bitmaps are configured into Label, Frame, Button,
 etc. widgets (Canvas widgets are another story however see question [10.1]
 below). In order to emphasize the bitmap option itself let us assume we were
 specifying a bitmap for a Label with a call like:

     $main->Label(-bitmap => 'bitmap-name')->pack;

 Where bitmap-name could be any of the built in Tk bitmaps: error,
 gray25, gray50, hourglass, info, question, questhead,
 warning (see the widget demo for a full list).

 In order to use some of the bitmaps in the perl5/Tk/demos/images/
 directory you would specify a fuller path name like:

 the $tk_library variable imported by use Tk;). If you wanted to
 specify a file called foobar.xbm in the directory where you were running
 the script then either:


 should work just fine. In another directory however that would be a problem.
 So something like:

 will help someone who has an img/foobar.xbm file in their $HOME
 directory. If you don't mind the non-portability then hard-wiring in the full
 path name will help as well. (Or if you have write access then put your files
 in Tk/demos/images/ e.g.)


 9.9. How do I display an image?

 You will want to get a "Photo" handle on the file as in the following
 example where 'imggif' is the Photo handle for a gif file that is
 distributed with Tk-b#:

     #!/usr/bin/perl -w
     use strict;
     use Tk;
     my $main = new MainWindow;

     $main ->Label(-text => 'Main')->pack;
     $main -> Photo('imggif',
                    -file => "$Tk::tk_library/demos/images/earth.gif");
     my $l = $main->Label('-image' => 'imggif')->pack;

     $main->Button(-text => 'close',
                   -command => sub{destroy $main}
                   )->pack(-side => 'left');
     $main->Button(-text => 'exit',
                   -command => [sub{exit}]
                   )->pack(-side => 'right');

 (Canvas widgets are another story however see question [10.3] below).


 9.10. What Image types are available?

 In addition to the Tk builtin bitmaps there is support for reading images
 from files in formats such as: X11 Bitmaps (.xbm), X Pixmaps (.xpm), and
 Graphics Inline Format (.gif). See the CrtPhImgFmt man page for more
 info (if you have Tk 4.X installed). (In order to support other formats you
 might also consider running through a netpbm filter.)

 For perl generation of images see the question on the GD module.


 9.11. Is there any way to have more than one Listbox contain a selection?

 To allow more than one Listbox to contain a "selection", (or at least a
 highlighted item - which need not be the actual selection) specify the
 configuration option:

     -exportselection => 0

 which will dis-associate Listbox's selection from X selection (only one
 window can have X selection at a time).


 9.12. How do I select a range of tags in a Text widget?

 A question arose concerning getting a range of selections from a Text
 widget. Nick Ing-Simmons' answer mentions several possibilities including:

 Keyboard Copy/Paste 'is' implemented of course...

 Subj:   RE: $Text->tag('ranges', 'sel') - does this work?

 On Fri, 29 Dec 1995 14:57:42 -0500

 >I was writing a little tk perl today, and i decided to try to
 >implement a copy/paste using the 'sel' tag
 >I enabled exportselection, and then try to probe the select
 >region with:
 >    $buffer = $text->tag('ranges', 'sel');
 >$buffer comes back with one entry, the end of the selection.

 That is to be expected - the scalar gets assigned the last element of the list.

 >i tried:

 >same difference.

 This seems to work for me:

     ($start,$end) = $text->tagRanges('sel');

 In perl/Tk ->tagRanges(...) is an alias for

 The following subroutine can also probe and print the tagRanges:

     sub showsel  

        print "start=$info[0] end=$info[1]\n"


 9.13. How do I group Radiobuttons together?

 Specify the -variable option on each one. Here is an example pulled
 from the demo script:

      $letters = '';
      my $w_frame_left_b3 = $w_frame_left->Radiobutton(

          -variable => \$letters,
          -value    => 'full',
      my $w_frame_left_b4 = $w_frame_left->Radiobutton(

          -variable => \$letters,
          -value    => 'empty',


 9.14. How do I specify fonts?

 The quick answer is to specify the font configuration option of your widget
 as in:

     #!/usr/local/bin/perl -w
     use Tk;
     $main = MaiWindow->new();
     $labl = $main -> Label('-text' => "Foo", '-font' => "fixed");
     $labl -> pack;

 The long answer involves figuring out what fonts you have access to locally.
 The Unix programs xlsfonts and xfontsel are useful in this regard.

 The perl/Tk version of xfontsel was distributed as the font_test script in
 the Tk-b9.01 build directory.

 See also the later question [19] on international fonts.


 9.15. How do I get the entry in an Entry?

 You want to call get on the return value of the widget itself. Here is how it
 may be used in a simplified version of example 1.1 from the Tk::UserGuide
 where a Button is set up to call a sub where the call to get lies:

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

     my $main = MainWindow -> new();
     my $entry = $main -> Entry();
     $entry -> pack;
     $main->Button(-text => 'Print',
                   -command => sub{do_print($entry)}

     sub do_print {

         my $entered = $widget -> get();
         print "The string \"$entered\" was entered.\n";


 9.16. How do I obtain Menus that do not tear off?

 Nick Ing-Simmons outlined a couple of ways to achieve this result. The
 critical feature being the -tearoff => 0 configuration option of the
 Menu. In Nick's words:

     my $mb = $parent->Menubutton(...);    # The button
     my $menu = $mb->Menu(-tearoff => 0);  # Create a non-tearoff menu
     $mb->configure(-menu => $menu);       # Tell button to use it.

 Above is for clarity - you can loose $menu variable:

     my $mb = $parent->Menubutton(...);  
     $mb->configure(-menu => $mb->Menu(-tearoff => 0));  

Sun, 19 Jul 1998 03:00:00 GMT  
 [ 1 post ] 

 Relevant Pages 

1. FAQ part2 of 5

2. FAQ part2 of 5

3. FAQ part2 of 3

4. FAQ part2 of 5

5. FAQ part2 of 5

6. FAQ part2 of 3

7. FAQ part2 of 5

8. FAQ part2 of 5

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