man Gtk::cookbook () - Typical usage of the Gtk module


Gtk cookbook - Typical usage of the Gtk module


        use Gtk '-init';
        my $window = new Gtk::Window;
        my $button = new Gtk::Button("Quit");
        $button->signal_connect("clicked", sub {Gtk->main_quit});



The first thing you need to do to use the Gtk module in your perl program is to load the module and initialize it:

        use Gtk;
        init Gtk;

This can also be shortened to the one-liner:

        use Gtk '-init';

Now, how do you build the user interface? The basic components to build an interface are the widgets (derived from the Gtk::Widget package) and the containers (derived from Gtk::Container). Note that a container is a widget itself. So, you usually create a toplevel container (for example a Gtk::Window) and the widgets you plan to add to the container:

        my $window = new Gtk::Window;
        my $button = new Gtk::Button("Hello world");

Then, you actually add the widgets to the container and tell the toolkit to show them on the users' display:


Note that there are several different types of containers and each of them may lay out the widgets added to them in different ways. For example the Gtk::VBox container stacks the widgets one above the other, while the Gtk::Layout container requires you to explicitly set the position of the child widgets. Note also that some containers may contain only one widget (though that widget may be a container itself and hold other widgets).

As you may have noted there is a hierarchy of packages: the base package is Gtk::Object and Gtk::Widget derives from it. Here is a bit of ascii art that describes a part of the hierarchy:

        +- Gtk::Widget
          +- Gtk::Misc
          |  |
          |  +- Gtk::Label
          +- Gtk::Container
                 +- Gtk::Bin
                    +- Gtk::Window

A good introduction to object oriented programming in Perl is available in the perltoot manpage.

One fundamental concept in Gtk programming is signals: objects emit a signal whenever something interesting happens to them, for example when a button is clicked or when a window is closed, or when an item in a list is selected and so on (note that these signals have nothing to do with POSIX signals). Every kind of widgets has a set of signals and you can connect to a signal emission to run your code or even to stop the signal. In the same way methods are inherited by derived classes, signals may be defined in a base class and they are available to derived classes, too.

Event-driven programming concepts in Gtk

To take advantage of the Gtk module you also need to understand event-driven programming: this means that you tell the toolkit to wake your program up whenever something interesting happens and then let it wait for the events to happen in the so-called main loop (with a call to CWGtk->main()).

When you tell the library you are interested in an event, you hand it a subroutine to be called whenever that event happens. The subroutine (often called handler) will receive as arguments the data specific to the event (if any) and any additional data you passed in when installing the handler. See below for specific examples.

Note that you can use as handler a subroutine name, an anonymous subroutine or a subroutine reference.

There are several events that you may be interested in:

* Timeout events
To execute code after a certain amount of time.
        sub handler {
                my ($data) = @_;
                # do something here ...
                # return 1 if you want the handler to be called again later
                # return 0 to stop the handler from being called again
                return 0;
        my $data = "yadda";
        # execute handler after 1000 milliseconds
        my $id = Gtk->timeout_add(1000, \&handler, $data);
You may remove the timeout handler by returning a FALSE value from it or at any time by calling
where CW$id is the value returned by the CWGtk->timeout_add() call. Note that the first value to the CWGtk->timeout_add() call is the timeout in milliseconds.
* Idle events
To execute code whenever the toolkit is not busy handling other events.
        sub handler {
                my ($data) = @_;
                # do something here ...
                # return 1 if you want the handler to be called again
                # return 0 to stop the handler from being called again
                return 0;
        my $data = "yadda";
        # execute handler when there aren't other events to service
        my $id = Gtk->idle_add(\&handler, $data);
You may remove the idle handler by returning a FALSE value from it or at any time by calling
where CW$id is the value returned by the CWGtk->idle_add() call.
* I/O events
To execute code when a pipe or socket is ready to be read or written to.
        sub io_handler {
                my ($socket, $fd, $flags) = @_;
                # check here $flags to see if the events we are
                # actually interested in happened.
                # You probably also need a state machine (represented
                # here with the $need_read and $need_write scalars).
                if ($flags->{'read'} && $need_read) {
                        $socket->sysread($buffer, 1024);
                } elsif ($flags->{'write'} && $need_write) {
                        $socket->syswrite($buffer, length($buffer));
        my $socket = new IO::Socket::INET(PeerAddr => '');
        my $id = Gtk::Gdk->input_add($socket->fileno, ['read', 'write'], 
                \&io_handler, $socket);
You may remove an I/O handler at any time by calling
where CW$id is the value returned by the CWGtk::Gdk->input_add() call. Note that the I/O handler subroutine gets the additional data as first arguments and then the handler-specific data, unlike the other more common signal handlers the get the signal-specific data first. The first argument to input_add() is an integer file descriptor (see the fileno() function description in the perlfunc manpage). The second argument is an array reference that may contain the 'read', 'write' and/or 'exception' strings if you want your handler to be called when you can read or write or when you get an exception on the file descriptor, respectively.
* Perl scalar changes
To execute code whenever the value of a Perl scalar is changed.
* Gtk signals
To execute code when something happens in a Gtk::Object or Gtk::Widget (for example when a button is clicked). Every signal may have different signal-specific params that are handed over to the signal handler: these are detailed below where each specific signal is documented.
        $window = new Gtk::Window;
        $window->signal_connect('destroy', sub {
                print "Exiting...\n";

Widget descriptions


Gtk::Object is not really a widget, but it is the base class for Gtk::Widget and provides some useful methods and signals.

        # Get a notification when the object is destroyed
        $object->signal_connect('destroy', \&do_something);

        # Destroy an object (if it is a widget it will be removed from its parent)


Base class for all the widgets in Gtk.

        # show and hide a widget


        # create a toplevel window
        $window = new Gtk::Window;

        # set the title
        $window->set_title("My app - version 1-foo");

        # set the default size
        $window->set_default_size($width, $height);

        # the window should be managed by the window manager as
        # a client of $parent_window

        # Set some resizing behavior policy
        # all the values are boolean
        $window->set_policy($allow_shrink, $allow_grow, $auto_shrink);

        # Quit the main loop (and possibly exit the program) when
        # the user closes the window using the window manager
        $window->signal_connect('delete_event', sub {
                return 1;


This is a base class for Gtk::Label and Gtk::Pixmap widgets.

        # align the widget in it's allocated space
        # Values are in the range 0.0 .. 1.0
        $widget->set_alignment($xalign, $yalign);

        # set the space padding (values in pixel units)
        $widget->set_padding($xpad, $ypad);


        # create a label
        $label = new Gtk::Label("Text");

        # change the text in it (embed newlines to get multi-line labels)
        $label->set_text("Blah!\nSecond line");

        # justify the lines when there are multiple lines

        # align the label in it's allocated space
        # this is actually a method in the Gtk::Misc package
        # Values are in the range 0.0 .. 1.0
        $label->set_alignment($xalign, $yalign);


A single line text entry widget.

        # create a single line text entry
        $entry = new Gtk::Entry;

        # set the content
        $entry->set_text("Hello world");

        # get the content of the entry
        $text = $entry->get_text;

        # hide the text (useful for password entry)

        # do not allow the user to edit the text


A text entry with a drop down list. This is a compound widget that contains a Gtk::Entry (CW$combo->entry) and a Gtk::List (CW$combo->list).

        # create a combo box
        $combo = new Gtk::Combo;

        # set the string to show in the list
        $combo->set_popdown_strings(qw(apples oranges bananas));

        # get a notification when the value changes
        $combo->entry->signal_conenct('changed', sub {
                my $entry = shift;
                warn "The text is: ", $entry->get_text, "\n";


This is a container, but usually it contains just a Gtk::Label, so, if you pass a string to the Gtk::Button::new() constructor it will also create a label in it with that text, otherwise you will have to create your own widget (maybe a Gtk::Pixmap) and add that to the button later.

        # create a button with a label
        $button = new Gtk::Button("Ok");

        # create a button and later add a Gtk::Pixmap widget to it
        $button = new Gtk::Button;

        # connect to the "clicked" signal
        $button->signal_connect("clicked", sub {
                print "Hello world\n";

        # do something with the child widget
        $widget = $button->child;


A simple container that adds a frame (and optionally a label) around its child.

        # create a frame
        $frame = new Gtk::Frame("Preferences");



        # create a table
        $table = new Gtk::Table($rows, $columns, $homogeneous);

        # add the child widget (in column $top_attach and row $left_attach)
        # note that the widget can span multiple rows and columns if
        # $right_attach-$left_attach or $bottom_attach-$top_attach are
        # different from 1
        $table->attach_defaults($child, $left_attach, $right_attach, 
                                                        $top_attach, $bottom_attach);


A Splash Screen


You want to create a splash page for your Gtk-Perl application.


Create a toplevel window, and use it as your splash page.

        sub splash {

                my ($splash, $pixmap ,$xpm, $splashimage $vbox );

                $splash = new Gtk::Window();
                $pixmap =
                $xpm = "splash.xpm");
                $splashimage = new Gtk::Pixmap($p,$xpm);

                # must realize the window to access it

                $splash->{$vbox} = new Gtk::VBox(0,0);
                $splash->{'statusbar'} = new Gtk::Statusbar;


                while (Gtk->events_pending) {
                return $splash;

        sub app_init {
                my $splash = splash();
                $splash->{'statusbar'->push(1,"Doing Something ...");

                while (Gtk->events_pending) {


                $splash->{'statusbar'}->push(1,"Doing Something Else ...");

                while (Gtk->events_pending) {


                while (Gtk->events_pending) {

                $splash->{'statusbar'}->push(1,"Creating main window ...");
                while (Gtk->events_pending) {



By creating a top level window, and loading it with a pixmap (the splash image) and a status bar, you've created a splash page. You can then write updates to the status bar as you do things.

When you're done initializing, you can create the main window then delete the splash page.

Validating input in a text entry


You want to restrict the characters inserted in a text entry field to a set of valid ones.


Connect to the CWtext-insert signal, check the text for invalid characters and insert only the valid ones.

        use Gtk -init;

        my $window = new Gtk::Window;
        $window->signal_connect('destroy', sub {Gtk->main_quit});
        my $entry = new Gtk::Entry;
        # we store the signal connection id in the entry for later
        $entry->{signalid} = $entry->signal_connect (
                insert_text => \&validate);

        # We allow only uppercase characters in the entry
        sub validate {
                my ($entry, $text, $len, $pos) = @_;
                my $newtext = uc $text;
                $newtext =~ s/[^A-Z]//g;
                if (length($newtext)) {
                        # we temporarily block this handler to avoid recursion
                        $pos = $entry->insert_text($newtext, $pos);
                # we already inserted the text if it was valid: no need
                # for the entry to process this signal emission
                # return the new position of the cursor here


Paolo Molaro