=head1 NAME

Gtk cookbook - Typical usage of the Gtk module

=head1 SINOPSYS

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

=head1 DESCRIPTION

=head2 Introduction

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:

	$window->add($button);
	$window->show_all;

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::Object
	|
	+- 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 I<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 this 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.

=head2 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 Gtk->main()).

When you tell the library you are intersted 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:

=over 4

=item * 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 

	Gtk->timeout_remove($id);

where $id is the value returned by the Gtk->timeout_add() call.

Note that the first value to the Gtk->timeout_add() call is the timeout in
milliseconds.

=item * 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 

	Gtk->idle_remove($id);

where $id is the value returned by the Gtk->idle_add() call.

=item * 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 => 'www.gtk.org:80');
	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

	Gtk::Gdk->input_remove($id);

where $id is the value returned by the Gtk::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 you when you get
an exception on the file descriptor, respectively.

=item * Perl scalar changes

To execute code whenever the value of a Perl scalar is changed.

=item * 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";
		Gtk->exit(0);
	});

=back

=head1 Widget descriptions

=head2 Gtk::Object

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)
	$object->destroy;

=head2 Gtk::Widget

Base class for all the widgets in Gtk.

=head2 Gtk::Window

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

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

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

	# the window should be managed by the window manager as
	# a client of $parent_window
	$window->set_transient_for($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 {
		Gtk->main_quit;
		return 1;
	});

=head2 Gtk::Misc

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);

=head2 Gtk::Label

	# 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
	$label->set_justify('right');
	
	# 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);

=head2 Gtk::Entry

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)
	$entry->set_visibility(0);

	# do not allow the user to edit the text
	$entry->set_editable(0);

=head2 Gtk::Combo

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

=head2 Gtk::Button

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;
	$button->add($pixmap_widget);

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

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

=head2 Gtk::Frame

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

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

	#
	$frame->set_shadow_type();
	
=head2 Gtk::Table

	# create a table
	$table = new Gtk::Table($rows, $columns, $homogeneous);
	#
	$table->attach_defaults($child, $left_attach, $right_attach, $top_attach, $bottom_attach);

=head1 AUTHOR

Paolo Molaro