man Gtk2::api (3): Mapping the Gtk+ C API to perl

ABSTRACT

The Gtk2 module attempts to stick as close as is reasonable to the C API, to minimize the need to maintain documentation which is nearly a copy of the C API reference documentation. However, the world is not perfect, and the mappings between C and perl are not as clean and predictable as you might wish. Thus, this page described the basics of how to map the C API to the perl API, and lists various points in the API which follow neither the C API documentation nor the mapping principles.

DESCRIPTION

The canonical documentation is the C API reference at http://developer.gnome.org/doc/API/gtk/ and http://developer.gnome.org/doc/API/gdk/

There are two main sections: 'BINDING BASICS' describes the principles on which the bindings work; understanding these can lead you to guess the proper syntax to use for any given function described in the C API reference. The second section lists various specific points of difference which don't necessarily correspond with what you expect; this section is in three main parts: missing methods, renamed methods, and different call signatures.

BINDING BASICS

We avoid deprecated APIs. Many functions refer to C concepts which are alien to the bindings. Many things have replacements.

Deprecated Stuff Isn't Bound

Things that were marked as deprecated at gtk+ 2.0.0 do not appear in the bindings. This means that gtk+-1.x's GtkCList, GtkTree, and GtkText are not available. The notable exception is GtkList, which is available solely in support of GtkCombo (which was itself replaced by GtkComboBox in 2.4); it should not be used under any other circumstances. If you really need access to these old widgets, search the web for "Gtk2::Deprecated".

Some other things were deprecated during the gtk+ 2.x series, e.g. GtkOptionMenu was deprecated in favor of GtkComboBox in 2.4. Things that were marked as deprecated during the 2.x series will not be removed, basically because older versions do not have the replacements, and removing them would break backward compatibility.

Namespaces and Objects

The namespaces of the C libraries are mapped to perl packages according to scope, although in some cases the distinction may seem rather arbitrary:

 g_ => Glib  (the Glib module - distributed separately)
 gtk_ => Gtk2
 gdk_ => Gtk2::Gdk
 gdk_pixbuf_ => Gtk2::Gdk::Pixbuf
 pango_ => Gtk2::Pango

Objects get their own namespaces, in a way, as the concept of the GType is completely replaced in the perl bindings by the perl package name. This goes for GBoxed, GObject, and even things like Glib::String and Glib::Int (which are needed for specifying column types in the Gtk2::TreeModel). (Flags and enums are special --- see below.)

 GtkButton => Gtk2::Button
 GdkPixbuf => Gtk2::Gdk::Pixbuf
 GtkScrolledWindow => Gtk2::ScrolledWindow
 PangoFontDescription => Gtk2::Pango::FontDescription

With this package mapping and perl's built-in method lookup, the bindings can do object casting for you. This gives us a rather comfortably object-oriented syntax, using normal perl syntax semantics:

  in C:
    GtkWidget * b;
    b = gtk_check_button_new_with_mnemonic ("_Something");
    gtk_toggle_button_set_active (GTK_TOGGLE_BUTTON (b), TRUE);
    gtk_widget_show (b);
  in perl:
    my $b = Gtk2::CheckButton->new ('_Something');
    $b->set_active (1);
    $b->show;

You see from this that constructors for most widgets which allow mnemonics will use mnemonics by default in their ``new'' methods. For those who don't guess this right off, Gtk2::Button->new_with_mnemonic is also available. Cast macros are not necessary, and your code is a lot shorter.

Flags and Enums

Constants are handled as strings, because it's much more readable than numbers, and because it's automagical thanks to the GType system. Constants are referred to by their nicknames; basically, strip the common prefix, lower-case it, and optionally convert '_' to '-':

  GTK_WINDOW_TOPLEVEL => 'toplevel'
  GTK_BUTTONS_OK_CANCEL => 'ok-cancel' (or 'ok_cancel')

Flags are a special case. You can't (sensibly) bitwise-or these string-constants, so you provide a reference to an array of them instead. Anonymous arrays are useful here, and an empty anonymous array is a simple way to say 'no flags'.

  FOO_BAR_BAZ | FOO_BAR_QUU | FOO_BAR_QUUX => [qw/baz quu qux/]
  0 => []

In some cases you need to see if a bit is set in a bitfield; methods returning flags therefore return an overloaded object. See Glib for more details on which operations are allowed on these flag objects, but here is a quick example:

 in C:
  /* event->state is a bitfield */
  if (event->state & GDK_CONTROL_MASK) g_printerr ("control was down\n");
 in perl:
  # $event->state is a special object
  warn "control was down\n" if $event->state & "control-mask";

But this also works:

  warn "control was down\n" if $event->state * "control-mask";
  warn "control was down\n" if $event->state >= "control-mask";
  warn "control and shift were down\n"
                            if $event->state >= ["control-mask", "shift-mask"];

And treating it as an array of strings (as in older versions) is still supported:

  warn "control was down\n" if grep /control-mask/, @{ $event->state };

The gtk stock item stuff is a little different --- the GTK_STOCK_* constants are actually macros which evaluate to strings, so they aren't handled by the mechanism described above; you just specify the string, e.g., GTK_STOCK_OK => 'gtk-ok'. The full list of stock items can be found at http://developer.gnome.org/doc/API/2.0/gtk/gtk-Stock-Items.html

Memory Handling

The functions for ref'ing and unref'ing objects and free'ing boxed structures are not even mapped to perl, because it's all handled automagically by the bindings. I could write a treatise on how we're handling reference counts and object lifetimes, but all you need to know as perl developer is that it's handled for you, and the object will be alive so long as you have a perl scalar pointing to it or the object is referenced in another way, e.g. from a container.

The only thing you have to be careful about is the lifespan of non reference counted structures, which means most things derived from "Glib::Boxed". If it comes from a signal callback it might be good only until you return, or if it's the insides of another object then it might be good only while that object lives. If in doubt you can "copy". Structs from "copy" or "new" are yours and live as long as referred to from Perl.

Callbacks

Use normal perl callback/closure tricks with callbacks. The most common use you'll have for callbacks is with the Glib "signal_connect" method:

  $widget->signal_connect (event => \&event_handler, $user_data);
  $button->signal_connect (clicked => sub { warn "hi!\n" });

$user_data is optional, and with perl closures you don't often need it (see ``Persistent variables with closures'' in perlsub).

The userdata is held in a scalar, initialized from what you give in "signal_connect" etc. It's passed to the callback in usual Perl ``call by reference'' style which means the callback can modify its last argument, ie. $_[-1], to modify the held userdata. This is a little subtle, but you can use it for some ``state'' associated with the connection.

    $widget->signal_connect (activate => \&my_func, 1);
    sub my_func {
      print "activation count: $_[-1]\n";
      $_[-1] ++;
    }

Because the held userdata is a new scalar there's no change to the variable (etc) you originally passed to "signal_connect".

If you have a parent object in the userdata (or closure) you have to be careful about circular references preventing parent and child being destroyed. See ``Two-Phased Garbage Collection'' in perlobj about this generally. In Gtk2-Perl toplevel widgets like "Gtk2::Window" always need an explicit "$widget->destroy" so their "destroy" signal is a good place to break circular references. But for other widgets it's usually friendliest to avoid circularities in the first place, either by using weak references in the userdata, or possibly locating a parent dynamically with "$widget->get_ancestor".

A major change from gtk-perl (the bindings for Gtk+-1.x) is that callbacks take their arguments in the order proscribed by the C documentation, and only one value is available for user data. gtk-perl allowed you to pass multiple values for user_data, and always brought in the user_data immediately after the instance reference; this proved to be rather confusing, and did not follow the C API reference, so we decided not to do that for gtk2-perl.

Miscellaneous

In C you can only return one value from a function, and it is a common practice to modify pointers passed in to simulate returning multiple values. In perl, you can return lists; any functions which modify arguments have been changed to return them instead. A common idiom in gtk is returning gboolean, and modifying several arguments if the function returns TRUE; for such functions, the perl wrapper just returns an empty list on failure.

  in C:  foo_get_baz_and_quux (foo, &baz, &quux);
  in perl:  ($baz, $quux) = $foo->get_baz_and_quux;

Most things that take or return a GList, GSList, or array of values will use native perl arrays (or the argument stack) instead.

You don't need to specify string lengths, although string length parameters should still be available for functions dealing with binary strings. You can always use "substr" to pass different parts of a string.

Anything that uses GError in C will "croak" on failure, setting $@ to a magical exception object, which is overloaded to print as the returned error message. The ideology here is that GError is to be used for runtime exceptions, and "croak" is how you do that in perl. You can catch a croak very easily by wrapping the function in an eval:

   eval {
      my $pixbuf = Gtk2::Gdk::Pixbuf->new_from_file ($filename);
      $image->set_from_pixbuf ($pixbuf);
   };
   if ($@) {
      print "$@\n"; # prints the possibly-localized error message
      if (Glib::Error::matches ($@, 'Gtk2::Gdk::Pixbuf::Error',
                                    'unknown-format')) {
         change_format_and_try_again ();
      } elsif (Glib::Error::matches ($@, 'Glib::File::Error', 'noent')) {
         change_source_dir_and_try_again ();
      } else {
         # don't know how to handle this
         die $@;
      }
   }

This has the added advantage of letting you bunch things together as you would with a try/throw/catch block in C++ --- you get cleaner code. By using Glib::Error exception objects, you don't have to rely on string matching on a possibly localized error message; you can match errors by explicit and predictable conditions. See Glib::Error for more information.

MISSING METHODS

g_object_ref => no replacement
g_object_unref => no replacement
g_boxed_free => no replacement: The bindings do automatic memory management. You should never need to use these.
gtk_timeout_add => Glib::Timeout->add
gtk_timeout_remove => Glib::Source->remove
gtk_idle_add => Glib::Idle->add
gtk_idle_remove => Glib::Source->remove
gtk_input_add => Glib::IO->add_watch
gtk_input_remove => Glib::Source->remove: The gtk_* functions are deprecated in favor of the g_* ones. Gtk2::Helper has a wrapper for Glib::IO->add_watch which makes it behave more like gtk_input_add.
gtk_accel_group_from_accel_closure => no replacement: Because of the use of Perl subroutine references in place of GClosures, there is no way to preserve at the Perl level the one-to-one mapping between GtkAccelGroups and GClosures. Without that mapping, this function is useless.

RENAMED METHODS

gtk_aspect_frame_set => $aspect_frame->set_params: Avoid a clash with $gobject->set.

DIFFERENT CALL SIGNATURES OR SEMANTICS

As a general rule function that take a pair of parameters, a list and the number of elements in that list, will normally omit the number of elements and just accept a variable number of arguments that will be converted into the list and number of elements. In many instances parameters have been reordered so that this will work. See below for exceptions and specific cases that are not detailed in the generated Perl API reference.

Gtk2::ScrollBar vs. GtkScrollbar

These classes were incorrectly written with a capital "B" in version 1.00 and below. They have been renamed in version 1.01 and the old way to write them is deprecated, but supported.

Gtk2::ItemFactory::create_items

The n_entries parameter has been ommited and callback_data is accepted as the first parameter. All parameters after that are considered to be entries.

Gtk2::List::insert_items

Position and items parameters flipped order so that an open ended parameter list could be used. "$list->insert_items($position, $item1, $item2, ...)" (Note that GtkList and GtkListItem are deprecated and only included because GtkCombo still makes use of them, they are subject to removal at any point so you should not utilize them unless absolutely necessary.)

Gtk2::Notebook::append_page

Gtk2::Notebook::prepend_page

Gtk2::Notebook::insert_page

The C API for these functions requires a GtkWidget for the tab_label, since you can set any widget you like to be the tab label. However, the most common use is a plain Gtk2::Label; so these three functions will stringify anything passed to tab_label that's not a GtkWidget and wrap a Gtk2::Label around it for you.

Note that the "_menu" versions of these functions do not do this.

Gtk2::AccelGroup::connect

Gtk2::AccelGroup::disconnect

Where a GClosure is wanted by the C stuff, a perl subroutine reference suffices. However, because of this, there are a few subtle differences in sematics. a GClosure may be connected to only one GtkAccelGroup; however, a perl subroutine may be connected to many GtkAccelGroups (because at the binding level, a new GClosure is created on each call to ->connect). Thus, $accel_group->disconnect will disconnect the first group it finds to be connected to the given perl subroutine. To disconnect all groups attached to a subroutine, you can call disconnect with the same subroutine reference (or name) until it stops returning true.

Gtk2::Clipboard::set_with_data

Gtk2::Clipboard::set_with_owner

In C, these functions take an array of GtkTargetEntries and the number of elements in that array as the second and third parameters. In Perl, the number of target entries is implied by the number of items on the stack, and the target entries are supplied as a list at the end of the parameter list:

 $clipboard->set_with_data (\&get_func, \&clear_func, $user_data,
                            @target_entries);
 $clipboard->set_with_owner (\&get_func, \&clear_func, $owner,
                             @target_entries);

AUTHOR

muppet <scott at asofyet dot org>

COPYRIGHT

This library is free software; you can redistribute it and/or modify it under the terms of the GNU Library General Public License as published by the Free Software Foundation; either version 2.1 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Library General Public License for more details.

You should have received a copy of the GNU Library General Public License along with this library; if not, write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307 USA.