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Gtk2::Buildable - Interface for objects that can be built by Gtk2::Builder


  package Thing;
  use Gtk2;
  use Glib::Object::Subclass
      # The important bit -- add this GInterface to our class
      interfaces => [ Gtk2::Buildable:: ],

      # Some signals and properties on the object...
      signals => {
          exploderize => {},
      properties => [
          Glib::ParamSpec->int ('force', 'Force',
                                'Explosive force, in megatons',
                                0, 1000000, 5, ['readable', 'writable']),

  sub exploderize {
      my $self = shift;
      $self->signal_emit ('exploderize');

  # We can accept all defaults for Buildable; see the description
  # for details on custom XML.

  package main;
  use Gtk2 -init;
  my $builder = Gtk2::Builder->new ();
  $builder->add_from_string ('<interface>
      <object class="Thing" id="thing1">
          <property name="force">50</property>
          <signal name="exploderize" handler="do_explode" />
  $builder->connect_signals ();

  my $thing = $builder->get_object ('thing1');

  $thing->exploderize ();

  sub do_explode {
      my $thing = shift;
      printf "boom * %d!\n", $thing->get ('force');

  # This program prints "boom * 50!" on stdout.




In order to allow construction from a Gtk2::Builder UI description (, an object must implement the Gtk2::Buildable interface. The interface includes methods for setting names and properties of objects, parsing custom tags, and constructing child objects.

The Gtk2::Buildable interface is implemented by all widgets and many of the non-widget objects that are provided by GTK+. The main user of this interface is Gtk2::Builder, so there should be very little need for applications to call any of the Gtk2::Buildable methods.

So, instead of focusing on how to call the methods of a Gtk2::Buildable, this documentation deals with implementing a buildable object.


Since Gtk2::Widget implements the Gtk2::Buildable interface, all widgets get buildability gratis. If your widget requires no special markup syntax to express its configuration, and all properties can be handled through the standard mechanisms, you can simply add the name of your perl-derived Glib::Object types to the object tag in the builder UI description. You don't even have to do anything special in your class definition. For example, objects of this class:

  package My::Frame;
  use Gtk2;
  use Glib::Object::Subclass
      properties => [
          Glib::ParamSpec->int ('foo', ...),



could be expressed in a builder definition file like this:

  <object class="My__Frame" id="myframe">
    <property name="foo">15</property>

Notice that the '::' package separator has been replaced with '__' in the class attribute; this is because the ':' character is not valid for GType type names. The mapping from perl package names to GType names should, in general, be as simple as transliterating the colons.


Glib::Object does not implement Gtk2::Buildable by itself, so to get a builder UI file to create your custom Glib::Object subtypes, you'll have add the Gtk2::Buildable interface to your class's interfaces list.

  package My::Thing;
  use Gtk2; # to get Gtk2::Buildable
  use Glib::Object::Subclass
      interfaces => [ 'Gtk2::Buildable' ],

Again, if you have no special requirements, then that should be all you need to do.


In some cases, you need to override the default Buildable behavior. Maybe your objects already store their names, or you need some special markup tags to express configuration. In these cases, add the Gtk2::Buildable interface to your object declaration, and implement the following methods as necessary.

Note that in the current implementation the custom tags code doesn't chain up to any buildable interfaces in superclasses. This means for instance if you implement Gtk2::Buildable on a new widget subclass then you lose the <accelerator> and <accessibility> tags normally available from Gtk2::Widget. This will likely change in the future, probably by chaining up by default for unhandled tags, maybe with a way to ask deliberately not to chain.

SET_NAME ($self, $name)
This method should store $name in $self somehow. For example, Gtk2::Widget maps this to the Gtk2::Widget's name property. If you don't implement this method, the name will be attached in object data down in C code. Implement this method if your object has some notion of "name" and it makes sense to map the XML name attribute to that.
string = GET_NAME ($self)
If you implement SET_NAME, you need to implement this method to retrieve that name.
ADD_CHILD ($self, $builder, $child, $type)
ADD_CHILD will be called to add $child to $self. $type can be used to determine the kind of child. For example, Gtk2::Container implements this method to add a child widget to the container, and Gtk2::Notebook uses $type to distinguish between "page-label" and normal children. The value of $type comes directly from the type attribute of the XML child tag.

SET_BUILDABLE_PROPERTY ($self, $builder, $name, $value)
This will be called to set the object property $name on $self, directly from the property XML tag. It is not normally necessary to implement this method, as the fallback simply calls Glib::Object::set(). Gtk2::Window implements this method to delay showing itself (i.e., setting the "visible" property) until the whole interface is created. You can also use this to handle properties that are not wired up through the Glib::Object property system (though simply creating the property is easier).

parser or undef = CUSTOM_TAG_START ($self, $builder, $child, $tagname)
When Gtk2::Builder encounters an unknown tag while parsing the definition of $self, it will call CUSTOM_TAG_START to give your code a chance to do something with it. If $tagname was encountered inside a child tag, the corresponding object will be passed in $child; otherwise, $child will be undef.

Your CUSTOM_TAG_START method should decide whether it supports $tagname. If not, return undef. If you do support it, return a blessed perl object that implements three special methods to be used to parse that tag. (These methods are defined by GLib's GMarkupParser, which is a simple SAX-style setup.)

START_ELEMENT ($self, $context, $element_name, $attributes)

TEXT ($self, $context, $text)

END_ELEMENT ($self, $context, $element_name)
Any blessed perl object that implements these methods is valid as a parser. (Ain't duck-typing great?) Gtk2::Builder will hang on to this object until the parsing is complete, and will pass it to CUSTOM_TAG_END and CUSTOM_FINISHED, so you shouldn't have to worry about its lifetime.

CUSTOM_TAG_END ($self, $builder, $child, $tagname, $parser)
This method will be called (if it exists) when the close tag for $tagname is encountered. $parser will be the object you returned from CUSTOM_TAG_START. $child is the same object-or-undef as passed to CUSTOM_TAG_START.

CUSTOM_FINISHED ($self, $builder, $child, $tagname, $parser)
This method will be called (if it exists) when the parser finishes dealing with the custom tag $tagname. $parser will be the object you returned from CUSTOM_TAG_START. $child is the same object-or-undef as passed to CUSTOM_TAG_START.

PARSER_FINISHED ($self, $builder)
If this method exists, it will be invoked when the builder finishes parsing the description data. This method is handy if you need to defer any object initialization until all of the rest of the input is parsed, most likely because you need to refer to an object that is declared after $self or you need to perform special cleanup actions. It is not normally necessary to implement this method.

object or undef = GET_INTERNAL_CHILD ($self, $builder, $childname)
This will be called to fetch an internal child of $self. Implement this method if your buildable has internal children that need to be accessed from a UI definition. For example, Gtk2::Dialog implements this to give access to its internal vbox child.


Gtk2, Glib::Interface,, Gtk2::Buildable::ParseContext


Copyright (C) 2003-2008 by the gtk2-perl team.

This software is licensed under the LGPL. See Gtk2 for a full notice.

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