glib/docs/reference/gobject/tut_howto.xml

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               "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd" [
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<part label="IV">
  <title>Tutorial</title>
  <partintro>
    <para>
      This chapter tries to answer the real-life questions of users and presents
      the most common scenario use cases I could come up with.
      The use cases are presented from most likely to less likely.
    </para>
  </partintro>

<chapter id="howto-gobject">
  <title>How to define and implement a new GObject</title>
  
  <para>
    Clearly, this is one of the most common questions people ask: they just
    want to crank code and implement a subclass of a GObject. Sometimes because
    they want to create their own class hierarchy, sometimes because they want
    to subclass one of GTK+'s widget. This chapter will focus on the 
    implementation of a subtype of GObject.
  </para>

  <sect1 id="howto-gobject-header">
    <title>Boilerplate header code</title>
    
    <para>
      The first step before writing the code for your GObject is to write the
      type's header which contains the needed type, function and macro
      definitions. Each of these elements is nothing but a convention which
      is followed not only by GTK+'s code but also by most users of GObject.
      If you feel the need not to obey the rules stated below, think about it
      twice:
      <itemizedlist>
        <listitem><para>If your users are a bit accustomed to GTK+ code or any
        GLib code, they will be a bit surprised and getting used to the
        conventions you decided upon will take time (money) and will make them
        grumpy (not a good thing)</para></listitem>
        <listitem><para>You must assess the fact that these conventions might
        have been designed by both smart and experienced people: maybe they
        were at least partly right. Try  to put your ego aside.</para></listitem>
      </itemizedlist>
    </para>

    <para>
      Pick a name convention for your headers and source code and stick to it:
      <itemizedlist>
        <listitem><para>use a dash to separate the prefix from the typename:
        <filename>maman-bar.h</filename> and <filename>maman-bar.c</filename>
        (this is the convention used by Nautilus and most GNOME libraries).</para></listitem>
        <listitem><para>use an underscore to separate the prefix from the
        typename: <filename>maman_bar.h</filename> and
        <filename>maman_bar.c</filename>.</para></listitem>
        <listitem><para>Do not separate the prefix from the typename:
        <filename>mamanbar.h</filename> and <filename>mamanbar.c</filename>.
        (this is the convention used by GTK+)</para></listitem>
      </itemizedlist>
      Some people like the first two solutions better: it makes reading file
      names easier for those with poor eyesight.
    </para>

    <para>
      When you need some private (internal) declarations in several
      (sub)classes, you can define them in a private header file which
      is often named by appending the <emphasis>private</emphasis> keyword
      to the public header name. For example, one could use
      <filename>maman-bar-private.h</filename>,
      <filename>maman_bar_private.h</filename> or
      <filename>mamanbarprivate.h</filename>. Typically, such private header
      files are not installed.
    </para>

    <para>
      The basic conventions for any header which exposes a GType are described
      in <xref linkend="gtype-conventions"/>. Most GObject-based code also
      obeys one of of the following conventions: pick one and stick to it.
      <itemizedlist>
        <listitem><para>
            If you want to declare a type named bar with prefix maman, name the type instance
            <function>MamanBar</function> and its class <function>MamanBarClass</function>
            (name is case-sensitive). It is customary to declare them with code similar to the 
            following:
<programlisting>
/*
 * Copyright/Licensing information.
 */

/* inclusion guard */
#ifndef __MAMAN_BAR_H__
#define __MAMAN_BAR_H__

#include &lt;glib-object.h&gt;
/*
 * Potentially, include other headers on which this header depends.
 */

/*
 * Type macros.
 */
#define MAMAN_TYPE_BAR                  (maman_bar_get_type ())
#define MAMAN_BAR(obj)                  (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_TYPE_BAR, MamanBar))
#define MAMAN_IS_BAR(obj)               (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_TYPE_BAR))
#define MAMAN_BAR_CLASS(klass)          (G_TYPE_CHECK_CLASS_CAST ((klass), MAMAN_TYPE_BAR, MamanBarClass))
#define MAMAN_IS_BAR_CLASS(klass)       (G_TYPE_CHECK_CLASS_TYPE ((klass), MAMAN_TYPE_BAR))
#define MAMAN_BAR_GET_CLASS(obj)        (G_TYPE_INSTANCE_GET_CLASS ((obj), MAMAN_TYPE_BAR, MamanBarClass))

typedef struct _MamanBar        MamanBar;
typedef struct _MamanBarClass   MamanBarClass;

struct _MamanBar
{
  GObject parent_instance;

  /* instance members */
};

struct _MamanBarClass
{
  GObjectClass parent_class;

  /* class members */
};

/* used by MAMAN_TYPE_BAR */
GType maman_bar_get_type (void);

/*
 * Method definitions.
 */

#endif /* __MAMAN_BAR_H__ */
</programlisting>
          </para></listitem>
        <listitem><para>
            Most GTK+ types declare their private fields in the public header
            with a /* private */ comment, relying on their user's intelligence
            not to try to play with these fields. Fields not marked private
            are considered public by default. The /* protected */ comment
            (same semantics as those of C++) is also used, mainly in the GType
            library, in code written by Tim Janik.
<programlisting>
struct _MamanBar
{
  GObject parent_instance;

  /*&lt; private &gt;*/
  int hsize;
};
</programlisting>
          </para></listitem>
        <listitem><para>
            All of Nautilus code and a lot of GNOME libraries use private
            indirection members, as described by Herb Sutter in his Pimpl
            articles(see <ulink url="http://www.gotw.ca/gotw/024.htm">Compilation Firewalls</ulink>
            and <ulink url="http://www.gotw.ca/gotw/028.htm">The Fast Pimpl Idiom</ulink>:
            he summarizes the different issues better than I will).
<programlisting>
typedef struct _MamanBarPrivate MamanBarPrivate;

struct _MamanBar
{
  GObject parent_instance;
    
  /*&lt; private &gt;*/
  MamanBarPrivate *priv;
};
</programlisting>
            <note><simpara>Do not call this <varname>private</varname>, as
            that is a registered c++ keyword.</simpara></note>

            The private structure is then defined in the .c file, using the
            g_type_class_add_private() function to notify the presence of
            a private memory area for each instance and it can either
            be retrieved using <function>G_TYPE_INSTANCE_GET_PRIVATE()</function>
            each time is needed, or assigned to the <literal>priv</literal>
            member of the instance structure inside the object's
            <function>init</function> function.
<programlisting>
#define MAMAN_BAR_GET_PRIVATE(obj) (G_TYPE_INSTANCE_GET_PRIVATE ((obj), MAMAN_TYPE_BAR, MamanBarPrivate))

struct _MamanBarPrivate
{
  int hsize;
};

static void
maman_bar_class_init (MamanBarClass *klass)
{
  g_type_class_add_private (klass, sizeof (MamanBarPrivate));
}

static void
maman_bar_init (MamanBar *self)
{
  MamanBarPrivate *priv;

  self->priv = priv = MAMAN_BAR_GET_PRIVATE (self);

  priv->hsize = 42;
}
</programlisting>
          </para></listitem>

          <listitem><para>
            You don't need to free or allocate the private structure, only the
            objects or pointers that it may contain. Another advantage of this
            to the previous version is that is lessens memory fragmentation,
            as the public and private parts of the instance memory are
            allocated at once.
          </para></listitem>
      </itemizedlist>
    </para>

    <para>
      Finally, there are different header include conventions. Again, pick one
      and stick to it. I personally use indifferently any of the two, depending
      on the codebase I work on: the rule, as always, is consistency.
      <itemizedlist>
        <listitem><para>
            Some people add at the top of their headers a number of #include
            directives to pull in all the headers needed to compile client
            code. This allows client code to simply #include "maman-bar.h".
          </para></listitem>
        <listitem><para>
            Other do not #include anything and expect the client to #include
            themselves the headers they need before including your header. This
            speeds up compilation because it minimizes the amount of
            pre-processor work. This can be used in conjunction with the
            re-declaration of certain unused types in the client code to
            minimize compile-time dependencies and thus speed up compilation.
          </para></listitem>
      </itemizedlist>
    </para>
      
  </sect1>

  <sect1 id="howto-gobject-code">
    <title>Boilerplate code</title>

    <para>
      In your code, the first step is to #include the needed headers: depending
      on your header include strategy, this can be as simple as
      <literal>#include "maman-bar.h"</literal> or as complicated as tens
      of #include lines ending with <literal>#include "maman-bar.h"</literal>:
<programlisting>
/*
 * Copyright information
 */

#include "maman-bar.h"

/* If you use Pimpls, include the private structure 
 * definition here. Some people create a maman-bar-private.h header
 * which is included by the maman-bar.c file and which contains the
 * definition for this private structure.
 */
struct _MamanBarPrivate {
  int member_1;
  /* stuff */
};

/* 
 * forward definitions
 */
</programlisting>
    </para>

    <para>
      Call the <function>G_DEFINE_TYPE</function> macro using the name
      of the type, the prefix of the functions and the parent GType to
      reduce the amount of boilerplate needed. This macro will:

      <itemizedlist>
        <listitem><simpara>implement the <function>maman_bar_get_type</function>
        function</simpara></listitem>
        <listitem><simpara>define a parent class pointer accessible from
        the whole .c file</simpara></listitem>
      </itemizedlist>

<programlisting>
G_DEFINE_TYPE (MamanBar, maman_bar, G_TYPE_OBJECT);
</programlisting>
    </para>

    <para>
      It is also possible to use the
      <function>G_DEFINE_TYPE_WITH_CODE</function> macro to control the
      get_type function implementation - for instance, to add a call to
      <function>G_IMPLEMENT_INTERFACE</function> macro which will
      call the <function>g_type_implement_interface</function> function.
    </para>
  </sect1>

  <sect1 id="howto-gobject-construction">
    <title>Object Construction</title>

    <para>
      People often get confused when trying to construct their GObjects because of the
      sheer number of different ways to hook into the objects's construction process: it is
      difficult to figure which is the <emphasis>correct</emphasis>, recommended way.
    </para>

    <para>
      <xref linkend="gobject-construction-table"/> shows what user-provided functions
      are invoked during object instantiation and in which order they are invoked.
      A user looking for the equivalent of the simple C++ constructor function should use
      the instance_init method. It will be invoked after all the parent's instance_init
      functions have been invoked. It cannot take arbitrary construction parameters 
      (as in C++) but if your object needs arbitrary parameters to complete initialization,
      you can use construction properties.
    </para>

    <para>
      Construction properties will be set only after all instance_init functions have run.
      No object reference will be returned to the client of <function><link linkend="g-object-new">g_object_new</link></function>
      until all the construction properties have been set.
    </para>

    <para>
      As such, I would recommend writing the following code first:
<programlisting>
static void
maman_bar_init (MamanBar *self)
{
  self->priv = MAMAN_BAR_GET_PRIVATE (self); 

  /* initialize all public and private members to reasonable default values. */

  /* If you need specific construction properties to complete initialization,
   * delay initialization completion until the property is set. 
   */
}
</programlisting>
    </para>

    <para>
      Now, if you need special construction properties, install the properties in the class_init function,
      override the set and get methods and implement the get and set methods as described in 
      <xref linkend="gobject-properties"/>. Make sure that these properties use a construct only 
      <link linkend="GParamSpec"><type>GParamSpec</type></link> by setting the param spec's flag field to G_PARAM_CONSTRUCT_ONLY: this helps
      GType ensure that these properties are not set again later by malicious user code.
<informalexample><programlisting>
enum {
  PROP_0,

  PROP_MAMAN,

  N_PROPERTIES
};

static GParamSpec *obj_properties[N_PROPERTIES] = { NULL, };

static void
bar_class_init (MamanBarClass *klass)
{
  GObjectClass *gobject_class = G_OBJECT_CLASS (klass);

  gobject_class->set_property = bar_set_property;
  gobject_class->get_property = bar_get_property;

  obj_properties[PROP_MAMAN] =
    g_param_spec_string ("maman",
                         "Maman construct prop",
                         "Set maman's name",
                         "no-name-set" /* default value */,
                         G_PARAM_CONSTRUCT_ONLY | G_PARAM_READWRITE);

  g_object_class_install_properties (gobject_class,
                                     N_PROPERTIES,
                                     obj_properties);
}
</programlisting></informalexample>
      If you need this, make sure you can build and run code similar to the code shown above. Make sure
      your construct properties can set correctly during construction, make sure you cannot set them 
      afterwards and make sure that if your users do not call <function><link linkend="g-object-new">g_object_new</link></function>
      with the required construction properties, these will be initialized with the default values.
    </para>

    <para>
      I consider good taste to halt program execution if a construction property is set its
      default value. This allows you to catch client code which does not give a reasonable
      value to the construction properties. Of course, you are free to disagree but you
      should have a good reason to do so.
    </para>

    <para>
      Some people sometimes need to construct their object but only after
      the construction properties have been set. This is possible through
      the use of the constructor class method as described in
      <xref linkend="gobject-instantiation"/> or, more simply, using
      the constructed class method available since GLib 2.12.
    </para>
  </sect1>

  <sect1 id="howto-gobject-destruction">
    <title>Object Destruction</title>

    <para>
      Again, it is often difficult to figure out which mechanism to use to
      hook into the object's destruction process: when the last
      <function><link linkend="g-object-unref">g_object_unref</link></function>
      function call is made, a lot of things happen as described in
      <xref linkend="gobject-destruction-table"/>.
    </para>

    <para>
      The destruction process of your object might be split in two different
      phases: dispose and the finalize.
<programlisting>
#define MAMAN_BAR_GET_PRIVATE(obj) (G_TYPE_INSTANCE_GET_PRIVATE ((obj), MAMAN_TYPE_BAR, MamanBarPrivate))

struct _MamanBarPrivate
{
  GObject *an_object;

  gchar *a_string;
};

G_DEFINE_TYPE (MamanBar, maman_bar, G_TYPE_OBJECT);

static void
maman_bar_dispose (GObject *gobject)
{
  MamanBar *self = MAMAN_BAR (gobject);

  /* 
   * In dispose, you are supposed to free all types referenced from this
   * object which might themselves hold a reference to self. Generally,
   * the most simple solution is to unref all members on which you own a 
   * reference.
   */

  /* dispose might be called multiple times, so we must guard against
   * calling g_object_unref() on an invalid GObject.
   */
  if (self->priv->an_object)
    {
      g_object_unref (self->priv->an_object);

      self->priv->an_object = NULL;
    }

  /* Chain up to the parent class */
  G_OBJECT_CLASS (maman_bar_parent_class)->dispose (gobject);
}

static void
maman_bar_finalize (GObject *gobject)
{
  MamanBar *self = MAMAN_BAR (gobject);

  g_free (self->priv->a_string);

  /* Chain up to the parent class */
  G_OBJECT_CLASS (maman_bar_parent_class)->finalize (gobject);
}

static void
maman_bar_class_init (MamanBarClass *klass)
{
  GObjectClass *gobject_class = G_OBJECT_CLASS (klass);

  gobject_class->dispose = maman_bar_dispose;
  gobject_class->finalize = maman_bar_finalize;

  g_type_class_add_private (klass, sizeof (MamanBarPrivate));
}

static void
maman_bar_init (MamanBar *self);
{
  self->priv = MAMAN_BAR_GET_PRIVATE (self);

  self->priv->an_object = g_object_new (MAMAN_TYPE_BAZ, NULL);
  self->priv->a_string = g_strdup ("Maman");
}
</programlisting>
    </para>

    <para>
      Add similar code to your GObject, make sure the code still builds
      and runs: dispose and finalize must be called during the last unref.
    </para>

    <para>
      It is possible that object methods might be invoked after dispose is
      run and before finalize runs. GObject does not consider this to be a
      program error: you must gracefully detect this and neither crash nor
      warn the user.
    </para>
  </sect1>

  <sect1 id="howto-gobject-methods">
    <title>Object methods</title>

    <para>
      Just as with C++, there are many different ways to define object
      methods and extend them: the following list and sections draw on
      C++ vocabulary. (Readers are expected to know basic C++ buzzwords.
      Those who have not had to write C++ code recently can refer to e.g.
      <ulink url="http://www.cplusplus.com/doc/tutorial/"/> to refresh
      their memories.)
      <itemizedlist>
        <listitem><para>
            non-virtual public methods,
          </para></listitem>
        <listitem><para>
            virtual public methods and
          </para></listitem>
        <listitem><para>
            virtual private methods
          </para></listitem>
      </itemizedlist>
    </para>

    <sect2>
      <title>Non-virtual public methods</title>

      <para>
        These are the simplest: you want to provide a simple method which
        can act on your object. All you need to do is to provide a function
        prototype in the header and an implementation of that prototype
        in the source file.
<programlisting>
/* declaration in the header. */
void maman_bar_do_action (MamanBar *self, /* parameters */);

/* implementation in the source file */
void
maman_bar_do_action (MamanBar *self, /* parameters */)
{
  g_return_if_fail (MAMAN_IS_BAR (self));

  /* do stuff here. */
}
</programlisting>
      </para>

      <para>There is really nothing scary about this.</para>
    </sect2>

    <sect2>
      <title>Virtual public methods</title>

      <para>
        This is the preferred way to create polymorphic GObjects. All you
        need to do is to define the common method and its class function in
        the public header, implement the common method in the source file
        and re-implement the class function in each object which inherits
        from you.
<programlisting>
/* declaration in maman-bar.h. */
struct _MamanBarClass
{
  GObjectClass parent_class;

  /* stuff */
  void (*do_action) (MamanBar *self, /* parameters */);
};

void maman_bar_do_action (MamanBar *self, /* parameters */);

/* implementation in maman-bar.c */
void
maman_bar_do_action (MamanBar *self, /* parameters */)
{
  g_return_if_fail (MAMAN_IS_BAR (self));

  MAMAN_BAR_GET_CLASS (self)->do_action (self, /* parameters */);
}
</programlisting>
        The code above simply redirects the do_action call to the relevant
        class function. Some users, concerned about performance, do not
        provide the <function>maman_bar_do_action</function> wrapper function
        and require users to dereference the class pointer themselves. This
        is not such a great idea in terms of encapsulation and makes it
        difficult to change the object's implementation afterwards, should
        this be needed.
      </para>

      <para>
        Other users, also concerned by performance issues, declare
        the <function>maman_bar_do_action</function> function inline in the
        header file. This, however, makes it difficult to change the
        object's implementation later (although easier than requiring users
        to directly dereference the class function) and is often difficult
        to write in a portable way (the <emphasis>inline</emphasis> keyword
        is part of the C99 standard but not every compiler supports it).
      </para>

      <para>
        In doubt, unless a user shows you hard numbers about the performance
        cost of the function call, just implement <function>maman_bar_do_action</function>
        in the source file.
      </para>

      <para>
        Please, note that it is possible for you to provide a default
        implementation for this class method in the object's
        <function>class_init</function> function: initialize the
        klass-&gt;do_action field to a pointer to the actual implementation.
        You can also make this class method pure virtual by initializing
        the klass-&gt;do_action field to NULL:
<programlisting>
static void
maman_bar_real_do_action_two (MamanBar *self, /* parameters */)
{
  /* Default implementation for the virtual method. */
}

static void
maman_bar_class_init (BarClass *klass)
{
  /* pure virtual method: mandates implementation in children. */
  klass->do_action_one = NULL;

  /* merely virtual method. */
  klass->do_action_two = maman_bar_real_do_action_two;
}

void
maman_bar_do_action_one (MamanBar *self, /* parameters */)
{
  g_return_if_fail (MAMAN_IS_BAR (self));

  MAMAN_BAR_GET_CLASS (self)->do_action_one (self, /* parameters */);
}

void
maman_bar_do_action_two (MamanBar *self, /* parameters */)
{
  g_return_if_fail (MAMAN_IS_BAR (self));

  MAMAN_BAR_GET_CLASS (self)->do_action_two (self, /* parameters */);
}
</programlisting>
      </para>
    </sect2>

    <sect2>
      <title>Virtual private Methods</title>

      <para>
        These are very similar to Virtual Public methods. They just don't
        have a public function to call the function directly. The header
        file contains only a declaration of the class function:
<programlisting>
/* declaration in maman-bar.h. */
struct _MamanBarClass
{
  GObjectClass parent;

  /* stuff */
  void (* helper_do_specific_action) (MamanBar *self, /* parameters */);
};

void maman_bar_do_any_action (MamanBar *self, /* parameters */);
</programlisting>
        These class functions are often used to delegate part of the job
        to child classes:
<programlisting>
/* this accessor function is static: it is not exported outside of this file. */
static void 
maman_bar_do_specific_action (MamanBar *self, /* parameters */)
{
  MAMAN_BAR_GET_CLASS (self)->do_specific_action (self, /* parameters */);
}

void
maman_bar_do_any_action (MamanBar *self, /* parameters */)
{
  /* random code here */

  /* 
   * Try to execute the requested action. Maybe the requested action
   * cannot be implemented here. So, we delegate its implementation
   * to the child class:
   */
  maman_bar_do_specific_action (self, /* parameters */);

  /* other random code here */
}
</programlisting>
      </para>

      <para>
        Again, it is possible to provide a default implementation for this
        private virtual class function:
<programlisting>
static void
maman_bar_class_init (MamanBarClass *klass)
{
  /* pure virtual method: mandates implementation in children. */
  klass->do_specific_action_one = NULL;

  /* merely virtual method. */
  klass->do_specific_action_two = maman_bar_real_do_specific_action_two;
}
</programlisting>
      </para>

      <para>
        Children can then implement the subclass with code such as:
<programlisting>
static void
maman_bar_subtype_class_init (MamanBarSubTypeClass *klass)
{
  MamanBarClass *bar_class = MAMAN_BAR_CLASS (klass);

  /* implement pure virtual class function. */
  bar_class->do_specific_action_one = maman_bar_subtype_do_specific_action_one;
}
</programlisting>
      </para>
    </sect2>
  </sect1>

  <sect1 id="howto-gobject-chainup">
    <title>Chaining up</title>
    
    <para>Chaining up is often loosely defined by the following set of
    conditions:
      <itemizedlist>
        <listitem><para>Parent class A defines a public virtual method named <function>foo</function> and 
        provides a default implementation.</para></listitem>
        <listitem><para>Child class B re-implements method <function>foo</function>.</para></listitem>
        <listitem><para>In the method B::foo, the child class B calls its parent class method A::foo.</para></listitem>
      </itemizedlist>
      There are many uses to this idiom:
      <itemizedlist>
        <listitem><para>You need to change the behaviour of a class without modifying its code. You create
          a subclass to inherit its implementation, re-implement a public virtual method to modify the behaviour
          slightly and chain up to ensure that the previous behaviour is not really modified, just extended.
          </para></listitem>
        <listitem><para>You are lazy, you have access to the source code of the parent class but you don't want 
          to modify it to add method calls to new specialized method calls: it is faster to hack the child class
          to chain up than to modify the parent to call down.</para></listitem>
        <listitem><para>You need to implement the Chain Of Responsibility pattern: each object of the inheritance
          tree chains up to its parent (typically, at the beginning or the end of the method) to ensure that
          they each handler is run in turn.</para></listitem>
      </itemizedlist>
      I am personally not really convinced any of the last two uses are really a good idea but since this
      programming idiom is often used, this section attempts to explain how to implement it.
    </para>

    <para>
      To explicitly chain up to the implementation of the virtual method in the parent class, 
      you first need a handle to the original parent class structure. This pointer can then be used to 
      access the original class function pointer and invoke it directly.
      <footnote>
        <para>
          The <emphasis>original</emphasis> adjective used in this sentence is not innocuous. To fully 
          understand its meaning, you need to recall how class structures are initialized: for each object type,
          the class structure associated to this object is created by first copying the class structure of its 
          parent type (a simple <function>memcpy</function>) and then by invoking the class_init callback on 
          the resulting class structure. Since the class_init callback is responsible for overwriting the class structure
          with the user re-implementations of the class methods, we cannot merely use the modified copy of the parent class
          structure stored in our derived instance. We want to get a copy of the class structure of an instance of the parent 
          class.
        </para>
      </footnote>
    </para>
    
    <para>The function <function><link linkend="g-type-class-peek-parent">g_type_class_peek_parent</link></function> is used to access the original parent 
    class structure. Its input is a pointer to the class of the derived object and it returns a pointer
    to the original parent class structure. The code below shows how you could use it:
<programlisting>
static void
b_method_to_call (B *obj, int a)
{
  BClass *klass;
  AClass *parent_class;

  klass = B_GET_CLASS (obj);
  parent_class = g_type_class_peek_parent (klass);

  /* do stuff before chain up */

  parent_class->method_to_call (obj, a);

  /* do stuff after chain up */
}
</programlisting>
  </para>

  </sect1>

</chapter>
<!-- End Howto GObject -->

<chapter id="howto-interface">
  <title>How to define and implement interfaces</title>

  <sect1 id="howto-interface-define">
    <title>How to define interfaces</title>
  
  <para>
    The bulk of interface definition has already been shown in <xref linkend="gtype-non-instantiable-classed"/>
    but I feel it is needed to show exactly how to create an interface.
  </para>

  <para>
    As above, the first step is to get the header right:
<programlisting>
#ifndef __MAMAN_IBAZ_H__
#define __MAMAN_IBAZ_H__

#include &lt;glib-object.h&gt;

#define MAMAN_TYPE_IBAZ                 (maman_ibaz_get_type ())
#define MAMAN_IBAZ(obj)                 (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_TYPE_IBAZ, MamanIbaz))
#define MAMAN_IS_IBAZ(obj)              (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_TYPE_IBAZ))
#define MAMAN_IBAZ_GET_INTERFACE(inst)  (G_TYPE_INSTANCE_GET_INTERFACE ((inst), MAMAN_TYPE_IBAZ, MamanIbazInterface))


typedef struct _MamanIbaz               MamanIbaz; /* dummy object */
typedef struct _MamanIbazInterface      MamanIbazInterface;

struct _MamanIbazInterface
{
  GTypeInterface parent_iface;

  void (*do_action) (MamanIbaz *self);
};

GType maman_ibaz_get_type (void);

void maman_ibaz_do_action (MamanIbaz *self);

#endif /* __MAMAN_IBAZ_H__ */
</programlisting>
    This code is the same as the code for a normal <link linkend="GType"><type>GType</type></link>
    which derives from a <link linkend="GObject"><type>GObject</type></link> except for a few details:
    <itemizedlist>
      <listitem><para>
        The <function>_GET_CLASS</function> macro is called <function>_GET_INTERFACE</function>
                  and not implemented with <function><link linkend="G-TYPE-INSTANCE-GET-CLASS:CAPS">G_TYPE_INSTANCE_GET_CLASS</link></function>
                  but with <function><link linkend="G-TYPE-INSTANCE-GET-INTERFACE:CAPS">G_TYPE_INSTANCE_GET_INTERFACE</link></function>.
      </para></listitem>
      <listitem><para>
        The instance type, <type>MamanIbaz</type> is not fully defined: it is
        used merely as an abstract type which represents an instance of
        whatever object which implements the interface.
      </para></listitem>
      <listitem><para>
        The parent of the <type>MamanIbazInterface</type> is not
        <type>GObjectClass</type> but <type>GTypeInterface</type>.
      </para></listitem>
    </itemizedlist>
  </para>

  <para>
    The implementation of the <type>MamanIbaz</type> type itself is trivial:
    <itemizedlist>
      <listitem><para><function>maman_ibaz_get_type</function> registers the
       type in the type system.
       </para></listitem>
      <listitem><para><function>maman_ibaz_base_init</function> is expected 
      to register the interface's signals if there are any (we will see a bit
      (later how to use them). Make sure to use a static local boolean variable
      to make sure not to run the initialization code twice (as described in
      <xref linkend="gtype-non-instantiable-classed-init"/>, 
      <function>base_init</function> is run once for each interface implementation 
      instantiation)</para></listitem>
      <listitem><para><function>maman_ibaz_do_action</function> dereferences
      the class structure to access its associated class function and calls it.
      </para></listitem>
    </itemizedlist>
<programlisting>
static void
maman_ibaz_base_init (gpointer g_class)
{
  static gboolean is_initialized = FALSE;

  if (!is_initialized)
    {
      /* add properties and signals to the interface here */

      is_initialized = TRUE;
    }
}

GType
maman_ibaz_get_type (void)
{
  static GType iface_type = 0;
  if (iface_type == 0)
    {
      const GTypeInfo info = {
        sizeof (MamanIbazInterface),
        maman_ibaz_base_init,   /* base_init */
        NULL,   /* base_finalize */
      };

      iface_type = g_type_register_static (G_TYPE_INTERFACE, "MamanIbaz",
                                           &amp;info, 0);
    }

  return iface_type;
}

void
maman_ibaz_do_action (MamanIbaz *self)
{
  g_return_if_fail (MAMAN_IS_IBAZ (self));

  MAMAN_IBAZ_GET_INTERFACE (self)->do_action (self);
}
</programlisting>
    </para>
  </sect1>
  
  <sect1 id="howto-interface-implement">
    <title>How To define implement an Interface?</title>
  
    <para>
      Once the interface is defined, implementing it is rather trivial.
    </para>
  
    <para>
      The first step is to define a normal GObject class, like:
<programlisting>
#ifndef __MAMAN_BAZ_H__
#define __MAMAN_BAZ_H__

#include &lt;glib-object.h&gt;

#define MAMAN_TYPE_BAZ             (maman_baz_get_type ())
#define MAMAN_BAZ(obj)             (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_TYPE_BAZ, Mamanbaz))
#define MAMAN_IS_BAZ(obj)          (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_TYPE_BAZ))
#define MAMAN_BAZ_CLASS(klass)     (G_TYPE_CHECK_CLASS_CAST ((klass), MAMAN_TYPE_BAZ, MamanbazClass))
#define MAMAN_IS_BAZ_CLASS(klass)  (G_TYPE_CHECK_CLASS_TYPE ((klass), MAMAN_TYPE_BAZ))
#define MAMAN_BAZ_GET_CLASS(obj)   (G_TYPE_INSTANCE_GET_CLASS ((obj), MAMAN_TYPE_BAZ, MamanbazClass))


typedef struct _MamanBaz        MamanBaz;
typedef struct _MamanBazClass   MamanBazClass;

struct _MamanBaz
{
  GObject parent_instance;

  int instance_member;
};

struct _MamanBazClass
{
  GObjectClass parent_class;
};

GType maman_baz_get_type (void);

#endif /* __MAMAN_BAZ_H__ */
</programlisting>
      There is clearly nothing specifically weird or scary about this header:
      it does not define any weird API or derives from a weird type.
    </para>
  
    <para>
      The second step is to implement <type>MamanBaz</type> by defining
      its GType. Instead of using <function>G_DEFINE_TYPE</function> we
      use <function>G_DEFINE_TYPE_WITH_CODE</function> and the
      <function>G_IMPLEMENT_INTERFACE</function> macros.
<programlisting>
static void maman_ibaz_interface_init (MamanIbazInterface *iface);

G_DEFINE_TYPE_WITH_CODE (MamanBar, maman_bar, G_TYPE_OBJECT,
                         G_IMPLEMENT_INTERFACE (MAMAN_TYPE_IBAZ,
                                                maman_ibaz_interface_init));
</programlisting>
      This definition is very much like all the similar functions we looked
      at previously. The only interface-specific code present here is the call
      to <function>G_IMPLEMENT_INTERFACE</function>. 
    </para>

    <note><para>Classes can implement multiple interfaces by using multiple
    calls to <function>G_IMPLEMENT_INTERFACE</function> inside the call
    to <function>G_DEFINE_TYPE_WITH_CODE</function>.</para></note>
  
    <para>
      <function>maman_baz_interface_init</function>, the interface
      initialization function: inside it every virtual method of the interface
      must be assigned to its implementation:
<programlisting>
static void
maman_baz_do_action (MamanBaz *self)
{
  g_print ("Baz implementation of IBaz interface Action: 0x%x.\n",
           self->instance_member);
}

static void
maman_ibaz_interface_init (MamanIbazInterface *iface)
{
  iface->do_action = baz_do_action;
}

static void
maman_baz_init (MamanBaz *self)
{
  MamanBaz *self = MAMAN_BAZ (instance);
  self->instance_member = 0xdeadbeaf;
}
</programlisting>
    </para>
  
  </sect1>
  
  <sect1>
    <title>Interface definition prerequisites</title>
  
    <para>
      To specify that an interface requires the presence of other interfaces
      when implemented, GObject introduces the concept of
      <emphasis>prerequisites</emphasis>: it is possible to associate
      a list of prerequisite interfaces to an interface. For example, if
      object A wishes to implement interface I1, and if interface I1 has a
      prerequisite on interface I2, A has to implement both I1 and I2.
    </para>
  
    <para>
      The mechanism described above is, in practice, very similar to
      Java's interface I1 extends interface I2. The example below shows
      the GObject equivalent:
<programlisting>
  /* inside the GType function of the MamanIbar interface */
  type = g_type_register_static (G_TYPE_INTERFACE, "MamanIbar", &amp;info, 0);

  /* Make the MamanIbar interface require MamanIbaz interface. */
  g_type_interface_add_prerequisite (type, MAMAN_TYPE_IBAZ);
</programlisting>
      The code shown above adds the MamanIbaz interface to the list of
      prerequisites of MamanIbar while the code below shows how an
      implementation can implement both interfaces and register their
      implementations:
<programlisting>
static void
maman_ibar_do_another_action (MamanIbar *ibar)
{
  MamanBar *self = MAMAN_BAR (ibar);

  g_print ("Bar implementation of IBar interface Another Action: 0x%x.\n",
           self->instance_member);
}

static void
maman_ibar_interface_init (MamanIbarInterface *iface)
{
  iface->do_another_action = maman_ibar_do_another_action;
}

static void
maman_ibaz_do_action (MamanIbaz *ibaz)
{
  MamanBar *self = MAMAN_BAR (ibaz);

  g_print ("Bar implementation of IBaz interface Action: 0x%x.\n",
           self->instance_member);
}

static void
maman_ibaz_interface_init (MamanIbazInterface *iface)
{
  iface->do_action = maman_ibaz_do_action;
}

static void
maman_bar_class_init (MamanBarClass *klass)
{

}

static void
maman_bar_init (MamanBar *self)
{
  self->instance_member = 0x666;
}

G_DEFINE_TYPE_WITH_CODE (MamanBar, maman_bar, G_TYPE_OBJECT,
                         G_IMPLEMENT_INTERFACE (MAMAN_TYPE_IBAZ,
                                                maman_ibaz_interface_init)
                         G_IMPLEMENT_INTERFACE (MAMAN_TYPE_IBAR,
                                                maman_ibar_interface_init));
</programlisting>
      It is very important to notice that the order in which interface
      implementations are added to the main object is not random:
      <function><link linkend="g-type-add-interface-static">g_type_add_interface_static</link></function>,
      which is called by <function>G_IMPLEMENT_INTERFACE</function>, must be
      invoked first on the interfaces which have no prerequisites and then on
      the others.
    </para>
  </sect1>
  
  <sect1 id="howto-interface-properties">
    <title>Interface Properties</title>
  
    <para>
      Starting from version 2.4 of GLib, GObject interfaces can also have
      properties. Declaration of the interface properties is similar to
      declaring the properties of ordinary GObject types as explained in
      <xref linkend="gobject-properties"/>, 
      except that <function><link linkend="g-object-interface-install-property">g_object_interface_install_property</link></function> is used to 
      declare the properties instead of <function><link linkend="g-object-class-install-property">g_object_class_install_property</link></function>.
    </para>
  
    <para>
      To include a property named 'name' of type <type>string</type> in the 
      <type>maman_ibaz</type> interface example code above, we only need to
      add one 
      <footnote>
        <para>
          That really is one line extended to six for the sake of clarity
        </para>
      </footnote>  
      line in the <function>maman_ibaz_base_init</function>
      <footnote>
        <para>
          The <function><link linkend="g-object-interface-install-property">g_object_interface_install_property</link></function>
          can also be called from <function>class_init</function> but it must
          not be called after that point.
        </para>
      </footnote>
      as shown below:
<programlisting>
static void
maman_ibaz_base_init (gpointer g_iface)
{
  static gboolean is_initialized = FALSE;

  if (!is_initialized)
    {
      g_object_interface_install_property (g_iface,
                                           g_param_spec_string ("name",
                                                                "Name",
                                                                "Name of the MamanIbaz",
                                                                "maman",
                                                                G_PARAM_READWRITE));
      is_initialized = TRUE;
    }
}
</programlisting>
    </para>
  
    <para>
      One point worth noting is that the declared property wasn't assigned an 
      integer ID. The reason being that integer IDs of properties are used
      only inside the get and set methods and since interfaces do not
      implement properties, there is no need to assign integer IDs to
      interface properties.
    </para>
    
    <para>
      An implementation shall declare and define it's properties in the usual
      way as explained in <xref linkend="gobject-properties"/>, except for one
      small change: it must declare the properties of the interface it
      implements using <function><link linkend="g-object-class-override-property">g_object_class_override_property</link></function>
      instead of <function><link linkend="g-object-class-install-property">g_object_class_install_property</link></function>.
      The following code snippet shows the modifications needed in the
      <type>MamanBaz</type> declaration and implementation above:
<programlisting>

struct _MamanBaz
{
  GObject parent_instance;

  gint instance_member;
  gchar *name;
};

enum
{
  PROP_0,

  PROP_NAME
};

static void
maman_baz_set_property (GObject      *object,
                        guint         property_id,
                        const GValue *value,
                        GParamSpec   *pspec)
{
  MamanBaz *baz = MAMAN_BAZ (object);
  GObject *obj;

  switch (prop_id)
    {
    case ARG_NAME:
      g_free (baz->name);
      baz->name = g_value_dup_string (value);
      break;

    default:
      G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
      break;
    }
}

static void
maman_baz_get_property (GObject    *object,
                        guint       prop_id,
                        GValue     *value,
                        GParamSpec *pspec)
{
  MamanBaz *baz = MAMAN_BAZ (object);

  switch (prop_id)
    {
    case ARG_NAME:
      g_value_set_string (value, baz->name);
      break;

    default:
      G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
      break;
    }
}

static void
maman_baz_class_init (MamanBazClass *klass)
{
  GObjectClass *gobject_class = G_OBJECT_CLASS (klass);

  gobject_class->set_property = maman_baz_set_property;
  gobject_class->get_property = maman_baz_get_property;

  g_object_class_override_property (gobject_class, PROP_NAME, "name");
}

</programlisting>
    </para>
  
  </sect1>
</chapter>
<!-- End Howto Interfaces -->

<chapter id="howto-signals">
  <title>How to create and use signals</title>

  <para>
    The signal system which was built in GType is pretty complex and
    flexible: it is possible for its users to connect at runtime any
    number of callbacks (implemented in any language for which a binding
    exists)
    <footnote>
      <para>A Python callback can be connected to any signal on any
      C-based GObject.
      </para>
    </footnote>
    to any signal and to stop the emission of any signal at any 
    state of the signal emission process. This flexibility makes it
    possible to use GSignal for much more than just emit signals which
    can be received by numerous clients. 
  </para>

  <sect1 id="howto-simple-signals">
    <title>Simple use of signals</title>

    <para>
      The most basic use of signals is to implement simple event
      notification: for example, if we have a MamanFile object, and
      if this object has a write method, we might wish to be notified
      whenever someone has changed something via our MamanFile instance.
      The code below shows how the user can connect a callback to the
      "changed" signal.
<programlisting>
file = g_object_new (MAMAN_FILE_TYPE, NULL);

g_signal_connect (file, "changed", G_CALLBACK (changed_event), NULL);

maman_file_write (file, buffer, strlen (buffer));
</programlisting>
    </para>
    
    <para>
      The <type>MamanFile</type> signal is registered in the class_init
      function:
<programlisting>
file_signals[CHANGED] = 
  g_signal_newv ("changed",
                 G_TYPE_FROM_CLASS (gobject_class),
                 G_SIGNAL_RUN_LAST | G_SIGNAL_NO_RECURSE | G_SIGNAL_NO_HOOKS,
                 NULL /* closure */,
                 NULL /* accumulator */,
                 NULL /* accumulator data */,
                 g_cclosure_marshal_VOID__VOID,
                 G_TYPE_NONE /* return_type */,
                 0     /* n_params */,
                 NULL  /* param_types */);
</programlisting>
      and the signal is emitted in <function>maman_file_write</function>:
<programlisting>
void
maman_file_write (MamanFile    *self,
                  const guchar *buffer,
                  gssize        size)
{
  /* First write data. */

  /* Then, notify user of data written. */
  g_signal_emit (self, file_signals[CHANGED], 0 /* details */);
}
</programlisting>
      As shown above, you can safely set the details parameter to zero if
      you do not know what it can be used for. For a discussion of what you
      could used it for, see <xref linkend="signal-detail"/>
    </para>

    <para>
      The signature of the signal handler in the above example is defined as
      <function>g_cclosure_marshal_VOID__VOID</function>. Its name follows
      a simple convention which encodes the function parameter and return value
      types in the function name. Specifically, the value in front of the
      double underscore is the type of the return value, while the value(s)
      after the double underscore denote the parameter types.
    </para>

    <para>
      The header <filename>gobject/gmarshal.h</filename> defines a set of
      commonly needed closures that one can use. If you want to have complex
      marshallers for your signals you should probably use glib-genmarshal
      to autogenerate them from a file containing their return and
      parameter types.
    </para>
  </sect1>

<!-- 
  this is utterly wrong and should be completely removed - or rewritten
  with a better example than writing a buffer using synchronous signals.

  <sect1>
    <title>How to provide more flexibility to users?</title>

    <para>
      The previous implementation does the job but the signal facility of
      GObject can be used to provide even more flexibility to this file
      change notification mechanism. One of the key ideas is to make the
      process of writing data to the file part of the signal emission
      process to allow users to be notified either before or after the
      data is written to the file.
    </para>
    
    <para>
      To integrate the process of writing the data to the file into the
      signal emission mechanism, we can register a default class closure
      for this signal which will be invoked during the signal emission,
      just like any other user-connected signal handler. 
    </para>
    
    <para>
      The first step to implement this idea is to change the signature of
      the signal: we need to pass around the buffer to write and its size.
      To do this, we use our own marshaller which will be generated
      through GLib's glib-genmarshal tool. We thus create a file named <filename>marshall.list</filename> which contains
      the following single line:
<programlisting>
VOID:POINTER,UINT
</programlisting>
      and use the Makefile provided in <filename>sample/signal/Makefile</filename> to generate the file named
      <filename>maman-file-complex-marshall.c</filename>. This C file is finally included in 
      <filename>maman-file-complex.c</filename>.
    </para>

    <para>
      Once the marshaller is present, we register the signal and its marshaller in the class_init function 
      of the object <type>MamanFileComplex</type> (full source for this object is included in 
      <filename>sample/signal/maman-file-complex.{h|c}</filename>):
<programlisting>
GClosure *default_closure;
GType param_types[2];

default_closure = g_cclosure_new (G_CALLBACK (default_write_signal_handler),
                                  (gpointer)0xdeadbeaf /* user_data */, 
                                  NULL /* destroy_data */);

param_types[0] = G_TYPE_POINTER;
param_types[1] = G_TYPE_UINT;
klass->write_signal_id = 
  g_signal_newv ("write",
                 G_TYPE_FROM_CLASS (g_class),
                 G_SIGNAL_RUN_LAST | G_SIGNAL_NO_RECURSE | G_SIGNAL_NO_HOOKS,
                 default_closure /* class closure */,
                 NULL /* accumulator */,
                 NULL /* accu_data */,
                 maman_file_complex_VOID__POINTER_UINT,
                 G_TYPE_NONE /* return_type */,
                 2     /* n_params */,
                 param_types /* param_types */);
</programlisting>
      The code shown above first creates the closure which contains the code to complete the file write. This
      closure is registered as the default class_closure of the newly created signal.
    </para>

    <para>
      Of course, you need to implement completely the code for the default closure since I just provided
      a skeleton:
<programlisting>
static void
default_write_signal_handler (GObject *obj, guint8 *buffer, guint size, gpointer user_data)
{
  g_assert (user_data == (gpointer)0xdeadbeaf);
  /* Here, we trigger the real file write. */
  g_print ("default signal handler: 0x%x %u\n", buffer, size);
}
</programlisting>
    </para>

    <para>
      Finally, the client code must invoke the <function>maman_file_complex_write</function> function which 
      triggers the signal emission:
<programlisting>
void maman_file_complex_write (MamanFileComplex *self, guint8 *buffer, guint size)
{
  /* trigger event */
  g_signal_emit (self,
                 MAMAN_FILE_COMPLEX_GET_CLASS (self)->write_signal_id,
                 0, /* details */
                 buffer, size);
}
</programlisting>
    </para>
    
    <para>
      The client code (as shown in <filename>sample/signal/test.c</filename> and below) can now connect signal handlers before 
      and after the file write is completed: since the default signal handler which does the write itself runs during the 
      RUN_LAST phase of the signal emission, it will run after all handlers connected with <function><link linkend="g-signal-connect">g_signal_connect</link></function>
      and before all handlers connected with <function><link linkend="g-signal-connect-after">g_signal_connect_after</link></function>. If you intent to write a GObject
      which emits signals, I would thus urge you to create all your signals with the G_SIGNAL_RUN_LAST such that your users
      have a maximum of flexibility as to when to get the event. Here, we combined it with G_SIGNAL_NO_RECURSE and 
      G_SIGNAL_NO_HOOKS to ensure our users will not try to do really weird things with our GObject. I strongly advise you
      to do the same unless you really know why (in which case you really know the inner workings of GSignal by heart and
      you are not reading this).
    </para>
    
    <para>
<programlisting>
static void complex_write_event_before (GObject *file, guint8 *buffer, guint size, gpointer user_data)
{
  g_assert (user_data == NULL);
  g_print ("Complex Write event before: 0x%x, %u\n", buffer, size);
}

static void complex_write_event_after (GObject *file, guint8 *buffer, guint size, gpointer user_data)
{
  g_assert (user_data == NULL);
  g_print ("Complex Write event after: 0x%x, %u\n", buffer, size);
}

static void test_file_complex (void)
{
  guint8 buffer[100];
  GObject *file;

  file = g_object_new (MAMAN_FILE_COMPLEX_TYPE, NULL);

  g_signal_connect (G_OBJECT (file), "write",
                    (GCallback)complex_write_event_before,
                    NULL);

  g_signal_connect_after (G_OBJECT (file), "write",
                          (GCallback)complex_write_event_after,
                          NULL);

  maman_file_complex_write (MAMAN_FILE_COMPLEX (file), buffer, 50);

  g_object_unref (G_OBJECT (file));
}
</programlisting>
      The code above generates the following output on my machine:
<programlisting>
Complex Write event before: 0xbfffe280, 50
default signal handler: 0xbfffe280 50
Complex Write event after: 0xbfffe280, 50
</programlisting>
    </para>

-->

<!--
  this is also utterly wrong on so many levels that I don't even want
  to enumerate them. it's also full of completely irrelevant footnotes
  about personal preferences demonstrating a severe lack of whatsoever
  clue. the whole idea of storing the signal ids inside the Class
  structure is so fundamentally flawed that I'll require a frontal
  lobotomy just to forget I've ever seen it.

    <sect2>
    <title>How most people do the same thing with less code</title>
    
      <para>For many historic reasons related to how the ancestor of GObject used to work in GTK+ 1.x versions,
        there is a much <emphasis>simpler</emphasis> 
        <footnote>
          <para>I personally think that this method is horribly mind-twisting: it adds a new indirection
          which unnecessarily complicates the overall code path. However, because this method is widely used
          by all of GTK+ and GObject code, readers need to understand it. The reason why this is done that way
          in most of GTK+ is related to the fact that the ancestor of GObject did not provide any other way to
          create a signal with a default handler than this one. Some people have tried to justify that it is done
          that way because it is better, faster (I am extremely doubtful about the faster bit. As a matter of fact,
          the better bit also mystifies me ;-). I have the feeling no one really knows and everyone does it
          because they copy/pasted code from code which did the same. It is probably better to leave this 
          specific trivia to hacker legends domain...
          </para>
        </footnote>
        way to create a signal with a default handler than to create 
        a closure by hand and to use the <function><link linkend="g-signal-newv">g_signal_newv</link></function>.
      </para>
    
      <para>For example, <function><link linkend="g-signal-new">g_signal_new</link></function> can be used to create a signal which uses a default 
        handler which is stored in the class structure of the object. More specifically, the class structure 
        contains a function pointer which is accessed during signal emission to invoke the default handler and
        the user is expected to provide to <function><link linkend="g-signal-new">g_signal_new</link></function> the offset from the start of the
        class structure to the function pointer.
          <footnote>
            <para>I would like to point out here that the reason why the default handler of a signal is named everywhere
             a class_closure is probably related to the fact that it used to be really a function pointer stored in
             the class structure.
            </para>
          </footnote>
      </para>
    
      <para>The following code shows the declaration of the <type>MamanFileSimple</type> class structure which contains
        the <function>write</function> function pointer.
<programlisting>
struct _MamanFileSimpleClass {
  GObjectClass parent;
        
  guint write_signal_id;

  /* signal default handlers */
  void (*write) (MamanFileSimple *self, guint8 *buffer, guint size);
};
</programlisting>
        The <function>write</function> function pointer is initialized in the class_init function of the object
        to <function>default_write_signal_handler</function>:
<programlisting>
static void
maman_file_simple_class_init (gpointer g_class,
                               gpointer g_class_data)
{
  GObjectClass *gobject_class = G_OBJECT_CLASS (g_class);
  MamanFileSimpleClass *klass = MAMAN_FILE_SIMPLE_CLASS (g_class);

  klass->write = default_write_signal_handler;
</programlisting>
        Finally, the signal is created with <function><link linkend="g-signal-new">g_signal_new</link></function> in the same class_init function:
<programlisting>
klass->write_signal_id = 
 g_signal_new ("write",
               G_TYPE_FROM_CLASS (g_class),
               G_SIGNAL_RUN_LAST | G_SIGNAL_NO_RECURSE | G_SIGNAL_NO_HOOKS,
               G_STRUCT_OFFSET (MamanFileSimpleClass, write),
               NULL /* accumulator */,
               NULL /* accu_data */,
               maman_file_complex_VOID__POINTER_UINT,
               G_TYPE_NONE /* return_type */,
               2     /* n_params */,
               G_TYPE_POINTER,
               G_TYPE_UINT);
</programlisting>
        Of note, here, is the 4th argument to the function: it is an integer calculated by the <function><link linkend="G-STRUCT-OFFSET">G_STRUCT_OFFSET</link></function>
        macro which indicates the offset of the member <emphasis>write</emphasis> from the start of the 
        <type>MamanFileSimpleClass</type> class structure.
        <footnote>
          <para>GSignal uses this offset to create a special wrapper closure 
           which first retrieves the target function pointer before calling it.
          </para>
        </footnote>
     </para>

     <para>
       While the complete code for this type of default handler looks less cluttered as shown in 
       <filename>sample/signal/maman-file-simple.{h|c}</filename>, it contains numerous subtleties.
       The main subtle point which everyone must be aware of is that the signature of the default 
       handler created that way does not have a user_data argument: 
       <function>default_write_signal_handler</function> is different in 
       <filename>sample/signal/maman-file-complex.c</filename> and in 
       <filename>sample/signal/maman-file-simple.c</filename>.
     </para>

     <para>If you have doubts about which method to use, I would advise you to use the second one which
       involves <function><link linkend="g-signal-new">g_signal_new</link></function> rather than <function><link linkend="g-signal-newv">g_signal_newv</link></function>: 
       it is better to write code which looks like the vast majority of other GTK+/GObject code than to
       do it your own way. However, now, you know why.
     </para>

   </sect2>

  </sect1>
-->

<!--
  yet another pointless section. if we are scared of possible abuses
  from the users then we should not be mentioning it inside a tutorial
  for beginners. but, obviously, there's nothing to be afraid of - it's
  just that this section must be completely reworded.

  <sect1>
    <title>How users can abuse signals (and why some think it is good)</title>

    <para>Now that you know how to create signals to which the users can connect easily and at any point in
      the signal emission process thanks to <function><link linkend="g-signal-connect">g_signal_connect</link></function>, 
      <function><link linkend="g-signal-connect-after">g_signal_connect_after</link></function> and G_SIGNAL_RUN_LAST, it is time to look into how your
      users can and will screw you. This is also interesting to know how you too, can screw other people.
      This will make you feel good and eleet.
    </para>
    
    <para>
      The users can:
      <itemizedlist>
         <listitem><para>stop the emission of the signal at anytime</para></listitem>
         <listitem><para>override the default handler of the signal if it is stored as a function
           pointer in the class structure (which is the preferred way to create a default signal handler,
           as discussed in the previous section).</para></listitem>
       </itemizedlist> 
    </para>
    
    <para>
      In both cases, the original programmer should be as careful as possible to write code which is
      resistant to the fact that the default handler of the signal might not able to run. This is obviously
      not the case in the example used in the previous sections since the write to the file depends on whether
      or not the default handler runs (however, this might be your goal: to allow the user to prevent the file 
      write if he wishes to).
    </para>
    
    <para>
      If all you want to do is to stop the signal emission from one of the callbacks you connected yourself,
      you can call <function><link linkend="g-signal-stop-by-name">g_signal_stop_by_name</link></function>. Its use is very simple which is why I won't detail 
      it further.
    </para>
    
    <para>
      If the signal's default handler is just a class function pointer, it is also possible to override 
      it yourself from the class_init function of a type which derives from the parent. That way, when the signal
      is emitted, the parent class will use the function provided by the child as a signal default handler.
      Of course, it is also possible (and recommended) to chain up from the child to the parent's default signal 
      handler to ensure the integrity of the parent object.
    </para>
    
    <para>
      Overriding a class method and chaining up was demonstrated in <xref linkend="howto-gobject-methods"/> 
      which is why I won't bother to show exactly how to do it here again.
    </para>

  </sect1>

-->

</chapter>

<!--
  <sect2>
    <title>Warning on signal creation and default closure</title>

    <para>
      Most of the existing code I have seen up to now (in both GTK+, GNOME libraries and
      many GTK+ and GNOME applications) using signals uses a small
      variation of the default handler pattern I have shown in the previous section.
    </para>

    <para>
      Usually, the <function><link linkend="g-signal-new">g_signal_new</link></function> function is preferred over
      <function><link linkend="g-signal-newv">g_signal_newv</link></function>. When <function><link linkend="g-signal-new">g_signal_new</link></function>
      is used, the default closure is exported as a class function. For example,
      <filename>gobject.h</filename> contains the declaration of <link linkend="GObjectClass"><type>GObjectClass</type></link>
      whose notify class function is the default handler for the <emphasis>notify</emphasis>
      signal:
<programlisting>
struct  _GObjectClass
{
  GTypeClass   g_type_class;

  /* class methods and other stuff. */

  /* signals */
  void (*notify) (GObject     *object,
                  GParamSpec  *pspec);
};
</programlisting>
     </para>

     <para>
       <filename>gobject.c</filename>'s <function><link linkend="g-object-do-class-init">g_object_do_class_init</link></function> function
       registers the <emphasis>notify</emphasis> signal and initializes this class function
       to NULL:
<programlisting>
static void
g_object_do_class_init (GObjectClass *class)
{

  /* Stuff */

  class->notify = NULL;

  gobject_signals[NOTIFY] =
    g_signal_new ("notify",
                  G_TYPE_FROM_CLASS (class),
                  G_SIGNAL_RUN_FIRST | G_SIGNAL_NO_RECURSE | G_SIGNAL_DETAILED | G_SIGNAL_NO_HOOKS,
                  G_STRUCT_OFFSET (GObjectClass, notify),
                  NULL, NULL,
                  g_cclosure_marshal_VOID__PARAM,
                  G_TYPE_NONE,
                  1, G_TYPE_PARAM);
}
</programlisting>
       <function><link linkend="g-signal-new">g_signal_new</link></function> creates a <link linkend="GClosure"><type>GClosure</type></link> which dereferences the
       type's class structure to access the class function pointer and invoke it if it not NULL. The
       class function is ignored it is set to NULL.
     </para>

     <para>
       To understand the reason for such a complex scheme to access the signal's default handler, 
       you must remember the whole reason for the use of these signals. The goal here is to delegate
       a part of the process to the user without requiring the user to subclass the object to override
       one of the class functions. The alternative to subclassing, that is, the use of signals
       to delegate processing to the user, is, however, a bit less optimal in terms of speed: rather
       than just dereferencing a function pointer in a class structure, you must start the whole
       process of signal emission which is a bit heavyweight.
     </para>

     <para>
       This is why some people decided to use class functions for some signal's default handlers:
       rather than having users connect a handler to the signal and stop the signal emission
       from within that handler, you just need to override the default class function which is
       supposedly more efficient.
     </para>

    </sect2>
-->
</part>