docs/reference/gobject/tut_howto.xml

<?xml version='1.0' encoding="ISO-8859-1"?>
<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.  The sample source code
    associated with this section can be found in the documentation's source tarball, in the 
    <filename>sample/gobject</filename> directory:
    <itemizedlist>
      <listitem><para><filename>maman-bar.{h|c}</filename>: this is the source for a object which derives from 
      <type><link linkend="GObject">GObject</link></type> and which shows how to declare different types of methods on the object.
      </para></listitem>
      <listitem><para><filename>maman-subbar.{h|c}</filename>: this is the source for a object which derives from 
      <type>MamanBar</type> and which shows how to override some of its parent's methods.
      </para></listitem>
      <listitem><para><filename>maman-foo.{h|c}</filename>: this is the source for an object which derives from 
      <type><link linkend="GObject">GObject</link></type> and which declares a signal.
      </para></listitem>
      <listitem><para><filename>test.c</filename>: this is the main source which instantiates an instance of
      type and exercises their API.
      </para></listitem>
    </itemizedlist>
  </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>
      I personally like the first solution better: it makes reading file names easier for those with poor
      eyesight like me.
    </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.
 */

#ifndef MAMAN_BAR_H
#define MAMAN_BAR_H

/*
 * Potentially, include other headers on which this header depends.
 */

/*
 * Type macros.
 */

typedef struct _MamanBar MamanBar;
typedef struct _MamanBarClass MamanBarClass;

struct _MamanBar {
  GObject parent;
  /* instance members */
};

struct _MamanBarClass {
  GObjectClass parent;
  /* class members */
};

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

/*
 * Method definitions.
 */

#endif
</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;

  /*&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;
    
  /*&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, instantiated in the object's
            <function>init</function> function and destroyed in the object's <function>finalize</function> function.
<programlisting>
static void
maman_bar_finalize (GObject *object) {
  MamanBar *self = MAMAN_BAR (object);
  /* do stuff */
  g_free (self->priv);
}

static void
maman_bar_init (GTypeInstance *instance, gpointer g_class) {
  MamanBar *self = MAMAN_BAR (instance);
  self->priv = g_new0 (MamanBarPrivate,1);
  /* do stuff */
}
</programlisting>
          </para></listitem>

          <listitem><para>
            A similar alternative, available since GLib version 2.4, is to define a private structure in the .c file,
            declare it as a private structure in <function>maman_bar_class_init</function> using 
            <function><link linkend="g-type-class-add-private">g_type_class_add_private</link></function>.
            Instead of allocating memory in <function>maman_bar_init</function> a pointer to the private memory area is
            stored in the instance to allow convenient access to this structure.
            A private structure will then be attached to each newly created object by the GObject system.
            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.
<programlisting>
typedef struct _MamanBarPrivate MamanBarPrivate;

struct _MamanBarPrivate {
  int private_field;
};

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

static void
maman_bar_init (GTypeInstance *instance, gpointer g_class) {
  MamanBar *self = MAMAN_BAR (instance);
  self->priv = G_TYPE_INSTANCE_GET_PRIVATE (self, MAMAN_TYPE_BAR, MamanBarPrivate);
  /* do stuff */
}
</programlisting>
          </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 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 #include "maman-bar.h" or as complicated as tens of #include lines ending with 
      #include "maman-bar.h":
<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>
      Implement <function>maman_bar_get_type</function> and make sure the code compiles:
<programlisting>
GType
maman_bar_get_type (void)
{
  static GType type = 0;
  if (type == 0) {
    static const GTypeInfo info = {
      sizeof (MamanBarClass),
      NULL,   /* base_init */
      NULL,   /* base_finalize */
      NULL,   /* class_init */
      NULL,   /* class_finalize */
      NULL,   /* class_data */
      sizeof (MamanBar),
      0,      /* n_preallocs */
      NULL    /* instance_init */
      };
      type = g_type_register_static (G_TYPE_OBJECT,
                                     "MamanBarType",
                                     &amp;info, 0);
    }
    return type;
}
</programlisting>
    </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 (GTypeInstance   *instance,
                gpointer         g_class)
{
  MamanBar *self = (MamanBar *)instance;
  self->private = g_new0 (MamanBarPrivate, 1);

  /* 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>
      And make sure that you set <function>maman_bar_init</function> as the type's instance_init function
      in <function>maman_bar_get_type</function>. Make sure the code builds and runs: create an instance 
      of the object and make sure <function>maman_bar_init</function> is called (add a 
      <function><link linkend="g-print">g_print</link></function> call in it).
    </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 
      <type><link linkend="GParamSpec">GParamSpec</link></type> 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.
<programlisting>
static void
bar_class_init (MamanBarClass *klass)
{
  GObjectClass *gobject_class = G_OBJECT_CLASS (klass);
  GParamSpec *maman_param_spec;

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

  maman_param_spec = 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_property (gobject_class,
                                   PROP_MAMAN,
                                   maman_param_spec);
}
</programlisting>
      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"/>. However, I have yet to see <emphasis>any</emphasis> reasonable
      use of this feature. As such, to initialize your object instances, use by default the base_init function
      and construction properties.
      </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 must be split is two different phases: you must override
      both the dispose and the finalize class methods.
<programlisting>
struct _MamanBarPrivate {
  gboolean dispose_has_run;
};

static GObjectClass parent_class = NULL;

static void
bar_dispose (GObject *obj)
{
  MamanBar *self = (MamanBar *)obj;

  if (self->priv->dispose_has_run) {
   /* If dispose did already run, return. */
    return;
  }
  /* Make sure dispose does not run twice. */
  object->priv->dispose_has_run = TRUE;

  /* 
   * 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.
   */

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

static void
bar_finalize (GObject *obj)
{
  MamanBar *self = (MamanBar *)obj;

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

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

  gobject_class->dispose = bar_dispose;
  gobject_class->finalize = bar_finalize;

  parent_class = g_type_class_peek_parent (klass);
  g_type_class_add_private(klass,sizeof(MamanBarPrivate));
}

static void
maman_bar_init (GTypeInstance   *instance,
                gpointer         g_class)
{
  MamanBar *self = (MamanBar *)instance;
  self->priv = G_TYPE_INSTANCE_GET_PRIVATE(self, BT_TYPE_PATTERN, BtPatternPrivate);
  self->priv->dispose_has_run = FALSE;

}
</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.
      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. To do this, you need something like the following code at the start of each object method, to make
      sure the object's data is still valid before manipulating it:
<programlisting>
if (self->private->dispose_has_run) {
  /* Dispose has run. Data is not valid anymore. */
  return;
}
</programlisting>
    </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 */)
{
  /* 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;

  /* 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 */)
{
  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 not part of the C standard).
      </para>

      <para>
        In doubt, unless a user shows you hard numbers about the performance cost of the function call,
        just <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 class_init function: initialize the klass->do_action field to a pointer to the actual
        implementation. You can also make this class method pure virtual by initializing the klass->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 */)
{
  MAMAN_BAR_GET_CLASS (self)->do_action_one (self, /* parameters */);
}
void maman_bar_do_action_two (MamanBar *self, /* parameters */)
{
  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>
   A lot of people who use this idiom in GTK+ store the parent class structure pointer in a global static 
   variable to avoid the costly call to <function><link linkend="g-type-class-peek-parent">g_type_class_peek_parent</link></function> for each function call.
   Typically, the class_init callback initializes the global static variable. <filename>gtk/gtkhscale.c</filename>
   does this.
  </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. The sample source code
    associated to this section can be found in the documentation's source tarball, in the 
    <filename>sample/interface/maman-ibaz.{h|c}</filename> file.
  </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;

  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 <type><link linkend="GType">GType</link></type>
    which derives from a <type><link linkend="GObject">GObject</link></type> 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">G_TYPE_INSTANCE_GET_CLASS</link></function>
                  but with <function><link linkend="G_TYPE_INSTANCE_GET_INTERFACE">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>
    </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 initialized = FALSE;

  if (!initialized) {
    /* create interface signals here. */
    initialized = TRUE;
  }
}

GType
maman_ibaz_get_type (void)
{
  static GType type = 0;
  if (type == 0) {
    static const GTypeInfo info = {
      sizeof (MamanIbazInterface),
      maman_ibaz_base_init,   /* base_init */
      NULL,   /* base_finalize */
      NULL,   /* class_init */
      NULL,   /* class_finalize */
      NULL,   /* class_data */
      0,
      0,      /* n_preallocs */
      NULL    /* instance_init */
    };
    type = g_type_register_static (G_TYPE_INTERFACE, "MamanIbaz", &amp;info, 0);
  }
  return type;
}

void maman_ibaz_do_action (MamanIbaz *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. Source code showing how to do this
      for the <type>IBaz</type> interface defined in the previous section is located in 
      <filename>sample/interface/maman-baz.{h|c}</filename>.
    </para>
  
    <para>
      The first step is to define a normal GType. Here, we have decided to use a GType which derives from
      GObject. Its name is <type>MamanBaz</type>:
<programlisting>
#ifndef MAMAN_BAZ_H
#define MAMAN_BAZ_H

#include &lt;glib-object.h>

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


typedef struct _MamanBaz MamanBaz;
typedef struct _MamanBazClass MamanBazClass;

struct _MamanBaz {
  GObject parent;
  int instance_member;
};

struct _MamanBazClass {
  GObjectClass parent;
};

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 <function>maman_baz_get_type</function>:
<programlisting>
GType 
maman_baz_get_type (void)
{
  static GType type = 0;
  if (type == 0) {
    static const GTypeInfo info = {
      sizeof (MamanBazClass),
      NULL,   /* base_init */
      NULL,   /* base_finalize */
      NULL,   /* class_init */
      NULL,   /* class_finalize */
      NULL,   /* class_data */
      sizeof (MamanBaz),
      0,      /* n_preallocs */
      baz_instance_init    /* instance_init */
    };
    static const GInterfaceInfo ibaz_info = {
      (GInterfaceInitFunc) baz_interface_init,    /* interface_init */
      NULL,               /* interface_finalize */
      NULL                /* interface_data */
    };
    type = g_type_register_static (G_TYPE_OBJECT,
                                   "MamanBazType",
                                   &amp;info, 0);
    g_type_add_interface_static (type,
                                 MAMAN_TYPE_IBAZ,
                                 &amp;ibaz_info);
  }
  return type;
}
</programlisting>
      This function is very much like all the similar functions we looked at previously. The only interface-specific
      code present here is the call to <function><link linkend="g-type-add-interface-static">g_type_add_interface_static</link></function> which is used to inform
      the type system that this just-registered <type><link linkend="GType">GType</link></type> also implements the interface 
      <function>MAMAN_TYPE_IBAZ</function>.
    </para>
  
    <para>
      <function>baz_interface_init</function>, the interface initialization function, is also pretty simple:
<programlisting>
static void baz_do_action (MamanBaz *self)
{
  g_print ("Baz implementation of IBaz interface Action: 0x%x.\n", self->instance_member);
}
static void
baz_interface_init (gpointer   g_iface,
                    gpointer   iface_data)
{
  MamanIbazInteface *iface = (MamanIbazInteface *)g_iface;
  iface->do_action = (void (*) (MamanIbaz *self))baz_do_action;
}
static void
baz_instance_init (GTypeInstance   *instance,
                   gpointer         g_class)
{
  MamanBaz *self = MAMAN_BAZ(instance);
  self->instance_member = 0xdeadbeaf;
}
</programlisting>
      <function>baz_interface_init</function> merely initializes the interface methods to the implementations
      defined by <type>MamanBaz</type>: <function>maman_baz_do_action</function> does nothing very useful 
      but it could :)
    </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>
  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 ibar_do_another_action (MamanBar *self)
{
  g_print ("Bar implementation of IBar interface Another Action: 0x%x.\n", self->instance_member);
}

static void
ibar_interface_init (gpointer   g_iface,
                     gpointer   iface_data)
{
  MamanIbarInterface *iface = (MamanIbarInterface *)g_iface;
  iface->do_another_action = (void (*) (MamanIbar *self))ibar_do_another_action;
}


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

static void
ibaz_interface_init (gpointer   g_iface,
                    gpointer   iface_data)
{
  MamanIbazInterface *iface = (MamanIbazInterface *)g_iface;
  iface->do_action = (void (*) (MamanIbaz *self))ibaz_do_action;
}

static void
bar_instance_init (GTypeInstance   *instance,
                   gpointer         g_class)
{
  MamanBar *self = (MamanBar *)instance;
  self->instance_member = 0x666;
}

GType 
maman_bar_get_type (void)
{
  static GType type = 0;
  if (type == 0) {
    static const GTypeInfo info = {
      sizeof (MamanBarClass),
      NULL,   /* base_init */
      NULL,   /* base_finalize */
      NULL,   /* class_init */
      NULL,   /* class_finalize */
      NULL,   /* class_data */
      sizeof (MamanBar),
      0,      /* n_preallocs */
      bar_instance_init    /* instance_init */
    };
    static const GInterfaceInfo ibar_info = {
      (GInterfaceInitFunc) ibar_interface_init,   /* interface_init */
      NULL,                                       /* interface_finalize */
      NULL                                        /* interface_data */
    };
    static const GInterfaceInfo ibaz_info = {
      (GInterfaceInitFunc) ibaz_interface_init,   /* interface_init */
      NULL,                                       /* interface_finalize */
      NULL                                        /* interface_data */
    };
    type = g_type_register_static (G_TYPE_OBJECT,
                                   "MamanBarType",
                                   &amp;info, 0);
    g_type_add_interface_static (type,
                                 MAMAN_TYPE_IBAZ,
                                 &amp;ibaz_info);
    g_type_add_interface_static (type,
                                 MAMAN_TYPE_IBAR,
                                 &amp;ibar_info);
  }
  return type;
}
</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> must be invoked first on the interfaces which have
      no prerequisites and then on the others.
    </para>
  
    <para>
      Complete source code showing how to define the MamanIbar interface which requires MamanIbaz and how to 
      implement the MamanIbar interface is located in <filename>sample/interface/maman-ibar.{h|c}</filename> 
      and <filename>sample/interface/maman-bar.{h|c}</filename>.
    </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 initialized = FALSE;

  if (!initialized) {
    /* create interface signals here. */

    g_object_interface_install_property (g_iface,
                g_param_spec_string ("name",
                    "maman_ibaz_name",
                    "Name of the MamanIbaz",
                    "maman",
                    G_PARAM_READWRITE));
    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 utilized 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>
      The story for the implementers of the interface is also quite trivial. 
      An implementer shall declare and define it's properties in the usual way as 
      explained in <xref linkend="gobject-properties"/>, except for one small
      change: it shall 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;
  gint instance_member;
  gchar *name;        /* placeholder for property */
};

enum
{
  ARG_0,
  ARG_NAME
};

GType 
maman_baz_get_type (void)
{
  static GType type = 0;
  if (type == 0) {
    static const GTypeInfo info = {
      sizeof (MamanBazClass),
      NULL,   /* base_init */
      NULL,   /* base_finalize */
      baz_class_init, /* class_init */
      NULL,   /* class_finalize */
      NULL,   /* class_data */
      sizeof (MamanBaz),
      0,      /* n_preallocs */
      baz_instance_init    /* instance_init */
    };
    static const GInterfaceInfo ibaz_info = {
      (GInterfaceInitFunc) baz_interface_init,    /* interface_init */
      NULL,               /* interface_finalize */
      NULL                /* interface_data */
    };
    type = g_type_register_static (G_TYPE_OBJECT,
                                   "MamanBazType",
                                   &amp;info, 0);
    g_type_add_interface_static (type,
                                 MAMAN_TYPE_IBAZ,
                                 &amp;ibaz_info);
  }
  return type;
}

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

  gobject_class = (GObjectClass *) klass;

  parent_class = g_type_class_ref (G_TYPE_OBJECT);

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

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

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

  /* it's not null if we got it, but it might not be ours */
  g_return_if_fail (G_IS_MAMAN_BAZ (object));

  baz = MAMAN_BAZ (object);

  switch (prop_id) {
    case ARG_NAME:
      baz->name = g_value_get_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;

  /* it's not null if we got it, but it might not be ours */
  g_return_if_fail (G_IS_TEXT_PLUGIN (object));

  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;
  }
}

</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 events 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 
      uses this method. The code below shows how the user can connect a callback to the write signal. Full code
      for this simple example is located in <filename>sample/signal/maman-file.{h|c}</filename> and
      in <filename>sample/signal/test.c</filename>
<programlisting>
file = g_object_new (MAMAN_FILE_TYPE, NULL);

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

maman_file_write (file, buffer, 50);
</programlisting>
    </para>
    
    <para>
      The <type>MamanFile</type> signal is registered in the class_init function:
<programlisting>
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,
                 NULL /* class closure */,
                 NULL /* accumulator */,
                 NULL /* accu_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, guint8 *buffer, guint32 size)
{
  /* First write data. */
  /* Then, notify user of data written. */
  g_signal_emit (self, MAMAN_FILE_GET_CLASS (self)->write_signal_id,
                 0 /* details */, 
                 NULL);
}
</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.
      The header <filename>gobject/gmarshal.h</filename> defines a set of commonly
      needed closures that one can use.
    </para>
  </sect1>

  <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 genmarshall 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>


    <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>


  <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 <type><link linkend="GObjectClass">GObjectClass</link></type>
      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 <type><link linkend="GClosure">GClosure</link></type> 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>
-->