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<div class="chapter" title="How to define and implement interfaces">
<div class="titlepage"><div><div><h2 class="title">
<a name="howto-interface"></a>How to define and implement interfaces</h2></div></div></div>
<div class="toc"><dl>
<dt><span class="sect1"><a href="howto-interface.html#howto-interface-define">How to define interfaces</a></span></dt>
<dt><span class="sect1"><a href="howto-interface-implement.html">How To define implement an Interface?</a></span></dt>
<dt><span class="sect1"><a href="ch06s03.html">Interface definition prerequisites</a></span></dt>
<dt><span class="sect1"><a href="howto-interface-properties.html">Interface Properties</a></span></dt>
</dl></div>
<div class="sect1" title="How to define interfaces">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="howto-interface-define"></a>How to define interfaces</h2></div></div></div>
<p>
The bulk of interface definition has already been shown in <a class="xref" href="gtype-non-instantiable-classed.html" title="Non-instantiable classed types: interfaces">the section called “Non-instantiable classed types: interfaces”</a>
but I feel it is needed to show exactly how to create an interface.
</p>
<p>
As above, the first step is to get the header right:
</p>
<pre class="programlisting">
#ifndef __MAMAN_IBAZ_H__
#define __MAMAN_IBAZ_H__
#include <glib-object.h>
#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__ */
</pre>
<p>
This code is the same as the code for a normal <span class="type"><a class="link" href="gobject-Type-Information.html#GType" title="GType">GType</a></span>
which derives from a <span class="type"><a class="link" href="gobject-The-Base-Object-Type.html#GObject">GObject</a></span> except for a few details:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p>
The <code class="function">_GET_CLASS</code> macro is called <code class="function">_GET_INTERFACE</code>
and not implemented with <code class="function">G_TYPE_INSTANCE_GET_CLASS</code>
but with <code class="function">G_TYPE_INSTANCE_GET_INTERFACE</code>.
</p></li>
<li class="listitem"><p>
The instance type, <span class="type">MamanIbaz</span> is not fully defined: it is
used merely as an abstract type which represents an instance of
whatever object which implements the interface.
</p></li>
<li class="listitem"><p>
The parent of the <span class="type">MamanIbazInterface</span> is not
<span class="type">GObjectClass</span> but <span class="type">GTypeInterface</span>.
</p></li>
</ul></div>
<p>
</p>
<p>
The implementation of the <span class="type">MamanIbaz</span> type itself is trivial:
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p><code class="function">maman_ibaz_get_type</code> registers the
type in the type system.
</p></li>
<li class="listitem"><p><code class="function">maman_ibaz_base_init</code> 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
<a class="xref" href="gtype-non-instantiable-classed.html#gtype-non-instantiable-classed-init" title="Interface Initialization">the section called “Interface Initialization”</a>,
<code class="function">base_init</code> is run once for each interface implementation
instantiation)</p></li>
<li class="listitem"><p><code class="function">maman_ibaz_do_action</code> dereferences
the class structure to access its associated class function and calls it.
</p></li>
</ul></div>
<p>
</p>
<pre class="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)
{
static const GTypeInfo info = {
sizeof (MamanIbazInterface),
maman_ibaz_base_init, /* base_init */
NULL, /* base_finalize */
};
iface_type = g_type_register_static (G_TYPE_INTERFACE, "MamanIbaz",
&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);
}
</pre>
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