glib/docs/reference/gio/migrating-gdbus.xml

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<chapter>
<title>Migrating to GDBus</title>
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<section>
<title>Conceptual differences</title>
<para>
The central concepts of D-Bus are modelled in a very similar way
in dbus-glib and GDBus. Both have a objects representing connections,
proxies and method invocations. But there are some important
differences:
<itemizedlist>
<listitem><para>
dbus-glib uses libdbus, GDBus doesn't. Instead, it relies on GIO
streams as transport layer, and has its own implementation for the
the D-Bus connection setup and authentication. Apart from using
streams as transport, avoiding libdbus also lets GDBus avoid some
thorny multithreading issues.
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</para></listitem>
<listitem><para>
dbus-glib uses the GObject type system for method arguments and
return values, including a homegrown container specialization
mechanism. GDBus relies uses the #GVariant type system which is
explicitly designed to match D-Bus types.
</para></listitem>
<listitem><para>
The typical way to export an object in dbus-glib involves generating
glue code from XML introspection data using <command>dbus-binding-tool</command>. GDBus does not (yet?) use code generation; you are expected to
embed the introspection data in your application code.
</para></listitem>
</itemizedlist>
</para>
</section>
<section>
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<title>API comparison</title>
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<table id="dbus-glib-vs-gdbus">
<title>dbus-glib APIs and their GDBus counterparts</title>
<tgroup cols="2">
<thead>
<row><entry>dbus-glib</entry><entry>GDBus</entry></row>
</thead>
<tbody>
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<row><entry>#DBusGConnection</entry><entry>#GDBusConnection</entry></row>
<row><entry>#DBusGProxy</entry><entry>#GDBusProxy</entry></row>
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<row><entry>#DBusGMethodInvocation</entry><entry>#GDBusMethodInvocation</entry></row>
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<row><entry>dbus_g_bus_get()</entry><entry>g_bus_get_sync(), also see
g_bus_get()</entry></row>
<row><entry>dbus_g_proxy_new_for_name()</entry><entry>g_dbus_proxy_new_sync() and
g_dbus_proxy_new_for_bus_sync(), also see g_dbus_proxy_new()</entry></row>
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<row><entry>dbus_g_proxy_add_signal()</entry><entry>not needed, use the generic #GDBusProxy::g-signal</entry></row>
<row><entry>dbus_g_proxy_connect_signal()</entry><entry>use g_signal_connect() with #GDBusProxy::g-signal</entry></row>
<row><entry>dbus_g_connection_register_g_object()</entry><entry>g_dbus_connection_register_object()</entry></row>
<row><entry>dbus_g_connection_unregister_g_object()</entry><entry>g_dbus_connection_unregister_object()</entry></row>
<row><entry>dbus_g_object_type_install_info()</entry><entry>introspection data is installed while registering
an object, see g_dbus_connection_register_object()</entry></row>
<row><entry>dbus_g_proxy_begin_call()</entry><entry>g_dbus_proxy_call()</entry></row>
<row><entry>dbus_g_proxy_end_call()</entry><entry>g_dbus_proxy_call_finish()</entry></row>
<row><entry>dbus_g_proxy_call()</entry><entry>g_dbus_proxy_call_sync()</entry></row>
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<row><entry>dbus_g_error_domain_register()</entry><entry>g_dbus_error_register_error_domain()</entry></row>
<row><entry>dbus_g_error_has_name()</entry><entry>no direct equivalent, see g_dbus_error_get_remote_error()</entry></row>
<row><entry>dbus_g_method_return()</entry><entry>g_dbus_method_invocation_return_value()</entry></row>
<row><entry>dbus_g_method_return_error()</entry><entry>g_dbus_method_invocation_return_error() and variants</entry></row>
<row><entry>dbus_g_method_get_sender()</entry><entry>g_dbus_method_invocation_get_sender()</entry></row>
</tbody>
</tgroup>
</table>
</section>
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<section>
<title>Owning bus names</title>
<para>
Using dbus-glib, you typically call RequestName manually
to own a name, like in the following excerpt:
<informalexample><programlisting><![CDATA[
error = NULL;
res = dbus_g_proxy_call (system_bus_proxy,
"RequestName",
&error,
G_TYPE_STRING, NAME_TO_CLAIM,
G_TYPE_UINT, DBUS_NAME_FLAG_ALLOW_REPLACEMENT,
G_TYPE_INVALID,
G_TYPE_UINT, &result,
G_TYPE_INVALID);
if (!res)
{
if (error != NULL)
{
g_warning ("Failed to acquire %s: %s",
NAME_TO_CLAIM, error->message);
g_error_free (error);
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}
else
{
g_warning ("Failed to acquire %s", NAME_TO_CLAIM);
}
goto out;
}
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if (result != DBUS_REQUEST_NAME_REPLY_PRIMARY_OWNER)
{
if (error != NULL)
{
g_warning ("Failed to acquire %s: %s",
NAME_TO_CLAIM, error->message);
g_error_free (error);
}
else
{
g_warning ("Failed to acquire %s", NAME_TO_CLAIM);
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}
exit (1);
}
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dbus_g_proxy_add_signal (system_bus_proxy, "NameLost",
G_TYPE_STRING, G_TYPE_INVALID);
dbus_g_proxy_connect_signal (system_bus_proxy, "NameLost",
G_CALLBACK (on_name_lost), NULL, NULL);
/* further setup ... */
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]]>
</programlisting></informalexample>
</para>
<para>
While you can do things this way with GDBus too, using
g_dbus_proxy_call_sync(), it is much nicer to use the high-level API
for this:
<informalexample><programlisting><![CDATA[
static void
on_name_acquired (GDBusConnection *connection,
const gchar *name,
gpointer user_data)
{
/* further setup ... */
}
/* ... */
owner_id = g_bus_own_name (G_BUS_TYPE_SYSTEM,
NAME_TO_CLAIM,
G_BUS_NAME_OWNER_FLAGS_ALLOW_REPLACEMENT,
on_bus_acquired,
on_name_acquired,
on_name_lost,
NULL,
NULL);
g_main_loop_run (loop);
g_bus_unown_name (owner_id);
]]>
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</programlisting></informalexample>
Note that g_bus_own_name() works asynchronously and requires
you to enter your mainloop to await the on_name_aquired()
callback. Also note that in order to avoid race conditions (e.g.
when your service is activated by a method call), you have to export
your manager object <emphasis>before</emphasis> acquiring the
name. The on_bus_acquired() callback is the right place to do
such preparations.
</para>
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</section>
<section>
<title>Creating proxies for well-known names</title>
<para>
dbus-glib lets you create proxy objects for well-known names, like the
following example:
<informalexample><programlisting><![CDATA[
proxy = dbus_g_proxy_new_for_name (system_bus_connection,
"org.freedesktop.Accounts",
"/org/freedesktop/Accounts",
"org.freedesktop.Accounts");
]]>
</programlisting></informalexample>
For a #DBusGProxy constructed like this, method calls will be sent to
the current owner of the name, and that owner can change over time.
</para>
<para>
The same can be achieved with #GDBusProxy:
<informalexample><programlisting><![CDATA[
error = NULL;
proxy = g_dbus_proxy_new_for_bus_sync (G_BUS_TYPE_SYSTEM,
G_DBUS_PROXY_FLAGS_NONE,
NULL, /* GDBusInterfaceInfo */
"org.freedesktop.Accounts",
"/org/freedesktop/Accounts",
"org.freedesktop.Accounts",
NULL, /* GCancellable */
&error);
]]>
</programlisting></informalexample>
For an added layer of safety, you can specify what D-Bus
interface the proxy is expected to conform to by using the
#GDBusInterfaceInfo type.
</para>
<para>
Additionally, #GDBusProxy loads, caches and tracks changes to
the D-Bus properties on the remote object. It also sets up match
rules so D-Bus signals from the remote object are delivered
locally.
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</para>
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</section>
<section>
<title>Client-side GObject bindings</title>
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<para>
dbus-glib comes with <command>dbus-binding-tool</command>, which
can produce somewhat nice client-side wrappers for a D-Bus interface.
GDBus does not have code-generation at this point, but #GDBusProxy
is designed to allow the creating of client-side wrappers by
subclassing #GDBusProxy.
</para>
<para>
For an example of a #GDBusProxy-derived class that wraps a D-Bus
interface in a type-safe way, see <xref
linkend="gdbus-example-proxy-subclass"/>. The comparison is as
follows:
<table id="gdbus-example-type-safe-proxy">
<title>Wrapping the org.freedesktop.Accounts.User D-Bus interface in the AccountUser GObject type</title>
<tgroup cols="2">
<thead>
<row><entry>D-Bus concept</entry><entry>GObject concept</entry></row>
</thead>
<tbody>
<row>
<entry>AutomaticLogin property</entry>
<entry>
<para><literal>AccountsUser:automatic-login</literal> GObject property</para>
<para>C getter: accounts_user_get_automatic_login()</para>
<para>Watch changes via the <literal>notify::automatic-login</literal> signal</para>
</entry>
</row>
<row>
<entry>RealName property</entry>
<entry>
<para><literal>AccountsUser:real-name</literal> GObject property</para>
<para>C getter: accounts_user_get_real_name()</para>
<para>Watch changes via the <literal>notify::real-name signal</literal></para>
</entry>
</row>
<row>
<entry>UserName property</entry>
<entry>
<para><literal>AccountsUser:user-name</literal> GObject property</para>
<para>C getter: accounts_user_get_user_name()</para>
<para>Watch changes via the <literal>notify::user-name</literal> signal</para>
</entry>
</row>
<row>
<entry>Changed signal</entry>
<entry>
<para><literal>AccountsUser::changed</literal> GObject signal</para>
<para>Watch via e.g. g_signal_connect()</para>
</entry>
</row>
<row>
<entry>Frobnicate method</entry>
<entry>
<para>Use accounts_user_frobnicate() + accounts_user_frobnicate_finish() or accounts_user_frobnicate_sync() to invoke</para>
</entry>
</row>
</tbody>
</tgroup>
</table>
</para>
<example id="gdbus-example-proxy-subclass"><title>GDBusProxy subclass example</title><programlisting><xi:include xmlns:xi="http://www.w3.org/2001/XInclude" parse="text" href="../../../../gio/tests/gdbus-example-proxy-subclass.c"><xi:fallback>FIXME: MISSING XINCLUDE CONTENT</xi:fallback></xi:include></programlisting></example>
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</section>
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<section>
<title>Exporting objects</title>
<para>
With dbus-glib, exporting an object over D-Bus works by generating
a bunch of glue code from your introspection XML with
<command>dbus-binding-tool</command>. The glue code gets included in
your source, and you need to call
<informalexample><programlisting>
dbus_g_object_type_install_info (TYPE_MYOBJECT,
&amp;dbus_glib_myobject_object_info);
</programlisting></informalexample>
in your class_init() function to tell dbus-glib about your type.
To actually export an instance, you call
<informalexample><programlisting>
dbus_g_connection_register_g_object (system_bus_connection,
my_object_path,
G_OBJECT (my_object));
</programlisting></informalexample>
</para>
<para>
The GDBus way of exporting an object works by embedding the
introspection XML in the source, creating introspection data
structures from it with g_dbus_node_info_new_for_xml(), and
passing that along when you register the object:
<informalexample><programlisting><![CDATA[
static const gchar introspection_xml[] =
"<node>"
" <interface name='org.gtk.GDBus.TestPeerInterface'>"
" <method name='HelloWorld'>"
" <arg type='s' name='greeting' direction='in'/>"
" <arg type='s' name='response' direction='out'/>"
" </method>"
" </interface>"
"</node>";
/* parse introspection data */
introspection_data = g_dbus_node_info_new_for_xml (introspection_xml, NULL);
/
id = g_dbus_connection_register_object (connection,
"/org/gtk/GDBus/TestObject",
"org.gtk.GDBus.TestPeerInterface",
introspection_data->interfaces[0],
&interface_vtable,
NULL, /* user_data */
NULL, /* user_data_free_func */
NULL); /* GError** */
]]>
</programlisting></informalexample>
</para>
<para>
The actual implementation of the exported object is done by specifying
a #GDBusInterfaceVTable that has method_call(), get_property() and
set_property() methods. There is no direct support beyond that for
exporting #GObjects, so there is quite a bit of manual work involved,
as you can see in the following example.
</para>
<para>
Since the VTable methods don't have any direct #GObject support, we
pass the exported object as @user_data. Also note that we have to handle
the emission of the PropertiesChanged signal ourselves, by connecting
to ::notify.
</para>
<example id="gdbus-export"><title>Exporting a GObject</title><programlisting><xi:include xmlns:xi="http://www.w3.org/2001/XInclude" parse="text" href="../../../../gio/tests/gdbus-example-export.c"><xi:fallback>FIXME: MISSING XINCLUDE CONTENT</xi:fallback></xi:include></programlisting></example>
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</section>
</chapter>