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

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<chapter>
  <title>Migrating to GDBus</title>

  <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 objects representing connections,
      proxies and method invocations. But there are some important
      differences:
      <itemizedlist>
        <listitem><para>
          dbus-glib uses the <ulink
          url="http://www.freedesktop.org/wiki/Software/dbus#ReferenceImplementation.28dbus-daemonandlibdbus.29">libdbus
          reference implementation</ulink>, GDBus doesn't. Instead, it
          relies on GIO streams as transport layer, and has its own
          implementation for the D-Bus connection setup and
          authentication. Apart from using streams as transport,
          avoiding libdbus also lets GDBus avoid some thorny
          multithreading issues.
        </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 on the #GVariant type system which is
          explicitly designed to match D-Bus types.
        </para></listitem>
        <listitem><para>
          dbus-glib models only D-Bus interfaces and does not provide
          any types for objects. GDBus models both D-Bus interfaces
          (via the #GDBusInterface, #GDBusProxy and
          #GDBusInterfaceSkeleton types) and objects (via the
          #GDBusObject, #GDBusObjectSkeleton and #GDBusObjectProxy types).
        </para></listitem>
        <listitem><para>
          GDBus includes native support for the <ulink url="http://dbus.freedesktop.org/doc/dbus-specification.html#standard-interfaces-properties">org.freedesktop.DBus.Properties</ulink> (via the #GDBusProxy type) and <ulink url="http://dbus.freedesktop.org/doc/dbus-specification.html#standard-interfaces-objectmanager">org.freedesktop.DBus.ObjectManager</ulink> D-Bus interfaces, dbus-glib doesn't.
        </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 provides a
          similar tool called <command><link
          linkend="gdbus-codegen">gdbus-codegen</link></command> that
          can also generate Docbook D-Bus interface documentation.
        </para></listitem>
        <listitem><para>
          dbus-glib doesn't provide any convenience API for owning and
          watching bus names, GDBus provides the g_bus_own_name() and
          g_bus_watch_name() family of convenience functions.
        </para></listitem>
        <listitem><para>
          GDBus provides API to parse, generate and work with <link
          linkend="gio-D-Bus-Introspection-Data">Introspection
          XML</link>, dbus-glib doesn't.
        </para></listitem>
        <listitem><para>
          GTestDBus provides API to create isolated unit tests <link
          linkend="gio-D-Bus-Test-Scaffolding">GDBus Test Scaffolding</link>.
        </para></listitem>
      </itemizedlist>
    </para>
  </section>

  <section>
    <title>API comparison</title>

    <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>
          <row><entry>#DBusGConnection</entry><entry>#GDBusConnection</entry></row>
          <row><entry>#DBusGProxy</entry><entry>#GDBusProxy, #GDBusInterface - also see #GDBusObjectProxy</entry></row>
          <row><entry>#DBusGObject</entry><entry>#GDBusInterfaceSkeleton, #GDBusInterface - also see #GDBusObjectSkeleton</entry></row>
          <row><entry>#DBusGMethodInvocation</entry><entry>#GDBusMethodInvocation</entry></row>
          <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>
          <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() - also see g_dbus_object_manager_server_export()</entry></row>
          <row><entry>dbus_g_connection_unregister_g_object()</entry><entry>g_dbus_connection_unregister_object() - also see g_dbus_object_manager_server_unexport()</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>
          <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>

  <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);
        }
      else
        {
          g_warning ("Failed to acquire %s", NAME_TO_CLAIM);
        }
      goto out;
    }

  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);
        }
      exit (1);
    }

  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 ... */
]]>
      </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);
]]>
    </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>
  </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.  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.
    </para>
    <para>
      The #GDBusProxy type normally isn't used directly - instead
      proxies subclassing #GDBusProxy generated by <command><link
      linkend="gdbus-codegen">gdbus-codegen</link></command> is used, see <xref linkend="gdbus-example-gdbus-codegen"/>
    </para>
  </section>

  <section id="gdbus-example-gdbus-codegen">
    <title>Using gdbus-codegen</title>

    <para>
      dbus-glib comes with <command>dbus-binding-tool</command>, which
      can produce somewhat nice client- and server-side wrappers for a D-Bus interface.
      With GDBus, <command><link
      linkend="gdbus-codegen">gdbus-codegen</link></command> is used and like
      its counterpart, it also takes D-Bus Introspection XML as input:
    </para>
    <example id="gdbus-example-codegen-input"><title>Example D-Bus Introspection XML</title><programlisting><xi:include xmlns:xi="http://www.w3.org/2001/XInclude" parse="text" href="../gdbus-example-objectmanager.xml"><xi:fallback>FIXME: MISSING XINCLUDE CONTENT</xi:fallback></xi:include></programlisting></example>
    <para>
      If this XML is processed like this
<informalexample><programlisting><![CDATA[
gdbus-codegen --interface-prefix org.gtk.GDBus.Example.ObjectManager. \
              --generate-c-code generated-code	                      \
              --c-namespace Example 				      \
              --c-generate-object-manager			      \
              --generate-docbook generated-docs                       \
              gdbus-example-objectmanager.xml
]]></programlisting></informalexample>
      then two files <filename>generated-code.h</filename> and
      <filename>generated-code.c</filename> are
      generated. Additionally, two XML files
      <filename>generated-docs-org.gtk.GDBus.Example.ObjectManager.Animal</filename> and
      <filename>generated-docs-org.gtk.GDBus.Example.ObjectManager.Cat</filename>
      with Docbook XML are generated.
    </para>
    <para>
      While the contents of <filename>generated-code.h</filename> and
      <filename>generated-code.c</filename> are best described by the
      <command><link
      linkend="gdbus-codegen">gdbus-codegen</link></command> manual
      page, brief examples of how this generated code can be used can be found in
      <xref linkend="gdbus-example-codegen-server"/>
      and <xref linkend="gdbus-example-codegen-client"/>.
    </para>

  </section>

</chapter>