## Exporting objects

With dbus-glib, exporting an object over D-Bus works by generating a bunch of glue code from your introspection XML with dbus-binding-tool. The glue code gets included in your source, and you need to call

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    dbus_g_object_type_install_info (TYPE_MYOBJECT,
                                     &dbus_glib_myobject_object_info);

in your `class_init()` function to tell dbus-glib about your type. To actually export an instance, you call

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    dbus_g_connection_register_g_object (system_bus_connection,
                                         my_object_path,
                                         G_OBJECT (my_object));

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:

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    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** */

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.

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.

Example 19. Exporting a GObject

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    #include <gio/gio.h>
    #include <stdlib.h>

    /* ---------------------------------------------------------------------------------------------------- */

    /* The object we want to export */
    typedef struct _MyObjectClass MyObjectClass;
    typedef struct _MyObject MyObject;

    struct _MyObjectClass
    {
      GObjectClass parent_class;
    };

    struct _MyObject
    {
      GObject parent_instance;

      gint count;
      gchar *name;
    };

    enum
    {
      PROP_0,
      PROP_COUNT,
      PROP_NAME
    };

    G_DEFINE_TYPE (MyObject, my_object, G_TYPE_OBJECT);

    static void
    my_object_finalize (GObject *object)
    {
      MyObject *myobj = (MyObject*)object;

      g_free (myobj->name);

      G_OBJECT_CLASS (my_object_parent_class)->finalize (object);
    }

    static void
    my_object_init (MyObject *object)
    {
      object->count = 0;
      object->name = NULL;
    }

    static void
    my_object_get_property (GObject    *object,
                            guint       prop_id,
                            GValue     *value,
                            GParamSpec *pspec)
    {
      MyObject *myobj = (MyObject*)object;

      switch (prop_id)
        {
        case PROP_COUNT:
          g_value_set_int (value, myobj->count);
          break;

        case PROP_NAME:
          g_value_set_string (value, myobj->name);
          break;

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

    static void
    my_object_set_property (GObject      *object,
                            guint         prop_id,
                            const GValue *value,
                            GParamSpec   *pspec)
    {
      MyObject *myobj = (MyObject*)object;

      switch (prop_id)
        {
        case PROP_COUNT:
          myobj->count = g_value_get_int (value);
          break;

        case PROP_NAME:
          g_free (myobj->name);
          myobj->name = g_value_dup_string (value);
          break;

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

    static void
    my_object_class_init (MyObjectClass *class)
    {
      GObjectClass *gobject_class = G_OBJECT_CLASS (class);

      gobject_class->finalize = my_object_finalize;
      gobject_class->set_property = my_object_set_property;
      gobject_class->get_property = my_object_get_property;

      g_object_class_install_property (gobject_class,
                                       PROP_COUNT,
                                       g_param_spec_int ("count",
                                                         "Count",
                                                         "Count",
                                                         0, 99999, 0,
                                                         G_PARAM_READWRITE));

      g_object_class_install_property (gobject_class,
                                       PROP_NAME,
                                       g_param_spec_string ("name",
                                                            "Name",
                                                            "Name",
                                                            NULL,
                                                            G_PARAM_READWRITE));
    }

    /* A method that we want to export */
    void
    my_object_change_count (MyObject *myobj,
                            gint      change)
    {
      myobj->count = 2 * myobj->count + change;

      g_object_notify (G_OBJECT (myobj), "count");
    }

    /* ---------------------------------------------------------------------------------------------------- */

    static GDBusNodeInfo *introspection_data = NULL;

    /* Introspection data for the service we are exporting */
    static const gchar introspection_xml[] =
      "<node>"
      "  <interface name='org.myorg.MyObject'>"
      "    <method name='ChangeCount'>"
      "      <arg type='i' name='change' direction='in'/>"
      "    </method>"
      "    <property type='i' name='Count' access='read'/>"
      "    <property type='s' name='Name' access='readwrite'/>"
      "  </interface>"
      "</node>";

    static void
    handle_method_call (GDBusConnection       *connection,
                        const gchar           *sender,
                        const gchar           *object_path,
                        const gchar           *interface_name,
                        const gchar           *method_name,
                        GVariant              *parameters,
                        GDBusMethodInvocation *invocation,
                        gpointer               user_data)
    {
      MyObject *myobj = user_data;

      if (g_strcmp0 (method_name, "ChangeCount") == 0)
        {
          gint change;
          g_variant_get (parameters, "(i)", &change);

          my_object_change_count (myobj, change);

          g_dbus_method_invocation_return_value (invocation, NULL);
        }
    }

    static GVariant *
    handle_get_property (GDBusConnection  *connection,
                         const gchar      *sender,
                         const gchar      *object_path,
                         const gchar      *interface_name,
                         const gchar      *property_name,
                         GError          **error,
                         gpointer          user_data)
    {
      GVariant *ret;
      MyObject *myobj = user_data;

      ret = NULL;
      if (g_strcmp0 (property_name, "Count") == 0)
        {
          ret = g_variant_new_int32 (myobj->count);
        }
      else if (g_strcmp0 (property_name, "Name") == 0)
        {
          ret = g_variant_new_string (myobj->name ? myobj->name : "");
        }

      return ret;
    }

    static gboolean
    handle_set_property (GDBusConnection  *connection,
                         const gchar      *sender,
                         const gchar      *object_path,
                         const gchar      *interface_name,
                         const gchar      *property_name,
                         GVariant         *value,
                         GError          **error,
                         gpointer          user_data)
    {
      MyObject *myobj = user_data;

      if (g_strcmp0 (property_name, "Count") == 0)
        {
          g_object_set (myobj, "count", g_variant_get_int32 (value), NULL);
        }
      else if (g_strcmp0 (property_name, "Name") == 0)
        {
          g_object_set (myobj, "name", g_variant_get_string (value, NULL), NULL);
        }

      return TRUE;
    }

    /* for now */
    static const GDBusInterfaceVTable interface_vtable =
    {
      handle_method_call,
      handle_get_property,
      handle_set_property
    };

    static void
    send_property_change (GObject         *obj,
                          GParamSpec      *pspec,
                          GDBusConnection *connection)
    {
      GVariantBuilder *builder;
      GVariantBuilder *invalidated_builder;
      MyObject *myobj = (MyObject *)obj;

      builder = g_variant_builder_new (G_VARIANT_TYPE_ARRAY);
      invalidated_builder = g_variant_builder_new (G_VARIANT_TYPE ("as"));

      if (g_strcmp0 (pspec->name, "count") == 0)
        g_variant_builder_add (builder,
                               "{sv}",
                               "Count", g_variant_new_int32 (myobj->count));
      else if (g_strcmp0 (pspec->name, "name") == 0)
        g_variant_builder_add (builder,
                               "{sv}",
                               "Name", g_variant_new_string (myobj->name ? myobj->name : ""));

      g_dbus_connection_emit_signal (connection,
                                     NULL,
                                     "/org/myorg/MyObject",
                                     "org.freedesktop.DBus.Properties",
                                     "PropertiesChanged",
                                     g_variant_new ("(sa{sv}as)",
                                                    "org.myorg.MyObject",
                                                    builder,
                                                    invalidated_builder),
                                     NULL);
    }

    static void
    on_bus_acquired (GDBusConnection *connection,
                     const gchar     *name,
                     gpointer         user_data)
    {
      MyObject *myobj = user_data;
      guint registration_id;

      g_signal_connect (myobj, "notify",
                        G_CALLBACK (send_property_change), connection);
      registration_id = g_dbus_connection_register_object (connection,
                                                           "/org/myorg/MyObject",
                                                           introspection_data->interfaces[0],
                                                           &interface_vtable,
                                                           myobj,
                                                           NULL,  /* user_data_free_func */
                                                           NULL); /* GError** */
      g_assert (registration_id > 0);
    }

    static void
    on_name_acquired (GDBusConnection *connection,
                      const gchar     *name,
                      gpointer         user_data)
    {
    }

    static void
    on_name_lost (GDBusConnection *connection,
                  const gchar     *name,
                  gpointer         user_data)
    {
      exit (1);
    }

    int
    main (int argc, char *argv[])
    {
      guint owner_id;
      GMainLoop *loop;
      MyObject *myobj;

      g_type_init ();

      /* We are lazy here - we don't want to manually provide
       * the introspection data structures - so we just build
       * them from XML.
       */
      introspection_data = g_dbus_node_info_new_for_xml (introspection_xml, NULL);
      g_assert (introspection_data != NULL);

      myobj = g_object_new (my_object_get_type (), NULL);

      owner_id = g_bus_own_name (G_BUS_TYPE_SESSION,
                                 "org.myorg.MyObject",
                                 G_BUS_NAME_OWNER_FLAGS_NONE,
                                 on_bus_acquired,
                                 on_name_acquired,
                                 on_name_lost,
                                 myobj,
                                 NULL);

      loop = g_main_loop_new (NULL, FALSE);
      g_main_loop_run (loop);

      g_bus_unown_name (owner_id);

      g_dbus_node_info_unref (introspection_data);

      g_object_unref (myobj);

      return 0;
    }