## The TpSvc* interfaces

The TpSvc* interfaces — How to export Telepathy objects

The GInterfaces whose names start with TpSvc are generated automatically from the Telepathy specification, and can be used to make it easier to export methods and signals onto D-Bus. By implementing these GInterfaces you can avoid needing to generate any "glue" using the dbus-glib tools - this is all done internally inside telepathy-glib.

The media session interface makes a convenient example because it only has two methods (Error() and Ready()) and one signal (NewStreamHandler), and media session handlers aren't expected to implement any other interfaces.

The first thing to do is pre-declare the interface init function, and define the type you'll be using, declaring it to implement the media stream handler interface:

    static void stream_handler_iface_init (gpointer, gpointer);

    G_DEFINE_TYPE_WITH_CODE(GabbleMediaStream,
        gabble_media_stream,
        G_TYPE_OBJECT,
        G_IMPLEMENT_INTERFACE (TP_TYPE_SVC_MEDIA_STREAM_HANDLER,
          stream_handler_iface_init)
        )

Here we're using a subclass of G_TYPE_OBJECT. You can of course subclass any type.

If you're implementing more than one interface on the same object, define more than one init function, and call G_IMPLEMENT_INTERFACE more than once. The interface init functions can even be extern if you want to separate off chunks of functionality into a different .c file. For instance, here's GabbleConnection:

    /* in header files */
    void conn_aliasing_iface_init (gpointer, gpointer);
    void conn_avatars_iface_init (gpointer, gpointer);
    void conn_presence_iface_init (gpointer, gpointer);

    /* in gabble-connection.c */
    static void conn_iface_init (gpointer, gpointer);
    static void capabilities_iface_init (gpointer, gpointer);

    G_DEFINE_TYPE_WITH_CODE(GabbleConnection,
        gabble_connection,
        TP_TYPE_BASE_CONNECTION,
        G_IMPLEMENT_INTERFACE (TP_TYPE_SVC_CONNECTION,
          conn_iface_init);
        G_IMPLEMENT_INTERFACE (TP_TYPE_SVC_CONNECTION_INTERFACE_ALIASING,
          conn_aliasing_iface_init);
        G_IMPLEMENT_INTERFACE (TP_TYPE_SVC_CONNECTION_INTERFACE_AVATARS,
          conn_avatars_iface_init);
        G_IMPLEMENT_INTERFACE (TP_TYPE_SVC_CONNECTION_INTERFACE_CAPABILITIES,
          capabilities_init);
        G_IMPLEMENT_INTERFACE (TP_TYPE_SVC_CONNECTION_INTERFACE_PRESENCE,
          conn_presence_iface_init);
        )

The _class_init, _init etc. functions are just like normal, so I won't describe them here. One thing to note, though, is that for signals which are defined by the GInterface, you do not need to do anything in the _class_init - the GInterface has already set the signal up for you.

For each exported D-Bus method, there's a typedef ending with _impl giving the signature you should use for your method implementation. For example, here's the signature for the Error method on the media session handler interface:

            void (*tp_svc_media_session_handler_error_impl)
              (TpSvcMediaSessionHandler *self, guint errno, const char *message,
               DBusGMethodInvocation *context);

and here's the beginning of the corresponding implementation:

    static void
    gabble_media_session_error (TpSvcMediaSessionHandler *iface,
                                guint errno,
                                const char *message,
                                DBusGMethodInvocation *context)
    {
      GabbleMediaSession *self = GABBLE_MEDIA_SESSION (iface);

      /* do stuff with self here */

All service methods in telepathy-glib are asynchronous - you can of course implement them synchronously if you like, but you have to return the result or error to D-Bus by calling a callback rather than by returning from a function.

The method implementation's last parameter is a DBusGMethodInvocation. To send the reply, you must either call dbus_g_method_return_error (for a failure), dbus_g_method_return (for a successful return), or an inline function whose name contains "_return_from_" provided by the TpSvc interface. For example, for Error there's an inline function tp_svc_media_session_handler_return_from_error(). These inline functions are just a simple wrapper around dbus_g_method_return() to make it type-safe - it's recommended that you use them where possible.

For instance, Error doesn't return anything, so tp_svc_media_session_handler_return_from_error() doesn't take any parameters apart from the DBusGMethodInvocation:

    static void
    gabble_media_session_error (TpSvcMediaSessionHandler *iface,
                                guint errno,
                                const char *message,
                                DBusGMethodInvocation *context)
    {
      GabbleMediaSession *self = GABBLE_MEDIA_SESSION (iface);

      /* do stuff with self here */

      tp_svc_media_session_handler_return_from_error (context);
    }

As for signals, they're named as dictated by dbus-glib. This normally gives you a sensible lower-case name - for instance NewStreamHandler is mapped to "new-stream-handler".

To emit a signal, the generated code contains another convenience function whose name contains _emit_. This is prototyped to take the correct arguments for the signal, and emits it efficiently:

    tp_svc_media_session_handler_emit_new_stream_handler (session,
      object_path, id, media_type, TP_MEDIA_STREAM_DIRECTION_BIDIRECTIONAL);

Finally, the interface init function needs to be written. Normally you'd set the fields of a vtable to be pointers to your method implementations. However, we couldn't do this in telepathy-glib because that would mean breaking the ABI every time we added methods to an interface. Instead, you call functions, with pointers to your method implementations as a parameter:

    static void
    session_handler_iface_init (gpointer g_iface, gpointer iface_data)
    {
      TpSvcMediaSessionHandlerClass *klass =
        (TpSvcMediaSessionHandlerClass *)g_iface;

      tp_svc_media_session_handler_implement_error (klass,
          gabble_media_session_error);
      tp_svc_media_session_handler_implement_ready (klass,
          gabble_media_session_ready);
    }

This is obviously quite repetitive if there are a lot of methods, so the convention I've used in telepathy-glib, Gabble and telepathy-sofiasip is to define a temporary macro called IMPLEMENT:

    static void
    session_handler_iface_init (gpointer g_iface, gpointer iface_data)
    {
      TpSvcMediaSessionHandlerClass *klass =
        (TpSvcMediaSessionHandlerClass *)g_iface;

    #define IMPLEMENT(x) tp_svc_media_session_handler_implement_##x (\
        klass, gabble_media_session_##x)
      IMPLEMENT(error);
      IMPLEMENT(ready);
    #undef IMPLEMENT
    }

If you're implementing many interfaces, just write many similar interface init functions.