# Migrating from libunique to GApplication or GtkApplication

libunique offers 'unique application' support as well as ways to communicate with a running application instance. This is implemented in various ways, either using D-Bus, or socket-based communication.

Starting with GLib 2.26, D-Bus support has been integrated into GIO in the form of GDBus, and #GApplication has been added to provide the same level of application support as libunique.

A unique application

Here is a simple application using libunique:

    int
    main (int argc, char *argv[])
    {
      UniqueApp *app;
      GtkWidget *window;

      gtk_init (&argc, &argv);

      app = unique_app_new ("org.gtk.TestApplication", NULL);

      if (unique_app_is_running (app))
        {
          UniqueResponse response;

          response = unique_app_send_message (app, UNIQUE_ACTIVATE, NULL);
          g_object_unref (app);

          return response == UNIQUE_RESPONSE_OK ? 0 : 1;
        }

      window = create_my_window ();

      unique_app_watch_window (app, GTK_WINDOW (window));

      gtk_widget_show (window);

      gtk_main ();

      g_object_unref (app);

      return 0;
    }

The same application using GtkApplication:

    static void
    activate (GtkApplication *app)
    {
      GList *list;
      GtkWidget *window;

      list = gtk_application_get_windows (app);

      if (list)
        {
          gtk_window_present (GTK_WINDOW (list->data));
        }
      else
        {
          window = create_my_window ();
          gtk_window_set_application (GTK_WINDOW (window), app);
          gtk_widget_show (window);
        }
    }

    int
    main (int argc, char *argv[])
    {
      GtkApplication *app;
      gint status;

      app = gtk_application_new ("org.gtk.TestApplication", 0);
      g_signal_connect (app, "activate", G_CALLBACK (activate), NULL);

      status = g_application_run (app);

      g_object_unref (app);

      return status;
    }

## Uniqueness

Instead of creating a #UniqueApp with unique_app_new(), create a #GApplication with g_application_new() or a #GtkApplication with gtk_application_new(). The @name that was used with unique_app_new() is very likely usable as the @application_id for g_application_new() without any changes, and GtkApplication passes the `DESKTOP_STARTUP_ID` environment variable automatically.

While libunique expects you to check for an already running instance yourself and activate it manually, GApplication handles all this on its own in g_application_run(). If you still need to find out if there is a running instance of your application, use g_application_get_is_remote() instead of unique_app_is_running().

## Commands and Messages

libunique lets you send messages with commands to a running instance using unique_app_send_message(). The commands can be either predefined or custom. Some of the predefined libunique commands have equivalents in GApplication. Instead of sending the %UNIQUE_ACTIVATE command, call g_application_activate(), instead of sending the %UNIQUE_OPEN command, call g_application_open(). The %UNIQUE_NEW and %UNIQUE_CLOSE and user-defined commands don't have direct replacement at this time.

As a replacement for custom commands, GApplication implements the #GActionGroup interface and lets you add a group of actions with g_application_set_action_group(). The actions can then be invoked, either by using the D-Bus interface for #GAction directly, or by calling g_action_group_activate_action() from another instance of the GApplication. The #GApplication documentation contains an example for using GApplication with actions.

For more complex needs, GApplication supports passing entire commandlines to the running instance.