GApplication: Add a way to replace a unique instance

While uniqueness is great, sometimes you want to restart
a newer version of the same app. These two flags make that
possible.

We also add a ::name-lost signal, that is emitted when it
happens. The default handler for this signal just calls
g_application_quit(), but applications may want to connect
and do cleanup or state-saving here.

gio/gapplication.c

@@ -212,6 +212,7 @@
  *     the @dbus_register vfunc. Since: 2.34
  * @handle_local_options: invoked locally after the parsing of the commandline
  *  options has occurred. Since: 2.40
+ * @name_lost: invoked when another instance is taking over the name. Since: 2.60
  *
  * Virtual function table for #GApplication.
  *
@@ -277,6 +278,7 @@ enum
   SIGNAL_ACTION,
   SIGNAL_COMMAND_LINE,
   SIGNAL_HANDLE_LOCAL_OPTIONS,
+  SIGNAL_NAME_LOST,
   NR_SIGNALS
 };
 
@@ -476,6 +478,7 @@ g_application_parse_command_line (GApplication   *application,
 {
   gboolean become_service = FALSE;
   gchar *app_id = NULL;
+  gboolean replace = FALSE;
   GVariantDict *dict = NULL;
   GOptionContext *context;
   GOptionGroup *gapplication_group;
@@ -557,6 +560,18 @@ g_application_parse_command_line (GApplication   *application,
       g_option_group_add_entries (gapplication_group, entries);
     }
 
+  /* Allow replacing if the application allows it */
+  if (application->priv->flags & G_APPLICATION_ALLOW_REPLACEMENT)
+    {
+      GOptionEntry entries[] = {
+        { "gapplication-replace", '\0', 0, G_OPTION_ARG_NONE, &replace,
+          N_("Replace the running instance") },
+        { NULL }
+      };
+
+      g_option_group_add_entries (gapplication_group, entries);
+    }
+
   /* Now we parse... */
   if (!g_option_context_parse_strv (context, arguments, error))
     goto out;
@@ -569,6 +584,10 @@ g_application_parse_command_line (GApplication   *application,
   if (app_id)
     g_application_set_application_id (application, app_id);
 
+  /* Check for --gapplication-replace */
+  if (replace)
+    application->priv->flags |= G_APPLICATION_REPLACE;
+
   dict = g_variant_dict_new (NULL);
   if (application->priv->packed_options)
     {
@@ -1177,6 +1196,13 @@ g_application_real_dbus_unregister (GApplication    *application,
 {
 }
 
+static gboolean
+g_application_real_name_lost (GApplication *application)
+{
+  g_application_quit (application);
+  return TRUE;
+}
+
 /* GObject implementation stuff {{{1 */
 static void
 g_application_set_property (GObject      *object,
@@ -1434,6 +1460,7 @@ g_application_class_init (GApplicationClass *class)
   class->add_platform_data = g_application_real_add_platform_data;
   class->dbus_register = g_application_real_dbus_register;
   class->dbus_unregister = g_application_real_dbus_unregister;
+  class->name_lost = g_application_real_name_lost;
 
   g_object_class_install_property (object_class, PROP_APPLICATION_ID,
     g_param_spec_string ("application-id",
@@ -1628,6 +1655,25 @@ g_application_class_init (GApplicationClass *class)
                   g_application_handle_local_options_accumulator, NULL, NULL,
                   G_TYPE_INT, 1, G_TYPE_VARIANT_DICT);
 
+  /**
+   * GApplication::name-lost:
+   * @application: the application
+   *
+   * The ::name-lost signal is emitted only on the registered primary instance
+   * when a new instance has taken over. This can only happen if the application
+   * is using the %G_APPLICATION_ALLOW_REPLACEMENT flag.
+   *
+   * The default handler for this signal calls g_application_quit().
+   *
+   * Returns: %TRUE if the signal has been handled
+   *
+   * Since: 2.60
+   */
+  g_application_signals[SIGNAL_NAME_LOST] =
+    g_signal_new (I_("name-lost"), G_TYPE_APPLICATION, G_SIGNAL_RUN_LAST,
+                  G_STRUCT_OFFSET (GApplicationClass, name_lost),
+                  g_signal_accumulator_true_handled, NULL, NULL,
+                  G_TYPE_BOOLEAN, 0);
 }
 
 /* Application ID validity {{{1 */