GtkApplication: use resources for loading menus

Use the new ::resource-base-path property on #GApplication to attempt to
load the menu layout of the application.

We look first at gtk/menus-appmenu.ui or gtk/menus-traditional.ui
depending on the setting of gtk_application_prefers_app_menu().  Failing
that, we fall back to the common case of gtk/menus.ui (which should
always be given).  This provides a convenient way for application
authors to provide a different set of menus, depending on the desktop
environment they find themselves in.

As is the intention with other resources, if the resource base path is
unset, nothing will be loaded.  Additionally, if the expected files are not
found, it is not an error -- just nothing happens.

https://bugzilla.gnome.org/show_bug.cgi?id=722092

gtk/gtkapplication.c

@@ -70,20 +70,25 @@
  * associated with #GtkApplicationWindow and to the “activate” and
  * 'open' #GApplication methods.
  *
- * To set an application menu for a GtkApplication, use
- * gtk_application_set_app_menu(). The #GMenuModel that this function
- * expects is usually constructed using #GtkBuilder, as seen in the
- * following example. To specify a menubar that will be shown by
- * #GtkApplicationWindows, use gtk_application_set_menubar(). Use the base
- * #GActionMap interface to add actions, to respond to the user
- * selecting these menu items.
+ * #GtkApplication will automatically load menus from the #GtkBuilder
+ * file located at "gtk/menus.ui", relative to the application's
+ * resource base path (see g_application_set_resource_base_path()).  The
+ * menu with the ID "app-menu" is taken as the application's app menu
+ * and the menu with the ID "menubar" is taken as the application's
+ * menubar.  Additional menus (most interesting submenus) can be named
+ * and accessed via gtk_application_get_menu_by_id() which allows for
+ * dynamic population of a part of the menu structure.
  *
- * GTK+ displays these menus as expected, depending on the platform
- * the application is running on.
+ * If the files "gtk/menus-appmenu.ui" or "gtk/menus-traditional.ui" are
+ * present then these files will be used in preference, depending on the
+ * value of gtk_application_prefers_app_menu().
+ *
+ * It is also possible to provide the menus manually using
+ * gtk_application_set_app_menu() and gtk_application_set_menubar().
  *
  * ## A simple application ## {#gtkapplication}
  *
- * [A simple example](https://git.gnome.org/browse/gtk+/tree/examples/bloatpad.c)
+ * [A simple example](https://git.gnome.org/browse/gtk+/tree/examples/bp/bloatpad.c)
  *
  * GtkApplication optionally registers with a session manager
  * of the users session (if you set the #GtkApplication:register-session
@@ -460,6 +465,7 @@ struct _GtkApplicationPrivate
 
   gboolean register_session;
   GtkActionMuxer  *muxer;
+  GtkBuilder      *menus_builder;
 };
 
 G_DEFINE_TYPE_WITH_PRIVATE (GtkApplication, gtk_application, G_TYPE_APPLICATION)
@@ -498,6 +504,56 @@ gtk_application_focus_in_event_cb (GtkWindow      *window,
   return FALSE;
 }
 
+static void
+gtk_application_load_resources (GtkApplication *application)
+{
+  const gchar *base_path;
+
+  base_path = g_application_get_resource_base_path (G_APPLICATION (application));
+
+  if (base_path == NULL)
+    return;
+
+  /* Load the menus */
+  {
+    gchar *menuspath;
+
+    /* If the user has given a specific file for the variant of menu
+     * that we are looking for, use it with preference.
+     */
+    if (gtk_application_prefers_app_menu (application))
+      menuspath = g_strconcat (base_path, "/gtk/menus-appmenu.ui", NULL);
+    else
+      menuspath = g_strconcat (base_path, "/gtk/menus-traditional.ui", NULL);
+
+    if (g_resources_get_info (menuspath, G_RESOURCE_LOOKUP_FLAGS_NONE, NULL, NULL, NULL))
+      application->priv->menus_builder = gtk_builder_new_from_resource (menuspath);
+    g_free (menuspath);
+
+    /* If we didn't get the specific file, fall back. */
+    if (application->priv->menus_builder == NULL)
+      {
+        menuspath = g_strconcat (base_path, "/gtk/menus.ui", NULL);
+        if (g_resources_get_info (menuspath, G_RESOURCE_LOOKUP_FLAGS_NONE, NULL, NULL, NULL))
+          application->priv->menus_builder = gtk_builder_new_from_resource (menuspath);
+        g_free (menuspath);
+      }
+
+    if (application->priv->menus_builder)
+      {
+        GObject *menu;
+
+        menu = gtk_builder_get_object (application->priv->menus_builder, "app-menu");
+        if (menu != NULL && G_IS_MENU_MODEL (menu))
+          gtk_application_set_app_menu (application, G_MENU_MODEL (menu));
+        menu = gtk_builder_get_object (application->priv->menus_builder, "menubar");
+        if (menu != NULL && G_IS_MENU_MODEL (menu))
+          gtk_application_set_menubar (application, G_MENU_MODEL (menu));
+      }
+  }
+}
+
+
 static void
 gtk_application_startup (GApplication *g_application)
 {
@@ -512,6 +568,8 @@ gtk_application_startup (GApplication *g_application)
 
   application->priv->impl = gtk_application_impl_new (application, gdk_display_get_default ());
   gtk_application_impl_startup (application->priv->impl, application->priv->register_session);
+
+  gtk_application_load_resources (application);
 }
 
 static void
@@ -767,6 +825,7 @@ gtk_application_finalize (GObject *object)
 {
   GtkApplication *application = GTK_APPLICATION (object);
 
+  g_clear_object (&application->priv->menus_builder);
   g_clear_object (&application->priv->app_menu);
   g_clear_object (&application->priv->menubar);
   g_clear_object (&application->priv->muxer);
@@ -1627,3 +1686,20 @@ gtk_application_handle_window_map (GtkApplication *application,
 {
   gtk_application_impl_handle_window_map (application->priv->impl, window);
 }
+
+GMenu *
+gtk_application_get_menu_by_id (GtkApplication *application,
+                                const gchar    *id)
+{
+  GObject *object;
+
+  if (!application->priv->menus_builder)
+    return NULL;
+
+  object = gtk_builder_get_object (application->priv->menus_builder, id);
+
+  if (!object || !G_IS_MENU (object))
+    return NULL;
+
+  return G_MENU (object);
+}

gtk/gtkapplication.h

@@ -154,6 +154,10 @@ void             gtk_application_set_accels_for_action           (GtkApplication
 GDK_AVAILABLE_IN_3_14
 gboolean         gtk_application_prefers_app_menu                (GtkApplication       *application);
 
+GDK_AVAILABLE_IN_3_14
+GMenu *          gtk_application_get_menu_by_id                  (GtkApplication       *application,
+                                                                  const gchar          *id);
+
 G_END_DECLS
 
 #endif /* __GTK_APPLICATION_H__ */