[gtk/popover-menu-custom-child] popovermenubar: Allow adding custom items in ui files

[Matthias Clasen <matthiasc src gnome org>]

commit 33447e1e92ef9003c05bbc1f1f730f2f239a7584
Author: Matthias Clasen <mclasen redhat com>
Date:   Mon Oct 26 13:26:55 2020 -0400

    popovermenubar: Allow adding custom items in ui files
    
    Just as for popover menus, support <child type="ID"> to fill
    custom child slots.

 gtk/gtkpopovermenubar.c | 32 +++++++++++++++++++++++++++++++-
 1 file changed, 31 insertions(+), 1 deletion(-)
---
diff --git a/gtk/gtkpopovermenubar.c b/gtk/gtkpopovermenubar.c
index cb00bba68a..148d182e92 100644
--- a/gtk/gtkpopovermenubar.c
+++ b/gtk/gtkpopovermenubar.c
@@ -67,6 +67,7 @@
 #include "gtkwidgetprivate.h"
 #include "gtkmain.h"
 #include "gtknative.h"
+#include "gtkbuildable.h"
 
 #define GTK_TYPE_POPOVER_MENU_BAR_ITEM    (gtk_popover_menu_bar_item_get_type ())
 #define GTK_POPOVER_MENU_BAR_ITEM(obj)    (G_TYPE_CHECK_INSTANCE_CAST ((obj), 
GTK_TYPE_POPOVER_MENU_BAR_ITEM, GtkPopoverMenuBarItem))
@@ -350,7 +351,11 @@ enum
 
 static GParamSpec * bar_props[LAST_PROP];
 
-G_DEFINE_TYPE (GtkPopoverMenuBar, gtk_popover_menu_bar, GTK_TYPE_WIDGET)
+static void gtk_popover_menu_bar_buildable_iface_init (GtkBuildableIface *iface);
+
+G_DEFINE_TYPE_WITH_CODE (GtkPopoverMenuBar, gtk_popover_menu_bar, GTK_TYPE_WIDGET,
+                         G_IMPLEMENT_INTERFACE (GTK_TYPE_BUILDABLE,
+                                                gtk_popover_menu_bar_buildable_iface_init))
 
 static void
 tracker_remove (int      position,
@@ -611,6 +616,31 @@ gtk_popover_menu_bar_init (GtkPopoverMenuBar *bar)
   gtk_widget_add_controller (GTK_WIDGET (bar), controller);
 }
 
+static GtkBuildableIface *parent_buildable_iface;
+
+static void
+gtk_popover_menu_bar_buildable_add_child (GtkBuildable *buildable,
+                                          GtkBuilder   *builder,
+                                          GObject      *child,
+                                          const char   *type)
+{
+  if (GTK_IS_WIDGET (child))
+    {
+      if (!gtk_popover_menu_bar_add_child (GTK_POPOVER_MENU_BAR (buildable), GTK_WIDGET (child), type))
+        g_warning ("No such custom attribute: %s", type);
+    }
+  else
+    parent_buildable_iface->add_child (buildable, builder, child, type);
+}
+
+static void
+gtk_popover_menu_bar_buildable_iface_init (GtkBuildableIface *iface)
+{
+  parent_buildable_iface = g_type_interface_peek_parent (iface);
+
+  iface->add_child = gtk_popover_menu_bar_buildable_add_child;
+}
+
 /**
  * gtk_popover_menu_bar_new_from_model:
  * @model: (allow-none): a #GMenuModel, or %NULL