value: Allow automatic transforms to/from interfaces

Use the new g_type_interface_instantiable_prerequisite() to check
compatibility for transform functions.

In particular, this allows interfaces (in my case GDK_TYPE_PAINTABLE) to
be transformed to/from any GObject type (in my case G_TYPE_OBJECT) using
the transform function registered to tranform between any 2 objects
(g_value_object_transform_value() does a type check and uses NULL if the
types don't match).

And this in turn allows be to g_object_bind_property() a gobject-typed
generic property (GtkListItem::item) to a GtkImage::paintable.

1 files changed, 11 insertions, 2 deletions

diff --git a/gobject/gvalue.c b/gobject/gvalue.c
index c30501a6b..df7b4ee11 100644
--- a/gobject/gvalue.c
+++ b/gobject/gvalue.c
@@ -448,6 +448,15 @@ g_value_init_from_instance (GValue  *value,
     }
 }
 
+static GType
+tranform_lookup_get_parent_type (GType type)
+{
+  if (g_type_fundamental (type) == G_TYPE_INTERFACE)
+    return g_type_interface_instantiable_prerequisite (type);
+
+  return g_type_parent (type);
+}
+
 static GValueTransform
 transform_func_lookup (GType src_type,
 		       GType dest_type)
@@ -470,11 +479,11 @@ transform_func_lookup (GType src_type,
 		  g_type_value_table_peek (entry.src_type) == g_type_value_table_peek (src_type))
 		return e->func;
 	    }
-	  entry.dest_type = g_type_parent (entry.dest_type);
+	  entry.dest_type = tranform_lookup_get_parent_type (entry.dest_type);
 	}
       while (entry.dest_type);
       
-      entry.src_type = g_type_parent (entry.src_type);
+      entry.src_type = tranform_lookup_get_parent_type (entry.src_type);
     }
   while (entry.src_type);