[glib] value: Allow automatic transforms to/from interfaces

[Benjamin Otte <otte src gnome org>]

commit 5899c61ed2b4be3f02d0382d6fb0d1fc1bb7bc98
Author: Benjamin Otte <otte redhat com>
Date:   Mon Nov 25 19:58:59 2019 +0100

    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.

 gobject/gvalue.c | 13 +++++++++++--
 1 file 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);