Preliminary steps to adding update and event callbacks to StateSet, StateAttribute

and Uniform.

include/osg/StateAttribute

@@ -30,7 +30,8 @@
 namespace osg {
 
 
-// forward declare State & StateSet
+// forward declare NodeVisitor, State & StateSet
+class NodeVisitor;
 class State;
 class StateSet;
 
@@ -244,6 +245,39 @@ class OSG_EXPORT StateAttribute : public Object
             // default to no GLMode's associated with use of the StateAttribute.
             return false;
         }
+        
+        
+        struct Callback : public virtual osg::Object
+        {
+            Callback() {}
+
+            Callback(const Callback&,const CopyOp&) {}
+
+            META_Object(osg,Callback);
+
+            /** do customized update code.*/
+            virtual void operator () (NodeVisitor*, StateAttribute*) {}
+        };
+
+        /** Set the UpdateCallback which allows users to attach customize the updating of an object during the update traversal.*/
+        void setUpdateCallback(Callback* uc) { _updateCallback = uc; }
+
+        /** Get the non const UpdateCallback.*/
+        Callback* getUpdateCallback() { return _updateCallback.get(); }
+
+        /** Get the const UpdateCallback.*/
+        const Callback* getUpdateCallback() const { return _updateCallback.get(); }
+
+
+        /** Set the EventCallback which allows users to attach customize the updating of an object during the Event traversal.*/
+        void setEventCallback(Callback* ec) { _eventCallback = ec; }
+
+        /** Get the non const EventCallback.*/
+        Callback* getEventCallback() { return _eventCallback.get(); }
+
+        /** Get the const EventCallback.*/
+        const Callback* getEventCallback() const { return _eventCallback.get(); }
+
     
         /** apply the OpenGL state attributes. 
           * The global state for the current OpenGL context is passed
@@ -265,6 +299,8 @@ class OSG_EXPORT StateAttribute : public Object
     
         virtual ~StateAttribute() {}
 
+        ref_ptr<Callback>   _updateCallback;
+        ref_ptr<Callback>   _eventCallback;
 };
 
 }