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

and Uniform.

include/osg/StateSet

@@ -33,6 +33,9 @@
 
 namespace osg {
 
+// forward declare for the purposes of the UpdateCallback.
+class NodeVisitor;
+
 /** Stores a set of modes and attributes which respresent a set of OpenGL state.
   *  Notice that a \c StateSet contains just a subset of the whole OpenGL state.
   * <p>In OSG, each \c Drawable and each \c Node has a reference to a
@@ -354,6 +357,46 @@ class OSG_EXPORT StateSet : public Object
         inline const std::string& getBinName() const { return _binName; }
 
 
+        struct Callback : public virtual osg::Object
+        {
+            Callback() {}
+
+            Callback(const Callback&,const CopyOp&) {}
+
+            META_Object(osg,Callback);
+
+            /** do customized callback code.*/
+            virtual void operator() (NodeVisitor*, StateSet*) {}
+        };
+
+        /** Set the Update Callback which allows users to attach customize the updating of an object during the update traversal.*/
+        void setUpdateCallback(Callback* ac);
+
+        /** Get the non const Update Callback.*/
+        Callback* getUpdateCallback() { return _updateCallback.get(); }
+
+        /** Get the const Update Callback.*/
+        const Callback* getUpdateCallback() const { return _updateCallback.get(); }
+
+        /** Get the number of Objects of this StateSet which require Update traversal,
+          * since they have an Update Callback attached to them or their children.*/
+        inline unsigned int getNumObjectsRequiringUpdateTraversal() const { return _numObjectsRequiringUpdateTraversal; }
+
+
+        /** Set the Event Callback which allows users to attach customize the updating of an object during the event traversal.*/
+        void setEventCallback(Callback* ac);
+
+        /** Get the non const Event Callback.*/
+        Callback* getEventCallback() { return _eventCallback.get(); }
+
+        /** Get the const Event Callback.*/
+        const Callback* getEventCallback() const { return _eventCallback.get(); }
+
+        /** Get the number of Objects of this StateSet which require Event traversal,
+          * since they have an Eevnt Callback attached to them or their children.*/
+        inline unsigned int getNumObjectsRequiringEventTraversal() const { return _numObjectsRequiringEventTraversal; }
+
+
         /** call compile on all StateAttributes contained within this StateSet.*/
         void compileGLObjects(State& state) const;
 
@@ -406,6 +449,12 @@ class OSG_EXPORT StateSet : public Object
         RenderBinMode                       _binMode;
         int                                 _binNum;
         std::string                         _binName;
+        
+        ref_ptr<Callback>                   _updateCallback;
+        unsigned int                        _numObjectsRequiringUpdateTraversal;
+
+        ref_ptr<Callback>                   _eventCallback;
+        unsigned int                        _numObjectsRequiringEventTraversal;
 
 };