             layout.verticalScrollPosition = value;
     }
     
-    
+    //----------------------------------
+    //  horizontal,verticalScrollPositionDelta
+    //----------------------------------
+
+    public function horizontalScrollPositionDelta(unit:uint):Number
+    {
+        return (layout) ? layout.horizontalScrollPositionDelta(unit) : 0;     
+    }
+    
+    public function verticalScrollPositionDelta(unit:uint):Number
+    {
+        return (layout) ? layout.verticalScrollPositionDelta(unit) : 0;     
+    }
+    
+    //--------------------------------------------------------------------------
+    //
+    //  IViewport properties
+    //
+    //--------------------------------------------------------------------------        
+
     //----------------------------------
     //  contentWidth
     //---------------------------------- 

    /**
     * @private
     */
    public function set layout(value:ILayout):void
    {
        if (_layout == value)
            return;
        
        _layout = value;  
        if (_layout)
            ILayout(_layout).target = this;
        invalidateSize();
        invalidateDisplayList();
    }

        tabChildren = true;
        
        _layout = new BasicLayout();
        _layout.target = this;  
    }
    
    //--------------------------------------------------------------------------
