def dragDropFilter( self ):
        """
        Returns a drag and drop filter method.  If set, the method should \
        accept 2 arguments: a QWidget and a drag/drop event and process it.
        
        :usage      |from projexui.qt.QtCore import QEvent
                    |
                    |class MyWidget(QWidget):
                    |   def __init__( self, parent ):
                    |       super(MyWidget, self).__init__(parent)
                    |       
                    |       self._tree = XCalendarWidget(self)
                    |       self._tree.setDragDropFilter(MyWidget.handleDragDrop)
                    |   
                    |   @staticmethod
                    |   def handleDragDrop(object, event):
                    |       if ( event.type() == QEvent.DragEnter ):
                    |           event.acceptProposedActions()
                    |       elif ( event.type() == QEvent.Drop ):
                    |           print 'dropping'
        
        :return     <function> || <method> || None
        """
        filt = None
        if ( self._dragDropFilterRef ):
            filt = self._dragDropFilterRef()
        
        if ( not filt ):
            self._dragDropFilterRef = None
        
        return filt