为什么放置在DataTemplate中时不会评估BindingExpression类型的附加属性？

I have defined an attached property for Selector of type BindingExpression in order to catch a binding so I can clone it onto multiple properties. The attached property works fine when the Selector is defined directly in XAML. However when the Selector is defined inside a DataTemplate, the property changed handler never gets fired.I have defined an attached property for Selecto