说明：  将需要公开的接口用一个句柄类封装起来，调用者只通过封装的句柄类来调用实现， 这样，将调用与实现隔离开来，或者说只让调用看到它所需要看到的接口。这样的话，只要 保证接口不变，那么实现代码的中要添加一些变量，函数，将不会影响到调用者。
(will require an open interface with a handle Packaging category, called only by the Packaging category to handle Call, which would call and to achieve separation, or call only need to see it to see the interface. So, as long as the guarantee the same interface, then the realization of the code necessary to add some variables, function and will not affect the person to call.)