What is written:

//! Callback service receiving notifications about Media Library content changes. Methods called only from main thread.\n
//! Use library_callback_factory_t template to register.
class NOVTABLE library_callback : public service_base {
public:
//! Called when new items are added to the Media Library.
virtual void on_items_added(const pfc::list_base_const_t<metadb_handle_ptr> & p_data) = 0;
//! Called when some items have been removed from the Media Library.
virtual void on_items_removed(const pfc::list_base_const_t<metadb_handle_ptr> & p_data) = 0;
//! Called when some items in the Media Library have been modified.
virtual void on_items_modified(const pfc::list_base_const_t<metadb_handle_ptr> & p_data) = 0;

FB2K_MAKE_SERVICE_INTERFACE_ENTRYPOINT(library_callback);
};

How can I use this? Tried many variants, smth is wrong.