virtual void on_changed(); //value+pagesize corresponds to child upper
virtual void on_value_changed(); //value corresponds to child lower
protected: //update interface
virtual void update_child();
virtual void update_fromchild();
virtual void on_changed(); //value+pagesize corresponds to child upper
virtual void on_value_changed(); //value corresponds to child lower
protected: //update interface
virtual void update_child();
virtual void update_fromchild();