(void)scrollViewDidScroll:(UIScrollView *)scrollView; - (void)scrollViewDidZoom:(UIScrollView *)scrollView; - (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView; - (void)scrollViewWillBeginDecelerating:(UIScrollView *)scrollView; ໋໊نଇʹ͍ͭͯCocoa͚ίʔσΟϯάΨΠυϥΠϯ, p.15Λࢀর