iOS 如何获取UITableview的某个header的高度 或者 某个section的frame

- (CGRect)rectForSection:(NSInteger)section;                                   

// includes header, footer and all rows

- (CGRect)rectForHeaderInSection:(NSInteger)section;

- (CGRect)rectForFooterInSection:(NSInteger)section;

- (CGRect)rectForRowAtIndexPath:(NSIndexPath *)indexPath;