刷新指定的cell
NSIndexPath *indexPath = [NSIndexPath indexPathForRow:3 inSection:0];
[tableView reloadRowsAtIndexPaths:[NSArray arrayWithObjects:indexPath,nil] withRowAnimation:UITableViewRowAnimationNone];
刷新指定的section
NSIndexSet *indexSet = [[NSIndexSet alloc] initWithIndex:2];
[tableview reloadSections:indexSet withRowAnimation:UITableViewRowAnimationAutomatic];
