Animated scrolling for table view section index.

UITableViewDataSource.m

- (NSInteger)tableView:(UITableView *)tableView sectionForSectionIndexTitle:(NSString *)title atIndex:(NSInteger)index {
    
    NSUInteger section = ...
    CGRect rect = [self.tableView rectForSection:section];
    
    // Include content inset
    rect.origin.y -= self.tableView.contentInset.top;
    
    // Restore previous offset and animate to destination
    CGPoint offset = self.tableView.contentOffset;
    [[NSOperationQueue mainQueue] asynchronous:^{
        self.tableView.contentOffset = offset;
        
        [self.tableView setContentOffset:rect.origin animated:YES];
    }];
    
    return section;
}