NSFetchedResultsController with indexed UITableViewController and UILocalizedIndexedCollation

Question

I have a table that uses an NSFetchedResultsController. This gets me an index with the headers that are present

- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
    return [[_fetchedResultsController sections] count];
}

-(NSString *)tableView:(UITableView *)tableView titleForHeaderInSection:(NSInteger)section
{
    return [[[_fetchedResultsController sections] objectAtIndex:section] name];
}

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
    NSInteger numberOfRows = 0;
    NSArray *sections = _fetchedResultsController.sections;
    if(sections.count > 0)
    {
        id <NSFetchedResultsSectionInfo> sectionInfo = [sections objectAtIndex:section];
        numberOfRows = [sectionInfo numberOfObjects];
    }
    return numberOfRows;
}

- (NSArray *)sectionIndexTitlesForTableView:(UITableView *)tableView
{
    return [_fetchedResultsController sectionIndexTitles];
}

- (NSInteger)tableView:(UITableView *)tableView sectionForSectionIndexTitle:(NSString *)title atIndex:(NSInteger)index
{
    return [_fetchedResultsController sectionForSectionIndexTitle:title atIndex:index];
}

but I want to use UILocalizedIndexCollation to get the complete alphabet.

How do I wire up these methods to the NSFetchedResultsController?

I think I need to get the index titles from the current Collation

- (NSArray *)sectionIndexTitlesForTableView:(UITableView *)tableView
{
    return [[UILocalizedIndexedCollation currentCollation] sectionIndexTitles];
}

but I am lost on how to write this method

- (NSInteger)tableView:(UITableView *)tableView sectionForSectionIndexTitle:(NSString *)title atIndex:(NSInteger)index
{
    //How do I translate index here to be the index of the _fetchedResultsController?
    return [_fetchedResultsController sectionForSectionIndexTitle:title atIndex:index];
}    

Answer 1

I think you have to traverse the fetched results controller sections and find a matching section for the given title, for example:

- (NSInteger)tableView:(UITableView *)tableView sectionForSectionIndexTitle:(NSString *)title atIndex:(NSInteger)index
{
    NSInteger section = 0;
    for (id <NSFetchedResultsSectionInfo> sectionInfo in [_fetchedResultsController sections]) {
        if ([sectionInfo.indexTitle compare:title] >= 0)
            break;
        section++;
    }
    return section;
}

For section index titles that to not have a matching section, you have to decide if you want to jump to a "lower" or "higher" section. The above method jumps to the next higher section.

Answer 2

The solution I went with, after inspiration from Martin.

    - (NSInteger)tableView:(UITableView *)tableView sectionForSectionIndexTitle:(NSString *)title atIndex:(NSInteger)index
{
    NSInteger localizedIndex = [[UILocalizedIndexedCollation currentCollation] sectionForSectionIndexTitleAtIndex:index];
    NSArray *localizedIndexTitles = [[UILocalizedIndexedCollation currentCollation] sectionIndexTitles];
    for(int currentLocalizedIndex = localizedIndex; currentLocalizedIndex > 0; currentLocalizedIndex--) {
        for(int frcIndex = 0; frcIndex < [[_fetchedResultsController sections] count]; frcIndex++) {
            id<NSFetchedResultsSectionInfo> sectionInfo = [[_fetchedResultsController sections] objectAtIndex:frcIndex];
            NSString *indexTitle = sectionInfo.indexTitle;
            if([indexTitle isEqualToString:[localizedIndexTitles objectAtIndex:currentLocalizedIndex]]) {
                return frcIndex;
            }
        }
    }
    return 0;
}