Custom NSSortDescriptor for NSFetchedResultsController

I have a custom "sort" that I currently perform on a set of data. I now need to perform this sort on the fetchedObjects in the NSFetchedResultsController. All of our table data works hand in hand with core data fetched results so replacing the data source with a generic array has been problematic.

Since NSFetchedResultsController can take NSSortDescriptors it seems like that is the best route. The problem is I don't know how to convert this sort algorithm into an custom comparator.

How do I convert this into a custom comparator (if possible)? (if not how do I get the desired sorted result while using NSFetchedResultsController). In essence the field 'priority' can be either 'high' 'normal' or 'low' and the list needs to be sorted in that order.

+(NSArray*)sortActionItemsByPriority:(NSArray*)listOfActionitemsToSort
{
    NSMutableArray *sortedArray = [[NSMutableArray alloc]initWithCapacity:listOfActionitemsToSort.count];
    NSMutableArray *highArray = [[NSMutableArray alloc]init];
    NSMutableArray *normalArray = [[NSMutableArray alloc]init];
    NSMutableArray *lowArray = [[NSMutableArray alloc]init];

for (int x = 0; x < listOfActionitemsToSort.count; x++)
{
    ActionItem *item = [listOfActionitemsToSort objectAtIndex:x];

    if ([item.priority caseInsensitiveCompare:@"high"] == NSOrderedSame)
        [highArray addObject:item];
    else if ([item.priority caseInsensitiveCompare:@"normal"] == NSOrderedSame)
        [normalArray addObject:item];
    else
        [lowArray addObject:item];
}

[sortedArray addObjectsFromArray:highArray];
[sortedArray addObjectsFromArray:normalArray];
[sortedArray addObjectsFromArray:lowArray];

return sortedArray;
}