Question

I have a list of geographical locations in Core Data (entity name is “Stops”).

I want to sort them by the current location, so I can show the user which locations are nearby. I’m using an NSFetchedResultsController so that the results can be easily displayed in a UITableView.

I am using the following code to attempt this sort:

NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"stop_lat" ascending:YES comparator:^NSComparisonResult(Stops *obj1, Stops *obj2) {
    CLLocation *currentLocation = locationManager.location;

    CLLocation *obj1Location = [[CLLocation alloc]initWithLatitude:[obj1.stop_lat doubleValue] longitude:[obj1.stop_lon doubleValue]];
    CLLocation *obj2Location = [[CLLocation alloc]initWithLatitude:[obj2.stop_lat doubleValue] longitude:[obj2.stop_lon doubleValue]];

    CLLocationDistance obj1Distance = [obj1Location distanceFromLocation:currentLocation];
    CLLocationDistance obj2Distance = [obj2Location distanceFromLocation:currentLocation];

    NSLog(@"Comparing %@ to %@.\n  Obj1 Distance: %f\n  Obj2 Distance: %f",obj1.stop_name, obj2.stop_name, obj1Distance, obj2Distance);

    if (obj1Distance > obj2Distance) {
        return (NSComparisonResult)NSOrderedDescending;
    }

    if (obj1Distance < obj2Distance) {
        return (NSComparisonResult)NSOrderedAscending;
    }

    return (NSComparisonResult)NSOrderedSame;
}];

NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[fetchRequest setSortDescriptors:sortDescriptors];
[fetchRequest setEntity:[NSEntityDescription entityForName:[Stops entityName] inManagedObjectContext:context]];

frcNearby = [[NSFetchedResultsController alloc]
          initWithFetchRequest:fetchRequest
          managedObjectContext:context
          sectionNameKeyPath:nil
          cacheName:nil];

NSError *error;
BOOL success = [frcNearby performFetch:&error];
if (error) NSLog(@"ERROR: %@ %@", error, [error userInfo]);

However, my NSFetchedResultsController is just returning all of my items sorted by the key I specified (“stop_lat”) rather than sorted by the user’s current location.

It looks like my comparator block isn’t ever getting called, because the NSLog in there never prints.

What am I missing here?

Answer 1

Objective-C based sort descriptors cannot be used with a fetch request.

From the “Core Data Programming Guide”:

… To summarize, though, if you execute a fetch directly,
you should typically not add Objective-C-based predicates or sort
descriptors to the fetch request. Instead you should apply these to
the results of the fetch.