How to use two or more NSFetchedResultsController?

Question

My app use CoreData. It has a UISegmentedControl and a tableview. The UISegmentedControl is used to update tableview based on different criteria.

The 1st option of UISegmentedControl will show all data in a single tableview section. So I use the following code in - (NSFetchedResultsController*)fetchedResultsController

frc = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest
                                          managedObjectContext:[self managedObjectContext] 
                                          sectionNameKeyPath:nil
                                          cacheName:@"Root"];

The 2nd option will show data in multiple tableview section. So I need NSSortDescriptor and set sectionNameKeyPath.

NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"city" 
                                                             ascending:YES];
NSArray *sortDescriptors = [[NSArray alloc] initWithObjects:sortDescriptor, nil];
[fetchRequest setSortDescriptors:sortDescriptors];

NSFetchedResultsController *frc = nil;
frc = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest 
                                          managedObjectContext:[self managedObjectContext] 
                                          sectionNameKeyPath:@"city" 
                                          cacheName:@"Root"];

Because sectionNameKeyPath is different which each of them need different NSSortDescriptor, I assume I need two different NSFetchedResultsController.

The code I use to reload tableview:

- (void) touchDownAtSegmentIndex:(NSUInteger)segmentIndex{

NSPredicate *predicate;
NSError *error;    

[NSFetchedResultsController deleteCacheWithName:@"Root"];

if(segmentIndex == 0)
{

}
else if(segmentIndex == 1)
{

}
else
{
    return;
}

[[[self fetchedResultsController] fetchRequest] setPredicate:predicate];
[[self fetchedResultsController] performFetch:&error];
[[self tableView] reloadData];
}

Anyone can provide me a sample of code that use two NSFetchedResultsController. Thank you.

SOLUTION:

In addition from @iTukker idea, I added more checking before returning fetchedResultsController. Without this checking, this method will return fetchedResultsController instead of executing switch .. case .. statement.

- (NSFetchedResultsController*)fetchedResultsController {

.....
    if ((fetchedResultsController!= nil) && [self.segmentChanged isEqualToString:@"false"])
    return fetchedResultsController;
.....

    if (frc == nil) {
        switch (segmentendControl.selectedSegmentIndex) {
           case 0: {
               frc = ... //first frc
               [frc retain];
               break;
           }
           case 1: {
               frc = ... //second frc
               [frc retain];
               break;
           }
        }
    }

....

}

iTukker iTukker · Accepted Answer · 2012-05-27T09:31:22

What you would do is something like this, assuming that frc is a instance variable of your controller

- (NSFetchedResultsController*)fetchedResultsController {
   if (frc == nil) {
      switch (segmentendControl.selectedSegmentIndex) {
         case 0: {
             frc = ... //first frc
             [frc retain];
             break;
         }
         case 1: {
             frc = ... //second frc
             [frc retain];
             break;
         }
      }
   }

   return frc;
}

And when the value of your segmentedController changes

[frc release];
frc = nil;
[tableView reloadData];

Hope this makes sence

How to use two or more NSFetchedResultsController?

1 Answers