Question

I’m currently working on a project where I use UITableView and NSFetchedResultsController to store my data. The problem I’m facing is the use of sections is not working.

The entity i want to be displayed as a tables is of the following character:

• Channel (String)
• Server (String)
• Name (String)

And I want to have sections based on the server and the rows in each section is the channel. The layout will look like this:

• Server1
  Channel1
  Channel2
• Server2
  Channel3
  Channel4
  etc..

The problem though is that when I try to insert/remove/change an element in the NSFetchedResultsController the application get an exception as the following:

CoreData: error: Serious application error. An exception was caught from the delegate of NSFetchedResultsController during a call to -controllerDidChangeContent:. Invalid update: invalid number of sections. The number of sections contained in the table view after the update (3) must be equal to the number of sections contained in the table view before the update (2), plus or minus the number of sections inserted or deleted (0 inserted, 0 deleted). with userInfo (null)

I have done some research and people say it’s because the UITableViewController Delegate fires first and tries to display an element/section that at the time does not exists. I have yet not found a solution and that is why I’m here.

This is my delegate methods for NSFetchedResultsController

- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller
{
    [self.tableView beginUpdates];
}  

- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject
   atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type
  newIndexPath:(NSIndexPath *)newIndexPath
{
    UITableView *tableView = self.tableView;
    NSLog(@"--- Core Data noticed change");
    switch(type) {
        case NSFetchedResultsChangeInsert:
            [tableView insertRowsAtIndexPaths:@[newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;

        case NSFetchedResultsChangeDelete:
            [tableView deleteRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;

        case NSFetchedResultsChangeUpdate:
            [self configureCell:[tableView cellForRowAtIndexPath:indexPath] atIndexPath:indexPath];
            break;

        case NSFetchedResultsChangeMove:
            [tableView deleteRowsAtIndexPaths:@[indexPath] withRowAnimation:UITableViewRowAnimationFade];
            [tableView insertRowsAtIndexPaths:@[newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;
    }
}  

- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller
{
    [self.tableView endUpdates];
}

Many thanks
Robert

Answer 1

You have to implement also the

 controller:didChangeSection:atIndex:forChangeType:

delegate function, otherwise the table view will not be notified of inserted or deleted sections in the Core Data store.

You find a sample implementation in the NSFetchedResultsControllerDelegate Protocol Reference.