NSFetchedResultsController delegate method calls on predicate change - ios

Question

I have a simple app, where I have countries which have cities, which in turn have people. I want to display list of countries in a table view. I use NSFetchedResultsController to get all the data. This is the setup :
-(void)initializeFetchedResultsController
{
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] initWithEntityName:#"Country"];
fetchRequest.sortDescriptors = #[[NSSortDescriptor sortDescriptorWithKey:#"name" ascending:YES]];
fetchRequest.fetchBatchSize = 30;
self.fetchResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest
managedObjectContext:[NSManagedObjectContext managedObjectContext]
sectionNameKeyPath:#"nameFirstLetter"
cacheName:nil];
self.fetchResultsController.delegate = self;
[self.fetchResultsController performFetch:nil];
}
I also added an ability to search by typing in the country name in the search bar, so I implemented UISearchBarDelegate method :
- (void)searchBar:(UISearchBar *)searchBar textDidChange:(NSString *)searchText
{
if([searchText isEqualToString:#""])
{
self.fetchResultsController.fetchRequest.predicate = nil;
}
else
{
self.fetchResultsController.fetchRequest.predicate = [NSPredicate predicateWithFormat:#"name BEGINSWITH[cd] %#", searchText];
}
[self.fetchResultsController performFetch:nil];
[self.tableView reloadData];
}
This works but not the way I want. I expected that on predicate change, my NSFetchResultsControllerDelegate delegate methods will be called again so I can insert or delete items / sections from my table view (i want animations) without having to figure out myself, what has been added and removed. But this is not the case, instead if I change the predicate, delegate methods are not called, and I must do a simple [self.tableView reloadData]. Am I doing something wrong or is it just the way it is supposed to work, and I cannot take this shortcut ?

Answer 1

You are not doing it wrong. This is the way it is and there's no shortcut to take. You'll have to implement your own code to animate the table view between fetches.

Answer 2

As #StianHøiland says, you need to do it yourself. Generally with delegate methods they are called as a result of an 'offline' / asynchronous change in order to notify you. They are not called as a result of a change you have explicitly requested.
You could think about using the fetchedObjects and the indexPathForObject: features of the FRC. Filter the fetchedObjects list (using your predicate). Get the index paths for the objects that have been removed and you can animate them out.