implementing NSFetchedResultsController - objective-c

Question

I'm having some problems implementing NSFetchedResultsController for something very simple. I'm just trying to learn how it works. All I want to do is display the firstName attribute of Person entities in a table view.
As far as I know I've implemented all the methods required but nothing is showing up in the table view. The method -tableView:cellForRowAtIndexPath: method isn't even being called.
Here is my code:
#implementation MyTableViewController
#synthesize managedObjectContext = _managedObjectContext;
#synthesize fetchedResultsController = _fetchedResultsController;
-(NSFetchedResultsController*) fetchedResultsController
{
if (!_fetchedResultsController)
{
NSFetchRequest *fetchRequest = [NSFetchRequest fetchRequestWithEntityName:#"Person"];
NSSortDescriptor *sortDescriptor = [NSSortDescriptor sortDescriptorWithKey:#"firstName" ascending:YES];
fetchRequest.sortDescriptors = [NSArray arrayWithObject:sortDescriptor];
_fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:nil];
}
return _fetchedResultsController;
}
- (id)initWithStyle:(UITableViewStyle)style
{
self = [super initWithStyle:style];
if (self) {
// Custom initialization
}
return self;
}
- (void)viewDidAppear:(BOOL)animated
{
[super viewDidAppear:animated];
NSError *error = nil;
[self.fetchedResultsController performFetch:&error];
}
#pragma mark - Table view data source
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
return [[[self.fetchedResultsController sections] objectAtIndex:section] numberOfObjects];
}
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
static NSString *CellIdentifier = #"cell";
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];
if (cell == nil) {
cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:CellIdentifier];
}
Person *person = [self.fetchedResultsController objectAtIndexPath:indexPath];
cell.textLabel.text = person.firstName;
return cell;
}
any help would be hugely appreciated! many thanks, Alex

Answer 1

You have to call [NSFetchedResultsController performFetch:] and implement its delegate
- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller {
// The fetch controller is about to start sending change notifications, so prepare the table view for updates.
[self.tableView beginUpdates];
}
- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath {
UITableView *tableView = self.tableView;
switch(type) {
case NSFetchedResultsChangeInsert:
[tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
break;
case NSFetchedResultsChangeDelete:
[tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
break;
case NSFetchedResultsChangeUpdate:
[self configureCell:[tableView cellForRowAtIndexPath:indexPath] atIndexPath:indexPath];
break;
case NSFetchedResultsChangeMove:
[tableView deleteRowsAtIndexPaths:[NSArray
arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
[tableView insertRowsAtIndexPaths:[NSArray
arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
break;
}
}
- (void)controller:(NSFetchedResultsController *)controller didChangeSection:(id )sectionInfo atIndex:(NSUInteger)sectionIndex forChangeType:(NSFetchedResultsChangeType)type {
switch(type) {
case NSFetchedResultsChangeInsert:
[self.tableView insertSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
break;
case NSFetchedResultsChangeDelete:
[self.tableView deleteSections:[NSIndexSet indexSetWithIndex:sectionIndex] withRowAnimation:UITableViewRowAnimationFade];
break;
}
}
- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller {
// The fetch controller has sent all current change notifications, so tell the table view to process all updates.
[self.tableView endUpdates];
}
See more at: http://www.raywenderlich.com/999/core-data-tutorial-for-ios-how-to-use-nsfetchedresultscontroller

Answer 2

Try getting your NSString from your Person object and via the code here:
NSString *firstName = ((Person *)[personArray objectAtIndex:indexPath.row]).firstName;
Mind you, my suggested line is assuming that you output your NSFetchRequest to a NS(Mutable)Array called personArray.
Hopefully that helps, otherwise let me know.