null indexPaths when deleting an object - ios

Question

I can't figure this out, but I seem to have a null indexPath when I delete an object from the NSFetchedResultsController.
When I delete my object, I do this:
- (void)tableView:(UITableView *)tableView commitEditingStyle:(UITableViewCellEditingStyle)editingStyle forRowAtIndexPath:(NSIndexPath *)indexPath {
if (editingStyle == UITableViewCellEditingStyleDelete) {
// Delete the object
NSManagedObjectContext *context = [self.fetchedResultsController managedObjectContext];
[context deleteObject:[self.fetchedResultsController objectAtIndexPath:indexPath]];
[self saveContext];
}
}
Setting up NSFetchedResultsController:
- (NSFetchedResultsController *)fetchedResultsController {
if (_fetchedResultsController != nil) {
return _fetchedResultsController;
}
NSFetchRequest *fetchRequest = [[NSFetchRequest alloc] init];
[fetchRequest setFetchBatchSize:20];
NSEntityDescription *entity = [NSEntityDescription entityForName:#"Route" inManagedObjectContext:self.managedObjectContext];
[fetchRequest setEntity:entity];
NSSortDescriptor *nameSort = [[NSSortDescriptor alloc] initWithKey:#"name" ascending:YES selector:#selector(localizedCaseInsensitiveCompare:)];
NSArray *sortDescriptors = #[nameSort];
[fetchRequest setSortDescriptors:sortDescriptors];
NSFetchedResultsController *aFetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:fetchRequest managedObjectContext:self.managedObjectContext sectionNameKeyPath:nil cacheName:#"Routes"];
self.fetchedResultsController = aFetchedResultsController;
self.fetchedResultsController.delegate = self;
NSError *error = nil;
if (![self.fetchedResultsController performFetch:&error]) {
NSLog(#"Unresolved error with NSFetchedResultsController: %#", [error description]);
abort();
}
return _fetchedResultsController;
}
This is where the failure occurs:
- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath {
switch (type) {
// Data was inserted - insert the data into the table view
case NSFetchedResultsChangeInsert: {
[self.savedRoutesTableView insertRowsAtIndexPaths:#[newIndexPath] withRowAnimation:UITableViewRowAnimationAutomatic];
break;
}
// Data was deleted - delete the data from the table view
case NSFetchedResultsChangeDelete: {
[self.savedRoutesTableView deleteRowsAtIndexPaths:#[indexPath] withRowAnimation:UITableViewRowAnimationAutomatic];
break;
}
case NSFetchedResultsChangeUpdate: {
SavedRoutesTableViewCell *cell = (SavedRoutesTableViewCell *)[self.savedRoutesTableView cellForRowAtIndexPath:indexPath];
[cell configureCellWithRoute:[controller objectAtIndexPath:newIndexPath]];
break;
}
case NSFetchedResultsChangeMove: {
[self.savedRoutesTableView deleteRowsAtIndexPaths:#[indexPath] withRowAnimation:UITableViewRowAnimationAutomatic];
[self.savedRoutesTableView insertRowsAtIndexPaths:#[newIndexPath] withRowAnimation:UITableViewRowAnimationAutomatic];
break;
}
default:
break;
}
}
In the didChangeObject method, both my indexPath and newIndexPath are nil. I can NSLog my object and I do see the entity. It crashes in the [self.savedRoutesTableView deleteRowsAtIndexPaths:#[indexPath] withRowAnimation:UITableViewRowAnimationAutomatic]; method with the exception:
CoreData: error: Serious application error. Exception was caught during Core Data change processing. This is usually a bug within an observer of NSManagedObjectContextObjectsDidChangeNotification. *** -[__NSPlaceholderArray initWithObjects:count:]: attempt to insert nil object from objects[0] with userInfo (null)
When I save this object, I save it like this:
self.route.name = routeName;
NSManagedObjectContext *tempContext = [self.route managedObjectContext];
[tempContext performBlock:^{
NSError *error = nil;
if (![tempContext save:&error]) {
NSLog(#"an error occurred: %#", [error localizedDescription]);
}
[self.managedObjectContext performBlock:^{
NSError *error = nil;
if (![_managedObjectContext save:&error]) {
NSLog(#"error in main context: %#", [error localizedDescription]);
}
}];
}];
I'm not really sure where else to debug this since the NSFetchedResultsController just isn't returning me the indexPath for the deleted object. Any thoughts? Thanks in advance.
Edit:
Well I found the culprit causing the error, but I'm not sure why it does. Basically I have a ViewController that receives either a Route entity from the main MOC if it's Editing the route, or it inserts a new one if you are creating a new route. So in that viewController, if I'm editing a route, because I am trying to use two MOCs, one temp, and one main for its parent so I can easily throw away stuff if the user decides to cancel and not create a new route, I needed to transfer over that route to the other context to make other code I have work. So that "transfer" looks like:
NSManagedObjectContext *moc = _route.managedObjectContext;
NSManagedObjectID *routeId = [_route objectID];
self.tempContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
self.tempContext.parentContext = moc;
NSManagedObject *localRoute = [self.tempContext objectWithID:routeId];
self.route = localRoute;
With this code, my adding on locations to an existing route works now that the locations are in the same contexts, but somehow it messes up deleting an existing route from the main MOC. Not sure why and what the best solution is.

Answer 1

Had the same problem and solved it by ensuring that NSFetchedResultsController always uses the same NSManagedContext. For example if u fetch object from a database with a fetch controller and later you want to delete that object, make sure that the fetch controller uses the same managed context it was using during fetching.
NSFetchedResultsController is optimised for working with UITableView and UITableView is user interface component, and user interface should always be handled by main thread, so there is no need to create new NSManagedContext every time you go into fetch... So implementing this code should fix this problem:
#synthesize fetchedResultsController = __fetchedResultsController;
#synthesize managedObjectContext = __managedObjectContext;
- (NSManagedObjectContext *)managedObjectContext
{
if (__managedObjectContext != nil)
{
return __managedObjectContext;
}
NSPersistentStoreCoordinator *coordinator = ap.persistentStoreCoordinator;
if (coordinator != nil)
{
__managedObjectContext = [[NSManagedObjectContext alloc] init];
[__managedObjectContext setPersistentStoreCoordinator:coordinator];
}
return __managedObjectContext;
}
ap is a pointer to AppDelegate:
ap = [[UIApplication sharedApplication] delegate];
What this code does is creates one instance of MOC and later reuses it. Warning: this is not threadsafe, and it doesn have to be (cause you should use it with main thread only), so if you missuse it it will not work...
Use that:
- (NSFetchedResultsController *)fetchedResultsController
{
if (__fetchedResultsController != nil)
{
return __fetchedResultsController;
}
//create __fetchedResultsController
// do some fetching
return __fetchedResultsController;
}
So when you want to populate table:
- (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
{
return [[self.fetchedResultsController sections] count];
}
- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
{
return [__fetchedResultsController.fetchedObjects count];
}
- (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];
}
NSManagedObject *managedObject = [__fetchedResultsController objectAtIndexPath:indexPath];
cell.textLabel.text = [managedObject valueForKey:#"smth."];
return cell;
}
If your dataset changes just:
__fetchedResultsController = nil;
[tableView reloadData]
And everything including NSFetchedResultsControllers delegat methods will work fine, no nil indexPaths and so on...
Hope I helped someone...

Answer 2

NSManagedObjectContext *moc = _route.managedObjectContext;
NSManagedObjectID *routeId = [_route objectID];
self.tempContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
self.tempContext.parentContext = moc;
NSManagedObject *localRoute = [self.tempContext objectWithID:routeId];
self.route = localRoute;
Had to get the context associated with the route.