NSFetchResultsController throws error during update two elements simultaneosly - ios

Question

I am trying two update elements in collection view. When I pass just one message it works nicely
func simulate() {
let delegate = UIApplication.shared.delegate as! AppDelegate
let context = delegate.persistentContainer.viewContext
FriendsController.createMessageWithText(text: "Here's a text message that was sent a few minutes ago...", friend: friend!, minutesAgo: 1, context: context)
//second message
FriendsController.createMessageWithText(text: "Another message to give you hard time.", friend: friend!, minutesAgo: 1, context: context)
do {
try context.save()
}
catch let error as NSError {
print("Error \(error.localizedDescription)")
}
}
but when I do same steps except passing two messages it throws error:
CoreData: error: Serious application error. Exception was caught
during Core Data change processing. This is usually a bug within an
observer of NSManagedObjectContextObjectsDidChangeNotification.
Invalid update: invalid number of items in section 0. The number of
items contained in an existing section after the update (8) must be
equal to the number of items contained in that section before the
update (6), plus or minus the number of items inserted or deleted from
that section (1 inserted, 0 deleted) and plus or minus the number of
items moved into or out of that section (0 moved in, 0 moved out).
with userInfo (null)
Maybe someone have any clue?
There are: NSFetchController and delegate function I implemented:
lazy var fetchedResultsController: NSFetchedResultsController = { () -> NSFetchedResultsController<Message> in
let fetchRequest = NSFetchRequest<Message>(entityName: "Message")
fetchRequest.sortDescriptors = [NSSortDescriptor(key:"date",ascending: true)]
fetchRequest.predicate = NSPredicate(format: "friend.name = %#", (self.friend?.name!)!)
let delegate = UIApplication.shared.delegate as! AppDelegate
let context = delegate.persistentContainer.viewContext
let frc = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: context, sectionNameKeyPath: nil, cacheName: nil)
frc.delegate = self
return frc
}()
func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange anObject: Any, at indexPath: IndexPath?, for type: NSFetchedResultsChangeType, newIndexPath: IndexPath?) {
if type == .insert {
collectionView?.insertItems(at: [newIndexPath!])
collectionView?.scrollToItem(at: newIndexPath!, at: .bottom, animated: true)
}
}

Answer 1

If you want to insert(or delete, reload) more than one cell, than you need to make it in performBatchUpdates method of UICollectionView instance. You need to call it in func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>).
Look at this example: https://github.com/NoFearJoe/Timmee/blob/master/Timmee/Timmee/Sources/Core%20layer/Database/StorageObservable.swift
or:
https://gist.github.com/tempire/debbabb2cccafa90320e