CoreData with NSFetchResultController :Invalid update with with a property in fetchObjects

Question

I have a serious problem reported by Xcode :

1. I have a NSFetchResultsController

var fetchedResultsController: NSFetchedResultsController {
    if _fetchedResultsController != nil {
        return _fetchedResultsController!
    }

    let fetchRequest = NSFetchRequest()
    // Edit the entity name as appropriate.
    let entity = NSEntityDescription.entityForName("Choice", inManagedObjectContext: NSManagedObjectContext.MR_defaultContext())
    fetchRequest.entity = entity

    // Set the batch size to a suitable number.
    fetchRequest.fetchBatchSize = 10

    // Edit the sort key as appropriate.
    let sortDescriptor = NSSortDescriptor(key: "id", ascending: false)
    let sortDescriptors = [sortDescriptor]

    fetchRequest.sortDescriptors = sortDescriptors

    //NSPredicate
    let predicate = NSPredicate(format: "decision.id = %@", decision.id!)
    fetchRequest.predicate = predicate

    // Edit the section name key path and cache name if appropriate.
    // nil for section name key path means "no sections".
    let aFetchedResultsController = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: NSManagedObjectContext.MR_defaultContext(), sectionNameKeyPath: nil, cacheName: nil)
    aFetchedResultsController.delegate = self
    _fetchedResultsController = aFetchedResultsController

    do{
        try _fetchedResultsController?.performFetch()
    }catch let error as NSError {
        print(error)
    }

    return _fetchedResultsController!
}

2. And I have a button do add action:

@IBAction func didTouchAddChoiceButton(sender: UIButton) {
    let choice = Choice.MR_createEntity() as! Choice
    choice.id = GDUtils.CMUUID()
    choice.decision = decision
    NSManagedObjectContext.MR_defaultContext().MR_saveToPersistentStoreAndWait()
}

3. After adding this Entity. I have a controller to handle updating tableView like this

func controller(controller: NSFetchedResultsController, didChangeObject anObject: AnyObject, atIndexPath indexPath: NSIndexPath?, forChangeType type: NSFetchedResultsChangeType, newIndexPath: NSIndexPath?) {
    switch(type){
    case .Insert:
        tableView.insertRowsAtIndexPaths([newIndexPath!], withRowAnimation: .Top)

    case .Delete:
        tableView.deleteRowsAtIndexPaths([indexPath!], withRowAnimation: .Fade)

    case .Update:
        tableView.cellForRowAtIndexPath(indexPath!)

    case .Move:
        tableView.deleteRowsAtIndexPaths([indexPath!], withRowAnimation: .Left)
        tableView.insertRowsAtIndexPaths([newIndexPath!], withRowAnimation: .Fade)
    }
}

4.But the problem happened : every time I try to change a property of an entity from fetchedObjects :

let chosenChoice = fetchedResultsController.objectAtIndexPath(currentIndextPath!) as! Choice
    chosenChoice.name = tableCell.choiceName.text

I got this message :

CoreData: error: Serious application error. An exception was caught from the delegate of NSFetchedResultsController during a call to -controllerDidChangeContent:. Invalid update: invalid number of rows in section 0. The number of rows contained in an existing section after the update (1) must be equal to the number of rows contained in that section before the update (1), plus or minus the number of rows inserted or deleted from that section (1 inserted, 0 deleted) and plus or minus the number of rows moved into or out of that section (0 moved in, 0 moved out). with userInfo (null)

Can anyone help me to figure out what happened ?

Answer 1

There's a bug in NSFetchedResultsController on iOS8, where FRC calls .Insert instead of .Update. I solved it the way, that I'm reloading the table completely, when .Insert is called on iOS8.

case .Insert:
    guard let newIndexPath = newIndexPath else { return }

    // iOS8 bug when FRC calls insert instead of Update
    if #available(iOS 9, *) {
        // insert item normally
    } else {
        // reload everything
    }