无效更新：使用 NSFetchedResultsController 的第 0 节中的行数无效

Question

I have 2 ViewControllers:

VC1 populates its tableView based on CoreData attribute isPicked, which is bool and show only items with true state. VC2 is a second Modal (not Full Screen) View Controller which allow user to change the state of isPicked attribute: check and uncheck item (make it true or false). The idea is user picks needed items end dismiss the VC2 and the items should appear in VC1.

I have implemented NSFetchedResultsController to both VC1 and VC2. And as soon as I press on first item (ie change isPicked state to true) I receive the error from VC1:

[error] fault: 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 (2) 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 (0 inserted, 0 deleted) and plus or minus the number of rows moved into or out of that section (0 moved in, 0 moved out).

Here is how I change a state of item in VC2 (true\false):

    override func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
    guard let currencyArray = fetchedResultsController.fetchedObjects else { return }
    let cell = tableView.cellForRow(at: indexPath) as! PickCurrencyTableViewCell
    
    for currency in currencyArray {
        if currency.shortName == cell.shortName.text {
           currency.isPicked = !currency.isPicked
            coreDataManager.save()
        }
    }
}

Here is my VC1 NSFetchedResultsController implementation:

    override func viewDidLoad() {
    super.viewDidLoad()
    setupFetchedResultsController()
}

func setupFetchedResultsController() {
    let predicate = NSPredicate(format: "isForConverter == YES")
    fetchedResultsController = createCurrencyFetchedResultsController(and: predicate)
    fetchedResultsController.delegate = self
    try? fetchedResultsController.performFetch()
}

func createCurrencyFetchedResultsController(with request: NSFetchRequest<Currency> = Currency.fetchRequest(), and predicate: NSPredicate? = nil, sortDescriptor: [NSSortDescriptor] = [NSSortDescriptor(key: "shortName", ascending: true)]) -> NSFetchedResultsController<Currency> {
    request.predicate = predicate
    request.sortDescriptors = sortDescriptor
    
    return NSFetchedResultsController(fetchRequest: request, managedObjectContext: context, sectionNameKeyPath: nil, cacheName: nil)
}

Here is my VC1 NSFetchedResultsController delegate:

extension VC1: NSFetchedResultsControllerDelegate {
func controllerWillChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
    tableView.beginUpdates()
}

func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
    tableView.endUpdates()
}

func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange anObject: Any, at indexPath: IndexPath?, for type: NSFetchedResultsChangeType, newIndexPath: IndexPath?) {
    if let indexPath = indexPath, let newIndexPath = newIndexPath {
        switch type {
        case .update:
            tableView.reloadRows(at: [indexPath], with: .none)
        case .move:
            tableView.moveRow(at: indexPath, to: newIndexPath)
        case .delete:
            tableView.deleteRows(at: [indexPath], with: .none)
        case .insert:
            tableView.insertRows(at: [indexPath], with: .none)
        default:
            tableView.reloadData()
        }
    }
}
}

When I reload the app, picked item shows itself in VC1 (which caused crash). But every change in VC2 crashes the app again with the same error. I don't have any methods to delete items in VC1 or so. I need that VC1 tableView just show or hide items according to isPicked state made from VC2.

What I missed in my code?

UPDATE: my VC1 TableView methods

    override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    let currency = fetchedResultsController.sections![section]
    return currency.numberOfObjects
}

override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCell(withIdentifier: "converterCell", for: indexPath) as! ConverterTableViewCell
    
    let currency = fetchedResultsController.object(at: indexPath)
    cell.shortName.text = currency.shortName
    cell.fullName.text = currency.fullName
    
    return cell
}

Answer 1

Doc states:

controller(_:didChange:at:for:newIndexPath:)

indexPath

The index path of the changed object (this value is nil for insertions).

So, you should change the code like this:

    func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange anObject: Any, at indexPath: IndexPath?, for type: NSFetchedResultsChangeType, newIndexPath: IndexPath?) {
      switch type {
            case .update:
                if let indexPath = indexPath {
                   tableView.reloadRows(at: [indexPath], with: .none)
              }
            case .move:
               if let indexPath = indexPath {
                  tableView.moveRow(at: indexPath, to: newIndexPath)
               }
           case .delete:
                if let indexPath = indexPath {
                   tableView.deleteRows(at: [indexPath], with: .none)
                   tableView.reloadData()   // << May be needed ?
               }
           case .insert:
                if let newIndexPath = newIndexPath {
                   tableView.insertRows(at: [newIndexPath], with: .none)
                   tableView.reloadData()   // << May be needed ?
               }
           default:
                tableView.reloadData()
            }
    }
}