NSFetchedResultsController crash when Section index changes - swift

Question

I'm writing my app in Swift 3 (converted) in Xcode 8.
NSFetchedResultsController is causing a Serious Application Error for me.
My main table view is sectioned by a text identifier called "yearText" which is being set on any given Event record (NSManagedObject) when the user changes the "Event Date" with a date picker. When the picker is changed or dismissed, the year is stripped from the date, converted to text, and stored in the Event object. The managed object context is then saved.
If a date is picked for which there is already a section in existence (i.e. the year "2020") an error is thrown that says:
[error] 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 (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). with userInfo (null)
As long as the date chosen is not within a year that already has a section named after it, it all works fine.
Here is my relevant code for updating the database and tableview:
var fetchedResultsController: NSFetchedResultsController<NSFetchRequestResult> {
if _fetchedResultsController != nil {
return _fetchedResultsController!
}
// Fetch the default object (Event)
let fetchRequest = NSFetchRequest<NSFetchRequestResult>()
let entity = NSEntityDescription.entity(forEntityName: "Event", in: managedObjectContext!)
fetchRequest.entity = entity
// Set the batch size to a suitable number.
fetchRequest.fetchBatchSize = 60
// Edit the sort key as appropriate.
let sortDescriptor = NSSortDescriptor(key: "date", ascending: false)
fetchRequest.sortDescriptors = [sortDescriptor]
// 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: managedObjectContext!, sectionNameKeyPath: "yearText", cacheName: nil)
aFetchedResultsController.delegate = self
_fetchedResultsController = aFetchedResultsController
do {
try _fetchedResultsController!.performFetch()
} catch {
// Implement error handling code here.
abort()
}
return _fetchedResultsController!
}
var _fetchedResultsController: NSFetchedResultsController<NSFetchRequestResult>?
// MARK: - UITableViewDelegate
extension EventListViewController: UITableViewDelegate {
func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
let cell = tableView.cellForRow(at: indexPath) as! EventCell
cell.isSelected = true
configureCell(withCell: cell, atIndexPath: indexPath)
}
func tableView(_ tableView: UITableView, didDeselectRowAt indexPath: IndexPath) {
let cell = tableView.cellForRow(at: indexPath) as! EventCell
cell.isSelected = false
configureCell(withCell: cell, atIndexPath: indexPath)
}
}
// MARK: - UITableViewDataSource
extension EventListViewController: UITableViewDataSource {
func numberOfSections(in tableView: UITableView) -> Int {
return fetchedResultsController.sections?.count ?? 0
}
func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
let sectionInfo = fetchedResultsController.sections![section]
return sectionInfo.numberOfObjects
}
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCell(withIdentifier: "EventCell", for: indexPath) as! EventCell
configureCell(withCell: cell, atIndexPath: indexPath)
return cell
}
func configureCell(withCell cell: EventCell, atIndexPath indexPath: IndexPath) {
// bunch of stuff to make the cell pretty and display the data
}
func tableView(_ tableView: UITableView, canEditRowAt indexPath: IndexPath) -> Bool {
// Return false if you do not want the specified item to be editable.
return true
}
func tableView(_ tableView: UITableView, commit editingStyle: UITableViewCellEditingStyle, forRowAt indexPath: IndexPath) {
if editingStyle == .delete {
let context = fetchedResultsController.managedObjectContext
context.delete(fetchedResultsController.object(at: indexPath) as! NSManagedObject)
do {
try context.save()
} catch {
// Replace this implementation with code to handle the error appropriately.
// abort() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
//print("Unresolved error \(error), \(error.userInfo)")
abort()
}
}
}
func tableView(_ tableView: UITableView, titleForHeaderInSection section: Int) -> String? {
let sectionInfo = fetchedResultsController.sections![section]
return sectionInfo.name
}
func tableView(_ tableView: UITableView, willDisplayHeaderView view: UIView, forSection section: Int) {
// make the section header look good
view.tintColor = kWPPTintColor
let header = view as! UITableViewHeaderFooterView
header.textLabel?.textColor = kWPPDarkColor
header.textLabel?.font = UIFont.preferredFont(forTextStyle: UIFontTextStyle.subheadline)
}
}
// MARK: - NSFetchedResultsControllerDelegate
extension EventListViewController: NSFetchedResultsControllerDelegate {
func controllerWillChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
tableView.beginUpdates()
}
func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange sectionInfo: NSFetchedResultsSectionInfo, atSectionIndex sectionIndex: Int, for type: NSFetchedResultsChangeType) {
switch type {
case .insert:
tableView.insertSections(IndexSet(integer: sectionIndex), with: .fade)
case .delete:
tableView.deleteSections(IndexSet(integer: sectionIndex), with: .fade)
default:
return
}
}
func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange anObject: Any, at indexPath: IndexPath?, for type: NSFetchedResultsChangeType, newIndexPath: IndexPath?) {
switch type {
case .insert:
tableView.insertRows(at: [newIndexPath!], with: .fade)
case .delete:
tableView.deleteRows(at: [indexPath!], with: .fade)
case .update:
configureCell(withCell: tableView.cellForRow(at: indexPath!)! as! EventCell, atIndexPath: indexPath!)
case .move:
tableView.moveRow(at: indexPath!, to: newIndexPath!)
}
}
func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
tableView.endUpdates()
}
}
I hope you can offer me some suggestions. Thank you.
EDIT: Took out some code that was just getting in the way and revised .move to use .moveRow
EDIT 2: Added FRC generation code.

Answer 1

I met the same error when I update some properties on my Core Data managed objects.
Here is my controller func:
func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange anObject: Any, at indexPath: IndexPath?, for type: NSFetchedResultsChangeType, newIndexPath: IndexPath?) {
switch type {
case .insert:
self.tableView.insertRows(at: [newIndexPath!], with: .fade)
case .delete:
self.tableView.deleteRows(at: [indexPath!], with: .fade)
case .update:
self.tableView.reloadRows(at: [indexPath!], with: .fade)
case .move:
self.tableView.insertRows(at: [newIndexPath!], with: .fade)
self.tableView.deleteRows(at: [indexPath!], with: .fade)
}
}
Before I used newIndexPath for the update case, but I found this will cause some section rows mismatch issue when fetch result controller do some update action. Instead, using indexPath for update case is fine.