Animate deleted row with fetched results controller - ios

Question

I’m looking to animate the removal of an item from my table view. I’m using the fetched results controller with CoreData. The issue I’m having is an exception getting raised about how the number of rows must match the number of items.
Here’s the error i get when I delete:
reason: 'Invalid update: invalid number of rows in section 0. The
number of rows contained in an existing section after the update (4)
must be equal to the number of rows contained in that section before
the update (3), 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).'
And here’s the code:
override func tableView(_ tableView: UITableView, commit editingStyle: UITableViewCellEditingStyle, forRowAt indexPath: IndexPath) {
guard let appDelegate = UIApplication.shared.delegate as? AppDelegate else { return }
let context = appDelegate.persistentContainer.viewContext
if editingStyle == .delete {
// Delete the row from the data source
context.delete(fetchedResultsController.object(at: indexPath))
do {
try context.save()
// Animate deleted row
tableView.deleteRows(at: [indexPath], with: .fade)
} catch {
print("There was an error: \(error)")
}
}
}
Thank you.

Answer 1

You need to also delete the item from your array (or whatever the data source is).
Basically, tableView(_:numberOfRowsInSection:) has to return one less object (or however many items you deleted) when you animate deletion in your UITableView

Answer 2

The problems is that you are removing the row when you delete it and then you are removing it again when the NSFetchedResultsControllerDelegate is telling you do. You must only remove the row once. There are two possible solutions.
1) Remove tableView.deleteRows
The fetchedResultsController delegate will inform your viewController when the object is deleted from the context. So simply delete the object from core data and then trust that the content will be updated appropriately. If you are saving things in the background you may notice a slight delay.
2) Ignore the delegate callback
Set a bool right before deleting the object to ignore the fetchedResultsController callbacks and unset it right after you save. Apple suggest this approach in their documentation.
I personally use the first approach I find it simplest. I am also uncomfortable with ignoring fetchedResultsController updates because if they become out of sync the application can crash.