NFetchedResultsController delegate method didChangeContentWith is not called when deleted an item using UIDIffableDataSource tableview

Question

Created Nov ’19

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I am trying to implement my existing coredata project using the UITableViewDiffableDataSource. My tableview is coupled using the NSFetchedResultsController and the corresponding delegate methods. I am able to list the data in the tableview using diffabledatasource. My datasource is declared with the generic types as below

UITableViewDiffableDataSource<String, NSManagedObjectID>

For enabling the editing mode in the tableview I subclassed the UITableViewDiffableDataSource. I can delete the cell from the tableview but not from my coreData. The code for deleting the cell is as below

override func tableView(_ tableView: UITableView, commit editingStyle: UITableViewCell.EditingStyle, forRowAt indexPath: IndexPath) {
  if editingStyle == .delete {
  if let identifierToDelete = itemIdentifier(for: indexPath){
  var snapshot = self.snapshot()
  snapshot.deleteItems([identifierToDelete])
  apply(snapshot)
  }
 }

The below NSFetchedResultsControllerDelegate method is not called when I delete the cell.

func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChangeContentWith snapshot: NSDiffableDataSourceSnapshotReference)

I am not sure whether this is the correct way of coupling diffabledatasource with NSFetchedResultscontroller. Any help will be appreciated. Thanks in advance

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Answer 1

malc OP

May ’20

You shouldn't be editing the snapshot, instead edit the model. I.e. delete the managed Object as follows:

- (void)tableView:(UITableView *)tableView commitEditingStyle:(UITableViewCellEditingStyle)editingStyle forRowAtIndexPath:(NSIndexPath *)indexPath {
    if (editingStyle == UITableViewCellEditingStyleDelete) {
        NSManagedObjectContext *context = self.managedObjectContext;
        NSManagedObjectID *objectID = [self itemIdentifierForIndexPath:indexPath];
        Event *event = [context objectWithID:objectID];
        [context deleteObject:event];
        NSError *error = nil;
        if (![context save:&error]) {
            // 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.
            NSLog(@"Unresolved error %@, %@", error, error.userInfo);
            abort();
        }
    }
}

After deleting the object the

didChangeContentWith snapshot object will be called.

Note: You need to add a managedObjectContext property to your UITableViewDiffableDataSource subclass.

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