(Swift) Core data move rows - invalid update error - swift

Question

I am attempting to move rows containing core data objects in my table view. I get invalid update error.
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 (2), plus or minus the number of rows inserted or deleted from that section (0 inserted, 1 deleted) and plus or minus the number of rows moved into or out of that section (0 moved in, 0 moved out).'
I don't understand why I get this. I'm deleting the row I want to move at the source index path, then inserting it at the destination index path.
Here is my code:
// Move fetchedResultsController objects
override func tableView(tableView: UITableView, moveRowAtIndexPath sourceIndexPath: NSIndexPath, toIndexPath destinationIndexPath: NSIndexPath) {
let movedObject = self.fetchedResultsController.objectAtIndexPath(sourceIndexPath) as! NSManagedObject
controller(self.fetchedResultsController, didChangeObject: movedObject, atIndexPath: sourceIndexPath, forChangeType: .Delete, newIndexPath: sourceIndexPath)
controller(self.fetchedResultsController, didChangeObject: movedObject, atIndexPath: sourceIndexPath, forChangeType: .Insert, newIndexPath: destinationIndexPath)
}

Answer 1

A fetchedResultsController job is to monitor core data changes and tell your UI that it needs to update to match what is now in core data. You don't call controller( didChangeObject:.. methods; the fetchedResultsController calls them.
You should just update the object in core-data wait for the fetchedResultsController to inform your viewController to update.
A move it is more complicated. The tableView has already been updated and now you want to update core-data. But when you update core-data the fetchedResultsController will kick in an tell you to do the move - which you already did! So instead set fetchedResultsController to nil, then update core-data (this would be the field that the objects are stored by in your fetch, maybe an order property or something like that), and then reset the fetchedResultsController's delegate.

Answer 2

The code below is swift 4:
In the situation where you have a table with multiple sections your moveRowAt fromIndexPath: IndexPath, to toIndexPath: IndexPath function needs to calculate the correct row to move.
You can't just use the index path.row (from & to) as these don't take account of the other sections.
I use the following method to get the correct rows:-
// Do not trigger delegate methods when changes are made to core data by the user
fetchedResultsController.delegate = nil
var fromIndex = fromIndexPath.row
var toIndex = toIndexPath.row
// print ("from row ",fromIndexPath.row)
//work out correct row to remove at based on it's current row and section
for sectionIndex in 0..<fromIndexPath.section
{
fromIndex += fetchedResultsController.sections![sectionIndex].numberOfObjects
}
//print ("fromIndex ",fromIndex)
// remove the row at it's old position
toDoData.remove(at: fromIndex)
//work out the correct row to insert at based on which section it's going to & row in that section
for sectionIndex in 0..<toIndexPath.section
{
toIndex += fetchedResultsController.sections![sectionIndex].numberOfObjects
//print ("toIndex ",toIndex)
if sectionIndex == fromIndexPath.section
{
toIndex -= 1 // Remember, controller still thinks this item is in the old section
//print ("-= toIndex",toIndex)
}
}
// put the item back into he array at new position
toDoData.insert(item, at: toIndex)
Note: toDoData is my array of records from the fetchedResultsController
Note: that you also have to update the section assuming you are keeping a record of it in core data and run through the array to update your order. use :-
fetchedResultsController.sections![toIndexPath.section].name