How do you save new index location in persistentcontainer? - arrays

Question

I have a tableview where I have added the ability to move the location of rows. I am able to use the "swapAt" function to update my array and it works fine. The problem is that when I close and re-open the app, the rows continue to show in their old IndexPath. How can I also update the index location in the PersistentContainer so that when I open the app, it is updated with the new IndexPath?
I tried calling the save method of the context but it does not work.
import UIKit
import CoreData
class ViewController: UIViewController, UITableViewDataSource, UITableViewDelegate {
#IBOutlet var tableView: UITableView!
let context = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
var toDoList = [Item] ()
var count: Int {
toDoList.count
}
override func viewDidLoad() {
super.viewDidLoad()
tableView.dataSource = self
tableView.delegate = self
navigationItem.rightBarButtonItem = UIBarButtonItem(barButtonSystemItem: .add, target: self, action: #selector(addItem))
navigationItem.leftBarButtonItem = UIBarButtonItem(barButtonSystemItem: .edit, target: self, action: #selector(editItem))
title = "Todo"
loadItem()
}
func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
return toDoList.count
}
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCell(withIdentifier: "customCell", for: indexPath)
cell.textLabel?.text = toDoList [indexPath.row].title
if toDoList[indexPath.row].checkmark == true {
cell.accessoryType = .checkmark }
else {
cell.accessoryType = .none
}
return cell
}
func tableView(_ tableView: UITableView, didSelectRowAt indexPath: IndexPath) {
if toDoList[indexPath.row].checkmark == false {
toDoList[indexPath.row].checkmark = true } else {
toDoList[indexPath.row].checkmark = false
}
tableView.reloadData()
saveItem()
}
func tableView(_ tableView: UITableView, canMoveRowAt indexPath: IndexPath) -> Bool {
return true
}
func tableView(_ tableView: UITableView, moveRowAt sourceIndexPath: IndexPath, to destinationIndexPath: IndexPath) {
toDoList.swapAt(sourceIndexPath.row, destinationIndexPath.row)
saveItem()
}
#objc func addItem () {
let ac = UIAlertController(title: "Add new category", message: nil, preferredStyle: .alert)
var textField = UITextField()
let submitAction = UIAlertAction(title: "Add", style: .default) { (action) in
let newItem = Item(context: self.context)
newItem.title = textField.text
newItem.checkmark = false
newItem.order = Int64(self.count)
self.toDoList.append(newItem)
self.saveItem()
}
ac.addTextField { (alertTextField) in
alertTextField.placeholder = "Create new item"
textField = alertTextField
}
ac.addAction(submitAction)
present(ac, animated: true, completion: nil)
}
func saveItem () {
do {
try context.save()
} catch {
print("This is the \(error)")
}
self.tableView.reloadData()
}
func loadItem () {
let request: NSFetchRequest<Item> = Item.fetchRequest()
let sortRequest = NSSortDescriptor(key: "order", ascending: true)
request.sortDescriptors = [sortRequest]
do {
toDoList = try context.fetch(request)
} catch {
print("the error is \(error)")
}
tableView.reloadData()
}
#objc func editItem () {
if tableView.isEditing {
tableView.isEditing = false
} else {
tableView.isEditing = true
}}
func tableView(_ tableView: UITableView, commit editingStyle: UITableViewCell.EditingStyle, forRowAt indexPath: IndexPath) {
if editingStyle == .delete {
context.delete(toDoList[indexPath.row])
saveItem()
self.toDoList.remove(at: indexPath.row)
self.tableView.deleteRows(at: [indexPath], with: .automatic)
}
}
}

Answer 1

I would tackle this problem by having an index property in the Coredata entity; this index reflects the position of the element in the table view. In the swapAtFunction add the logic to switch the index properties of the two elements.
Then, when you fetch the entities, you add a predicate to your request, to sort the elements by the index property.
let fetchRequest = NSFetchRequest<EntityName>(entityName: "EntityName")
let sortDescripor = NSSortDescriptor(key: "index", ascending: true)
fetchRequest.sortDescriptors = [sortDescripor]
Then execute fetchRequest as you normally would, and assign the resulting array to the UICollectionView's Datasource.