尝试保存数组时CoreData出错。 '无法将'String'类型的值转换为预期的参数类型'NSManagedObject''

Question

I am using CoreData to save a tableView to the device. I am relatively new to CoreData, and cannot figure this out. I get the error:

'Cannot convert value of type 'String' to expected argument type 'NSManagedObject''

On the line:

favourites.append(addNewMemory.text!)

//MARK:- Core Data
    func save(name: String) {

      guard let appDelegate =
        UIApplication.shared.delegate as? AppDelegate else {
        return
      }

      // 1
      let managedContext =
        appDelegate.persistentContainer.viewContext

      // 2
      let entity =
        NSEntityDescription.entity(forEntityName: "Memory",
                                   in: managedContext)!

      let person = NSManagedObject(entity: entity,
                                   insertInto: managedContext)

      // 3
      person.setValue(name, forKeyPath: "name")

      // 4
      do {
        try managedContext.save()
        favourites.append(person)
      } catch let error as NSError {
        print("Could not save. \(error), \(error.userInfo)")
      }
    }

    var favourites: [NSManagedObject] = []


   func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return favourites.count
    }

    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
        let cell = UITableViewCell(style: UITableViewCell.CellStyle.default, reuseIdentifier: "Cell")
        /*cell.imageView?.image = UIImage(named: "applelogo")
        cell.imageView?.setRounded()
        cell.imageView?.clipsToBounds = true
        */

        let favMemory = favourites[indexPath.row]
        cell.textLabel?.text = favMemory.value(forKeyPath: "name") as? String

        return cell
    }
@IBAction func addButtonTapped(_ sender: UIButton) {
        insertNewCell()
    }

    func insertNewCell() {

        favourites.append(addNewMemory.text!)

        let indexPath = IndexPath(row: favourites.count - 1, section: 0)
        tableView.beginUpdates()
        tableView.insertRows(at: [indexPath], with: .automatic)
        tableView.endUpdates()

        addNewMemory.text = ""
        view.endEditing(true)
    }

I expected the app to save the string, but it does not work. How can I fix this?

Answer 1

You are mixing up the text property of the text field and the Core Data entity. Obviously favourites is declared as [NSManagedObject] so you can't append a string. That's what the error message is telling you.

You have to insert a new record in insertNewCell . The easiest solution is to call save and return a Bool from save to indicate that the insertion was successful.

And you are encouraged to use more contemporary API. If there is no Memory subclass create one

var favourites = [Memory]()

...

func save(name: String) -> Bool {
    let appDelegate = UIApplication.shared.delegate as! AppDelegate // force unwrapping is perfectly fine

    // 1
    let managedContext = appDelegate.persistentContainer.viewContext

    // 2
    let person = Memory(context: managedContext)

    // 3
    person.name = name

    // 4
    do {
      try managedContext.save()
      favourites.append(person)
      return true
    } catch let error as NSError {
      print("Could not save. \(error), \(error.userInfo)")
      return false
    }
}

and change insertNewCell to

func insertNewCell() {
    guard save(name: addNewMemory.text!) else { return }
    let indexPath = IndexPath(row: favourites.count - 1, section: 0)
    tableView.insertRows(at: [indexPath], with: .automatic)
    addNewMemory.text = ""
    view.endEditing(true)
}

beginUpdates/endUpdates is pointless.