Swift synchronise network request - ios

Question

Im getting user's input and creating CoreData entity for it, and put in to the tableView, but before i need to designate some properties for entity by making a couple network requests. And there is a problem. First entity is always nil (and create a empty cell for table view) but if i save context and open app again - here it is, entity is ok. If i create second (and so on) entity it works fine. Its also seems like table always create am empty cell for start and then delete it and create proper cell…
I cant get why completion handlers doesn't work properly and network requests makes asynchronously. Please help me to understand.
Creation metod starts in textFieldShouldReturn
import UIKit
import CoreData
class ViewController: UIViewController, UITextFieldDelegate {
var managedObjectContext: NSManagedObjectContext?
lazy var fetchedResultsController: NSFetchedResultsController<Word> = {
let fetchRequest: NSFetchRequest<Word> = Word.fetchRequest()
let createdSort = NSSortDescriptor(key: "created", ascending: false)
let idSort = NSSortDescriptor(key: "id", ascending: false)
fetchRequest.sortDescriptors = [createdSort, idSort]
var fetchedResultsController = NSFetchedResultsController(
fetchRequest: fetchRequest,
managedObjectContext: AppDelegate.viewContext,
sectionNameKeyPath: nil,
cacheName: nil)
fetchedResultsController.delegate = self
return fetchedResultsController
}()
private var hasWords: Bool {
guard let fetchedObjects = fetchedResultsController.fetchedObjects else { return false }
return fetchedObjects.count > 0
}
let creator = WordManager.shared
#IBOutlet weak var tableView: UITableView!
#IBOutlet weak var messageLabel: UILabel!
override func viewDidLoad() {
super.viewDidLoad()
managedObjectContext = AppDelegate.viewContext
fetchWords()
updateView()
}
func updateView() {
tableView.isHidden = !hasWords
messageLabel.isHidden = hasWords
}
............
func textFieldShouldReturn(_ textField: UITextField) -> Bool {
guard let managedObjectContext = managedObjectContext else { return false }
guard let title = textField.text, !title.isEmpty, !title.hasPrefix(" ") else {
showAllert(title: "Warning!", message: "Title may not to be nil!")
return false
}
let word = Word(context: managedObjectContext)
guard let fetchedWordsCount = fetchedResultsController.fetchedObjects?.count else { return false }
creator.createWord(title: title, counter: fetchedWordsCount, word: word)
self.updateView()
textField.text = nil
return true
}
func fetchWords() {
do {
try fetchedResultsController.performFetch()
} catch {
fatalError("Cant fetch words. Error \(error.localizedDescription)")
}
}
}
Creating entity
import CoreData
import UIKit
class WordManager {
static var shared = WordManager()
private init() {}
private var api = DictionaryAPI()
private var api2 = MeaningAPI()
func createWord(title: String, counter: Int, word: Word) {
self.api.fetch(word: title, completion: { translation in
self.api2.fetchTranscription(word: title, completion: { transcription in
word.id = Int16(counter) + 1
word.title = title
word.translation = translation
word.transcription = "[\(transcription)]"
word.created = NSDate()
})
})
}
}
Nothing interesting in tableView
extension ViewController: UITableViewDataSource {
func numberOfSections(in tableView: UITableView) -> Int {
guard let sections = fetchedResultsController.sections else { return 0 }
return sections.count
}
func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
guard let section = fetchedResultsController.sections?[section] else { return 0 }
return section.numberOfObjects
}
func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
let cell = tableView.dequeueReusableCell(withIdentifier: "mainCell", for: indexPath) as! MainCell
let word = fetchedResultsController.object(at: indexPath)
cell.configure(with: word)
return cell
}
}
Configure cell method
func configure(with word: Word) {
titleField.text = word.title
translationField.text = word.translation
transcriptionLabel.text = word.transcription
totalCounter.text = "\(word.showCounter)"
correctCounter.text = "\(word.correctCounter)"
wrongCounter.text = "\(word.wrongCounter)"
}
Fetched Results Delegate
extension ViewController: NSFetchedResultsControllerDelegate {
func controllerWillChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
tableView.beginUpdates()
}
func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
tableView.endUpdates()
updateView()
}
func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange anObject: Any, at indexPath: IndexPath?, for type: NSFetchedResultsChangeType, newIndexPath: IndexPath?) {
switch type {
case .delete:
guard let indexPath = indexPath else { return }
tableView.deleteRows(at: [indexPath], with: .fade)
case .insert:
guard let indexPath = newIndexPath else { return }
tableView.insertRows(at: [indexPath], with: .fade)
case .move:
guard let indexPath = indexPath else { return }
tableView.deleteRows(at: [indexPath], with: .fade)
guard let newIndexPath = newIndexPath else { return }
tableView.insertRows(at: [newIndexPath], with: .fade)
default:
break
}
}
}

Answer 1

Problem:
The data you have in core data is not in a state that is ready to be shown in the app.
The NSFetchedResultsController just happens to pick this data and shows it in the app.
Approach:
When a user creates a records, the user wants to see the changes immediately in the app and shouldn't have to wait for the app to update in the server to see the changes. So a user would still be able to create records even when there is no network connectivity.
Create a child context from view context
Use a background thread to upload to the server and when update records on to the child context.
Save child context only when the data is in a consistent state.
NSFetchedResultsController should be using the view context so would be unaware of the child context records.
When you save the child context, the NSFetchedResultsController would pick these records.