In my application, my main class is a UITableViewController. The JSON data is fetched using a ConnectionManager class and parsed with an ItemManager class. Inter-class communication is facilitated by the NotificationCenter. When there's a response from ConnectionManager, it sends a notification to ItemManager for parsing.

While ConnectionManager successfully posts notifications that are received by ItemManager, I'm facing an issue with my main class as it doesn't receive notifications from ItemManager after parsing completion.

• ConnectionManager -> posts, ItemManager -> receives -- OK
• ItemManager -> posts, UITableViewController -> receives -- NOK

Here's some code snippet:

class ConnectionManager {

  private let URL = "http://foo.json"
  private var response: Data?

  init () {
    getData()
  }

  func getData() {
    let req = NSMutableURLRequest(url: NSURL(string: URL)! as URL)
    req.httpMethod = "GET"
    URLSession.shared.dataTask(with: req as URLRequest) { data, response, error in
      if error != nil {
        //HTTP request failed
        NSLog(error?.localizedDescription ?? "Unknown Error")
        NotificationCenter.default.post(name: Notification.Name(NotificationCenterKeys.NOK), object: nil)
      } else {
        //HTTP request succeeded
        self.response = data
        NotificationCenter.default.post(name: Notification.Name(NotificationCenterKeys.OK), object: nil)
      }
      }.resume()
  }
}

class ItemManager {
  private var cm: ConnectionManager
  private var items = [Item]()

  init() {
    self.cm = ConnectionManager()
    NotificationCenter.default.addObserver(self, selector: #selector(self.parseResponse), name: Notification.Name(NotificationCenterKeys.OK), object: nil)
    NotificationCenter.default.addObserver(self, selector: #selector(self.noData), name: Notification.Name(NotificationCenterKeys.NOK), object: nil)
  }

  @objc private func noData() {
    NSLog("No data to parse")
  }

  @objc private func parseResponse() {
    do {
      if let data = cm.getResponse() {
        let items: NSDictionary = try JSONSerialization.jsonObject(with: data, options: JSONSerialization.ReadingOptions.mutableContainers) as! NSDictionary
        let rawItems = items["numbers"] as! [Int]
        
        self.items = parseItems(rawItems: rawItems)
        
        NotificationCenter.default.post(name: Notification.Name(NotificationCenterKeys.PARSED), object: nil)

      }
    } catch {
      print("Error deserializing JSON: \(error)")
    }
  }

Now, let's take a look at my UITableViewController:

class MainViewTableViewController: UITableViewController {

  var items = [Item]()
  var dataAvailable = false
  var im = ItemManager()
  var dataLabel = "Loading Data..."

  override func viewDidLoad() {
    super.viewDidLoad()
    NotificationCenter.default.addObserver(self, selector: #selector(self.updateTable), name: Notification.Name(NotificationCenterKeys.PARSED), object: nil)
    setTable()
  }

  @objc func updateTable() {
    if im.getItems()?.count != 0 {
      items = im.getItems()!
      dataAvailable = true
    } else {
      dataAvailable = false
      dataLabel = "No Data Available"
    }
    self.tableView.reloadData()
  }

  override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
  }

  func setTable() {
    title = "Test"
    tableView.rowHeight = 40
  }

  // MARK: - Table view data source

  override func numberOfSections(in tableView: UITableView) -> Int {
    var numOfSections = 0
    if dataAvailable {
      numOfSections = 4
      return numOfSections
    } else {
      let noDataLabel: UILabel = UILabel(frame: CGRect(x: 0, y: 0, width: tableView.bounds.size.width, height: tableView.bounds.size.height))
      noDataLabel.text = dataLabel
      noDataLabel.textColor = UIColor.black
      noDataLabel.textAlignment = .center
      tableView.backgroundView = noDataLabel
      tableView.separatorStyle = .none
    }
    return numOfSections
  }

  override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    return getNumOfItemsPerSection(section: section)
  }

  override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCell(withIdentifier: "Cell", for: indexPath) as! TableViewCell
    guard let sectionNumber = SectionNumber(rawValue: indexPath.section) else { return cell }
    let elements = self.items.filter({ $0.sectionNumber == sectionNumber })
    cell.nameLabel?.text = elements[indexPath.row].itemNumber?.description
    if elements[indexPath.row].checkmark {
      cell.accessoryType = .checkmark
    } else { cell.accessoryType = .none }
    return cell
  }

  func getNumOfItemsPerSection(section: Int) -> Int {
    guard let sectionNumber = SectionNumber(rawValue: section) else { return 0 }
    return self.items.filter({ $0.sectionNumber == sectionNumber }).count
  }

  override func tableView(_ tableView: UITableView, titleForHeaderInSection section: Int) -> String? {
    return "Section \(section + 1)"
  }
}