An example of an NSNotificationCenter driven background updater.

BackgroundModelUpdater.swift

/**
 * A protocol that handles listening to notifications and updating model data asycronously.
 *
 * To use: create a class that fulfills the protocol and fill in the updateData method.
 */
@objc protocol BackgroundModelUpdater {
    /**
     * A notification identifier to be used to tell the updater to begin processing updates.
     */
    var updateNotification: String { get set }
    /**
     * A notification identifier to be used to tell other instances that the models have been processed.
     */
    optional var updateCompleteNotification: String { get set }
    
    /**
     * This method should perform any updates or networked behavior, then call the callback.
     */
    func updateData(completionHandler: ()->())
}

extension BackgroundModelUpdater {
    
    func configure() {
        let notifications = NSNotificationCenter.defaultCenter()
        notifications.addObserverForName(updateNotification, object: nil, queue: nil) { notification in
            self.shouldUpdateData()
        }
    }
    
    private func shouldUpdateData() {
        updateData(notify)
    }
    
    private func notify() {
        if let completionNotificationName = updateCompleteNotification {
            let notifications = NSNotificationCenter.defaultCenter()
            notifications.postNotificationName(completionNotificationName, object: nil)
        }
    }
}

MyDataModelUpdater.swift

enum MyModelUpdaterNotifications: String {
    case updateNotification = "MyModelUpdater.updateNotification"
    case updateCompleteNotification = "MyModelUpdater.updateCompleteNotification"
}

class MyModelUpdater: BackgroundModelUpdater {
    
    @objc var updateNotification = MyModelUpdaterNotifications.updateNotification.rawValue
    @objc var updateCompleteNotification = MyModelUpdaterNotifications.updateCompleteNotification.rawValue
    
    @objc func updateData(completionHandler: () -> ()) {
      // Do all your networking, downloading, and persisting here.
      // When all of the data you need to download has finished, 
      // then call the completionHandler to let the rest of the app know.
      completionHandler()
    }
}

MyViewController.swift

/**
 * Your UI View Controller
 */
class MyTableViewController: UITableViewController {
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        refreshControl = UIRefreshControl()
        
        // Pull to Refresh will trigger the download.
        refreshControl?.addTarget(self,
                                  action: #selector(MyTableViewController.refreshData),
                                  forControlEvents: UIControlEvents.ValueChanged)
      
        // A completion notification will be fired to let the VC know that the updates are in the database.
        let notifications = NSNotificationCenter.defaultCenter()
        notifications.addObserver(self,
                                  selector: #selector(refreshTable),
                                  name: MyModelUpdaterNotifications.updateCompleteNotification.rawValue,
                                  object: nil)
        
        shouldRefreshData()
    }
    
    // MARK: Refresh Methods
    
    func shouldRefreshData() {
        let notifications = NSNotificationCenter.defaultCenter()
        notifications.postNotificationName(ActivityModelUpdaterNotifications.updateNotification.rawValue, object: self)
    }
    
    func refreshTable() {
        self.refreshControl?.endRefreshing()
        self.tableView.reloadData()
    }
    
    //...
}