Using generators with tableviews in swift

Generator.swift

protocol AsyncGeneratorType {  
    typealias Element
    typealias Fetch
    mutating func next(fetchNextBatch: Fetch, onFinish: ((Element) -> Void)?)
}

/// Generator is the class because struct is captured in asynchronous operations so offset won't update.
class PagingGenerator<T>: AsyncGeneratorType {  
    typealias Element = Array<T>
    typealias Fetch = (offset: Int, limit: Int, completion: (result: Element) -> Void) -> Void

    var offset:Int
    let limit: Int

    init(startOffset: Int = 0, limit: Int = 25) {
        self.offset = startOffset
        self.limit = limit
    }

    func next(fetchNextBatch: Fetch, onFinish: ((Element) -> Void)? = nil) {
        fetchNextBatch(offset: offset, limit: limit) { [unowned self] (items) in
            onFinish?(items)
            self.offset += items.count
        }
    }
}

TableView.swift

class ViewController: UIViewController {  
    @IBOutlet var tableView: UITableView!
    private var paging = PagingGenerator<Contact>(startOffset: 0, limit: 25)

    private var contacts = [Contact]() {
        didSet {
            tableView.reloadData()
        }
    }

    override func viewDidLoad() {
        super.viewDidLoad()
        paging.next(fetchNextBatch, onFinish: updateDataSource) // first page
    }
}

//MARK: Paging

extension ViewController {  
    private func fetchNextBatch(offset: Int, limit: Int, completion: (Array<Contact>) -> Void) -> Void {
        if let remotelyFetched = downloadGithubUsersPage(offset) {
            completion(remotelyFetched)
        }
    }

    private func updateDataSource(items: Array<Contact>) {
        self.contacts += items
    }
}

//MARK: UITableViewDataSource

extension ViewController: UITableViewDataSource {  
    func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return contacts.count
    }

    func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
        let cell = UITableViewCell(style: UITableViewCellStyle.Default, reuseIdentifier: nil)
        let contact = contacts[indexPath.row]
        cell.textLabel?.text = "\(contact.firstName) \(contact.lastName)"
        return cell
    }
}

//MARK: UITableViewDelegate

extension ViewController: UITableViewDelegate {  
    func tableView(tableView: UITableView, willDisplayCell cell: UITableViewCell, forRowAtIndexPath indexPath: NSIndexPath) {
        if indexPath.row == tableView.dataSource!.tableView(tableView, numberOfRowsInSection: indexPath.section) - 1 {
            paging.next(fetchNextBatch, onFinish: updateDataSource)
        }
    }
}