Cancelation Story about promises HOT 6 CLOSED

class Fetcher {
  var task: URLSessionDataTask?

  func fetch(url: URL) -> Promise<(Data?, URLResponse?)> {
    cancel()
    return wrap { (handler: @escaping (Data?, URLResponse?, Error?) -> Void) in
      self.task = URLSession.shared.dataTask(with: url, completionHandler: handler)
      self.task?.resume()
    }
  }

  func cancel() {
    task?.cancel()
  }
}

let fetcher = Fetcher()
fetcher.fetch(url: URL(string: "https://google.com")!).then { _, response in
  print(response)
}.catch { error in
  print(error)
}
DispatchQueue.main.asyncAfter(deadline: .now() + 0.1) {
  fetcher.cancel()
}

func fetch(url: URL) -> Promise<(Data?, URLResponse?)> {
  cancel()
  return wrap { (handler: @escaping (Data?, URLResponse?, Error?) -> Void) in
    self.task = URLSession.shared.dataTask(with: url, completionHandler: handler)
    self.task?.resume()
  }
}

    public static func download(token: Cancellation.Token? = nil) -> Promise<Data> {
        return Promise { fulfill, reject in
            // simulate download ..., typically data will come in chunks and will be cached ...  
            DispatchQueue.main.asyncAfter(deadline: .now() + 2, execute: {
                if let token = token, token.isCancellationRequested {
                    return reject(Error.cancel)
                } else {
                    return fulfill(Data())
                }
            })
        }
    }

    let source = Cancellation.Source()
    
    let promise = download(token: source.token)
        .then({ (data) in
            print("data -----> ", data)
        })
        .catch({ (error) in
            print("ERROR -> ", error)
        })
    
    DispatchQueue.main.asyncAfter(deadline: .now() + 1, execute: {
        // simulate user pressed stop downloading ...
        source.cancel()
    })