FileReader.swift

func read(byteRange: CountableRange<Int>, queue: DispatchQueue = .main, completionHandler: @escaping (DispatchData?) -> Void) {
  if let channel = channel {
    channel.read(offset: off_t(byteRange.startIndex), length: byteRange.count, queue: queue, ioHandler: { done, data, error in
      print(done, data?.count ?? -1, error)
      completionHandler(data)
    })
  }
  else {
    print("Channel is closed")
    completionHandler(nil)
  }
}