Storage.swift

final class Storage {
  private let queue = DispatchQueue(label: "myexample", attributes: .concurrent)
  private var _contents = [String: Data]()

  private var contents: [String: Data] {
    get {
      queue.sync { _contents }
    }
    set {
      queue.async(flags: .barrier) { self._contents = newValue }
    }
  }

  func store(_ data: Data, forIdentifier id: String) {
    contents[id] = data
  }
  // …
}