Persisting Change Tokens for CloudKit Sync

tokens.swift

// MARK: - Storing and Accessing Change Tokens

fileprivate extension UserDefaults {

  fileprivate func setUUID(_ uuid: UUID, using key: String) {
    self.set(uuid.uuidString, forKey: key)
  }

  fileprivate func uuid(matching key: String) -> UUID? {
    guard let str = UserDefaults.standard.string(forKey: key) else {
      os_log("** UUID not found: %@", log: log, key)
      return nil
    }

    guard let uuid = UUID(uuidString: str) else {
      os_log("** decoding UUID failed: %@",log: log, key)
      return nil
    }

    return uuid
  }

  fileprivate func setServerChangeToken(_ token: CKServerChangeToken, using key: String) {
    let coder = NSKeyedArchiver(requiringSecureCoding: true)
    token.encode(with: coder)
    self.set(coder.encodedData, forKey: key)
  }

  fileprivate func serverChangeToken(matching key: String) -> CKServerChangeToken? {
    guard let data = UserDefaults.standard.object(forKey: key) as? Data else {
      os_log("** server change token not found: %@", log: log, key)
      return nil
    }

    do {
      let coder = try NSKeyedUnarchiver(forReadingFrom: data)
      coder.requiresSecureCoding = true
      return CKServerChangeToken(coder: coder)
    } catch {
      os_log("** decoding server change token failed: ( %@, %@ )",
             log: log, key, error as CVarArg)
      return nil
    }
  }

}