Add Extended Attributes to the file

Introduction

On iOS, you can get the value corresponding to the attribute key FileAttributeKey for the file on the device. It contains information such as the date the file was created and modified, and the size of the file. You can get the value defined in FileAttributeKey through the method attributesOfItem (atPath :) provided by FileManager. However, you can also set your own attributes to save / retrieve additional information. As an example of the usage scene, when the file on S3 is downloaded locally, the information about the update time to the file [LastModified](https://docs.aws.amazon.com/sdkfornet1/latest/apidocs/html/P_Amazon_S3_Model_S3Object_LastModified. You can think of things like assigning a value of htm) and using it for file synchronization management from the next time onwards.

Implementation example

For the sake of simplicity, the information to be saved in the code below is String. I think it's okay to target types that conform to Codable instead of String.

extension URL {
  ///Get extended attributes
  func extendedAttribute(for name: String) -> String?  {
    let result = withUnsafeFileSystemRepresentation { fileSystemPath -> String? in
      //Get the size of the extended attribute data corresponding to name
      let length = getxattr(fileSystemPath, name, nil, 0, 0, 0)
      guard length >= 0 else {
        return nil
      }
      //Secure an area to store data
      var data = Data(count: length)
      let result = data.withUnsafeMutableBytes { [count = data.count] in
        //Get extended attribute data corresponding to name
        getxattr(fileSystemPath, name, $0.baseAddress, count, 0, 0)
      }
      guard result >= 0 else {
        return nil
      }
      return String(data: data, encoding: .utf8)
    }
    return result
  }

  ///Save extended attributes
  @discardableResult
  func setExtendedAttribute(value: String, for name: String) -> Bool {
    guard let data = value.data(using: .utf8) else {
      return false
    }
    let result = withUnsafeFileSystemRepresentation { fileSystemPath -> Bool in
      let result = data.withUnsafeBytes {
        //Save extended attribute data corresponding to name
        setxattr(fileSystemPath, name, $0.baseAddress, data.count, 0, 0)
      }
      return result >= 0
    }
    return result
  }
}

Check the operation

//The name of the extended attribute to set
let key = "com.example.extended_attribute"
//File location to set extended attributes
let url = FileManager.default.temporaryDirectory.appendingPathComponent("example_file")

//If the file does not exist, create it and add extended attributes
if !FileManager.default.fileExists(atPath: url.path) {
  let data = "example_content".data(using: .utf8)!
  try! data.write(to: url)
  //Grant extended attributes
  url.setExtendedAttribute(value: "example_value", for: key)
}

//Check if extended attributes can be obtained
print(url.extendedAttribute(for: key))

Console output

Optional("example_value")

Even if you restart the application, you can see that the value saved by setExtendedAttribute can be obtained and the value associated with the file can be persisted.

reference

• Stack Overflow - Write extend file attributes swift example --For implementation, refer to here. In the code example given as an example, the content of the error is crushed for the sake of simplicity, but if you need to understand the error, you should throw the error as in the implementation of the link destination. There are also code examples such as getting the names of extended attributes in a list and deleting extended attributes, so please refer to them if you need them.
• Apple Developer Documentation - withUnsafeFileSystemRepresentation(_:)
• Mac OS X Man Pages - getxattr
  • On success, the size of the extended attribute data is returned. On failure, -1 is returned...
• Mac OS X Man Pages - setxattr
  • On success, 0 is returned. On failure, -1 is returned...