如何在iphone的xml文件中保存和更新值？

import Foundation
import XMLParser

let xmlDocument = XMLDocument()
let rootElement = XMLElement(name: "root")
xmlDocument.rootElement = rootElement

let element = XMLElement(name: "element")
element.addAttribute(XMLNode.attribute(withName: "attributeName", stringValue: "attributeValue") as! XMLNode)
rootElement.addChild(element)

let fileManager = FileManager.default
let documentsURL = fileManager.urls(for: .documentDirectory, in: .userDomainMask)[0]
let fileURL = documentsURL.appendingPathComponent("file.xml")
xmlDocument.xmlData.write(to: fileURL, atomically: true)

func updateXMLFile(fileURL: URL, completion: @escaping (XMLDocument?) -> Void) {
    DispatchQueue.global(qos: .background).async {
        do {
            let xmlDocument = try XMLDocument(contentsOf: fileURL, options: [])
            let rootElement = xmlDocument.rootElement()
            
            // 更新元素和属性的值
            if let element = rootElement?.elements(forName: "element").first {
                element.addAttribute(XMLNode.attribute(withName: "attributeName", stringValue: "newAttributeValue") as! XMLNode)
            }
            
            // 将更新后的XML文件保存到本地存储中
            xmlDocument.xmlData.write(to: fileURL, atomically: true)
            
            completion(xmlDocument)
        } catch {
            completion(nil)
        }
    }
}

let fileManager = FileManager.default
let documentsURL = fileManager.urls(for: .documentDirectory, in: .userDomainMask)[0]
let fileURL = documentsURL.appendingPathComponent("file.xml")

updateXMLFile(fileURL: fileURL) { (xmlDocument) in
    if let xmlDocument = xmlDocument {
        print("XML文件更新成功：\(xmlDocument)")
    } else {
        print("XML文件更新失败")
    }
}