在Swift中，我有一个函数可以递归地复制文件夹，并使用异步调用。我想添加一个完成处理程序。有什么优雅的解决方案吗？

import Foundation

func copyFolder(at srcURL: URL, to dstURL: URL, completion: @escaping () -> Void) {
    let fileManager = FileManager.default
    let group = DispatchGroup()

    func copyItem(at srcURL: URL, to dstURL: URL) {
        group.enter()
        DispatchQueue.global(qos: .userInitiated).async {
            do {
                if fileManager.fileExists(atPath: dstURL.path) {
                    try fileManager.removeItem(at: dstURL)
                }
                try fileManager.copyItem(at: srcURL, to: dstURL)
            } catch {
                print("Failed to copy item at \(srcURL) to \(dstURL): \(error)")
            }
            group.leave()
        }
    }

    func copyFolderContents(at srcURL: URL, to dstURL: URL) {
        do {
            let contents = try fileManager.contentsOfDirectory(at: srcURL, includingPropertiesForKeys: nil, options: [])
            for item in contents {
                let dstItemURL = dstURL.appendingPathComponent(item.lastPathComponent)
                if item.hasDirectoryPath {
                    try fileManager.createDirectory(at: dstItemURL, withIntermediateDirectories: true, attributes: nil)
                    copyFolderContents(at: item, to: dstItemURL)
                } else {
                    copyItem(at: item, to: dstItemURL)
                }
            }
        } catch {
            print("Failed to copy folder contents at \(srcURL) to \(dstURL): \(error)")
        }
    }

    group.enter()
    DispatchQueue.global(qos: .userInitiated).async {
        do {
            if !fileManager.fileExists(atPath: dstURL.path) {
                try fileManager.createDirectory(at: dstURL, withIntermediateDirectories: true, attributes: nil)
            }
            copyFolderContents(at: srcURL, to: dstURL)
        } catch {
            print("Failed to create destination folder at \(dstURL): \(error)")
        }
        group.leave()
    }

    group.notify(queue: .main) {
        completion()
    }
}

// 使用示例
let srcFolderURL = URL(fileURLWithPath: "/path/to/source/folder")
let dstFolderURL = URL(fileURLWithPath: "/path/to/destination/folder")

copyFolder(at: srcFolderURL, to: dstFolderURL) {
    print("Folder copied successfully.")
}