Use a semaphore to block

sema.swift

func xmlDocument(url: String) -> Document? {
    var doc: Document?
    let semaphore = DispatchSemaphore(value: 0)

    AF.request(url).response { response in
        if let data = response.data, let xml = String(data: data, encoding: .utf8) {
            do {
                doc = try SwiftSoup.parse(xml, "", Parser.xmlParser())
            } catch {
                print("error: \(error.localizedDescription)")
            }
            semaphore.signal()
        }
    }
    
    // This blocks until the other thread calls .signal()
    semaphore.wait()
    return doc
}