Issue an ACS user access token in Swift

issue_token.swift

// Script to issue an ACS user token via HMAC auth as described in this doc:
// https://docs.microsoft.com/en-us/azure/communication-services/concepts/authentication?tabs=csharp#sign-an-http-request
import UIKit
import Foundation
import CryptoKit

struct FetchTokenResult: Codable {
    var identity:String
    var token:String
    var expiresIn: Int
}

func fetchToken(endpoint: String, accessKey: String, identity: String? = nil, completionHandler: @escaping (Result<FetchTokenResult, Error>) -> Void) {
    // create the URL
    let url = URL(string: "\(endpoint)/identity/token?api-version=2020-07-20-preview1")!
    
    // create the JSON body
    var jsonObject: [String: Any] = ["scopes": ["chat", "voip"]]
    if let identity = identity {
        jsonObject["identity"] = identity
    }
    let bodyBytes = try! JSONSerialization.data(withJSONObject: jsonObject)
    
    // create an SHA256 digest of the body
    let digest = SHA256.hash(data: bodyBytes)
    let bodyDigest = Data(digest).base64EncodedString()
    
    // ISO UTC Timestamp fot eh date header
    let dateFormatter = DateFormatter()
    dateFormatter.dateFormat = "E, dd MMM yyyy HH:mm:ss z"
    dateFormatter.timeZone = TimeZone(abbreviation: "UTC")
    let date = dateFormatter.string(from: Date())
    
    // HMAC for the Authorization header
    let stringToSign = "POST\n\(url.path)?\(url.query!)\n\(date);\(url.host!);\(bodyDigest)"
    let key = SymmetricKey.init(data: Data(base64Encoded: accessKey)!)
    let code = HMAC<SHA256>.authenticationCode(for: stringToSign.data(using: .utf8)!, using: key)
    let authHeader = "HMAC-SHA256 SignedHeaders=date;host;x-ms-content-sha256&Signature=\(Data(code).base64EncodedString())"

    // configure the URL request
    var request = URLRequest(url: url)
    request.httpMethod = "POST" //set http method as POST
    request.httpBody = bodyBytes
    request.addValue("application/json", forHTTPHeaderField: "Content-Type")
    request.addValue(date, forHTTPHeaderField: "Date")
    request.addValue(bodyDigest, forHTTPHeaderField: "x-ms-content-sha256")
    request.addValue(authHeader, forHTTPHeaderField: "Authorization")

    // submit the HTTP request
    let task = URLSession.shared.dataTask(with: request as URLRequest) { data, response, error in
        if let error = error {
            completionHandler(.failure(error))
            return
        }
        let decoder = JSONDecoder()
        let result = try! decoder.decode(FetchTokenResult.self, from: data!)
        completionHandler(.success(result))

    }
    task.resume()
}