swift – 用户代理在运行时更改

我正在尝试在我的应用上添加移动,桌面版网站功能.但问题是用户代理只在webView加载之前立即更改.在我的应用程序中,我添加了一个用于移动的按钮,第二个用于桌面版本站点UIWebView刚刚启动时选择用户代理.任何人都告诉我有没有办法在运行时更改用户代理.谢谢

var defaultUserAgent = String()
    @IBOutlet weak var webView: UIWebView!

    @IBOutlet weak var activityIndicator: UIActivityIndicatorView!

    override func viewDidLoad() {
        super.viewDidLoad()

        let url = NSURL(string: "https://www.youtube.com")
        let request = NSURLRequest(URL: url!)
        activityIndicator.hidesWhenStopped = true
        activityIndicator.startAnimating()
        webView.loadRequest(request)
    }

    @IBAction func mobile(sender: AnyObject) {
        NSUserDefaults.standardUserDefaults().registerDefaults(["UserAgent": defaultUserAgent])
        print(defaultUserAgent)

        let url = NSURL(string: "https://www.youtube.com")
        let request = NSURLRequest(URL: url!)
        activityIndicator.hidesWhenStopped = true
        activityIndicator.startAnimating()
        webView.loadRequest(request)

    }

    @IBAction func desktop(sender: AnyObject) {

        NSUserDefaults.standardUserDefaults().registerDefaults(["UserAgent": "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_10_5) AppleWebKit/537.36 (KHTML,like Gecko) Chrome/48.0.2564.109 Safari/537.36"])

        let url = NSURL(string: "https://www.youtube.com")
        let request = NSURLRequest(URL: url!)
        activityIndicator.hidesWhenStopped = true
        activityIndicator.startAnimating()
        webView.loadRequest(request)

    }

    func webViewDidFinishLoad(webView: UIWebView) {
        activityIndicator.stopAnimating()
        print(webView.stringByEvaluatingJavaScriptFromString("navigator.userAgent"))
        defaultUserAgent = webView.stringByEvaluatingJavaScriptFromString("navigator.userAgent")!
    }

if #available(iOS 9,*) {
            webViewObj.customUserAgent = "customUserAgentValue"
 }