swift：WKWEBViewにおけるJavascriptのalert,confirm,promptの表示について

WKWEBViewを利用して、特定のURLを表示する所謂ガワアプリを作成しようとしたところ、Javascriptのalert,confiem,promptが表示されません。
どの様にすればdelegateが作動するのかご指導頂けたら幸いです。

//ViewController.swift
import UIKit
import WebKit

class ViewController: UIViewController, WKUIDelegate {
@IBOutlet var webView1: WKWebView!
@IBOutlet weak var testText: UILabel!

override func viewDidLoad() {
let defaultURL = "https://sample.com"
super.viewDidLoad()

webView1.uiDelegate = self
if let targetURL = URL(string: defaultURL) {
  let request = NSURLRequest(url:targetURL as URL)
  webView1.load(request as URLRequest)
}

func webView(_ webView1: WKWebView, runJavaScriptAlertPanelWithMessage message: String, initiatedByFrame frame: WKFrameInfo, completionHandler: @escaping () -> Void) {
  testText.text = "Alert..."
  let alertController = UIAlertController(title: nil, message: message, preferredStyle: .actionSheet)
  alertController.addAction(UIAlertAction(title: "OK", style: .default, handler: { (action) in
    completionHandler()
  }))
  present(alertController, animated: true, completion: nil)
}

func webView(_ webView1: WKWebView, runJavaScriptConfirmPanelWithMessage message: String, initiatedByFrame frame: WKFrameInfo, completionHandler: @escaping (Bool) -> Void) {
  testText.text = "Confirm."
  let alertController = UIAlertController(title: nil, message: message, preferredStyle: .actionSheet)
  alertController.addAction(UIAlertAction(title: "Ok", style: .default, handler: { (action) in
    completionHandler(true)
  }))

  alertController.addAction(UIAlertAction(title: "Cancel", style: .default, handler: { (action) in
    completionHandler(false)
  }))
  self.present(alertController, animated: true, completion: nil)
}

func webView(_ webView1: WKWebView, runJavaScriptTextInputPanelWithPrompt prompt: String, defaultText: String?, initiatedByFrame frame: WKFrameInfo, completionHandler: @escaping (String?) -> Void) {
  testText.text = "Prompt..."
  let alertController = UIAlertController(title: nil, message: prompt, preferredStyle: .alert)

  alertController.addTextField { (textField) in
    textField.text = defaultText
  }

  alertController.addAction(UIAlertAction(title: "Ok", style: .default, handler: { (action) in
    if let text = alertController.textFields?.first?.text {
      completionHandler(text)
    } else {
      completionHandler(defaultText)
    }
  }))

  alertController.addAction(UIAlertAction(title: "Cancel", style: .default, handler: { (action) in
    completionHandler(nil)
  }))
  self.present(alertController, animated: true, completion: nil)
}