Shows a ViewController that contains a WKWebView and UIActivityIndicatorView. This is a pruned down snippet from working code that may or may not work exactly as-is, but should give the basic idea of how to show/hide an activity indicator while a WKWebView is navigating to a URL. The original project this was adapted from was done in XCode 8, an…

WebViewWithActivityIndicator.swift

import Foundation
import UIKit
import WebKit

class ViewController: UIViewController, WKNavigationDelegate, WKUIDelegate {
	
	var webView: WKWebView!
	var activityIndicator: UIActivityIndicatorView!
	@IBOutlet var webViewContainer: UIView!
	
	override func viewDidLoad() {
		webView = WKWebView(frame: CGRect.zero)
		webView.navigationDelegate = self
		webView.uiDelegate = self
		
		webViewContainer.addSubview(webView)
		
		activityIndicator = UIActivityIndicatorView()
		activityIndicator.center = self.view.center
		activityIndicator.hidesWhenStopped = true
		activityIndicator.activityIndicatorViewStyle = UIActivityIndicatorViewStyle.gray

		webViewContainer.addSubview(activityIndicator)
		
		webView.load(URLRequest(url: URL(string: "http://google.com")!))
	}
	
	func showActivityIndicator(show: Bool) {
		if show {
			activityIndicator.startAnimating()
		} else {
			activityIndicator.stopAnimating()
		}
	}
	
	func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
		showActivityIndicator(show: false)
	}
	
	func webView(_ webView: WKWebView, didStartProvisionalNavigation navigation: WKNavigation!) {
		showActivityIndicator(show: true)
	}

	func webView(_ webView: WKWebView, didFail navigation: WKNavigation!, withError error: Error) {
		showActivityIndicator(show: false)
	}
}

But at viewDidLoad() self.view.center is (0, 0), isn't it?

Line 21: A few things got renames, now it's

activityIndicator.style = UIActivityIndicatorView.Style.gray