How to monitor WKWebView page load progress in Swift?

This should work properly on Swift 5:

@IBOutlet weak var webView: WKWebView!
@IBOutlet weak var progressView: UIProgressView!

private var observation: NSKeyValueObservation? = nil

override func viewDidLoad() {
    super.viewDidLoad()

    // load website or HTML page
    webView.load(NSURLRequest(url: URL(string: "https://www.apple.com")!) as URLRequest)

    // add observer to update estimated progress value
    observation = webView.observe(\.estimatedProgress, options: [.new]) { _, _ in
        self.progressView.progress = Float(self.webView.estimatedProgress)
    }
}

deinit {
    observation = nil
}

Swift 4.0

@IBOutlet weak var webView: WKWebView!
@IBOutlet weak var progressView: UIProgressView!
override func viewDidLoad() {
    super.viewDidLoad()

    // load website or HTML page
    self.webView.load(NSURLRequest(url: URL(string: "https://www.apple.com")!) as URLRequest);

    //add observer to get estimated progress value
    self.webView.addObserver(self, forKeyPath: "estimatedProgress", options: .new, context: nil);
}

// Observe value
override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
    if keyPath == "estimatedProgress" {
        print(self.webView.estimatedProgress);
        self.progressView.progress = Float(self.webView.estimatedProgress);
    }
}

New Swift KVO approach

class ViewController: UIViewController {

    private lazy var webView = WKWebView()
    private var observation: NSKeyValueObservation?

    override func viewDidLoad() {
        super.viewDidLoad()

        observation = webView.observe(\WKWebView.estimatedProgress, options: .new) { _, change in
            print("Loaded: \(change)")
        }
    }

    deinit {
        self.observation = nil
    }
}