Show progress of WKWebView in UIProgressBar that is attached to an UINavigationBar

ViewController.swift

//
//  ViewController.swift
//
//  Created by Felix Mau on 06.01.18.
//  Copyright © 2018 Felix Mau. All rights reserved.
//

import UIKit
import WebKit

class ViewController: UIViewController {

    /// WebView from interface builder
    @IBOutlet weak var webView: WKWebView!

    /// Progress view showing the current loading progress of the web view
    let progressView = UIProgressView(progressViewStyle: .default)
    
    override func viewDidLoad() {
        super.viewDidLoad()

        navigationItem.title = "WKWebView - Sample App"

        setupProgressView()
        setupWebview(withUrl: "https://github.com/fxm90")
    }

    deinit {
        webView.removeObserver(self, forKeyPath: #keyPath(WKWebView.estimatedProgress))
        webView.navigationDelegate = nil
    }

    // MARK: - UIProgressView

    func setupProgressView() {
        guard let navigationBar = navigationController?.navigationBar else { return }
        navigationBar.addSubview(progressView)

        progressView.autoresizingMask = [.flexibleWidth, .flexibleTopMargin]
        progressView.alpha = 0.0

        progressView.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            progressView.leadingAnchor.constraint(equalTo: navigationBar.leadingAnchor),
            progressView.trailingAnchor.constraint(equalTo: navigationBar.trailingAnchor),

            progressView.bottomAnchor.constraint(equalTo: navigationBar.bottomAnchor),
            progressView.heightAnchor.constraint(equalToConstant: 2.0)
        ])

        webView.addObserver(self,
                            forKeyPath: #keyPath(WKWebView.estimatedProgress),
                            options: .new,
                            context: nil)
    }

    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        guard object as? WKWebView === webView, keyPath == #keyPath(WKWebView.estimatedProgress) else { return }

        progressView.progress = Float(webView.estimatedProgress)
    }

    // MARK: - WKWebView

    func setupWebview(withUrl urlAsString: String) {
        guard let url = URL(string: urlAsString) else { return }

        let request = URLRequest(url: url)

        webView.navigationDelegate = self
        webView.load(request)
    }
}

// MARK: - WKNavigationDelegate

extension ViewController: WKNavigationDelegate {
    func webView(_ webView: WKWebView, didStartProvisionalNavigation navigation: WKNavigation!) {
        UIApplication.shared.isNetworkActivityIndicatorVisible = true

        progressView.alpha = 0.0
        UIView.animate(withDuration: 0.33, delay: 0.0, options: .curveEaseInOut, animations: {
            self.progressView.alpha = 1.0
        })
    }

    func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
        UIApplication.shared.isNetworkActivityIndicatorVisible = false

        progressView.alpha = 1.0
        UIView.animate(withDuration: 0.33, delay: 0.0, options: .curveEaseInOut, animations: {
            self.progressView.alpha = 0.0
        })
    }
}