DetailViewController.swift

import UIKit
import WebKit

class DetailViewController: UIViewController, WKNavigationDelegate {
    
    var articleURLString: String!
    let webView = WKWebView()
    
    override func loadView() {
        self.view = webView
        webView.navigationDelegate = self
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        webView.addObserver(self, forKeyPath: #keyPath(WKWebView.estimatedProgress), options: .new, context: nil)
        
        if let url = URL(string: articleURLString) {
            let request = URLRequest(url: url)
            webView.load(request)
            webView.allowsBackForwardNavigationGestures = true
        }
    }
    
    override class func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        if keyPath == "estimatedProgress" {
            print(Float(webView.estimatedProgress))
        }
    }
    
}