title: No calls occurring for observeValueForKeyPath for UIProgressView in Swift
body: <p>With the help of <a href="http://www.appcoda.com/webkit-framework-intro/" rel="nofollow noreferrer">this tutorial</a> I was adding a progress bar to my view with Swift but the observer does not get any calls of itself.</p>
  
  <pre><code>//  ViewController.swift
  
  import UIKit
  import WebKit
  
  class ViewController: UIViewController, WKNavigationDelegate {
  
      var webView: WKWebView?
  
      @IBOutlet dynamic var progressView: UIProgressView!
  
      override func viewDidLoad() {
          super.viewDidLoad()
  
          setupBrowser()
  
          self.addObserver(self, forKeyPath: "estimatedProgress", options: .New, context: nil)
  
          let website = "https://google.com"
          let site = NSURL(string: website)
  
          loadURLRequest(NSURLRequest(URL: site!))
      }
  
      func setupBrowser() {
          let wkWebView = WKWebView(frame: view.bounds)
          wkWebView.navigationDelegate = self
          webView = wkWebView
  
          let wkProgressView = UIProgressView(frame: view.bounds)
          progressView = wkProgressView
  
          self.view.insertSubview(wkWebView, belowSubview: wkProgressView)
      }
  
      func loadURLRequest(request: NSURLRequest) {
          webView?.loadRequest(request)
      }
  
      func webView(webView: WKWebView, didFinishNavigation navigation: WKNavigation!) {
          print("Finished navigating")
      }
  
      override func observeValueForKeyPath(keyPath: String?, ofObject object: AnyObject?, change: [String : AnyObject]?, context: UnsafeMutablePointer&lt;Void&gt;) {
          print(1)
          if (keyPath == "estimatedProgress") {
              // TODO
          }
      }
  
  }
  </code></pre>
  
  <p>Taking a look on <a href="https://stackoverflow.com/questions/25378034/swift-kvo-in-xcode-6-beta-6-observevalueforkeypath-no-longer-called">SWIFT - KVO in xcode 6 beta 6.. observeValueForKeyPath no longer called</a> or <a href="https://stackoverflow.com/questions/2728638/observevalueforkeypath-not-being-called">observeValueForKeyPath not being called</a> didn't help. Is UIProgressView not observable? The console shows after every page load <code>Finished navigating</code> but not once <code>1</code>.</p>