title: How to listen Zoom in/out event of WKWebView
body: <p>I want to hide/show navigationBar of a <code>UINavigationController</code> when the <code>WKWebView</code> zooms out/in.</p>
  
  <p>To hide or show a <code>UINavigationBar</code> is quite easy as follows:</p>
  
  <pre><code>self.navigationController?.navigationBarHidden = true
  </code></pre>
  
  <p>But the problem is that I don't know where to put the code. </p>
  
  <p>I am thinking to intercept the zoom event of <code>WKWebView</code>. May be there are other ways, any comments are welcome.</p>