title: Is there any way to control views inside NavigationView header?
body: <p>As the title says, I want to know if there is any way to control views inside the <code>NavigationView</code> header? (Except for adding or removing header.)</p>
  
  <p>For example: In the header, I have a user avatar. By default, it displays a guest image, but after the user logs in, the real avatar will be showed. </p>
  
  <p>How can this be accomplished?</p>