title: iOS Swift: a view controller as the root view controller
body: <p>I know how to make a view controller the root view controller of the app
  by this code:</p>
  
  <pre><code>window.rootViewController = (your root view controller())
  </code></pre>
  
  <p>but I saw also this:</p>
  
  <pre><code> instantiateInitialViewController
  </code></pre>
  
  <p>and this:</p>
  
  <pre><code>instantiateViewController(withIdentifier:)
  </code></pre>
  
  <p>are they the same as rootViewController or they have different functionalities?</p>