title: UICollectionViewController flow layout with readable content width
body: <p>I have a <code>UICollectionView</code> using flow layout, and I'm trying to achieve the same margins as a <code>UITableViewController</code> with readable content width.</p>
  
  <p>The closest I've come to matching this layout behavior is to embed a <code>UICollectionViewController</code> within a <code>UIViewController</code> and have the embedded view "Follow Readable Width".</p>
  
  <p><a href="https://i.stack.imgur.com/FcQfv.png" rel="noreferrer"><img src="https://i.stack.imgur.com/FcQfv.png" alt="UICollectionViewController embedded in UIViewController"></a></p>
  
  <p>Here the teal color is the <code>UIViewController</code> and the salmon color is the <code>UICollectionViewController</code>.  The problem is the teal area doesn't scroll the <code>UICollectionView</code> and the scroll indicators are also not along the edge of the screen like you would expect.</p>
  
  <p>My question is:</p>
  
  <blockquote>
    <p>How can I achieve this layout without having to embed a <code>UICollectionViewController</code>? </p>
  </blockquote>
  
  <p>My guess is that I can somehow set the <code>UICollectionView</code> left and right section insets to match the readable content guide margins and update them by overriding <code>viewWillTransition(to size: with coordinator:)</code> and observing <code>UIContentSizeCategoryDidChange</code> notifications, but I'm not sure how to go about this. </p>