title: UICollectionViewDelegateFlowLayout insets does not work
body: <p>I have implemented <code>UICollectionViewDelegateFlowLayout</code> in my ViewController, and specify insets using the following code:</p>
  
  <blockquote>
  <pre><code>func collectionView(_ collectionView: UICollectionView,
                                 layout collectionViewLayout: UICollectionViewLayout,
                                        insetForSectionAt section: Int) -&gt; UIEdgeInsets {
      return sectionInsets
  }
  </code></pre>
  </blockquote>
  
  <p>But this function is never executed when I use breakpoints to debug.
  As I have already specified the delegate of the UICollectionView to the ViewController, I do not know what is going wrong.</p>