title: Collection view within another collection view: section header or cell?
body: <p>I already have a <code>UICollectionView</code> which scrolls vertically and shows a collection of custom <code>UICollectionViewCell</code>s which have a fixed size.</p>
  
  <p>Now I've been asked for showing another <code>UICollectionView</code> in top of all other cells, which should <strong>scroll horizontally</strong> and whose <strong>cells size is dynamic</strong> (I'll only know the size after at an async network call completion). In addition, this inner collection view <strong>may not be always necessary to be shown</strong> (it depends on the data received from the network call), but if it is, it should be shown only once (on top of everything).</p>
  
  <p>My question is: how the best way to deal with this second and inner collection view should be? Should I add it to the outer view controller as a different kind of cell of it, or maybe as a section header?
  Maybe another approach to layout this would be better?</p>
  
  <p><strong>EDIT:</strong> More considerations: </p>
  
  <ul>
  <li>I'd need to animate the inner collection view when I'm going to show it</li>
  <li>The whole thing should be vertically scrollable, this inner collection view should not stick to the top of the screen</li>
  </ul>