title: Update one UICollectionView when cell is selected in second CollectionView in same ViewController
body: <p>I'm working in Swift and have one ViewController serving as the data source and delegate for 2 UICollectionView instances. I want to change which data source is used for collectionView2 based on which item in collectionView1 is selected, and refresh the items displayed in collectionView2 when a new selection is made in collectionView1.</p>
  
  <p>I assume the user's selection will propagate more-or-less like so:</p>
  
  <p>CollectionView1Cell -> ViewController -> CollectionView2</p>
  
  <p>How can the ViewController be alerted that a new selection has been made in the first CollectionView so it can update the cells of the second ViewController?</p>