title: UICollectionView add gap between cells
body: <p>I'm using <code>ALAssetsLibrary</code> to access system photo library to preview and multiple select photos,which is like the origin system photo library and I can scroll to preview photos.I implement it by <code>UICollectionView</code> and <code>UICollectionViewFlowLayout</code>.Everything goes well except a little issue.</p>
  
  <p>When I use iOS system photo library and scroll to preview photos,there is a <strong>Gap</strong> between two photos like below.
  <a href="https://i.stack.imgur.com/cDNVe.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/cDNVe.jpg" alt="enter image description here"></a></p>
  
  <p>While in my app,there is is no <strong>Gap</strong> between the two photos when scrolling.I set <code>UICollectionView</code> paged and <code>UICollectionViewFlowLayout</code> cell space(<code>minimumLineSpacing</code> and <code>minimumInteritemSpacing</code>) to zero both.
  <a href="https://i.stack.imgur.com/Z86bV.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Z86bV.jpg" alt="enter image description here"></a></p>
  
  <p>How can I make my photo preview like system photo library with a <strong>Gap</strong> between two scrolling photos?I am new to <code>UICollectionView</code>,and I am not sure if the <strong>Gap</strong> is implemented by <code>SupplementaryView</code> or anything else.</p>