title: Paging UICollectionView by cells, not screen
body: <p>I have <code>UICollectionView</code> with horizontal scrolling and there are always 2 cells side-by-side per the entire screen. I need the scrolling to stop at the begining of a cell. With paging enabled, the collection view scrolls the whole page, which is 2 cells at once, and then it stops.</p>
  
  <p>I need to enable scrolling by a single cell, or scrolling by multiple cells with stopping at the edge of the cell.</p>
  
  <p>I tried to subclass <code>UICollectionViewFlowLayout</code> and to implement the method <code>targetContentOffsetForProposedContentOffset</code>, but so far I was only able to break my collection view and it stopped scrolling. Is there any easier way to achieve this and how, or do I really need to implement all methods of <code>UICollectionViewFlowLayout</code> subclass? Thanks.</p>