title: programmatically add custom flow layout to UICollectionView
body: <p>I have a custom flow layout that I want to use for my <code>UICollectionView</code>, but I don't know how to do it programmatically. </p>
  
  <p>More details: my <code>UICollectionView</code> is a subview in my view controller that conforms to <code>UICollectionViewDataSource</code> and <code>UICollectionViewDelegateFlowLayout</code>protocols.</p>