title: How can I change the duration of a layout change using collectionView setCollectionViewLayout:animated?
body: <p>I have this (where <code>layout</code> is a <code>UICollectionViewFlowLayout</code> subclass):</p>
  
  <pre><code>[layout invalidateLayout];
  [self.collectionView setCollectionViewLayout:layout animated:YES]
  </code></pre>
  
  <p>But it seems the duration is fixed. What's the simplest way to change the duration?</p>