title: UICollectionView layout update - item size not changing
body: <p>I'm having a problem with collectionView's item size. I want to display 3 items in row for Portrait mode and 6 for landscape. I've set up -layoutSubviews like this:</p>
  
  <pre><code>- (void)layoutSubviews {
      [super layoutSubviews];
  
      if ([[UIApplication sharedApplication] statusBarOrientation] == UIInterfaceOrientationPortrait || ([[UIApplication sharedApplication] statusBarOrientation] == UIInterfaceOrientationPortraitUpsideDown)) {
          //Portrait orientation
          self.flowLayoutPortrait.itemSize = CGSizeMake(106, 106);
  
      } else if ([[UIApplication sharedApplication] statusBarOrientation] == UIInterfaceOrientationLandscapeLeft || ([[UIApplication sharedApplication] statusBarOrientation] == UIInterfaceOrientationLandscapeRight)) {
          //Landscape orientation
          self.flowLayoutPortrait.itemSize = CGSizeMake(93, 93);
      }
      [self.collectionView.collectionViewLayout invalidateLayout];
  }
  </code></pre>
  
  <p>But cell size does not get updated, it always stay the same for both orientations. 
  If I create 2 flowLayouts and the use:</p>
  
  <pre><code>[self.collectionView setCollectionViewLayout:self.flowLayoutLandscape];
  </code></pre>
  
  <p>Everything works, but I really don't like how they are changed. Animation is really bad. 
  And because itemSize is the only property that needs to be updated, using 1 layout seems like best option. </p>
  
  <p>How can I tell collectionView to update layout?</p>