SO Question 34980383: http://stackoverflow.com/questions/34980383/could-not-dequeue-a-view-of-kind-uicollectionelementkindcell-with-identifier

so_question_34980383.m

/**
Assume there are two collection views: collectionViewA and collectionViewB and both have the same dataSource (although, they probably shouldn't...)
collectionViewA has registered a UICollectionViewCell class with the reuseIdentifier "CellA".
collectionViewB has registered a UICollectionViewCell class with the reuseIdentifier "CellB".

When dequeueing a cell with a given reuseIdentifier, you should be confident that the collectionView has the reuseIdentifier
you're attempting to dequeue registered to it. That is, ONLY collectionViewA can dequeue a cell with reuseIdentifier "CellA" and ONLY collectionViewB can do the same for "CellB"
Otherwise, a crash will occur.

Notice, how one of the arguments is a collectionView in the UICollectionViewDataSource method below? That is the collectionView that is asking the datasource for a cell which
means it could be either collectionViewA or collectionViewB in this scenario.

Thus, you need to check which collecitonView it is before dequeueing as follows...
*/
func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {

      if collectionView == collectionViewA {
        // At this point, collectionView is certain to be collectionViewA
        let cellA = collectionView.dequeueReusableCellWithReuseIdentifier("CellA", forIndexPath: indexPath)
        // Configure your cell here...
        return cellA
      }
      else if collectionView == collectionViewB {

        let cellB = collectionView.dequeueReusableCellWithReuseIdentifier("CellB", forIndexPath: indexPath)
        // Configure your cell here...
        return cellB
      }
    }