Collection cell that fills entire width of screen and scrolls only one at a time in swift?

Question

I have a UICollectionView that has 20 cells and 1 section. I made each cell 320 in width and 304 in height.

I scroll the collection view programmatically using two buttons at the bottom of the collection view using scrollToItemAtIndexPath(currentIndex + 1). I only scroll them 1 by 1.

This works fine in iPhone 4s and iPhone 5/5s. The problem appears when using an iPhone 6/6 Plus.

When I scrollToItemAtIndexPath it scrolls 2 cells at a time. How can I prevent this from happening? I tried to make the cell fit the width of the screen but just one cell appeared, and the rest of the UICollectionView was black.

EDIT:

Here is the datasource code:

extension ViewController: UICollectionViewDataSource {

    func numberOfSectionsInCollectionView(collectionView: UICollectionView) -> Int {
    return 1
    }

    func collectionView(collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    return 32
    }

    func collectionView(collectionView: UICollectionView, cellForItemAtIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCellWithReuseIdentifier("collectionCell", forIndexPath: indexPath) as! CollectionCell
    cell.backgroundColor = UIColor.whiteColor()

    self.current = indexPath

    self.configureCell(cell, atIndexPath: indexPath) as! CollectionCell
    return cell
    }

    func configureCell(cell: CollectionCell, atIndexPath indexPath: NSIndexPath) -> UICollectionViewCell {
    let string = textArray[indexPath.item % 20] as String
    cell.textView.text = string
    cell.textView.backgroundColor = UIColor.cloudsColor()
    return cell
    }
}

And here is how I scroll them:

func buttonSelected(sender: UIButton) {
    switch sender.tag {
      case 0:
      let previousItem: NSIndexPath = NSIndexPath(forItem: self.current.item - 1, inSection: self.current.section)
      self.collectionView?.scrollToItemAtIndexPath(previousItem, atScrollPosition:UICollectionViewScrollPosition.CenteredHorizontally, animated:true)
      case 1:
      let nextItem: NSIndexPath = NSIndexPath(forItem: self.current.item + 1, inSection: self.current.section)
      self.collectionView?.scrollToItemAtIndexPath(nextItem, atScrollPosition:UICollectionViewScrollPosition.CenteredHorizontally, animated:true)
      default: break
      }
}

Answer 1

I've solved this using a custom FlowLayout in the collection view. Here it is:

    class CenterFlowLayout: UICollectionViewFlowLayout {

    override func targetContentOffsetForProposedContentOffset(proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
      if let cv = self.collectionView {
         let cvBounds = cv.bounds
         let halfWidth = cvBounds.size.width * 0.5
         let proposedContentOffsetCenterX = proposedContentOffset.x + halfWidth
      if let attributesForVisibleCells = self.layoutAttributesForElementsInRect(cvBounds) as [UICollectionViewLayoutAttributes]! {
         var candidateAttributes: UICollectionViewLayoutAttributes?
         for attributes in attributesForVisibleCells {
         // == Skip comparison with non-cell items (headers and footers) == //
            if attributes.representedElementCategory != UICollectionElementCategory.Cell {
    continue
            }
            if let candAttrs = candidateAttributes {
               let a = attributes.center.x - proposedContentOffsetCenterX
               let b = candAttrs.center.x - proposedContentOffsetCenterX
            if fabsf(Float(a)) < fabsf(Float(b)) {
               candidateAttributes = attributes
            }
      } else { // == First time in the loop == //
        candidateAttributes = attributes
        continue
      }
   }
   return CGPoint(x : candidateAttributes!.center.x - halfWidth, y : proposedContentOffset.y)
   }
 }
    // Fallback
    return super.targetContentOffsetForProposedContentOffset(proposedContentOffset)
    }
  }