SadCollectionViewFlowLayout.swift

class SadCollectionViewFlowLayout: UICollectionViewFlowLayout {

  private let sideItemScale: CGFloat = 0.9
  private let sideItemAlpha: CGFloat = 0.9
  private let sideItemShift: CGFloat = 0.0

  override open func prepare() {
    super.prepare()
    guard let collectionSize = collectionView?.bounds.size else { return }
    let yInset = (collectionSize.height - itemSize.height) / 2
    let xInset = (collectionSize.width - itemSize.width) / 2
    self.sectionInset = UIEdgeInsets(top: yInset, left: xInset, bottom: yInset, right: xInset)
    let side = itemSize.width
    let scaledItemOffset = (side - side * 0.9) / 2
    let fullSizeSideItemOverlap = 16.0 + scaledItemOffset
    let inset = xInset
    self.minimumLineSpacing = inset - fullSizeSideItemOverlap
  }

  override open func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
    return true
  }

  override open func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
    guard
      let superAttributes = super.layoutAttributesForElements(in: rect),
      let attributes = NSArray(array: superAttributes, copyItems: true) as? [UICollectionViewLayoutAttributes]
      else { return nil }
    return attributes.map { [unowned self] attribute in self.transformLayoutAttributes(attribute) }
  }

  private func transformLayoutAttributes(_ attributes: UICollectionViewLayoutAttributes) -> UICollectionViewLayoutAttributes {
    guard let collectionView = collectionView else { return attributes }
    let collectionCenter = collectionView.frame.size.width / 2
    let offset = collectionView.contentOffset.x
    let normalizedCenter = attributes.center.x - offset

    let maxDistance = itemSize.width + minimumLineSpacing
    let distance = min(abs(collectionCenter - normalizedCenter), maxDistance)
    let ratio = (maxDistance - distance)/maxDistance

    let alpha = ratio * (1 - sideItemAlpha) + sideItemAlpha
    let scale = ratio * (1 - sideItemScale) + sideItemScale
    let shift = (1 - ratio) * sideItemShift
    attributes.transform3D = CATransform3DScale(CATransform3DIdentity, scale, scale, 1)
    attributes.zIndex = Int(alpha * 10)
    attributes.center.y += shift
    return attributes
  }

  override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {

    let collectionViewSize = collectionView!.bounds.size
    let proposedContentOffsetCenterX = proposedContentOffset.x + collectionViewSize.width * 0.5
    var proposedRect = collectionView!.bounds

    // comment this out if you don't want it to scroll so quickly
    proposedRect = CGRect(x: proposedContentOffset.x, y: 0, width: collectionViewSize.width, height: collectionViewSize.height)

    var candidateAttributes: UICollectionViewLayoutAttributes?
    for attributes in layoutAttributesForElements(in: proposedRect)! {
      // == Skip comparison with non-cell items (headers and footers) == //
      if attributes.representedElementCategory != .cell {
        continue
      }

      // Get collectionView current scroll position
      let currentOffset = collectionView!.contentOffset

      // Don't even bother with items on opposite direction
      // You'll get at least one, or else the fallback got your back
      // swiftlint:disable:next line_length
      if (attributes.center.x <= (currentOffset.x + collectionViewSize.width * 0.5) && velocity.x > 0) || (attributes.center.x >= (currentOffset.x + collectionViewSize.width * 0.5) && velocity.x < 0) {

        continue
      }

      // First good item in the loop
      if candidateAttributes == nil {
        candidateAttributes = attributes
        continue
      }

      // Save constants to improve readability
      let lastCenterOffset = candidateAttributes!.center.x - proposedContentOffsetCenterX
      let centerOffset = attributes.center.x - proposedContentOffsetCenterX

      if fabsf(Float(centerOffset)) < fabsf(Float(lastCenterOffset)) {
        candidateAttributes = attributes
      }
    }

    if candidateAttributes != nil {
      // Great, we have a candidate
      let x = candidateAttributes!.center.x - collectionViewSize.width * 0.5
      return CGPoint(x: x, y: proposedContentOffset.y)
    } else {
      // Fallback
      let offset = proposedContentOffset
      return super.targetContentOffset(forProposedContentOffset: offset)
    }
  }
}