KeepingContentOffsetCollectionViewLayout.swift

import UIKit

final class KeepingContentOffsetCollectionViewLayout : UICollectionViewFlowLayout {

  private var oldOffset: CGFloat?

  typealias Completion = () -> Void

  /// Perform passed closure with keeping current contentOffset.
  ///
  /// - Parameter closure: You should call completion closure when operation completed.
  func performWithKeepingContentOffset(_ closure: (@escaping Completion) -> Void) {

    guard let collectionView = self.collectionView else { return }

    let oldOffsetY = collectionViewContentSize.height - collectionView.contentOffset.y
    performWithKeeping(contentOffsetY: oldOffsetY, closure)
  }

  func performWithKeeping(contentOffsetY: CGFloat, _ closure: (@escaping Completion) -> Void) {

    assert(oldOffset == nil, "Completion did not be called last time")

    oldOffset = contentOffsetY
    closure({
      self.oldOffset = nil
    })
  }

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

    guard let oldOffset = oldOffset else {
      return proposedContentOffset
    }

    var offset = proposedContentOffset
    offset.y = collectionViewContentSize.height - oldOffset

    return offset
  }

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

    return proposedContentOffset
  }
}