UICollectionView insert cells above maintaining position (like Messages.app)

Question

By default Collection View maintains content offset while inserting cells. On the other hand I\'d like to insert cells above the currently displaying ones so that they appea

Answer 1

Here's a slightly tweaked version of Peter's solution (subclassing flow layout, no upside-down, lightweight approach). It's Swift 3. Note UIView.animate with zero duration - that's to allow the animation of the even/oddness of the cells (what's on a row) animate, but stop the animation of the viewport offset changing (which would look terrible)

Usage:

        let layout = self.collectionview.collectionViewLayout as! ContentSizePreservingFlowLayout
        layout.isInsertingCellsToTop = true
        self.collectionview.performBatchUpdates({
            if let deletionIndexPaths = deletionIndexPaths, deletionIndexPaths.count > 0 {
                self.collectionview.deleteItems(at: deletionIndexPaths.map { return IndexPath.init(item: $0.item+twitterItems, section: 0) })
            }
            if let insertionIndexPaths = insertionIndexPaths, insertionIndexPaths.count > 0 {
                self.collectionview.insertItems(at: insertionIndexPaths.map { return IndexPath.init(item: $0.item+twitterItems, section: 0) })
            }
        }) { (finished) in
            completionBlock?()
        }

Here's ContentSizePreservingFlowLayout in its entirety:

    class ContentSizePreservingFlowLayout: UICollectionViewFlowLayout {
        var isInsertingCellsToTop: Bool = false {
            didSet {
                if isInsertingCellsToTop {
                    contentSizeBeforeInsertingToTop = collectionViewContentSize
                }
            }
        }
        private var contentSizeBeforeInsertingToTop: CGSize?

        override func prepare() {
            super.prepare()
            if isInsertingCellsToTop == true {
                if let collectionView = collectionView, let oldContentSize = contentSizeBeforeInsertingToTop {
                    UIView.animate(withDuration: 0, animations: {
                        let newContentSize = self.collectionViewContentSize
                        let contentOffsetY = collectionView.contentOffset.y + (newContentSize.height - oldContentSize.height)
                        let newOffset = CGPoint(x: collectionView.contentOffset.x, y: contentOffsetY)
                        collectionView.contentOffset = newOffset
                    })
                }
                contentSizeBeforeInsertingToTop = nil
                isInsertingCellsToTop = false
            }
        }
    }