Populating UICollectionView in reverse order

Swift 4.2

I found a simple solution and worked for me to show last item first of a collection view:

Inside viewDidLoad() method:

collectionView.transform = CGAffineTransform.init(rotationAngle: (-(CGFloat)(Double.pi)))

and inside collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) method before returning the cell:

cell.transform = CGAffineTransform(rotationAngle: CGFloat.pi)

(optional) Below lines will be necessary to auto scroll and show new item with smooth scroll.

Add below lines after loading new data:

   if self.dataCollection.count > 0 {
      self.collectionView.scrollToItem(at: //scroll collection view to indexpath
      NSIndexPath.init(row:(self.collectionView?.numberOfItems(inSection: 0))!-1, //get last item of self collectionview (number of items -1)
      section: 0) as IndexPath //scroll to bottom of current section
      , at: UICollectionView.ScrollPosition.bottom, //right, left, top, bottom, centeredHorizontally, centeredVertically
      animated: true)
   }

Just click on UICollectionView in storyboard, in inspector menu under view section change semantic to Force Right-to-Left

I have attach an image to show how to do it in the inspector menu:

I'm surprised that Apple scares people away from writing their own UICollectionViewLayout in the documentation. It's really very straightforward. Here's an implementation that I just used in an app that will do exactly what are asking. New items appear at the bottom, and the while there is not enough content to fill up the screen the the items are bottom justified, like you see in message apps. In other words item zero in your data source is the lowest item in the stack.

This code assumes that you have multiple sections, each with items of a fixed height and no spaces between items, and the full width of the collection view. If your layout is more complicated, such as different spacing between sections and items, or variable height items, Apple's intention is that you use the prepare() callback to do the heavy lifting and cache size information for later use.

This code uses Swift 3.0.

//
//  Created by John Lyon-Smith on 1/7/17.
//  Copyright © 2017 John Lyon-Smith. All rights reserved.
//

import Foundation
import UIKit

class InvertedStackLayout: UICollectionViewLayout {
    let cellHeight: CGFloat = 100.00 // Your cell height here...

    override func prepare() {
        super.prepare()
    }

    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        var layoutAttrs = [UICollectionViewLayoutAttributes]()

        if let collectionView = self.collectionView {
            for section in 0 ..< collectionView.numberOfSections {
                if let numberOfSectionItems = numberOfItemsInSection(section) {
                    for item in 0 ..< numberOfSectionItems {
                        let indexPath = IndexPath(item: item, section: section)
                        let layoutAttr = layoutAttributesForItem(at: indexPath)

                        if let layoutAttr = layoutAttr, layoutAttr.frame.intersects(rect) {
                            layoutAttrs.append(layoutAttr)
                        }
                    }
                }
            }
        }

        return layoutAttrs
    }

    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        let layoutAttr = UICollectionViewLayoutAttributes(forCellWith: indexPath)
        let contentSize = self.collectionViewContentSize

        layoutAttr.frame = CGRect(
            x: 0, y: contentSize.height - CGFloat(indexPath.item + 1) * cellHeight,
            width: contentSize.width, height: cellHeight)

        return layoutAttr
    }

    func numberOfItemsInSection(_ section: Int) -> Int? {
        if let collectionView = self.collectionView,
            let numSectionItems = collectionView.dataSource?.collectionView(collectionView, numberOfItemsInSection: section)
        {
            return numSectionItems
        }

        return 0
    }

    override var collectionViewContentSize: CGSize {
        get {
            var height: CGFloat = 0.0
            var bounds = CGRect.zero

            if let collectionView = self.collectionView {
                for section in 0 ..< collectionView.numberOfSections {
                    if let numItems = numberOfItemsInSection(section) {
                        height += CGFloat(numItems) * cellHeight
                    }
                }

                bounds = collectionView.bounds
            }

            return CGSize(width: bounds.width, height: max(height, bounds.height))
        }
    }

    override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
        if let oldBounds = self.collectionView?.bounds,
            oldBounds.width != newBounds.width || oldBounds.height != newBounds.height
        {
            return true
        }

        return false
    }
}