Create a UICollectionView with a reversed flow layout - ios

Question

I want to create a view that resembles a chat view, where the newest message appears at the bottom of the screen. I would like the UICollectionViewCells to be 'pinned' to the bottom of the screen - the opposite of what the default flow layout does. How can I do this?
I've got this code from another stackoverflow answer, but it doesn't work with variable height items (which it needs to):
import Foundation
import UIKit
class InvertedStackLayout: UICollectionViewLayout {
let cellHeight: CGFloat = 100 // 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
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
}
}

Answer 1

Flip your collectionView with a CGAffineTransform so that it starts at the bottom.
collectionView.transform = CGAffineTransform(scaleX: 1, y: -1)
The only problem now is that all your cells display upside down. You then apply the transform to each cell to flip it back upright.
class InvertedCollectionViewFlowLayout: UICollectionViewFlowLayout {
// inverting the transform in the layout, rather than directly on the cell,
// is the only way I've found to prevent cells from flipping during animated
// cell updates
override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
let attrs = super.layoutAttributesForItem(at: indexPath)
attrs?.transform = CGAffineTransform(scaleX: 1, y: -1)
return attrs
}
override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
let attrsList = super.layoutAttributesForElements(in: rect)
if let list = attrsList {
for i in 0..<list.count {
list[i].transform = CGAffineTransform(scaleX: 1, y: -1)
}
}
return attrsList
}
}

Answer 2

UICollectionView with a reversed flow layout and dynamic cell and header height.
import Foundation
import UIKit
class InvertedFlowLayout: UICollectionViewFlowLayout {
override func prepare() {
super.prepare()
}
override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
guard super.layoutAttributesForElements(in: rect) != nil else { return nil }
var attributesArrayNew = [UICollectionViewLayoutAttributes]()
if let collectionView = self.collectionView {
for section in 0 ..< collectionView.numberOfSections {
for item in 0 ..< collectionView.numberOfItems(inSection: section) {
let indexPathCurrent = IndexPath(item: item, section: section)
if let attributeCell = layoutAttributesForItem(at: indexPathCurrent) {
if attributeCell.frame.intersects(rect) {
attributesArrayNew.append(attributeCell)
}
}
}
}
for section in 0 ..< collectionView.numberOfSections {
let indexPathCurrent = IndexPath(item: 0, section: section)
if let attributeKind = layoutAttributesForSupplementaryView(ofKind: UICollectionView.elementKindSectionHeader, at: indexPathCurrent) {
attributesArrayNew.append(attributeKind)
}
}
}
return attributesArrayNew
}
override func layoutAttributesForSupplementaryView(ofKind elementKind: String, at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
let attributeKind = UICollectionViewLayoutAttributes(forSupplementaryViewOfKind: elementKind, with: indexPath)
if let collectionView = self.collectionView {
var fullHeight: CGFloat = 0.0
for section in 0 ..< indexPath.section + 1 {
for item in 0 ..< collectionView.numberOfItems(inSection: section) {
let indexPathCurrent = IndexPath(item: item, section: section)
fullHeight += cellHeight(indexPathCurrent) + minimumLineSpacing
}
}
attributeKind.frame = CGRect(x: 0, y: collectionViewContentSize.height - fullHeight - CGFloat(indexPath.section + 1) * headerHeight(indexPath.section) - sectionInset.bottom + minimumLineSpacing/2, width: collectionViewContentSize.width, height: headerHeight(indexPath.section))
}
return attributeKind
}
override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
let attributeCell = UICollectionViewLayoutAttributes(forCellWith: indexPath)
if let collectionView = self.collectionView {
var fullHeight: CGFloat = 0.0
for section in 0 ..< indexPath.section + 1 {
for item in 0 ..< collectionView.numberOfItems(inSection: section) {
let indexPathCurrent = IndexPath(item: item, section: section)
fullHeight += cellHeight(indexPathCurrent) + minimumLineSpacing
if section == indexPath.section && item == indexPath.item {
break
}
}
}
attributeCell.frame = CGRect(x: 0, y: collectionViewContentSize.height - fullHeight + minimumLineSpacing - CGFloat(indexPath.section) * headerHeight(indexPath.section) - sectionInset.bottom, width: collectionViewContentSize.width, height: cellHeight(indexPath) )
}
return attributeCell
}
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 {
for item in 0 ..< collectionView.numberOfItems(inSection: section) {
let indexPathCurrent = IndexPath(item: item, section: section)
height += cellHeight(indexPathCurrent) + minimumLineSpacing
}
}
height += sectionInset.bottom + CGFloat(collectionView.numberOfSections) * headerHeight(0)
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
}
func cellHeight(_ indexPath: IndexPath) -> CGFloat {
if let collectionView = self.collectionView, let delegateFlowLayout = collectionView.delegate as? UICollectionViewDelegateFlowLayout {
let size = delegateFlowLayout.collectionView!(collectionView, layout: self, sizeForItemAt: indexPath)
return size.height
}
return 0
}
func headerHeight(_ section: Int) -> CGFloat {
if let collectionView = self.collectionView, let delegateFlowLayout = collectionView.delegate as? UICollectionViewDelegateFlowLayout {
let size = delegateFlowLayout.collectionView!(collectionView, layout: self, referenceSizeForHeaderInSection: section)
return size.height
}
return 0
}
}

Answer 3

you could rotate(transform) collectionView 180 degrees so top will be at the bottom and rotate(transform) each cell 180 to make them look normal.