Creating a custom UICollectionFlowLayout Programmatically

I am attempting to create a UICollectionView with a custom FlowLayout all programmatically.

I followed this tutorial on how to set-up a custom FlowLayout through a storyboard: https://octodev.net/custom-collectionviewlayout/

However I want to refrain from using the story board and implement this via pure logic in my code.

Currently my set-up of the collectionView is as follows in my main VC:

/** Collection View **/
        let flowLayout : UICollectionViewFlowLayout = UICollectionViewFlowLayout()
        flowLayout.sectionInset = UIEdgeInsets(top: 0, left: 0, bottom: 0, right: 0)
        flowLayout.scrollDirection = .vertical
        self.collectionView = UICollectionView(frame: CGRect(x: self.frame.width * 0, y: self.frame.height * 0, width: self.frame.width, height: self.frame.height), collectionViewLayout: flowLayout)
        collectionView.register(DiscoverCVC.self, forCellWithReuseIdentifier: cellId)
        collectionView.delegate = self
        collectionView.dataSource = self
        collectionView.backgroundColor = UIColor.clear
        self.addSubview(collectionView)

What I need to register programmatically is as follows on the StoryBoard

The Layout needs to be "Custom" and the class needs to be a special Class.

Here is the class:

import UIKit

private let kReducedHeightColumnIndex = 1
private let kItemHeightAspect: CGFloat = 2

class DiscoverCollection: BaseCollectionViewLayout {

    private var _itemSize : CGSize!
    private var _columnsXoffset : [CGFloat]!

    required init?(coder aDecoder: NSCoder) {
        super.init(coder: aDecoder)
        self.totalColumns = 3
    }

    private func isLastItemSingleInRow(_ indexPath: IndexPath) -> Bool {
        return indexPath.item == (totalItemsInSection - 1) && indexPath.item % totalColumns == 0
    }

    override func calculateItemSize() {
        let contentWidthWithoutIndents = collectionView!.bounds.width - contentInsets.left - contentInsets.right
        let itemWidth = (contentWidthWithoutIndents - (CGFloat(totalColumns) - 1) * interItemsSpacing) / CGFloat(totalColumns)

        let itemHeight = itemWidth * kItemHeightAspect
        _itemSize = CGSize(width: itemWidth, height: itemWidth)

        _columnsXoffset = []

        for columnIndex in 0...(totalColumns - 1) {
            _columnsXoffset.append(CGFloat(columnIndex) * (_itemSize.width + interItemsSpacing))
        }
    }

    override func columnIndexForItemAt(indexPath: IndexPath) -> Int {
        let columnIndex = indexPath.item % totalColumns
        return self.isLastItemSingleInRow(indexPath) ? kReducedHeightColumnIndex : columnIndex
    }

    override func calculateItemFrame(indexPath: IndexPath, columnIndex: Int, columnYoffset: CGFloat) -> CGRect {
        let rowIndex = indexPath.item / totalColumns
        let halfItemHeight = (_itemSize.height - interItemsSpacing) / 2

        var itemHeight = _itemSize.height

        if (rowIndex == 0 && columnIndex == kReducedHeightColumnIndex) || self.isLastItemSingleInRow(indexPath) {
            itemHeight = halfItemHeight
        }
        return CGRect(x: _columnsXoffset[columnIndex], y: columnYoffset, width: _itemSize.width, height: itemHeight)
    }
}

Here is the base collectionViewFlowLayout

import UIKit

class BaseCollectionViewLayout: UICollectionViewLayout {

    private var _layoutMap = [IndexPath : UICollectionViewLayoutAttributes]()
    private var _columnsYoffset : [CGFloat]!
    private var _contentSize : CGSize!

    private(set) var totalItemsInSection = 0

    var totalColumns = 0
    var interItemsSpacing : CGFloat = 8

    var contentInsets : UIEdgeInsets {
        return collectionView!.contentInset
    }

    override var collectionViewContentSize: CGSize {
        return _contentSize
    }

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

        for(_, layoutAttributes) in _layoutMap {
            if rect.intersects(layoutAttributes.frame) {
                layoutAttributesArray.append(layoutAttributes)
            }
        }

        return layoutAttributesArray
    }

    func columnIndexForItemAt(indexPath: IndexPath) -> Int {
        return indexPath.item % totalColumns
    }

    func calculateItemFrame(indexPath: IndexPath, columnIndex: Int, columnYoffset: CGFloat) -> CGRect {
        return CGRect.zero
    }

    func calculateItemSize(){}

    override func prepare() {
        _layoutMap.removeAll()
        _columnsYoffset = Array(repeating: 0, count: totalColumns)

        totalItemsInSection = collectionView!.numberOfItems(inSection: 0)

        if totalItemsInSection > 0 && totalColumns > 0 {
            self.calculateItemSize()
            var itemIndex = 0
            var contentSizeHeight : CGFloat = 0

            while itemIndex < totalItemsInSection {

                let indexPath = IndexPath(item: itemIndex, section: 0)
                let columnIndex = self.columnIndexForItemAt(indexPath: indexPath)

                let attributeRect = calculateItemFrame(indexPath: indexPath, columnIndex: columnIndex, columnYoffset: _columnsYoffset[columnIndex])
                let targetLayoutAttributes = UICollectionViewLayoutAttributes.init(forCellWith: indexPath)
                targetLayoutAttributes.frame = attributeRect

                contentSizeHeight = max(attributeRect.maxY, contentSizeHeight)
                _columnsYoffset[columnIndex] = attributeRect.maxY + interItemsSpacing
                _layoutMap[indexPath] = targetLayoutAttributes

                itemIndex += 1
            }

            _contentSize = CGSize(width: collectionView!.bounds.width - contentInsets.left - contentInsets.right, height: contentSizeHeight)
        }
    }
}

How do I go about this?

Add this additional method to DiscoverCollection:

override init() {
    super.init()
    self.totalColumns = 3
}

and then set your CollectionView's layout to your layout instance:

let customLayout = DiscoverCollection()
collectionView.collectionViewLayout = customLayout