import UIKit

private let reuseIdentifier = "Cell"

class BasicCollectionViewController: UICollectionViewController {
    
    var dataSource: [[UIColor]] = [
        (0..<20).map { _ in .red },
        (0..<20).map {  _ in .blue },
        (0..<20).map {  _ in .green }
    ]

    override func viewDidLoad() {
        super.viewDidLoad()
        self.collectionView!.register(UICollectionViewCell.self, forCellWithReuseIdentifier: reuseIdentifier)
    }
    // MARK: UICollectionViewDataSource

    override func numberOfSections(in collectionView: UICollectionView) -> Int {
        return dataSource.count
    }


    override func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return dataSource[section].count
    }

    override func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: reuseIdentifier, for: indexPath)
        cell.backgroundColor = dataSource[indexPath.section][indexPath.row]
        return cell
    }
}

    @IBOutlet weak var layout: UICollectionViewSplitLayout!
 
    override func viewDidLoad() {
        super.viewDidLoad()
        //...
        //...
        
        // margins for each section
        layout.minimumItemLineSpacing = 8
        layout.minimumInterItemSpacing = 8
        layout.sectionInset = UIEdgeInsets(top: 8, left: 8, bottom: 8, right: 8)
        
        // Column Setting
        layout.leftSideRatio = 0.4
        layout.splitSpacing = 8
       
        //...
        //...
    }

extension BasicCollectionViewController: UICollectionViewDelegateSectionSplitLayout {
    // Fix the size of each item as UICollectionViewDelegateFlowLayout does. calculateFixedWidthRaughly() is utility method UICollectionViewSplitLayout has.
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: IndexPath, width: CGFloat, side: UICollectionViewSplitLayoutSide) -> CGSize {
        let width = layout.calculateFixedWidthRaughly(
                               to: 3, 
                               of: side, 
                               minimumInterItemSpacing: layout.minimumInterItemSpacing, 
                               sectionInset: layout.sectionInset)
        return CGSize(width: width, height: width)
    }

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sideForSection section: Int) -> UICollectionViewSplitLayoutSide {
        // when a section number is odd, the items are placed into left side. 
        return section % 2 == 0 ? .left : .right
    }
}

leftSideRatio = 0.4

splitSpacing = 10

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sideForSection section: Int) -> UICollectionViewSplitLayoutSide

/// The minimum spacing to use between items in the same row.
open var minimumInterItemSpacing: CGFloat
/// The minimum spacing to use between lines of items in the grid.
open var minimumItemLineSpacing: CGFloat
/// The margins used to lay out content in a section
open var sectionInset: UIEdgeInsets

// section inset to each section
@objc optional func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAtIndex section: Int, side: UICollectionViewSplitLayoutSide) -> UIEdgeInsets
    
// minimumInterItemSpacing to each section
@objc optional func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumInterItemSpacingForSectionAtIndex section: Int, side: UICollectionViewSplitLayoutSide) -> CGFloat

// minimumItemLineSpacing to each section
@objc optional func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumItemLineSpacingForSectionAtIndex section: Int, side: UICollectionViewSplitLayoutSide) -> CGFloat

func collectionView(
    _ collectionView: UICollectionView, 
    layout collectionViewLayout: UICollectionViewLayout, 
    sizeForItemAtIndexPath indexPath: IndexPath, 
    width: CGFloat, 
    side: UICollectionViewSplitLayoutSide) -> CGSize

open func calculateFixedWidthRaughly(
    to num: Int, 
    of side: UICollectionViewSplitLayoutSide, 
    minimumInterItemSpacing: CGFloat, 
    sectionInset: UIEdgeInsets) -> CGFloat

// header
@objc optional func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, referenceSizeForHeaderInSection section: Int, width: CGFloat, side: UICollectionViewSplitLayoutSide) -> CGSize

// footer size
@objc optional func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, referenceSizeForFooterInSection section: Int, width: CGFloat, side: UICollectionViewSplitLayoutSide) -> CGSize

@objc optional func collectionView(
    _ collectionView: UICollectionView, 
    layout collectionViewLayout: UICollectionViewLayout,
    itemsBackgroundColorFor section: Int, 
    side: UICollectionViewSplitLayoutSide) -> UIColor?

open var isNormalizingLineHeight: Bool