This is an example of creating a custom UICollectionViewFlowLayout with a decoration view in order to give each section a border for iOS using Swift. This was in response to this stack overflow question: https://stackoverflow.com/questions/71552265/is-there-a-way-to-give-outer-border-in-every-section-of-uicollctionview?noredirect=1#comment126463…

CustomLayoutVC.swift

//
//  CustomLayoutVC.swift
//  TestApp
//
//  Created by Shawn Frank on 21/03/2022.
//

import UIKit

// This is used as the header and footer of each section
fileprivate class HeaderFooterView: UICollectionReusableView
{
    let title = UILabel()
    
    static let identifier = "HeaderFooterViewID"
    
    override init(frame: CGRect) {
        
        super.init(frame: frame)
        layoutInterface()
    }
    
    required init(coder aDecoder: NSCoder) {
        
        super.init(coder: aDecoder)!
        layoutInterface()
    }
    
    func layoutInterface() {
        
        backgroundColor = .clear
        title.translatesAutoresizingMaskIntoConstraints = false
        title.backgroundColor = .clear
        title.textAlignment = .center
        title.textColor = .black
        addSubview(title)
        
        addConstraints([
            
            title.leadingAnchor.constraint(equalTo: leadingAnchor),
            title.topAnchor.constraint(equalTo: topAnchor),
            title.trailingAnchor.constraint(equalTo: trailingAnchor),
            title.bottomAnchor.constraint(equalTo: bottomAnchor)
            
        ])
    }
}

// This is the decoration view behind every section
class SectionBackgroundView : UICollectionReusableView {
    
    static let DecorationViewKind = "SectionBackgroundIdentifier"
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        
        // Customize the settings to what you want
        backgroundColor = .clear
        layer.borderWidth = 5.0
        layer.borderColor = UIColor.blue.cgColor
    }
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

// This is the custom flow layout
class BorderedFlowLayout: UICollectionViewFlowLayout {
    
    override init() {
        super.init()
        
        // Register your decoration view for the layout
        register(SectionBackgroundView.self,
                 forDecorationViewOfKind: SectionBackgroundView.DecorationViewKind)
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override func layoutAttributesForDecorationView(ofKind elementKind: String,
                                                    at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
        if elementKind == SectionBackgroundView.DecorationViewKind {
            
            guard let collectionView = collectionView else { return nil }
            
            // Initialize a UICollectionViewLayoutAttributes for a DecorationView
            let decorationAttributes
                = UICollectionViewLayoutAttributes(forDecorationViewOfKind: SectionBackgroundView.DecorationViewKind,
                                                   with:indexPath)
            
            // Set it behind other views
            decorationAttributes.zIndex = 2
            
            let numberOfItemsInSection
                = collectionView.numberOfItems(inSection: indexPath.section)
            
            // Get the first and last item in the section
            let firstItem = layoutAttributesForItem(at: IndexPath(item: 0, section: indexPath.section))
            
            let lastItem = layoutAttributesForItem(at: IndexPath(item: (numberOfItemsInSection - 1),
                                                                 section: indexPath.section))
            
            // The difference between the maxY of the last item and
            // the the minY of the first item is the height of the section
            let height = lastItem!.frame.maxY - firstItem!.frame.minY
            
            // Set the frame of the decoration view for the section
            decorationAttributes.frame = CGRect(x: 0,
                                                y: firstItem!.frame.minY,
                                                width: collectionView.bounds.width,
                                                height: height)
            
            return decorationAttributes
        }
        
        return nil
    }
    
    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        
        // Get all the UICollectionViewLayoutAttributes for the current view port
        var attributes = super.layoutAttributesForElements(in: rect)
        
        // Filter to get all the different sections
        let sectionAttributes
            = attributes?.filter { $0.indexPath.item == 0 } ?? []
        
        // Loop through the different sections
        for sectionAttribute in sectionAttributes {
            
            // Create decoration attributes for the current section
            if let decorationAttributes
                = self.layoutAttributesForDecorationView(ofKind: SectionBackgroundView.DecorationViewKind,
                                                         at: sectionAttribute.indexPath) {
                
                // Add the decoration attributes for a section if it is in the current viewport
                if rect.intersects(decorationAttributes.frame) {
                    attributes?.append(decorationAttributes)
                }
            }
        }
        
        return attributes
    }
}

class CustomLayoutVC: UIViewController {
    
    private var collectionView: UICollectionView!
    private var cellType: AnyObject?
    
    let numberOfColumns = 3
    let horizontalPadding: CGFloat = 10
    
    override func viewDidLoad() {
        
        super.viewDidLoad()
        
        title = "Collection View"
        view.backgroundColor = .white
        
        configureCollectionView()
    }
    
    private func configureCollectionView() {
        
        collectionView = UICollectionView(frame: CGRect.zero,
                                          collectionViewLayout: createLayout())
        
        collectionView.backgroundColor = .white
        
        collectionView.register(UICollectionViewCell.self,
                                forCellWithReuseIdentifier: "cell")
        
        // You can ignore the header and footer views as you probably already did this
        collectionView.register(HeaderFooterView.self,
                                forSupplementaryViewOfKind: UICollectionView.elementKindSectionHeader,
                                withReuseIdentifier: HeaderFooterView.identifier)
        
        collectionView.register(HeaderFooterView.self,
                                forSupplementaryViewOfKind: UICollectionView.elementKindSectionFooter,
                                withReuseIdentifier: HeaderFooterView.identifier)
        
        collectionView.dataSource = self
        collectionView.delegate = self
        
        view.addSubview(collectionView)
        
        // Auto layout config to pin collection view to the edges of the view
        // You can ignore this for your solution
        collectionView.translatesAutoresizingMaskIntoConstraints = false
        
        collectionView.leadingAnchor
            .constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor,
                        constant: 0).isActive = true
        
        collectionView.topAnchor
            .constraint(equalTo: view.topAnchor,
                        constant: 0).isActive = true
        collectionView.trailingAnchor
            
            .constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor,
                        constant: 0).isActive = true
        collectionView.bottomAnchor
            
            .constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor,
                        constant: 0).isActive = true
    }

    private func createLayout() -> UICollectionViewFlowLayout {
        
        let flowLayout = BorderedFlowLayout()
        flowLayout.minimumLineSpacing = 10
        flowLayout.minimumInteritemSpacing = 10
        flowLayout.scrollDirection = .vertical
        flowLayout.sectionInset = UIEdgeInsets(top: 10,
                                               left: horizontalPadding,
                                               bottom: 10,
                                               right: horizontalPadding)
        
        return flowLayout
    }
}

extension CustomLayoutVC: UICollectionViewDataSource {
    
    func numberOfSections(in collectionView: UICollectionView) -> Int {
        return 4
    }
    
    func collectionView(_ collectionView: UICollectionView,
                        numberOfItemsInSection section: Int) -> Int {
        
        return 3 * (section + 1)
    }
    
    func collectionView(_ collectionView: UICollectionView,
                        cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
        
        let cell = collectionView
            .dequeueReusableCell(withReuseIdentifier:"cell",
                                 for: indexPath)
        
        cell.backgroundColor = .red
        
        return cell
    }
}

extension CustomLayoutVC: UICollectionViewDelegate {
    
    func collectionView(_ collectionView: UICollectionView,
                        viewForSupplementaryElementOfKind kind: String,
                        at indexPath: IndexPath) -> UICollectionReusableView {
        
        switch kind {
            
            case UICollectionView.elementKindSectionHeader:
                let headerView
                    = collectionView.dequeueReusableSupplementaryView(ofKind: kind,
                                                                      withReuseIdentifier: HeaderFooterView.identifier,
                                                                      for: indexPath) as! HeaderFooterView
                
                headerView.backgroundColor = UIColor.yellow
                headerView.title.text = "Header Section \(indexPath.section + 1)"
                return headerView
                
            case UICollectionView.elementKindSectionFooter:
                let footerView = collectionView.dequeueReusableSupplementaryView(ofKind: kind,
                                                                                 withReuseIdentifier: HeaderFooterView.identifier,
                                                                                 for: indexPath)  as! HeaderFooterView
                
                footerView.backgroundColor = UIColor.orange
                footerView.title.text = "Footer Section \(indexPath.section + 1)"
                return footerView
                
            default:
                assert(false, "Unexpected element kind")
        }
    }
    
}

extension CustomLayoutVC: UICollectionViewDelegateFlowLayout {
    
    func collectionView(_ collectionView: UICollectionView,
                        layout collectionViewLayout: UICollectionViewLayout,
                        sizeForItemAt indexPath: IndexPath) -> CGSize {
        
        if let collectionViewLayout = collectionViewLayout as? UICollectionViewFlowLayout {
            
            let totalHorizontalPadding = horizontalPadding * CGFloat(numberOfColumns)
            let interItemSpacing = collectionViewLayout.minimumInteritemSpacing * CGFloat(numberOfColumns - 1)
            let availableWidth = collectionView.bounds.size.width - totalHorizontalPadding - interItemSpacing
            let cellWidth = availableWidth / CGFloat(numberOfColumns)
            
            return CGSize(width: cellWidth, height: cellWidth)
        }
        
        return CGSize.zero
    }
    
    func collectionView(_ collectionView: UICollectionView,
                        layout collectionViewLayout: UICollectionViewLayout,
                        referenceSizeForHeaderInSection section: Int) -> CGSize {
        
        return CGSize(width: collectionView.frame.width, height: 100)
    }
    func collectionView(_ collectionView: UICollectionView,
                        layout collectionViewLayout: UICollectionViewLayout,
                        referenceSizeForFooterInSection section: Int) -> CGSize {
        return CGSize(width: collectionView.frame.width, height: 100)
    }
}