Complete.swift

import UIKit

enum Section {
    case section1
    case section2
}

struct Item: Hashable {
    let color: UIColor
}

class ViewController: UIViewController {
    @IBOutlet var collectionview: UICollectionView! {
        didSet {
            collectionview.delegate = self
        }
    }

    lazy var dataSource = UICollectionViewDiffableDataSource<Section, Item>(collectionView: self.collectionview,
                                                                            cellProvider: self.cellProvider)
    lazy var cellProvider: (UICollectionView, IndexPath, Item) -> UICollectionViewCell? = { collectionView, indexPath, item in
        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "Cell", for: indexPath)
        cell.backgroundColor = item.color
        return cell
    }
    
    override func viewDidLoad() {
       super.viewDidLoad()
        let items1 = [
            Item(color: .green),
            Item(color: .red),
            Item(color: .blue)
        ]
        
        let items2 = [
            Item(color: .yellow),
            Item(color: .orange),
            Item(color: .purple)
        ]
        
        addItems(items: items1, to: .section1)
        addItems(items: items2, to: .section2)
    }
}

extension ViewController {
    func addItems(items: [Item], to section: Section) {
        var snapshot = dataSource.snapshot()
        snapshot.appendSections([section])
        snapshot.appendItems(items, toSection: section)
        dataSource.apply(snapshot)
    }
}