iOS14-DiffableDataSource-SectionSnapshot.swift

import UIKit


enum OutlineItem: Hashable {
    case parent(Parent)
    case child(Child)
}
struct Child : Hashable{
    let item: String
}

struct Parent: Hashable {
    let item: String
    let childItems: [Child]
}

class ViewController: UIViewController {

    var collectionView : UICollectionView!
    private lazy var dataSource = makeDataSource()

    let hirerachicalData = [
            
        Parent(item: "First", childItems: Array(0...4).map { Child(item: String($0)) }),
        Parent(item: "Second", childItems: Array(5...10).map { Child(item: String($0)) }),
        Parent(item: "Third", childItems: Array(11...13).map { Child(item: String($0)) }),
    ]
    
    override func viewDidLoad() {
        super.viewDidLoad()

        let config = UICollectionLayoutListConfiguration(appearance: .insetGrouped)
        let layout = UICollectionViewCompositionalLayout.list(using: config)
        
        collectionView = UICollectionView(frame: .zero, collectionViewLayout: layout)
        self.view.addSubview(collectionView)
        
        collectionView.translatesAutoresizingMaskIntoConstraints = false
        collectionView.topAnchor.constraint(equalTo: view.topAnchor).isActive = true
        collectionView.leftAnchor.constraint(equalTo: view.leftAnchor).isActive = true
        collectionView.rightAnchor.constraint(equalTo: view.rightAnchor).isActive = true
        collectionView.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
        
        collectionView.dataSource = dataSource

        var sectionSnapshot = NSDiffableDataSourceSectionSnapshot<OutlineItem>()
        
        for data in hirerachicalData{
            
            let header = OutlineItem.parent(data)
            sectionSnapshot.append([header])
            sectionSnapshot.append(data.childItems.map { OutlineItem.child($0) }, to: header)

            sectionSnapshot.expand([header])
        }
        
        dataSource.apply(sectionSnapshot, to: "Root", animatingDifferences: false, completion: nil)
    }
    
    
    func makeDataSource() -> UICollectionViewDiffableDataSource<String, OutlineItem> {
         
        
        let parentRegistration = UICollectionView.CellRegistration<UICollectionViewListCell, Parent> { cell, indexPath, item in
            
            var content = cell.defaultContentConfiguration()
            content.text = item.item
            cell.contentConfiguration = content
            
            let headerDisclosureOption = UICellAccessory.OutlineDisclosureOptions(style: .header)
            cell.accessories = [.outlineDisclosure(options:headerDisclosureOption)]
            
        }
        
        
        let cellRegistration = UICollectionView.CellRegistration<UICollectionViewListCell, Child> { cell, indexPath, item in
            
            var content = cell.defaultContentConfiguration()
            content.text = item.item
            cell.indentationLevel = 2
            cell.contentConfiguration = content
        }
        
        return UICollectionViewDiffableDataSource<String, OutlineItem>(
                    collectionView: collectionView,
                    cellProvider: { collectionView, indexPath, item in
    
                        
                    switch item{
                        case .parent(let parentItem):
                            
                            let cell = collectionView.dequeueConfiguredReusableCell(
                                using: parentRegistration,
                                for: indexPath,
                                item: parentItem)
    
                            return cell

                        case .child(let childItem):
    
                            let cell = collectionView.dequeueConfiguredReusableCell(
                                using: cellRegistration,
                                for: indexPath,
                                item: childItem)
    
                            return cell
                    }
        })
    }
}