Diffable collection view for macos

import Cocoa
import With

class SectionList: NSView {
   lazy var collectionView: NSCollectionView = createCollectionView()
   lazy var dataSource: DiffableDataSource = configureDataSource()
   var currentSnapshot: DiffableSnapshot? // we use this as the one source of truth
   lazy var scrollView: NSScrollView = createScrollView()
   init() {
      super.init(frame: .zero)
      self.wantsLayer = true
      self.layer?.backgroundColor = NSColor.systemGreen.cgColor
      _ = scrollView
      configureHeader()
      addItems()
   }
   /**
    * Add items
    */
   func addItems() {
      let a: [Film] = [.init(name: "Joe"), .init(name: "Goa")]
      let b: [Film] = [.init(name: "Hi"), .init(name: "Cava"), .init(name: "Demi")]
      let sectionItems: [FilmSection] = [.init(headerItem: .init(titleHeader: "Pro"), items: a), .init(headerItem: .init(titleHeader: "Amateur"), items: b)]
      let payloadDatasource = DataSource(sections: sectionItems)
      currentSnapshot = DiffableSnapshot() // DiffableDataSource()
      payloadDatasource.sections.forEach {
         currentSnapshot?.appendSections([$0])
         currentSnapshot?.appendItems($0.items, toSection: $0)
      }
      self.dataSource.apply(currentSnapshot!, animatingDifferences: true)
   }
   /**
    * Clears one section
    */
   func clear() {
      let snapshot = self.dataSource.snapshot()
      currentSnapshot = snapshot//NSDiffableDataSourceSnapshot<SectionItem<Header, [Film]>, Film>() // DiffableDataSource()
      currentSnapshot?.deleteItems(snapshot.sectionIdentifiers[0].items)
      currentSnapshot?.deleteSections([snapshot.sectionIdentifiers[0]]) // []
      self.dataSource.apply(currentSnapshot!, animatingDifferences: true)
   }
   /**
    * Boilerplate
    */
   required init?(coder: NSCoder) {
      fatalError("init(coder:) has not been implemented")
   }
}
extension SectionList: NSCollectionViewDelegateFlowLayout {
   private func configureDataSource() -> DiffableDataSource {
      return NSCollectionViewDiffableDataSource.init(collectionView: collectionView, itemProvider: { collectionView, indexPath, film  in
         let cell: Cell = collectionView.makeItem(withIdentifier: .init(rawValue: "Cell"), for: indexPath) as! Cell
         let data = film // sectionData[indexPath.section].rows[indexPath.item] // - Fixme: ⚠️️ add some asserts here or?
         cell.setData(name: data.name)
         // cell.configureCell(with: film)
         return cell
      })
   }
   private func configureHeader() {
      self.dataSource.supplementaryViewProvider = { (collectionView: NSCollectionView, kind: NSCollectionView.SupplementaryElementKind, indexPath: IndexPath) -> NSSection? in
         Swift.print("supplementaryViewProvider kind: \(kind) indexPath: \(indexPath)")
         if kind == NSCollectionView.elementKindSectionHeader {
//            let numberOfSections = collectionView.dataSource?.numberOfSections?(in: collectionView)
            let view = collectionView.makeSupplementaryView(ofKind: kind, withIdentifier: .init("SectionHeader"), for: indexPath)
            if let section = self.currentSnapshot?.sectionIdentifiers[indexPath.section] {
               (view as? SectionHeader)?.titleTextField.stringValue = "\(section.headerItem.titleHeader)"
            }
            return view as? NSSection
         } else if kind == NSCollectionView.elementKindSectionFooter {
            return nil
         } else {
            return nil
         }
      }
   }
   /**
    * CollectionView
    */
   func createCollectionView() -> NSCollectionView {
      Swift.print("createCollectionView")
      let layout: NSCollectionViewFlowLayout = with(.init()) {
         $0.minimumLineSpacing = 0
         $0.minimumInteritemSpacing = 0
         $0.scrollDirection = .vertical // this might be default?
      }
      return with(.init()) {
         $0.delegate = self
         $0.collectionViewLayout = layout
         $0.backgroundColors = [.clear]
         $0.isSelectable = true
         $0.register(Cell.self, forItemWithIdentifier: .init(rawValue: "Cell"))
         $0.register(SectionHeader.self, forSupplementaryViewOfKind: NSCollectionView.elementKindSectionHeader, withIdentifier: .init(rawValue: "SectionHeader"))
      }
   }
   /**
    * ScrollView
    * - Note: configure wrapping scroll (necessary for support collection view's scrolling)
    */
   func createScrollView() -> NSScrollView {
      let scrollView = NSScrollView()
      scrollView.documentView = collectionView
      self.addSubview(scrollView)
      scrollView.anchorAndSize(to: self)
      return scrollView
   }
   /**
    * sizeForItemAt
    */
   func collectionView(_ collectionView: NSCollectionView, layout collectionViewLayout: NSCollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> NSSize {
      return .init(width: collectionView.frame.size.width, height: 40)
   }
   // header
   func collectionView(_ collectionView: NSCollectionView, layout collectionViewLayout: NSCollectionViewLayout, referenceSizeForHeaderInSection section: Int) -> NSSize {
      return NSSize(width: collectionView.frame.size.width, height: 20)
   }
   // footer, this is called
   func collectionView(_ collectionView: NSCollectionView, layout collectionViewLayout: NSCollectionViewLayout, referenceSizeForFooterInSection section: Int) -> NSSize {
      return .zero
   }
   /**
    * didSelectItemsAt
    */
   func collectionView(_ collectionView: NSCollectionView, didSelectItemsAt indexPaths: Set<IndexPath>) {
      Swift.print("didSelectItemsAt: \(indexPaths)")
      self.clear()
   }
   /**
    * React on selection
    */
   func collectionView(_ collectionView: NSCollectionView, didDeselectItemsAt indexPaths: Set<IndexPath>) {
      Swift.print("didDeselectItemsAt: \(indexPaths)")

   }
}
typealias FilmSection = SectionItem<Header, [Film]>
struct SectionItem<U: Hashable, T: Hashable>: Hashable {
   let headerItem: U
   let items: T
}
struct Film: Decodable, Hashable {
   let name: String
}
struct Header: Hashable {
   let titleHeader: String
}
struct DataSource<T: Hashable> {
   let sections: [T]
}
typealias DiffableDataSource = NSCollectionViewDiffableDataSource<SectionItem<Header, [Film]>, Film>
typealias DiffableSnapshot = NSDiffableDataSourceSnapshot<SectionItem<Header, [Film]>, Film>