UICollectionViewDiffableDataSource iOS 12

DataSource.swift

//
//  DataSource.swift
//
//  Created by Tomas Harkema on 06/06/2019.
//  Copyright © 2019 Tomas Harkema. All rights reserved.
//

import UIKit
import DeepDiff // https://github.com/onmyway133/DeepDiff

enum Section: Hashable {
  case empty
}

class DataSource<Item: Hashable & DiffAware>: NSObject {
  static func create(collectionView: UICollectionView, cellProvider: @escaping (UICollectionView, IndexPath, Item) -> UICollectionViewCell?) -> DataSource<Item> {
    if #available(iOS 13, *) {
      return NewDataSource(collectionView: collectionView, cellProvider: cellProvider)
    } else {
      return OldDataSource(collectionView: collectionView, cellProvider: cellProvider)
    }
  }

  func apply(newValue: [Item]) {
    fatalError()
  }

  func itemIdentifier(for indexPath: IndexPath) -> Item? {
    fatalError()
  }
}

@available(iOS 13, *)
class NewDataSource<Item: Hashable & DiffAware>: DataSource<Item> {
  private let dataSource: UICollectionViewDiffableDataSource<Section, Item>

  init(collectionView: UICollectionView, cellProvider: @escaping (UICollectionView, IndexPath, Item) -> UICollectionViewCell?) {
    dataSource = UICollectionViewDiffableDataSource<Section, Item>(collectionView: collectionView, cellProvider: cellProvider)
    super.init()
  }

  override func apply(newValue: [Item]) {
    let snapshot = NSDiffableDataSourceSnapshot<Section, Item>()
    snapshot.appendSections([.empty])
    snapshot.appendItems(newValue)
    dataSource.apply(snapshot)
  }

  override func itemIdentifier(for indexPath: IndexPath) -> Item? {
    return dataSource.itemIdentifier(for: indexPath)
  }
}

@available(iOS, deprecated: 12.4, obsoleted: 12.4)
class OldDataSource<Item: Hashable & DiffAware>: DataSource<Item>, UICollectionViewDataSource {
  private let collectionView: UICollectionView
  private var backingStore = [Item]()
  private let cellProvider: (UICollectionView, IndexPath, Item) -> UICollectionViewCell?

  init(collectionView: UICollectionView, cellProvider: @escaping (UICollectionView, IndexPath, Item) -> UICollectionViewCell?) {
    self.collectionView = collectionView
    self.cellProvider = cellProvider
    super.init()
    collectionView.dataSource = self
  }

  func numberOfSections(in collectionView: UICollectionView) -> Int {
    return 1
  }

  func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
    return backingStore.count
  }

  func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    return cellProvider(collectionView, indexPath, backingStore[indexPath.row])!
  }

  override func apply(newValue: [Item]) {
    DispatchQueue.main.async {
      let changes = diff(old: self.backingStore, new: newValue)
      self.collectionView.reload(changes: changes, updateData: {
        self.backingStore = newValue
      })
    }
  }

  override func itemIdentifier(for indexPath: IndexPath) -> Item? {
    return backingStore[indexPath.item]
  }
}