NSDiffableDataSourceSnapshot+Combine.swift

import UIKit
import Combine

protocol DiffableListDataSourceType {
  associatedtype SectionIdentifier: Hashable
  associatedtype ItemIdentifier: Hashable

  func apply(_ snapshot: NSDiffableDataSourceSnapshot<SectionIdentifier, ItemIdentifier>,
             animatingDifferences: Bool,
             completion: (() -> Void)?)
}

extension UICollectionViewDiffableDataSource: DiffableListDataSourceType {}
extension UITableViewDiffableDataSource: DiffableListDataSourceType {}

extension Subscribers {

  fileprivate class Apply<DataSource: DiffableListDataSourceType>: Subscriber, Cancellable {

    typealias Failure = Never
    private var subscription: Subscription?
    private let dataSource: DataSource
    private let animatingDifferences: Bool
    private let completion: (() -> Void)?

    init(dataSource: DataSource, animatingDifferences: Bool, completion: (() -> Void)?) {
      self.dataSource = dataSource
      self.animatingDifferences = animatingDifferences
      self.completion = completion
    }

    func receive(_ input: NSDiffableDataSourceSnapshot<DataSource.SectionIdentifier, DataSource.ItemIdentifier>) -> Subscribers.Demand {
      self.dataSource.apply(input, animatingDifferences: animatingDifferences, completion: completion)
      return .none
    }

    func receive(completion: Subscribers.Completion<Never>) {
    }

    func receive(subscription: Subscription) {
      self.subscription = subscription
      subscription.request(.unlimited)
    }

    func cancel() {
      subscription?.cancel()
    }

  }

}

extension Publisher where Failure == Never {

  func apply<DataSource: DiffableListDataSourceType>(to dataSource: DataSource,
                                                     animatingDifferences: Bool = true,
                                                     completion: (() -> Void)? = nil) ->
    AnyCancellable where Output == NSDiffableDataSourceSnapshot<DataSource.SectionIdentifier, DataSource.ItemIdentifier> {
      let apply = Subscribers.Apply<DataSource>(dataSource: dataSource,
                                                animatingDifferences: animatingDifferences,
                                                completion: completion)
      receive(subscriber: apply)
      return AnyCancellable(apply)
  }

}

viewModel.sectionSnapshot
  .receive(on: DispatchQueue.main)
  .apply(to: dataSource)
  .store(in: &cancellables)