Re: Unable to cast NSDiffableDataSourceSnapshotReference

This doesn't look ready, yet. The example is clearly wrong, as the cast operates on unrelated types and always fails when you add the missing generic paramaters, given the following class declarations in beta 1:

open class NSDiffableDataSourceSnapshotReference<sectionidentifiertype, itemidentifiertype=""> : NSObject, NSCopying where SectionIdentifierType : AnyObject, ItemIdentifierType : AnyObject {
public class NSDiffableDataSourceSnapshot<sectionidentifiertype, itemidentifiertype=""> where SectionIdentifierType : Hashable, ItemIdentifierType : Hashable {

Can confirm this issue when I try to use this on a UITableView. I hope Apple addresses this in a future beta. Everyone send a bugreport!

Maybe this question is related: What is the difference between NSDiffableDataSourceSnapshot and NSDiffableDataSourceSnapshotReference?

It looks as if this isn't supported directly yet. However I did manage to get it to work by pulling out the associated data. This assumes a single section so you'll need to adjust for your own implenentation however it does work and appears to update, animate etc as expected. Given the Apple sample code I was also able to add the publisher approach for cell field updates and also remove the dataSource methods

for numberOfItems, etc...

<nsmanagedobjectid, nsstring=""><section, tag="">

func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChangeContentWith snapshot: NSDiffableDataSourceSnapshotReference<NSManagedObjectID, NSString>) {
         // NOTE: Section.main is an enum that represents my single section
        let diffable = NSDiffableDataSourceSnapshot<Section, Tag>()
        diffable.appendSections([.main])
        diffable.appendItems(dataProvider.fetchedResultsController.fetchedObjects ?? [], toSection: .main)
        dataSource.apply(diffable, animatingDifferences: true)
    }

I'm also running into this issue, but a bit of type finagling I have it mostly working. My remaining issue is that I can't figure out how to properly populate the collection view on load.

My type order is, as IngmarStein suggests, string before object ID,

    typealias Snapshot = NSDiffableDataSourceSnapshotReference<NSString, NSManagedObjectID>
    typealias DataSource = UICollectionViewDiffableDataSourceReference<NSString, NSManagedObjectID>

extension ViewController: NSFetchedResultsControllerDelegate {
    func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChangeContentWith snapshot: NSDiffableDataSourceSnapshotReference<NSManagedObjectID, NSString>) {
        let newSnapshot = Snapshot()
        newSnapshot.appendSections(withIdentifiers: snapshot.sectionIdentifiers as [AnyObject] as! [NSString])
        newSnapshot.appendItems(withIdentifiers: snapshot.itemIdentifiers as [AnyObject] as! [NSManagedObjectID])
        self.dataSource.applySnapshot(newSnapshot, animatingDifferences: true)
    }
}

The secionIdentifiers are constant each launch, but I can't see how to get them out of NSFetchedResultsController without calling didChangeContent. Notably, section.name is nil. I can populated on view load by inventing temporary section titles

        let snapshot = Snapshot()
        if let sections = self.fetched.sections {
            let sectionMap = [NSString: NSFetchedResultsSectionInfo](uniqueKeysWithValues: sections.map {section -> (NSString, NSFetchedResultsSectionInfo) in return (UUID().description as NSString, section)})
            snapshot.appendSections(withIdentifiers: sectionMap.compactMap {$0.key})
            for section in sectionMap {
                snapshot.appendItems(withIdentifiers: (section.value.objects as! [NSManagedObject]).map {$0.objectID}, intoSectionWithIdentifier: section.key)
            }
        }
        self.dataSource.applySnapshot(snapshot, animatingDifferences: false)

These IDs will be replaced on first change. I don't know what effect this might have on a more complicated fetch, but so far it seems to not animate a large rearrangement of existing objects into a new section.

I have also submitted a feedback request, FB6162037.

It's a straightforward cast:

func controller(_ controller: NSFetchedResultsController, 
  didChangeContentWith snapshot: NSDiffableDataSourceSnapshotReference) {
    let snapshot = snapshot as NSDiffableDataSourceSnapshot<string,nsmanagedobjectid>
    self.ds.apply(snapshot, animatingDifferences: false)
}

I'm assuming here that your diffable data source is typed with generic resolutions String, NSManagedObjectID. In your cell population function you go back to the fetched results controller, fetch out the actual object by index path, and populate the cell from its properties.