Don't use NSFetchedResultsController for items list

Reader/Screens/Home/HomeViewController.swift

@@ -189,6 +189,22 @@ extension HomeViewController {
             return req
         }
         
+        var idFetchRequest: NSFetchRequest<NSManagedObjectID> {
+            let req = NSFetchRequest<NSManagedObjectID>(entityName: "Item")
+            req.resultType = .managedObjectIDResultType
+            switch self {
+            case .unread:
+                req.predicate = NSPredicate(format: "read = NO")
+            case .all:
+                break
+            case .group(let group):
+                req.predicate = NSPredicate(format: "feed in %@", group.feeds!)
+            case .feed(let feed):
+                req.predicate = NSPredicate(format: "feed = %@", feed)
+            }
+            return req
+        }
+        
         var countFetchRequest: NSFetchRequest<Reader.Item>? {
             let req = Reader.Item.fetchRequest()
             switch self {
@@ -229,7 +245,7 @@ extension HomeViewController: UICollectionViewDelegate {
         guard let item = dataSource.itemIdentifier(for: indexPath) else {
             return
         }
-        let vc = ItemsViewController(fetchRequest: item.fetchRequest, fervorController: fervorController)
+        let vc = ItemsViewController(fetchRequest: item.idFetchRequest, fervorController: fervorController)
         vc.title = item.title
         vc.delegate = itemsDelegate
         show(vc, sender: nil)
@@ -241,7 +257,7 @@ extension HomeViewController: UICollectionViewDelegate {
             return nil
         }
         return UIContextMenuConfiguration(identifier: nil, previewProvider: {
-            return ItemsViewController(fetchRequest: item.fetchRequest, fervorController: self.fervorController)
+            return ItemsViewController(fetchRequest: item.idFetchRequest, fervorController: self.fervorController)
         }, actionProvider: nil)
     }
     

Reader/Screens/Items/ItemsViewController.swift

@@ -18,14 +18,16 @@ class ItemsViewController: UIViewController {
     weak var delegate: ItemsViewControllerDelegate?
     
     let fervorController: FervorController
-    let fetchRequest: NSFetchRequest<Item>
+    let fetchRequest: NSFetchRequest<NSManagedObjectID>
     
     private var collectionView: UICollectionView!
-    private var resultsController: NSFetchedResultsController<Item>!
+    private var dataSource: UICollectionViewDiffableDataSource<Section, NSManagedObjectID>!
     
     private var batchUpdates: [() -> Void] = []
     
-    init(fetchRequest: NSFetchRequest<Item>, fervorController: FervorController) {
+    init(fetchRequest: NSFetchRequest<NSManagedObjectID>, fervorController: FervorController) {
+        precondition(fetchRequest.entityName == "Item")
+        
         self.fervorController = fervorController
         self.fetchRequest = fetchRequest
         
@@ -47,12 +49,11 @@ class ItemsViewController: UIViewController {
         configuration.backgroundColor = .clear
         let layout = UICollectionViewCompositionalLayout.list(using: configuration)
         collectionView = UICollectionView(frame: view.bounds, collectionViewLayout: layout)
-        collectionView.delegate = self
-        collectionView.dataSource = self
         collectionView.translatesAutoresizingMaskIntoConstraints = false
-        collectionView.register(ItemCollectionViewCell.self, forCellWithReuseIdentifier: "itemCell")
         view.addSubview(collectionView)
         
+        dataSource = createDataSource()
+        
         NSLayoutConstraint.activate([
             collectionView.leadingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor),
             collectionView.trailingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor),
@@ -61,69 +62,116 @@ class ItemsViewController: UIViewController {
         ])
         
         fetchRequest.sortDescriptors = [NSSortDescriptor(key: "published", ascending: false)]
-        fetchRequest.fetchBatchSize = 20
+        do {
-        resultsController = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: fervorController.persistentContainer.viewContext, sectionNameKeyPath: nil, cacheName: nil)
+            let ids = try fervorController.persistentContainer.viewContext.fetch(fetchRequest)
-        resultsController.delegate = self
-        try! resultsController.performFetch()
-    }
             
-}
+            var snapshot = NSDiffableDataSourceSnapshot<Section, NSManagedObjectID>()
+            snapshot.appendSections([.items])
+            snapshot.appendItems(ids, toSection: .items)
+            dataSource.apply(snapshot, animatingDifferences: false)
             
-extension ItemsViewController: UICollectionViewDataSource {
+            NotificationCenter.default.addObserver(self, selector: #selector(managedObjectsDidChange), name: .NSManagedObjectContextObjectsDidChange, object: fervorController.persistentContainer.viewContext)
-    func numberOfSections(in collectionView: UICollectionView) -> Int {
-        return 1
-    }
-    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
-        return resultsController.fetchedObjects?.count ?? 0
-    }
-    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
-        let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "itemCell", for: indexPath) as! ItemCollectionViewCell
-        cell.delegate = self
-        cell.scrollView = collectionView
-        cell.updateUI(item: resultsController.fetchedObjects![indexPath.row])
-        return cell
-    }
-}
             
-extension ItemsViewController: NSFetchedResultsControllerDelegate {
+        } catch {
-    func controllerWillChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
+            let alert = UIAlertController(title: "Error fetching items", message: error.localizedDescription, preferredStyle: .alert)
-        batchUpdates = []
+            alert.addAction(UIAlertAction(title: "Ok", style: .default, handler: nil))
-    }
+            present(alert, animated: true)
-    func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
-        collectionView.performBatchUpdates {
-            for update in self.batchUpdates {
-                update()
-            }
         }
-        // clear to prevent retain cycles
-        batchUpdates = []
     }
-    func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange anObject: Any, at indexPath: IndexPath?, for type: NSFetchedResultsChangeType, newIndexPath: IndexPath?) {
+    
-        guard let indexPath = indexPath else {
+    private func createDataSource() -> UICollectionViewDiffableDataSource<Section, NSManagedObjectID> {
-            return
+        let itemCell = UICollectionView.CellRegistration<ItemCollectionViewCell, NSManagedObjectID> { [unowned self] cell, indexPath, itemIdentifier in
+            cell.delegate = self
+            cell.scrollView = self.collectionView
+            let item = self.fervorController.persistentContainer.viewContext.object(with: itemIdentifier) as! Item
+            cell.updateUI(item: item)
         }
-        batchUpdates.append {
+        return UICollectionViewDiffableDataSource(collectionView: collectionView) { collectionView, indexPath, itemIdentifier in
-            switch type {
+            return collectionView.dequeueConfiguredReusableCell(using: itemCell, for: indexPath, item: itemIdentifier)
-            case .insert:
+        }
-                self.collectionView.insertItems(at: [indexPath])
+    }
-            case .delete:
+    
-                self.collectionView.deleteItems(at: [indexPath])
+    @objc private func managedObjectsDidChange(_ notification: Notification) {
-            case .move:
+        let inserted = notification.userInfo?[NSInsertedObjectsKey] as? NSSet
-                if let newIndexPath = newIndexPath {
+        let updated = notification.userInfo?[NSUpdatedObjectsKey] as? NSSet
-                    self.collectionView.moveItem(at: indexPath, to: newIndexPath)
+        let deleted = notification.userInfo?[NSDeletedObjectsKey] as? NSSet
+        
+        // managed objectss from the notification are tied to the thread it was delivered on
+        // so get the published dates and evaluate the predicate here
+        let insertedItems = inserted?.compactMap { $0 as? Item }.filter { fetchRequest.predicate?.evaluate(with: $0) ?? true }.map { ($0, $0.published) }
+        
+        var snapshot = self.dataSource.snapshot()
+        
+        if let updated = updated {
+            snapshot.reconfigureItems(updated.compactMap { ($0 as? Item)?.objectID })
+        }
+        if let deleted = deleted {
+            snapshot.deleteItems(deleted.compactMap { ($0 as? Item)?.objectID })
+        }
+        
+        if let insertedItems = insertedItems {
+            self.fervorController.persistentContainer.performBackgroundTask { ctx in
+                
+                // for newly inserted items, the ctx doesn't have the published date so we check the data we got from the notification
+                func publishedForItem(_ id: NSManagedObjectID) -> Date? {
+                    if let (_, pub) = insertedItems.first(where: { $0.0.objectID == id }) {
+                        return pub
+                    } else {
+                        return (ctx.object(with: id) as! Item).published
+                    }
                 }
-            case .update:
+                
-                self.collectionView.reloadItems(at: [indexPath])
+                // todo: this feels inefficient
-            @unknown default:
+                for (inserted, insertedPublished) in insertedItems {
-                break
+                    // todo: uhh, what does sql do if the published date is nil?
+                    guard let insertedPublished = insertedPublished else {
+                        snapshot.insertItems([inserted.objectID], beforeItem: snapshot.itemIdentifiers.first!)
+                        continue
+                    }
+                    
+                    var index = 0
+                    while index < snapshot.itemIdentifiers.count,
+                          let pub = publishedForItem(snapshot.itemIdentifiers[index]),
+                          insertedPublished < pub {
+                        index += 1
+                    }
+                    
+                    // index is the index of the first item which the inserted one was published after
+                    // (i.e., the item that should appear immediately after inserted in the list)
+                    
+                    if index == snapshot.itemIdentifiers.count {
+                        snapshot.appendItems([inserted.objectID], toSection: .items)
+                    } else {
+                        snapshot.insertItems([inserted.objectID], beforeItem: snapshot.itemIdentifiers[index])
+                    }
+                }
+                
+                DispatchQueue.main.async {
+                    self.dataSource.apply(snapshot, animatingDifferences: true)
+                }
+            }
+        } else {
+            DispatchQueue.main.async {
+                self.dataSource.apply(snapshot, animatingDifferences: true)
             }
         }
     }
+
+}
+
+extension ItemsViewController {
+    enum Section: Hashable {
+        case items
+    }
 }
 
 extension ItemsViewController: UICollectionViewDelegate {
     func collectionView(_ collectionView: UICollectionView, contextMenuConfigurationForItemAt indexPath: IndexPath, point: CGPoint) -> UIContextMenuConfiguration? {
-        let item = resultsController.fetchedObjects![indexPath.row]
+        guard let id = dataSource.itemIdentifier(for: indexPath) else {
+            return nil
+        }
+        let item = fervorController.persistentContainer.viewContext.object(with: id) as! Item
+        //        let item = resultsController.fetchedObjects![indexPath.row]
         return UIContextMenuConfiguration(identifier: nil, previewProvider: {
             ReadViewController(item: item, fervorController: self.fervorController)
         }, actionProvider: { _ in