Move item count fetching to background task

Reader/ItemListType.swift

@@ -9,7 +9,7 @@ import Foundation
 import CoreData
 import Persistence
 
-enum ItemListType: Hashable {
+enum ItemListType: Hashable, Equatable {
     case unread
     case all
     case group(Group)
@@ -52,7 +52,9 @@ enum ItemListType: Hashable {
         case .all:
             return nil
         case .group(let group):
-            req.predicate = NSPredicate(format: "read = NO AND feed in %@", group.feeds!)
+            // use the feed ids, because passing the NSSet of feeds into the predicate results in a multithreading violation if the request is used on a different context
+            let feedIDs = group.feeds!.map { ($0 as! Feed).objectID }
+            req.predicate = NSPredicate(format: "read = NO AND feed in %@", feedIDs)
         case .feed(let feed):
             req.predicate = NSPredicate(format: "read = NO AND feed = %@", feed)
         }

Reader/Screens/Home/HomeCollectionViewCell.swift

@@ -7,9 +7,16 @@
 
 import UIKit
 import Fervor
+import Persistence
+import OSLog
+
+private let signposter = OSSignposter(subsystem: "net.shadowfacts.Reader", category: "HomeCollectionViewCell")
 
 class HomeCollectionViewCell: UICollectionViewListCell {
     
+    private var currentItemListType: ItemListType?
+    private var itemCount: Int?
+    
     #if !targetEnvironment(macCatalyst)
     override func updateConfiguration(using state: UICellConfigurationState) {
         var backgroundConfig = UIBackgroundConfiguration.listGroupedCell().updated(for: state)
@@ -22,4 +29,45 @@ class HomeCollectionViewCell: UICollectionViewListCell {
     }
     #endif
     
+    override func prepareForReuse() {
+        super.prepareForReuse()
+        itemCount = nil
+    }
+    
+    func updateUI(item: ItemListType, persistentContainer: PersistentContainer) {
+        self.currentItemListType = item
+        
+        var config = UIListContentConfiguration.valueCell()
+        config.text = item.title
+        if let itemCount {
+            config.secondaryText = itemCount.formatted(.number)
+        }
+        config.secondaryTextProperties.color = .tintColor
+        self.contentConfiguration = config
+        
+        Task(priority: .userInitiated) {
+            let state = signposter.beginInterval("fetch count", id: signposter.makeSignpostID())
+            if let count = await fetchCount(item: item, in: persistentContainer),
+               self.currentItemListType == item {
+                self.itemCount = count
+                config.secondaryText = count.formatted(.number)
+                self.contentConfiguration = config
+            }
+            signposter.endInterval("fetch count", state)
+        }
+    }
+    
+    private func fetchCount(item: ItemListType, in persistentContainer: PersistentContainer) async -> Int? {
+        guard let request = item.countFetchRequest else {
+            return nil
+        }
+        return await withCheckedContinuation({ continuation in
+            let context = persistentContainer.backgroundContext
+            context.perform {
+                let count = try? context.count(for: request)
+                continuation.resume(returning: count)
+            }
+        })
+    }
+    
 }

Reader/Screens/Home/HomeViewController.swift

@@ -126,15 +126,8 @@ class HomeViewController: UIViewController {
             config.text = section.title
             supplementaryView.contentConfiguration = config
         }
-        let listCell = UICollectionView.CellRegistration<HomeCollectionViewCell, ItemListType> { cell, indexPath, item in
-            var config = UIListContentConfiguration.valueCell()
-            config.text = item.title
-            if let req = item.countFetchRequest,
-               let count = try? self.fervorController.persistentContainer.viewContext.count(for: req) {
-                config.secondaryText = "\(count)"
-                config.secondaryTextProperties.color = .tintColor
-            }
-            cell.contentConfiguration = config
+        let listCell = UICollectionView.CellRegistration<HomeCollectionViewCell, ItemListType> { [unowned self] cell, indexPath, item in
+            cell.updateUI(item: item, persistentContainer: self.fervorController.persistentContainer)
             
             cell.accessories = [.disclosureIndicator()]
         }