fix: table reload in the background cannot keep scroll position issue

Mastodon/Scene/HomeTimeline/HomeTimelineViewModel+Diffable.swift

@@ -155,8 +155,14 @@ extension HomeTimelineViewModel {
     ) -> Difference<T>? {
         guard let sourceIndexPath = (tableView.indexPathsForVisibleRows ?? []).sorted().first else { return nil }
         let rectForSourceItemCell = tableView.rectForRow(at: sourceIndexPath)
-        let sourceDistanceToTableViewTopEdge = tableView.convert(rectForSourceItemCell, to: nil).origin.y - tableView.safeAreaInsets.top
+        let sourceDistanceToTableViewTopEdge: CGFloat = {
-        
+            if tableView.window != nil {
+                return tableView.convert(rectForSourceItemCell, to: nil).origin.y - tableView.safeAreaInsets.top
+            } else {
+                return rectForSourceItemCell.origin.y - tableView.contentOffset.y - tableView.safeAreaInsets.top
+            }
+        }()
+
         guard sourceIndexPath.section < oldSnapshot.numberOfSections,
               sourceIndexPath.row < oldSnapshot.numberOfItems(inSection: oldSnapshot.sectionIdentifiers[sourceIndexPath.section])
         else { return nil }

Mastodon/Scene/Thread/ThreadViewModel+Diffable.swift

@@ -139,130 +139,6 @@ extension ThreadViewModel {
             }   // end Task
         }
         .store(in: &disposeBag)
-
-        
-//        Publishers.CombineLatest3(
-//            rootItem.removeDuplicates(),
-//            ancestorItems.removeDuplicates(),
-//            descendantItems.removeDuplicates()
-//        )
-//        .receive(on: RunLoop.main)
-//        .sink { [weak self] rootItem, ancestorItems, descendantItems in
-//            guard let self = self else { return }
-//            var items: [Item] = []
-//            rootItem.flatMap { items.append($0) }
-//            items.append(contentsOf: ancestorItems)
-//            items.append(contentsOf: descendantItems)
-//            self.updateDeletedStatus(for: items)
-//        }
-//        .store(in: &disposeBag)
-//        
-//        Publishers.CombineLatest4(
-//            rootItem,
-//            ancestorItems,
-//            descendantItems,
-//            existStatusFetchedResultsController.objectIDs
-//        )
-//        .debounce(for: .milliseconds(100), scheduler: RunLoop.main)       // some magic to avoid jitter
-//        .sink { [weak self] rootItem, ancestorItems, descendantItems, existObjectIDs in
-//            guard let self = self else { return }
-//            guard let tableView = self.tableView,
-//                  let navigationBar = self.contentOffsetAdjustableTimelineViewControllerDelegate?.navigationBar()
-//            else { return }
-//            
-//            guard let diffableDataSource = self.diffableDataSource else { return }
-//            let oldSnapshot = diffableDataSource.snapshot()
-//            
-//            var newSnapshot = NSDiffableDataSourceSnapshot<StatusSection, Item>()
-//            newSnapshot.appendSections([.main])
-//            
-//            let currentState = self.loadThreadStateMachine.currentState
-//            
-//            // reply to
-//            if self.rootNode.value?.replyToID != nil, !(currentState is LoadThreadState.NoMore) {
-//                newSnapshot.appendItems([.topLoader], toSection: .main)
-//            }
-//            
-//            let ancestorItems = ancestorItems.filter { item in
-//                guard case let .reply(statusObjectID, _) = item else { return false }
-//                return existObjectIDs.contains(statusObjectID)
-//            }
-//            newSnapshot.appendItems(ancestorItems, toSection: .main)
-//            
-//            // root
-//            if let rootItem = rootItem,
-//               case let .root(objectID, _) = rootItem,
-//               existObjectIDs.contains(objectID) {
-//                newSnapshot.appendItems([rootItem], toSection: .main)
-//            }
-//            
-//            // leaf
-//            if !(currentState is LoadThreadState.NoMore) {
-//                newSnapshot.appendItems([.bottomLoader], toSection: .main)
-//            }
-//            
-//            let descendantItems = descendantItems.filter { item in
-//                switch item {
-//                case .leaf(let statusObjectID, _):
-//                    return existObjectIDs.contains(statusObjectID)
-//                default:
-//                    return true
-//                }
-//            }
-//            newSnapshot.appendItems(descendantItems, toSection: .main)
-//            
-//            // difference for first visible item exclude .topLoader
-//            guard let difference = self.calculateReloadSnapshotDifference(navigationBar: navigationBar, tableView: tableView, oldSnapshot: oldSnapshot, newSnapshot: newSnapshot) else {
-//                diffableDataSource.apply(newSnapshot)
-//                return
-//            }
-//
-//            // additional margin for .topLoader
-//            let oldTopMargin: CGFloat = {
-//                let marginHeight = TimelineTopLoaderTableViewCell.cellHeight
-//                if oldSnapshot.itemIdentifiers.contains(.topLoader) {
-//                    return marginHeight
-//                }
-//                if !ancestorItems.isEmpty {
-//                    return marginHeight
-//                }
-//                
-//                return .zero
-//            }()
-//            
-//            let oldRootCell: UITableViewCell? = {
-//                guard let rootItem = rootItem else { return nil }
-//                guard let index = oldSnapshot.indexOfItem(rootItem) else { return nil }
-//                guard let cell = tableView.cellForRow(at: IndexPath(row: index, section: 0)) else { return nil }
-//                return cell
-//            }()
-//            // save height before cell reuse
-//            let oldRootCellHeight = oldRootCell?.frame.height
-//            
-//            diffableDataSource.reloadData(snapshot: newSnapshot) {
-//                guard let _ = rootItem else {
-//                    return
-//                }
-//                if let oldRootCellHeight = oldRootCellHeight {
-//                    // set bottom inset. Make root item pin to top (with margin).
-//                    let bottomSpacing = tableView.safeAreaLayoutGuide.layoutFrame.height - oldRootCellHeight - oldTopMargin
-//                    tableView.contentInset.bottom = max(0, bottomSpacing)
-//                }
-//
-//                // set scroll position
-//                tableView.scrollToRow(at: difference.targetIndexPath, at: .top, animated: false)
-//                let contentOffsetY: CGFloat = {
-//                    var offset: CGFloat = tableView.contentOffset.y - difference.offset
-//                    if tableView.contentInset.bottom != 0.0 && descendantItems.isEmpty {
-//                        // needs restore top margin if bottom inset adjusted AND no descendantItems
-//                        offset += oldTopMargin
-//                    }
-//                    return offset
-//                }()
-//                tableView.setContentOffset(CGPoint(x: 0, y: contentOffsetY), animated: false)
-//            }
-//        }
-//        .store(in: &disposeBag)
     }
     
 }
@@ -379,7 +255,13 @@ extension ThreadViewModel {
         let sourceIndexPath = IndexPath(row: index, section: 0)
 
         let rectForSourceItemCell = tableView.rectForRow(at: sourceIndexPath)
-        let sourceDistanceToTableViewTopEdge = tableView.convert(rectForSourceItemCell, to: nil).origin.y - tableView.safeAreaInsets.top
+        let sourceDistanceToTableViewTopEdge: CGFloat = {
+            if tableView.window != nil {
+                return tableView.convert(rectForSourceItemCell, to: nil).origin.y - tableView.safeAreaInsets.top
+            } else {
+                return rectForSourceItemCell.origin.y - tableView.contentOffset.y - tableView.safeAreaInsets.top
+            }
+        }()
 
         guard sourceIndexPath.section < oldSnapshot.numberOfSections,
               sourceIndexPath.row < oldSnapshot.numberOfItems(inSection: oldSnapshot.sectionIdentifiers[sourceIndexPath.section])
@@ -403,33 +285,3 @@ extension ThreadViewModel {
         )
     }
 }
-
-//extension ThreadViewModel {
-//    private func updateDeletedStatus(for items: [Item]) {
-//        let parentManagedObjectContext = context.managedObjectContext
-//        let managedObjectContext = NSManagedObjectContext(concurrencyType: .privateQueueConcurrencyType)
-//        managedObjectContext.parent = parentManagedObjectContext
-//        managedObjectContext.perform {
-//            var statusIDs: [Status.ID] = []
-//            for item in items {
-//                switch item {
-//                case .root(let objectID, _):
-//                    guard let status = managedObjectContext.object(with: objectID) as? Status else { continue }
-//                    statusIDs.append(status.id)
-//                case .reply(let objectID, _):
-//                    guard let status = managedObjectContext.object(with: objectID) as? Status else { continue }
-//                    statusIDs.append(status.id)
-//                case .leaf(let objectID, _):
-//                    guard let status = managedObjectContext.object(with: objectID) as? Status else { continue }
-//                    statusIDs.append(status.id)
-//                default:
-//                    continue
-//                }
-//            }
-//            DispatchQueue.main.async { [weak self] in
-//                guard let self = self else { return }
-//                self.existStatusFetchedResultsController.statusIDs.value = statusIDs
-//            }
-//        }
-//    }
-//}