fix : maintain contentOffset after refresh timeline

Mastodon.xcodeproj/project.pbxproj

@@ -14,6 +14,7 @@
 		2D32EAAC25CB96DC00C9ED86 /* TimelineMiddleLoaderTableViewCell.swift in Sources */ = {isa = PBXBuildFile; fileRef = 2D32EAAB25CB96DC00C9ED86 /* TimelineMiddleLoaderTableViewCell.swift */; };
 		2D32EABA25CB9B0500C9ED86 /* UIView.swift in Sources */ = {isa = PBXBuildFile; fileRef = 2D32EAB925CB9B0500C9ED86 /* UIView.swift */; };
 		2D32EADA25CBCC3300C9ED86 /* PublicTimelineViewModel+LoadMiddleState.swift in Sources */ = {isa = PBXBuildFile; fileRef = 2D32EAD925CBCC3300C9ED86 /* PublicTimelineViewModel+LoadMiddleState.swift */; };
+		2D38F1C625CD37F400561493 /* ContentOffsetAdjustableTimelineViewControllerDelegate.swift in Sources */ = {isa = PBXBuildFile; fileRef = 2D38F1C525CD37F400561493 /* ContentOffsetAdjustableTimelineViewControllerDelegate.swift */; };
 		2D42FF6125C8177C004A627A /* ActiveLabel in Frameworks */ = {isa = PBXBuildFile; productRef = 2D42FF6025C8177C004A627A /* ActiveLabel */; };
 		2D42FF6B25C817D2004A627A /* MastodonContent.swift in Sources */ = {isa = PBXBuildFile; fileRef = 2D42FF6A25C817D2004A627A /* MastodonContent.swift */; };
 		2D42FF7E25C82218004A627A /* ActionToolBarContainer.swift in Sources */ = {isa = PBXBuildFile; fileRef = 2D42FF7D25C82218004A627A /* ActionToolBarContainer.swift */; };
@@ -161,6 +162,7 @@
 		2D32EAAB25CB96DC00C9ED86 /* TimelineMiddleLoaderTableViewCell.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = TimelineMiddleLoaderTableViewCell.swift; sourceTree = "<group>"; };
 		2D32EAB925CB9B0500C9ED86 /* UIView.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = UIView.swift; sourceTree = "<group>"; };
 		2D32EAD925CBCC3300C9ED86 /* PublicTimelineViewModel+LoadMiddleState.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = "PublicTimelineViewModel+LoadMiddleState.swift"; sourceTree = "<group>"; };
+		2D38F1C525CD37F400561493 /* ContentOffsetAdjustableTimelineViewControllerDelegate.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ContentOffsetAdjustableTimelineViewControllerDelegate.swift; sourceTree = "<group>"; };
 		2D42FF6A25C817D2004A627A /* MastodonContent.swift */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.swift; path = MastodonContent.swift; sourceTree = "<group>"; };
 		2D42FF7D25C82218004A627A /* ActionToolBarContainer.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ActionToolBarContainer.swift; sourceTree = "<group>"; };
 		2D42FF8425C8224F004A627A /* HitTestExpandedButton.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = HitTestExpandedButton.swift; sourceTree = "<group>"; };
@@ -371,6 +373,7 @@
 			isa = PBXGroup;
 			children = (
 				2D69CFF325CA9E2200C3A1B2 /* LoadMoreConfigurableTableViewContainer.swift */,
+				2D38F1C525CD37F400561493 /* ContentOffsetAdjustableTimelineViewControllerDelegate.swift */,
 			);
 			path = Protocol;
 			sourceTree = "<group>";
@@ -1099,6 +1102,7 @@
 				2D42FF7E25C82218004A627A /* ActionToolBarContainer.swift in Sources */,
 				DB0140A125C40C0600F9F3CF /* MastodonPinBasedAuthenticationViewController.swift in Sources */,
 				DB8AF55025C13703002E6C99 /* MainTabBarController.swift in Sources */,
+				2D38F1C625CD37F400561493 /* ContentOffsetAdjustableTimelineViewControllerDelegate.swift in Sources */,
 				DB8AF54525C13647002E6C99 /* NeedsDependency.swift in Sources */,
 				DB45FADD25CA6F6B005A8AC7 /* APIService+CoreData+MastodonUser.swift in Sources */,
 				2D32EABA25CB9B0500C9ED86 /* UIView.swift in Sources */,

Mastodon/Protocol/ContentOffsetAdjustableTimelineViewControllerDelegate.swift

@@ -0,0 +1,13 @@
+//
+//  ContentOffsetAdjustableTimelineViewControllerDelegate.swift
+//  Mastodon
+//
+//  Created by sxiaojian on 2021/2/5.
+//
+
+import UIKit
+
+protocol ContentOffsetAdjustableTimelineViewControllerDelegate: class {
+    func navigationBar() -> UINavigationBar?
+}
+

Mastodon/Scene/PublicTimeline/PublicTimelineViewController.swift

@@ -70,6 +70,7 @@ extension PublicTimelineViewController {
         ])
         
         viewModel.tableView = tableView
+        viewModel.contentOffsetAdjustableTimelineViewControllerDelegate = self
         tableView.delegate = self
         viewModel.setupDiffableDataSource(
             for: tableView,
@@ -122,6 +123,14 @@ extension PublicTimelineViewController: UITableViewDelegate {
         viewModel.cellFrameCache.setObject(NSValue(cgRect: frame), forKey: NSNumber(value: key))
     }
 }
+
+// MARK: - ContentOffsetAdjustableTimelineViewControllerDelegate
+extension PublicTimelineViewController: ContentOffsetAdjustableTimelineViewControllerDelegate {
+    func navigationBar() -> UINavigationBar? {
+        return navigationController?.navigationBar
+    }
+}
+
 // MARK: - LoadMoreConfigurableTableViewContainer
 extension PublicTimelineViewController: LoadMoreConfigurableTableViewContainer {
     typealias BottomLoaderTableViewCell = TimelineBottomLoaderTableViewCell

Mastodon/Scene/PublicTimeline/PublicTimelineViewModel+State.swift

@@ -68,7 +68,6 @@ extension PublicTimelineViewModel.State {
                         break
                     }
                 } receiveValue: { response in
-                    viewModel.isFetchingLatestTimeline.value = false
                     let resposeTootIDs = response.value.compactMap { $0.id }
                     var newTootsIDs = resposeTootIDs
                     let oldTootsIDs = viewModel.tootIDs.value

Mastodon/Scene/PublicTimeline/PublicTimelineViewModel.swift

@@ -27,6 +27,9 @@ class PublicTimelineViewModel: NSObject {
     let loadMiddleSateMachineList = CurrentValueSubject<[String: GKStateMachine], Never>([:])
     
     weak var tableView: UITableView?
+    
+    weak var contentOffsetAdjustableTimelineViewControllerDelegate: ContentOffsetAdjustableTimelineViewControllerDelegate?
+    
     //
     var tootIDsWhichHasGap = [String]()
     // output
@@ -74,6 +77,9 @@ class PublicTimelineViewModel: NSObject {
             .sink { [weak self] items in
                 guard let self = self else { return }
                 guard let diffableDataSource = self.diffableDataSource else { return }
+                guard let navigationBar = self.contentOffsetAdjustableTimelineViewControllerDelegate?.navigationBar() else { return }
+                guard let tableView = self.tableView else { return }
+                let oldSnapshot = diffableDataSource.snapshot()
                 os_log("%{public}s[%{public}ld], %{public}s: items did change", (#file as NSString).lastPathComponent, #line, #function)
 
                 var snapshot = NSDiffableDataSourceSnapshot<TimelineSection, Item>()
@@ -87,7 +93,21 @@ class PublicTimelineViewModel: NSObject {
                         break
                     }
                 }
-                diffableDataSource.apply(snapshot, animatingDifferences: !items.isEmpty)
+
+                DispatchQueue.main.async {
+
+                    guard let difference = self.calculateReloadSnapshotDifference(navigationBar: navigationBar, tableView: tableView, oldSnapshot: oldSnapshot, newSnapshot: snapshot) else {
+                        diffableDataSource.apply(snapshot)
+                        self.isFetchingLatestTimeline.value = false
+                        return
+                    }
+                    
+                    diffableDataSource.apply(snapshot, animatingDifferences: false) {
+                        tableView.scrollToRow(at: difference.targetIndexPath, at: .top, animated: false)
+                        tableView.contentOffset.y = tableView.contentOffset.y - difference.offset
+                        self.isFetchingLatestTimeline.value = false
+                    }
+                }
             }
             .store(in: &disposeBag)
         
@@ -109,4 +129,37 @@ class PublicTimelineViewModel: NSObject {
     deinit {
         os_log("%{public}s[%{public}ld], %{public}s", (#file as NSString).lastPathComponent, #line, #function)
     }
+    
+    private struct Difference<T> {
+        let item: T
+        let sourceIndexPath: IndexPath
+        let targetIndexPath: IndexPath
+        let offset: CGFloat
+    }
+    
+    private func calculateReloadSnapshotDifference<T: Hashable>(
+        navigationBar: UINavigationBar,
+        tableView: UITableView,
+        oldSnapshot: NSDiffableDataSourceSnapshot<TimelineSection, T>,
+        newSnapshot: NSDiffableDataSourceSnapshot<TimelineSection, T>
+    ) -> Difference<T>? {
+        guard oldSnapshot.numberOfItems != 0 else { return nil }
+        
+        // old snapshot not empty. set source index path to first item if not match
+        let sourceIndexPath = UIViewController.topVisibleTableViewCellIndexPath(in: tableView, navigationBar: navigationBar) ?? IndexPath(row: 0, section: 0)
+        
+        guard sourceIndexPath.row < oldSnapshot.itemIdentifiers(inSection: .main).count else { return nil }
+        
+        let timelineItem = oldSnapshot.itemIdentifiers(inSection: .main)[sourceIndexPath.row]
+        guard let itemIndex = newSnapshot.itemIdentifiers(inSection: .main).firstIndex(of: timelineItem) else { return nil }
+        let targetIndexPath = IndexPath(row: itemIndex, section: 0)
+        
+        let offset = UIViewController.tableViewCellOriginOffsetToWindowTop(in: tableView, at: sourceIndexPath, navigationBar: navigationBar)
+        return Difference(
+            item: timelineItem,
+            sourceIndexPath: sourceIndexPath,
+            targetIndexPath: targetIndexPath,
+            offset: offset
+        )
+    }
 }