Merge pull request #8 from tootsuite/feature/MiddleLoader

feat: add MiddleLoader

Mastodon.xcodeproj/project.pbxproj

@@ -11,6 +11,10 @@
 		2D04F42525C255B9003F936F /* APIService+PublicTimeline.swift in Sources */ = {isa = PBXBuildFile; fileRef = 2D04F42425C255B9003F936F /* APIService+PublicTimeline.swift */; };
 		2D152A8C25C295CC009AA50C /* TimelinePostView.swift in Sources */ = {isa = PBXBuildFile; fileRef = 2D152A8B25C295CC009AA50C /* TimelinePostView.swift */; };
 		2D152A9225C2980C009AA50C /* UIFont.swift in Sources */ = {isa = PBXBuildFile; fileRef = 2D152A9125C2980C009AA50C /* UIFont.swift */; };
+		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 */; };
@@ -155,6 +159,10 @@
 		2D04F42425C255B9003F936F /* APIService+PublicTimeline.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = "APIService+PublicTimeline.swift"; sourceTree = "<group>"; };
 		2D152A8B25C295CC009AA50C /* TimelinePostView.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = TimelinePostView.swift; sourceTree = "<group>"; };
 		2D152A9125C2980C009AA50C /* UIFont.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = UIFont.swift; sourceTree = "<group>"; };
+		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>"; };
@@ -365,6 +373,7 @@
 			isa = PBXGroup;
 			children = (
 				2D69CFF325CA9E2200C3A1B2 /* LoadMoreConfigurableTableViewContainer.swift */,
+				2D38F1C525CD37F400561493 /* ContentOffsetAdjustableTimelineViewControllerDelegate.swift */,
 			);
 			path = Protocol;
 			sourceTree = "<group>";
@@ -375,6 +384,7 @@
 				2D76316425C14BD100929FB9 /* PublicTimelineViewController.swift */,
 				2D76317C25C14DF400929FB9 /* PublicTimelineViewController+StatusProvider.swift */,
 				2D76316A25C14D4C00929FB9 /* PublicTimelineViewModel.swift */,
+				2D32EAD925CBCC3300C9ED86 /* PublicTimelineViewModel+LoadMiddleState.swift */,
 				2D76318225C14E8F00929FB9 /* PublicTimelineViewModel+Diffable.swift */,
 				2D45E5BE25C9549700A6D639 /* PublicTimelineViewModel+State.swift */,
 			);
@@ -423,6 +433,7 @@
 				2D7631A725C1535600929FB9 /* TimelinePostTableViewCell.swift */,
 				2DA7D04325CA52B200804E11 /* TimelineLoaderTableViewCell.swift */,
 				2DA7D04925CA52CB00804E11 /* TimelineBottomLoaderTableViewCell.swift */,
+				2D32EAAB25CB96DC00C9ED86 /* TimelineMiddleLoaderTableViewCell.swift */,
 			);
 			path = TableviewCell;
 			sourceTree = "<group>";
@@ -698,6 +709,7 @@
 				DB45FAB525CA5485005A8AC7 /* UIAlertController.swift */,
 				2D42FF8E25C8228A004A627A /* UIButton.swift */,
 				DB45FAD625CA6C76005A8AC7 /* UIBarButtonItem.swift */,
+				2D32EAB925CB9B0500C9ED86 /* UIView.swift */,
 			);
 			path = Extension;
 			sourceTree = "<group>";
@@ -1090,8 +1102,10 @@
 				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 */,
 				DB98334725C8056600AD9700 /* AuthenticationViewModel.swift in Sources */,
 				DB0140CF25C42AEE00F9F3CF /* OSLog.swift in Sources */,
 				2D76319F25C1521200929FB9 /* TimelineSection.swift in Sources */,
@@ -1101,6 +1115,7 @@
 				DB98338725C945ED00AD9700 /* Strings.swift in Sources */,
 				DB45FAB625CA5485005A8AC7 /* UIAlertController.swift in Sources */,
 				DB98336B25C9420100AD9700 /* APIService+App.swift in Sources */,
+				2D32EADA25CBCC3300C9ED86 /* PublicTimelineViewModel+LoadMiddleState.swift in Sources */,
 				DB8AF54425C13647002E6C99 /* SceneCoordinator.swift in Sources */,
 				DB45FAF925CA80A2005A8AC7 /* APIService+CoreData+MastodonAuthentication.swift in Sources */,
 				2D04F42525C255B9003F936F /* APIService+PublicTimeline.swift in Sources */,
@@ -1117,6 +1132,7 @@
 				DB01409625C40B6700F9F3CF /* AuthenticationViewController.swift in Sources */,
 				DB427DD825BAA00100D1B89D /* SceneDelegate.swift in Sources */,
 				DB0140AE25C40C7300F9F3CF /* MastodonPinBasedAuthenticationViewModel.swift in Sources */,
+				2D32EAAC25CB96DC00C9ED86 /* TimelineMiddleLoaderTableViewCell.swift in Sources */,
 				DB45FB0F25CA87D0005A8AC7 /* AuthenticationService.swift in Sources */,
 			);
 			runOnlyForDeploymentPostprocessing = 0;

Mastodon/Diffiable/Item/Item.swift

@@ -15,6 +15,8 @@ enum Item {
     // normal list
     case toot(objectID: NSManagedObjectID)
 
+    // loader
+    case middleLoader(tootID: String)
     case bottomLoader
 }
 
@@ -25,6 +27,8 @@ extension Item: Equatable {
             return objectIDLeft == objectIDRight
         case (.bottomLoader, .bottomLoader):
             return true
+        case (.middleLoader(let upperLeft), .middleLoader(let upperRight)):
+            return upperLeft == upperRight
         default:
             return false
         }
@@ -36,6 +40,9 @@ extension Item: Hashable {
         switch self {
         case .toot(let objectID):
             hasher.combine(objectID)
+        case .middleLoader(let upper):
+            hasher.combine(String(describing: Item.middleLoader.self))
+            hasher.combine(upper)
         case .bottomLoader:
             hasher.combine(String(describing: Item.bottomLoader.self))
         }

Mastodon/Diffiable/Section/TimelineSection.swift

@@ -21,9 +21,10 @@ extension TimelineSection {
         dependency: NeedsDependency,
         managedObjectContext: NSManagedObjectContext,
         timestampUpdatePublisher: AnyPublisher<Date, Never>,
-        timelinePostTableViewCellDelegate: TimelinePostTableViewCellDelegate
+        timelinePostTableViewCellDelegate: TimelinePostTableViewCellDelegate,
+        timelineMiddleLoaderTableViewCellDelegate: TimelineMiddleLoaderTableViewCellDelegate?
     ) -> UITableViewDiffableDataSource<TimelineSection, Item> {
-        UITableViewDiffableDataSource(tableView: tableView) { [weak timelinePostTableViewCellDelegate] tableView, indexPath, item -> UITableViewCell? in
+        UITableViewDiffableDataSource(tableView: tableView) { [weak timelinePostTableViewCellDelegate, weak timelineMiddleLoaderTableViewCellDelegate] tableView, indexPath, item -> UITableViewCell? in
             guard let timelinePostTableViewCellDelegate = timelinePostTableViewCellDelegate else { return UITableViewCell() }
 
             switch item {
@@ -33,10 +34,15 @@ extension TimelineSection {
                 // configure cell
                 managedObjectContext.performAndWait {
                     let toot = managedObjectContext.object(with: objectID) as! Toot
-                    TimelineSection.configure(cell: cell,timestampUpdatePublisher: timestampUpdatePublisher, toot: toot)
+                    TimelineSection.configure(cell: cell, timestampUpdatePublisher: timestampUpdatePublisher, toot: toot)
                 }
                 cell.delegate = timelinePostTableViewCellDelegate
                 return cell
+            case .middleLoader(let upperTimelineTootID):
+                let cell = tableView.dequeueReusableCell(withIdentifier: String(describing: TimelineMiddleLoaderTableViewCell.self), for: indexPath) as! TimelineMiddleLoaderTableViewCell
+                cell.delegate = timelineMiddleLoaderTableViewCellDelegate
+                timelineMiddleLoaderTableViewCellDelegate?.configure(cell: cell, upperTimelineTootID: upperTimelineTootID)
+                return cell
             case .bottomLoader:
                 let cell = tableView.dequeueReusableCell(withIdentifier: String(describing: TimelineBottomLoaderTableViewCell.self), for: indexPath) as! TimelineBottomLoaderTableViewCell
                 cell.activityIndicatorView.startAnimating()

Mastodon/Extension/UIView.swift

@@ -0,0 +1,26 @@
+//
+//  UIView.swift
+//  Mastodon
+//
+//  Created by sxiaojian on 2021/2/4.
+//
+
+import UIKit
+
+extension UIView {
+    
+    static var separatorLine: UIView {
+        let line = UIView()
+        line.backgroundColor = .separator
+        return line
+    }
+    
+    static func separatorLineHeight(of view: UIView) -> CGFloat {
+        return 1.0 / view.traitCollection.displayScale
+    }
+    
+    static var floatyButtonBottomMargin: CGFloat {
+        return 16
+    }
+    
+}

Mastodon/Generated/Assets.swift

@@ -22,6 +22,9 @@ internal typealias AssetImageTypeAlias = ImageAsset.Image
 // swiftlint:disable identifier_name line_length nesting type_body_length type_name
 internal enum Asset {
   internal static let accentColor = ColorAsset(name: "AccentColor")
+  internal enum Arrows {
+    internal static let arrowTriangle2Circlepath = ImageAsset(name: "Arrows/arrow.triangle.2.circlepath")
+  }
   internal enum Colors {
     internal static let tootDark = ColorAsset(name: "Colors/Toot.Dark")
     internal static let tootGray = ColorAsset(name: "Colors/Toot.Gray")

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/Resources/Assets.xcassets/Arrows/Contents.json

@@ -0,0 +1,9 @@
+{
+  "info" : {
+    "author" : "xcode",
+    "version" : 1
+  },
+  "properties" : {
+    "provides-namespace" : true
+  }
+}

Mastodon/Resources/Assets.xcassets/Arrows/arrow.triangle.2.circlepath.imageset/Contents.json

@@ -0,0 +1,21 @@
+{
+  "images" : [
+    {
+      "filename" : "arrow.triangle.2.circlepath.pdf",
+      "idiom" : "universal",
+      "scale" : "1x"
+    },
+    {
+      "idiom" : "universal",
+      "scale" : "2x"
+    },
+    {
+      "idiom" : "universal",
+      "scale" : "3x"
+    }
+  ],
+  "info" : {
+    "author" : "xcode",
+    "version" : 1
+  }
+}

Mastodon/Resources/Assets.xcassets/Arrows/arrow.triangle.2.circlepath.imageset/arrow.triangle.2.circlepath.pdf

@@ -0,0 +1,193 @@
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Mastodon/Scene/PublicTimeline/PublicTimelineViewController.swift

@@ -7,6 +7,7 @@
 
 import AVKit
 import Combine
+import CoreData
 import CoreDataStack
 import GameplayKit
 import os.log
@@ -24,6 +25,7 @@ final class PublicTimelineViewController: UIViewController, NeedsDependency, Tim
     lazy var tableView: UITableView = {
         let tableView = UITableView()
         tableView.register(TimelinePostTableViewCell.self, forCellReuseIdentifier: String(describing: TimelinePostTableViewCell.self))
+        tableView.register(TimelineMiddleLoaderTableViewCell.self, forCellReuseIdentifier: String(describing: TimelineMiddleLoaderTableViewCell.self))
         tableView.register(TimelineBottomLoaderTableViewCell.self, forCellReuseIdentifier: String(describing: TimelineBottomLoaderTableViewCell.self))
         tableView.rowHeight = UITableView.automaticDimension
         tableView.separatorStyle = .none
@@ -68,11 +70,13 @@ extension PublicTimelineViewController {
         ])
         
         viewModel.tableView = tableView
+        viewModel.contentOffsetAdjustableTimelineViewControllerDelegate = self
         tableView.delegate = self
         viewModel.setupDiffableDataSource(
             for: tableView,
             dependency: self,
-            timelinePostTableViewCellDelegate: self
+            timelinePostTableViewCellDelegate: self,
+            timelineMiddleLoaderTableViewCellDelegate: self
         )
     }
 
@@ -119,6 +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
@@ -127,3 +139,66 @@ extension PublicTimelineViewController: LoadMoreConfigurableTableViewContainer {
     var loadMoreConfigurableTableView: UITableView { return tableView }
     var loadMoreConfigurableStateMachine: GKStateMachine { return viewModel.stateMachine }
 }
+
+// MARK: - TimelineMiddleLoaderTableViewCellDelegate
+extension PublicTimelineViewController: TimelineMiddleLoaderTableViewCellDelegate {
+    
+    func configure(cell: TimelineMiddleLoaderTableViewCell, upperTimelineTootID: String) {
+        viewModel.loadMiddleSateMachineList
+            .receive(on: DispatchQueue.main)
+            .sink { [weak self] ids in
+                guard let _ = self else { return }
+                if let stateMachine = ids[upperTimelineTootID] {
+                    guard let state = stateMachine.currentState else {
+                        assertionFailure()
+                        return
+                    }
+
+                    // make success state same as loading due to snapshot updating delay
+                    let isLoading = state is PublicTimelineViewModel.LoadMiddleState.Loading || state is PublicTimelineViewModel.LoadMiddleState.Success
+                    cell.loadMoreButton.isHidden = isLoading
+                    if isLoading {
+                        cell.activityIndicatorView.startAnimating()
+                    } else {
+                        cell.activityIndicatorView.stopAnimating()
+                    }
+                } else {
+                    cell.loadMoreButton.isHidden = false
+                    cell.activityIndicatorView.stopAnimating()
+                }
+            }
+            .store(in: &cell.disposeBag)
+        
+        var dict = viewModel.loadMiddleSateMachineList.value
+        if let _ = dict[upperTimelineTootID] {
+            // do nothing
+        } else {
+            let stateMachine = GKStateMachine(states: [
+                PublicTimelineViewModel.LoadMiddleState.Initial(viewModel: viewModel, upperTimelineTootID: upperTimelineTootID),
+                PublicTimelineViewModel.LoadMiddleState.Loading(viewModel: viewModel, upperTimelineTootID: upperTimelineTootID),
+                PublicTimelineViewModel.LoadMiddleState.Fail(viewModel: viewModel, upperTimelineTootID: upperTimelineTootID),
+                PublicTimelineViewModel.LoadMiddleState.Success(viewModel: viewModel, upperTimelineTootID: upperTimelineTootID),
+            ])
+            stateMachine.enter(PublicTimelineViewModel.LoadMiddleState.Initial.self)
+            dict[upperTimelineTootID] = stateMachine
+            viewModel.loadMiddleSateMachineList.value = dict
+        }
+    }
+    
+    func timelineMiddleLoaderTableViewCell(_ cell: TimelineMiddleLoaderTableViewCell, loadMoreButtonDidPressed button: UIButton) {
+        guard let diffableDataSource = viewModel.diffableDataSource else { return }
+        guard let indexPath = tableView.indexPath(for: cell) else { return }
+        guard let item = diffableDataSource.itemIdentifier(for: indexPath) else { return }
+
+        switch item {
+        case .middleLoader(let upper):
+            guard let stateMachine = viewModel.loadMiddleSateMachineList.value[upper] else {
+                assertionFailure()
+                return
+            }
+            stateMachine.enter(PublicTimelineViewModel.LoadMiddleState.Loading.self)
+        default:
+            assertionFailure()
+        }
+    }
+}

Mastodon/Scene/PublicTimeline/PublicTimelineViewModel+Diffable.swift

@@ -14,7 +14,8 @@ extension PublicTimelineViewModel {
     func setupDiffableDataSource(
         for tableView: UITableView,
         dependency: NeedsDependency,
-        timelinePostTableViewCellDelegate: TimelinePostTableViewCellDelegate
+        timelinePostTableViewCellDelegate: TimelinePostTableViewCellDelegate,
+        timelineMiddleLoaderTableViewCellDelegate: TimelineMiddleLoaderTableViewCellDelegate
     ) {
         let timestampUpdatePublisher = Timer.publish(every: 1.0, on: .main, in: .common)
             .autoconnect()
@@ -26,7 +27,8 @@ extension PublicTimelineViewModel {
             dependency: dependency,
             managedObjectContext: fetchedResultsController.managedObjectContext,
             timestampUpdatePublisher: timestampUpdatePublisher,
-            timelinePostTableViewCellDelegate: timelinePostTableViewCellDelegate
+            timelinePostTableViewCellDelegate: timelinePostTableViewCellDelegate,
+            timelineMiddleLoaderTableViewCellDelegate: timelineMiddleLoaderTableViewCellDelegate
         )
         items.value = []
         stateMachine.enter(PublicTimelineViewModel.State.Loading.self)
@@ -42,13 +44,20 @@ extension PublicTimelineViewModel: NSFetchedResultsControllerDelegate {
         let indexes = tootIDs.value
         let toots = fetchedResultsController.fetchedObjects ?? []
         guard toots.count == indexes.count else { return }
-        let items: [Item] = toots
+        let indexTootTuples: [(Int, Toot)] = toots
             .compactMap { toot -> (Int, Toot)? in
                 guard toot.deletedAt == nil else { return nil }
                 return indexes.firstIndex(of: toot.id).map { index in (index, toot) }
             }
             .sorted { $0.0 < $1.0 }
-            .map { Item.toot(objectID: $0.1.objectID) }
+        var items = [Item]()
+        for tuple in indexTootTuples {
+            items.append(Item.toot(objectID: tuple.1.objectID))
+            if tootIDsWhichHasGap.contains(tuple.1.id) {
+                items.append(Item.middleLoader(tootID: tuple.1.id))
+            }
+        }
+
         self.items.value = items
     }
 }

Mastodon/Scene/PublicTimeline/PublicTimelineViewModel+LoadMiddleState.swift

@@ -0,0 +1,115 @@
+//
+//  PublicTimelineViewModel+LoadMiddleState.swift
+//  Mastodon
+//
+//  Created by sxiaojian on 2021/2/4.
+//
+
+import CoreData
+import CoreDataStack
+import Foundation
+import GameplayKit
+import os.log
+
+extension PublicTimelineViewModel {
+    class LoadMiddleState: GKState {
+        weak var viewModel: PublicTimelineViewModel?
+        let upperTimelineTootID: String
+        
+        init(viewModel: PublicTimelineViewModel, upperTimelineTootID: String) {
+            self.viewModel = viewModel
+            self.upperTimelineTootID = upperTimelineTootID
+        }
+        
+        override func didEnter(from previousState: GKState?) {
+            os_log("%{public}s[%{public}ld], %{public}s: enter %s, previous: %s", (#file as NSString).lastPathComponent, #line, #function, self.debugDescription, previousState.debugDescription)
+            guard let viewModel = viewModel, let stateMachine = stateMachine else { return }
+            var dict = viewModel.loadMiddleSateMachineList.value
+            dict[self.upperTimelineTootID] = stateMachine
+            viewModel.loadMiddleSateMachineList.value = dict // trigger value change
+        }
+    }
+}
+
+extension PublicTimelineViewModel.LoadMiddleState {
+    class Initial: PublicTimelineViewModel.LoadMiddleState {
+        override func isValidNextState(_ stateClass: AnyClass) -> Bool {
+            return stateClass == Loading.self
+        }
+    }
+    
+    class Loading: PublicTimelineViewModel.LoadMiddleState {
+        override func isValidNextState(_ stateClass: AnyClass) -> Bool {
+            // guard let viewModel = viewModel else { return false }
+            return stateClass == Success.self || stateClass == Fail.self
+        }
+        
+        override func didEnter(from previousState: GKState?) {
+            super.didEnter(from: previousState)
+            
+            guard let viewModel = viewModel, let stateMachine = stateMachine else { return }
+            guard let activeMastodonAuthenticationBox = viewModel.context.authenticationService.activeMastodonAuthenticationBox.value else {
+                stateMachine.enter(Fail.self)
+                return
+            }
+            viewModel.context.apiService.publicTimeline(
+                domain: activeMastodonAuthenticationBox.domain,
+                maxID: upperTimelineTootID
+            )
+            .receive(on: DispatchQueue.main)
+            .sink { completion in
+                switch completion {
+                    case .failure(let error):
+                        os_log("%{public}s[%{public}ld], %{public}s: fetch tweets failed. %s", (#file as NSString).lastPathComponent, #line, #function, error.localizedDescription)
+                        stateMachine.enter(Fail.self)
+                    case .finished:
+                        break
+                }
+            } receiveValue: { response in
+                let toots = response.value
+                let addedToots = toots.filter { !viewModel.tootIDs.value.contains($0.id) }
+                
+                guard let gapIndex = viewModel.tootIDs.value.firstIndex(of: self.upperTimelineTootID) else { return }
+                let upToots = Array(viewModel.tootIDs.value[...gapIndex])
+                let downToots = Array(viewModel.tootIDs.value[(gapIndex + 1)...])
+                
+                // construct newTootIDs
+                var newTootIDs = upToots
+                newTootIDs.append(contentsOf: addedToots.map { $0.id })
+                newTootIDs.append(contentsOf: downToots)
+                // remove old gap from viewmodel
+                if let index = viewModel.tootIDsWhichHasGap.firstIndex(of: self.upperTimelineTootID) {
+                    viewModel.tootIDsWhichHasGap.remove(at: index)
+                }
+                // add new gap from viewmodel if need
+                let intersection = toots.filter { downToots.contains($0.id) }
+                if intersection.isEmpty {
+                    addedToots.last.flatMap { viewModel.tootIDsWhichHasGap.append($0.id) }
+                }
+                
+                viewModel.tootIDs.value = newTootIDs
+                os_log("%{public}s[%{public}ld], %{public}s: load %{public}ld tweets, %{public}%ld new tweets", (#file as NSString).lastPathComponent, #line, #function, toots.count, addedToots.count)
+                if addedToots.isEmpty {
+                    stateMachine.enter(Fail.self)
+                } else {
+                    stateMachine.enter(Success.self)
+                }
+            }
+            .store(in: &viewModel.disposeBag)
+        }
+    }
+    
+    class Fail: PublicTimelineViewModel.LoadMiddleState {
+        override func isValidNextState(_ stateClass: AnyClass) -> Bool {
+            // guard let viewModel = viewModel else { return false }
+            return stateClass == Loading.self
+        }
+    }
+    
+    class Success: PublicTimelineViewModel.LoadMiddleState {
+        override func isValidNextState(_ stateClass: AnyClass) -> Bool {
+            // guard let viewModel = viewModel else { return false }
+            return false
+        }
+    }
+}

Mastodon/Scene/PublicTimeline/PublicTimelineViewModel+State.swift

@@ -68,9 +68,21 @@ extension PublicTimelineViewModel.State {
                         break
                     }
                 } receiveValue: { response in
-                    viewModel.isFetchingLatestTimeline.value = false
+                    let resposeTootIDs = response.value.compactMap { $0.id }
-                    let tootsIDs = response.value.map { $0.id }
+                    var newTootsIDs = resposeTootIDs
-                    viewModel.tootIDs.value = tootsIDs
+                    let oldTootsIDs = viewModel.tootIDs.value
+                    var hasGap = true
+                    for tootID in oldTootsIDs {
+                        if !newTootsIDs.contains(tootID) {
+                            newTootsIDs.append(tootID)
+                        } else {
+                            hasGap = false
+                        }
+                    }
+                    if hasGap && oldTootsIDs.count > 0 {
+                        resposeTootIDs.last.flatMap { viewModel.tootIDsWhichHasGap.append($0) }
+                    }
+                    viewModel.tootIDs.value = newTootsIDs
                     stateMachine.enter(Idle.self)
                 }
                 .store(in: &viewModel.disposeBag)
@@ -149,7 +161,6 @@ extension PublicTimelineViewModel.State {
                 }
                 
                 viewModel.tootIDs.value = oldTootsIDs
-                
             }
             .store(in: &viewModel.disposeBag)
         }

Mastodon/Scene/PublicTimeline/PublicTimelineViewModel.swift

@@ -20,9 +20,18 @@ class PublicTimelineViewModel: NSObject {
     // input
     let context: AppContext
     let fetchedResultsController: NSFetchedResultsController<Toot>
+    
     let isFetchingLatestTimeline = CurrentValueSubject<Bool, Never>(false)
+    
+    // middle loader
+    let loadMiddleSateMachineList = CurrentValueSubject<[String: GKStateMachine], Never>([:])
+    
     weak var tableView: UITableView?
     
+    weak var contentOffsetAdjustableTimelineViewControllerDelegate: ContentOffsetAdjustableTimelineViewControllerDelegate?
+    
+    //
+    var tootIDsWhichHasGap = [String]()
     // output
     var diffableDataSource: UITableViewDiffableDataSource<TimelineSection, Item>?
 
@@ -68,6 +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>()
@@ -81,7 +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)
         
@@ -103,11 +129,37 @@ class PublicTimelineViewModel: NSObject {
     deinit {
         os_log("%{public}s[%{public}ld], %{public}s", (#file as NSString).lastPathComponent, #line, #function)
     }
-}
-
-extension PublicTimelineViewModel {
     
-    func loadMore() -> AnyPublisher<Mastodon.Response.Content<[Mastodon.Entity.Toot]>, Error> {
+    private struct Difference<T> {
-        return context.apiService.publicTimeline(domain: "mstdn.jp")
+        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
+        )
     }
 }

Mastodon/Scene/Share/View/TableviewCell/TimelineMiddleLoaderTableViewCell.swift

@@ -0,0 +1,48 @@
+//
+//  TimelineMiddleLoaderTableViewCell.swift
+//  Mastodon
+//
+//  Created by sxiaojian on 2021/2/4.
+//
+
+import Combine
+import CoreData
+import os.log
+import UIKit
+
+protocol TimelineMiddleLoaderTableViewCellDelegate: class {
+    func configure(cell: TimelineMiddleLoaderTableViewCell, upperTimelineTootID: String)
+    func timelineMiddleLoaderTableViewCell(_ cell: TimelineMiddleLoaderTableViewCell, loadMoreButtonDidPressed button: UIButton)
+}
+
+final class TimelineMiddleLoaderTableViewCell: TimelineLoaderTableViewCell {
+    weak var delegate: TimelineMiddleLoaderTableViewCellDelegate?
+    
+    override func _init() {
+        super._init()
+        
+        backgroundColor = Asset.Colors.tootDark.color
+        
+        let separatorLine = UIView.separatorLine
+        separatorLine.translatesAutoresizingMaskIntoConstraints = false
+        contentView.addSubview(separatorLine)
+        NSLayoutConstraint.activate([
+            separatorLine.leadingAnchor.constraint(equalTo: contentView.leadingAnchor),
+            contentView.trailingAnchor.constraint(equalTo: separatorLine.trailingAnchor),
+            contentView.bottomAnchor.constraint(equalTo: separatorLine.bottomAnchor),
+            separatorLine.heightAnchor.constraint(equalToConstant: UIView.separatorLineHeight(of: separatorLine))
+        ])
+        
+        loadMoreButton.isHidden = false
+        loadMoreButton.setImage(Asset.Arrows.arrowTriangle2Circlepath.image.withRenderingMode(.alwaysTemplate), for: .normal)
+        loadMoreButton.setInsets(forContentPadding: .zero, imageTitlePadding: 4)
+        loadMoreButton.addTarget(self, action: #selector(TimelineMiddleLoaderTableViewCell.loadMoreButtonDidPressed(_:)), for: .touchUpInside)
+    }
+}
+
+extension TimelineMiddleLoaderTableViewCell {
+    @objc private func loadMoreButtonDidPressed(_ sender: UIButton) {
+        os_log("%{public}s[%{public}ld], %{public}s", (#file as NSString).lastPathComponent, #line, #function)
+        delegate?.timelineMiddleLoaderTableViewCell(self, loadMoreButtonDidPressed: sender)
+    }
+}