Remove old and debug code

Pachyderm/Sources/Pachyderm/Request/RequestRange.swift

@@ -11,7 +11,9 @@ import Foundation
 public enum RequestRange {
     case `default`
     case count(Int)
+    /// Chronologically immediately before the given ID
     case before(id: String, count: Int?)
+    /// Chronologically immediately after the given ID
     case after(id: String, count: Int?)
 }
 

Tusker.xcodeproj/project.pbxproj

@@ -270,6 +270,7 @@
 		D6C99FCB24FADC91005C74D3 /* MainComposeTextView.swift in Sources */ = {isa = PBXBuildFile; fileRef = D6C99FCA24FADC91005C74D3 /* MainComposeTextView.swift */; };
 		D6CA6A92249FAD8900AD45C1 /* AudioSessionHelper.swift in Sources */ = {isa = PBXBuildFile; fileRef = D6CA6A91249FAD8900AD45C1 /* AudioSessionHelper.swift */; };
 		D6CA6A94249FADE700AD45C1 /* GalleryPlayerViewController.swift in Sources */ = {isa = PBXBuildFile; fileRef = D6CA6A93249FADE700AD45C1 /* GalleryPlayerViewController.swift */; };
+		D6D12B2F2925D66500D528E1 /* TimelineGapCollectionViewCell.swift in Sources */ = {isa = PBXBuildFile; fileRef = D6D12B2E2925D66500D528E1 /* TimelineGapCollectionViewCell.swift */; };
 		D6D3F4C424FDB6B700EC4A6A /* View+ConditionalModifier.swift in Sources */ = {isa = PBXBuildFile; fileRef = D6D3F4C324FDB6B700EC4A6A /* View+ConditionalModifier.swift */; };
 		D6D3FDE224F46A8D00FF50A5 /* ComposeUIState.swift in Sources */ = {isa = PBXBuildFile; fileRef = D6D3FDE124F46A8D00FF50A5 /* ComposeUIState.swift */; };
 		D6D4CC94250DB86A00FCCF8D /* ComposeTests.swift in Sources */ = {isa = PBXBuildFile; fileRef = D6D4CC93250DB86A00FCCF8D /* ComposeTests.swift */; };
@@ -632,6 +633,7 @@
 		D6C99FCA24FADC91005C74D3 /* MainComposeTextView.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = MainComposeTextView.swift; sourceTree = "<group>"; };
 		D6CA6A91249FAD8900AD45C1 /* AudioSessionHelper.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = AudioSessionHelper.swift; sourceTree = "<group>"; };
 		D6CA6A93249FADE700AD45C1 /* GalleryPlayerViewController.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = GalleryPlayerViewController.swift; sourceTree = "<group>"; };
+		D6D12B2E2925D66500D528E1 /* TimelineGapCollectionViewCell.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = TimelineGapCollectionViewCell.swift; sourceTree = "<group>"; };
 		D6D3F4C324FDB6B700EC4A6A /* View+ConditionalModifier.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = "View+ConditionalModifier.swift"; sourceTree = "<group>"; };
 		D6D3FDE124F46A8D00FF50A5 /* ComposeUIState.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ComposeUIState.swift; sourceTree = "<group>"; };
 		D6D4CC93250DB86A00FCCF8D /* ComposeTests.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ComposeTests.swift; sourceTree = "<group>"; };
@@ -924,6 +926,7 @@
 				D6BC9DD6232D7811002CA326 /* TimelinesPageViewController.swift */,
 				D66C900A28DAB7FD00217BF2 /* TimelineViewController.swift */,
 				D6ADB6E728E8C878009924AB /* PublicTimelineDescriptionCollectionViewCell.swift */,
+				D6D12B2E2925D66500D528E1 /* TimelineGapCollectionViewCell.swift */,
 			);
 			path = Timeline;
 			sourceTree = "<group>";
@@ -1832,6 +1835,7 @@
 				D6CA6A94249FADE700AD45C1 /* GalleryPlayerViewController.swift in Sources */,
 				D6A3BC802321B7E600FD64D5 /* FollowNotificationGroupTableViewCell.swift in Sources */,
 				D627944D23A9A03D00D38C68 /* ListTimelineViewController.swift in Sources */,
+				D6D12B2F2925D66500D528E1 /* TimelineGapCollectionViewCell.swift in Sources */,
 				D6945C3823AC739F005C403C /* InstanceTimelineViewController.swift in Sources */,
 				D625E4822588262A0074BB2B /* DraggableTableViewCell.swift in Sources */,
 				D68E525B24A3D77E0054355A /* TuskerRootViewController.swift in Sources */,

Tusker.xcodeproj/xcshareddata/xcschemes/Tusker.xcscheme

@@ -99,6 +99,11 @@
             value = "1"
             isEnabled = "NO">
          </EnvironmentVariable>
+         <EnvironmentVariable
+            key = "CG_CONTEXT_SHOW_BACKTRACE"
+            value = ""
+            isEnabled = "NO">
+         </EnvironmentVariable>
          <EnvironmentVariable
             key = "CG_NUMERICS_SHOW_BACKTRACE"
             value = ""

Tusker/Screens/Explore/TrendingStatusesViewController.swift

@@ -97,7 +97,7 @@ class TrendingStatusesViewController: UIViewController {
         do {
             statuses = try await mastodonController.run(Client.getTrendingStatuses()).0
         } catch {
-            var snapshot = NSDiffableDataSourceSnapshot<Section, Item>()
+            let snapshot = NSDiffableDataSourceSnapshot<Section, Item>()
             await dataSource.apply(snapshot)
             let config = ToastConfiguration(from: error, with: "Loading Trending Posts", in: self) { toast in
                 toast.dismissToast(animated: true)

Tusker/Screens/Profile/ProfileStatusesViewController.swift

@@ -25,8 +25,8 @@ class ProfileStatusesViewController: UIViewController, TimelineLikeCollectionVie
     private var older: RequestRange?
     private var cancellables = Set<AnyCancellable>()
     
-    var collectionView: UICollectionView {
+    var collectionView: UICollectionView! {
-        view as! UICollectionView
+        view as? UICollectionView
     }
     private(set) var dataSource: UICollectionViewDiffableDataSource<Section, Item>!
     private(set) var headerCell: ProfileHeaderCollectionViewCell?
@@ -157,7 +157,7 @@ class ProfileStatusesViewController: UIViewController, TimelineLikeCollectionVie
         }
         
         Task {
-            if case .notLoadedInitial = await controller.state {
+            if case .notLoadedInitial = controller.state {
                 await load()
             }
         }

Tusker/Screens/Timeline/TimelineGapCollectionViewCell.swift

@@ -0,0 +1,164 @@
+//
+//  TimelineGapCollectionViewCell.swift
+//  Tusker
+//
+//  Created by Shadowfacts on 11/16/22.
+//  Copyright © 2022 Shadowfacts. All rights reserved.
+//
+
+import UIKit
+
+class TimelineGapCollectionViewCell: UICollectionViewCell {
+    
+    private(set) var direction = TimelineGapDirection.above
+    
+    private let indicator = UIActivityIndicatorView(style: .medium)
+    private let chevronView = AnimatingChevronView()
+    
+    override var isHighlighted: Bool {
+        didSet {
+            backgroundColor = isHighlighted ? .systemFill : .systemGroupedBackground
+        }
+    }
+    
+    var showsIndicator: Bool = false {
+        didSet {
+            if showsIndicator {
+                indicator.isHidden = false
+                indicator.startAnimating()
+            } else {
+                indicator.isHidden = true
+                indicator.stopAnimating()
+            }
+        }
+    }
+    
+    override init(frame: CGRect) {
+        super.init(frame: frame)
+        
+        backgroundColor = .systemGroupedBackground
+        
+        indicator.isHidden = true
+        indicator.color = .tintColor
+        
+        let label = UILabel()
+        label.text = "Load more"
+        label.font = .preferredFont(forTextStyle: .headline)
+        label.textColor = .tintColor
+        
+        chevronView.update(direction: .above)
+        
+        let stack = UIStackView(arrangedSubviews: [
+            label,
+            chevronView,
+        ])
+        stack.axis = .horizontal
+        stack.spacing = 8
+        stack.translatesAutoresizingMaskIntoConstraints = false
+        contentView.addSubview(stack)
+        
+        indicator.translatesAutoresizingMaskIntoConstraints = false
+        contentView.addSubview(indicator)
+        
+        NSLayoutConstraint.activate([
+            stack.centerXAnchor.constraint(equalTo: contentView.centerXAnchor),
+            stack.centerYAnchor.constraint(equalTo: contentView.centerYAnchor),
+            indicator.trailingAnchor.constraint(equalTo: stack.leadingAnchor, constant: -8),
+            indicator.centerYAnchor.constraint(equalTo: contentView.centerYAnchor),
+            contentView.heightAnchor.constraint(equalToConstant: 44),
+        ])
+    }
+    
+    required init?(coder: NSCoder) {
+        fatalError("init(coder:) has not been implemented")
+    }
+    
+    override func didMoveToSuperview() {
+        super.didMoveToSuperview()
+        update()
+    }
+    
+    func update() {
+        guard let superview = superview as? UICollectionView else {
+            return
+        }
+        let yInParent = frame.minY - superview.contentOffset.y
+        let centerInParent = yInParent + bounds.height / 2
+        let centerOfParent = superview.frame.height / 2
+        
+        let newDirection: TimelineGapDirection
+        if centerInParent > centerOfParent {
+            newDirection = .above
+        } else {
+            newDirection = .below
+        }
+        
+        if newDirection != direction {
+            direction = newDirection
+            chevronView.update(direction: newDirection)
+        }
+    }
+    
+}
+
+private class AnimatingChevronView: UIView {
+    
+    override class var layerClass: AnyClass { CAShapeLayer.self }
+    var shapeLayer: CAShapeLayer { layer as! CAShapeLayer }
+    
+    override var intrinsicContentSize: CGSize { CGSize(width: 20, height: 25) }
+    
+    var animator: UIViewPropertyAnimator?
+    
+    let upPath: CGPath = {
+        let path = UIBezierPath()
+        let width: CGFloat = 20
+        let height: CGFloat = 25
+        path.move(to: CGPoint(x: 0, y: height / 2))
+        path.addLine(to: CGPoint(x: width / 2, y: height / 5))
+        path.addLine(to: CGPoint(x: width, y: height / 2))
+        return path.cgPath
+    }()
+    let downPath: CGPath = {
+        let path = UIBezierPath()
+        let width: CGFloat = 20
+        let height: CGFloat = 25
+        path.move(to: CGPoint(x: 0, y: height / 2))
+        path.addLine(to: CGPoint(x: width / 2, y: 4 * height / 5))
+        path.addLine(to: CGPoint(x: width, y: height / 2))
+        return path.cgPath
+    }()
+    
+    init() {
+        super.init(frame: .zero)
+        shapeLayer.fillColor = nil
+        shapeLayer.strokeColor = tintColor.cgColor
+        shapeLayer.lineCap = .round
+        shapeLayer.lineJoin = .round
+        shapeLayer.lineWidth = 3
+    }
+    
+    required init?(coder: NSCoder) {
+        fatalError("init(coder:) has not been implemented")
+    }
+    
+    override func tintColorDidChange() {
+        super.tintColorDidChange()
+        shapeLayer.strokeColor = tintColor.cgColor
+    }
+    
+    func update(direction: TimelineGapDirection) {
+        if animator?.isRunning == true {
+            animator!.stopAnimation(true)
+        }
+        animator = UIViewPropertyAnimator(duration: 0.1, curve: .linear) {
+            if direction == .below {
+                self.shapeLayer.path = self.upPath
+            } else {
+                self.shapeLayer.path = self.downPath
+            }
+        }
+        animator!.startAnimation()
+    }
+    
+}

Tusker/Screens/Timeline/TimelineViewController.swift

@@ -16,16 +16,14 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
     
     private(set) var controller: TimelineLikeController<TimelineItem>!
     let confirmLoadMore = PassthroughSubject<Void, Never>()
-    private var newer: RequestRange?
-    private var older: RequestRange?
     // stored separately because i don't want to query the snapshot every time the user scrolls
     private var isShowingTimelineDescription = false
     
-    var collectionView: UICollectionView {
+    private(set) var collectionView: UICollectionView!
-        view as! UICollectionView
-    }
     private(set) var dataSource: UICollectionViewDiffableDataSource<Section, Item>!
     
+    private var contentOffsetObservation: NSKeyValueObservation?
+    
     init(for timeline: Timeline, mastodonController: MastodonController!) {
         self.timeline = timeline
         self.mastodonController = mastodonController
@@ -42,7 +40,9 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
         fatalError("init(coder:) has not been implemented")
     }
     
-    override func loadView() {
+    override func viewDidLoad() {
+        super.viewDidLoad()
+        
         var config = UICollectionLayoutListConfiguration(appearance: .plain)
         config.leadingSwipeActionsConfigurationProvider = { [unowned self] in
             (collectionView.cellForItem(at: $0) as? TimelineStatusCollectionViewCell)?.leadingSwipeActions()
@@ -58,17 +58,24 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
             if item.hideSeparators {
                 config.topSeparatorVisibility = .hidden
                 config.bottomSeparatorVisibility = .hidden
-            }
+            } else {
-            if case .status(_, _) = item {
                 config.topSeparatorInsets = TimelineStatusCollectionViewCell.separatorInsets
                 config.bottomSeparatorInsets = TimelineStatusCollectionViewCell.separatorInsets
             }
             return config
         }
         let layout = UICollectionViewCompositionalLayout.list(using: config)
-        view = UICollectionView(frame: .zero, collectionViewLayout: layout)
+        collectionView = UICollectionView(frame: .zero, collectionViewLayout: layout)
         collectionView.delegate = self
         collectionView.dragDelegate = self
+        collectionView.translatesAutoresizingMaskIntoConstraints = false
+        view.addSubview(collectionView)
+        NSLayoutConstraint.activate([
+            collectionView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
+            collectionView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
+            collectionView.topAnchor.constraint(equalTo: view.topAnchor),
+            collectionView.bottomAnchor.constraint(equalTo: view.bottomAnchor),
+        ])
         
         registerTimelineLikeCells()
         collectionView.register(PublicTimelineDescriptionCollectionViewCell.self, forCellWithReuseIdentifier: "publicTimelineDescription")
@@ -79,10 +86,15 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
         collectionView.refreshControl = UIRefreshControl()
         collectionView.refreshControl!.addTarget(self, action: #selector(refresh), for: .valueChanged)
 #endif
-    }
+        
-    
+        contentOffsetObservation = collectionView.observe(\.contentOffset) { [weak self] _, _ in
-    override func viewDidLoad() {
+           if let indexPath = self?.dataSource.indexPath(for: .gap),
-        super.viewDidLoad()
+              let cell = self?.collectionView.cellForItem(at: indexPath) as? TimelineGapCollectionViewCell {
+               cell.update()
+           }
+        }
+        
+        NotificationCenter.default.addObserver(self, selector: #selector(sceneWillEnterForeground), name: UIScene.willEnterForegroundNotification, object: nil)
     }
     
     // separate method because InstanceTimelineViewController needs to be able to customize it
@@ -95,6 +107,9 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
         let statusCell = UICollectionView.CellRegistration<TimelineStatusCollectionViewCell, (String, StatusState)> { [unowned self] cell, indexPath, item in
             self.configureStatusCell(cell, id: item.0, state: item.1)
         }
+        let gapCell = UICollectionView.CellRegistration<TimelineGapCollectionViewCell, Void> { cell, indexPath, _ in
+            cell.showsIndicator = false
+        }
         let timelineDescriptionCell = UICollectionView.CellRegistration<PublicTimelineDescriptionCollectionViewCell, Item> { [unowned self] cell, indexPath, item in
             guard case .public(let local) = timeline else {
                 fatalError()
@@ -109,6 +124,8 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
             switch itemIdentifier {
             case .status(id: let id, state: let state):
                 return collectionView.dequeueConfiguredReusableCell(using: statusCell, for: indexPath, item: (id, state))
+            case .gap:
+                return collectionView.dequeueConfiguredReusableCell(using: gapCell, for: indexPath, item: ())
             case .loadingIndicator:
                 return loadingIndicatorCell(for: indexPath)
             case .confirmLoadMore:
@@ -140,7 +157,7 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
         }
         
         Task {
-            if case .notLoadedInitial = await controller.state {
+            if case .notLoadedInitial = controller.state {
                 await controller.loadInitial()
             }
         }
@@ -159,12 +176,6 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
         }
     }
     
-    override func viewDidDisappear(_ animated: Bool) {
-        super.viewDidDisappear(animated)
-        
-//        pruneOffscreenRows()
-    }
-    
     private func removeTimelineDescriptionCell() {
         var snapshot = dataSource.snapshot()
         snapshot.deleteSections([.header])
@@ -172,35 +183,30 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
         isShowingTimelineDescription = false
     }
     
-//    private func pruneOffscreenRows() {
+    @objc private func sceneWillEnterForeground(_ notification: Foundation.Notification) {
-//        guard let lastVisibleIndexPath = collectionView.indexPathsForVisibleItems.last else {
+        guard let scene = notification.object as? UIScene,
-//            return
+              // view.window is nil when the VC is not on screen
-//        }
+              view.window?.windowScene == scene else {
-//        var snapshot = dataSource.snapshot()
+            return
-//        guard snapshot.indexOfSection(.statuses) != nil else {
+        }
-//            return
+        Task {
-//        }
+            if case .idle = controller.state,
-//        let items = snapshot.itemIdentifiers(inSection: .statuses)
+               let presentItems = try? await loadInitial() {
-//        let pageSize = 20
+                insertPresentItemsIfNecessary(presentItems)
-//        let numberOfPagesToPrune = (items.count - lastVisibleIndexPath.row - 1) / pageSize
+            }
-//        if numberOfPagesToPrune > 0 {
+        }
-//            let itemsToRemove = Array(items.suffix(numberOfPagesToPrune * pageSize))
+    }
-//            snapshot.deleteItems(itemsToRemove)
-//
-//            dataSource.apply(snapshot, animatingDifferences: false)
-//
-//            if case .status(id: let id, state: _) = snapshot.itemIdentifiers(inSection: .statuses).last {
-//                older = .before(id: id, count: nil)
-//            }
-//        }
-//    }
     
     @objc func refresh() {
         Task {
-            if case .notLoadedInitial = await controller.state {
+            if case .notLoadedInitial = controller.state {
                 await controller.loadInitial()
             } else {
-                await controller.loadNewer()
+                // I'm not sure whether this should move into TimelineLikeController/TimelineLikeCollectionViewController
+                let (_, presentItems) = await (controller.loadNewer(), try? loadInitial())
+                if let presentItems {
+                    insertPresentItemsIfNecessary(presentItems)
+                }
             }
             #if !targetEnvironment(macCatalyst)
             collectionView.refreshControl?.endRefreshing()
@@ -208,6 +214,68 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
         }
     }
     
+    private func insertPresentItemsIfNecessary(_ presentItems: [String]) {
+        var snapshot = dataSource.snapshot()
+        let currentItems = snapshot.itemIdentifiers(inSection: .statuses)
+        if case .status(id: let firstID, state: _) = currentItems.first,
+           // if there's no overlap between presentItems and the existing items in the data source, insert the present items and prompt the user
+           !presentItems.contains(firstID) {
+            let applySnapshotBeforeScrolling: Bool
+            
+            // remove any existing gap, if there is one
+            if let index = currentItems.lastIndex(of: .gap) {
+                snapshot.deleteItems(Array(currentItems[index...]))
+                
+                let statusesSection = snapshot.indexOfSection(.statuses)!
+                if collectionView.indexPathsForVisibleItems.contains(IndexPath(row: index, section: statusesSection)) {
+                    // the gap cell is on screen
+                    applySnapshotBeforeScrolling = false
+                } else if let topMostVisibleCell = collectionView.indexPathsForVisibleItems.first(where: { $0.section == statusesSection }),
+                          index < topMostVisibleCell.row {
+                    // the gap cell is above the top, so applying the snapshot would remove the currently-viewed statuses
+                    applySnapshotBeforeScrolling = false
+                } else {
+                    // the gap cell is below the bottom of the screen
+                    applySnapshotBeforeScrolling = true
+                }
+            } else {
+                // there is no existing gap
+                applySnapshotBeforeScrolling = true
+            }
+            snapshot.insertItems([.gap], beforeItem: currentItems.first!)
+            snapshot.insertItems(presentItems.map { .status(id: $0, state: .unknown) }, beforeItem: .gap)
+            
+            if applySnapshotBeforeScrolling {
+                // use a snapshot of the collection view to hide the flicker as the content offset changes and then changes back
+                let snapshotView = collectionView.snapshotView(afterScreenUpdates: false)!
+                snapshotView.layer.zPosition = 1000
+                snapshotView.frame = view.bounds
+                view.addSubview(snapshotView)
+                
+                let bottomOffset = collectionView.contentSize.height - collectionView.contentOffset.y
+                self.dataSource.apply(snapshot, animatingDifferences: false) {
+                    self.collectionView.contentOffset = CGPoint(x: 0, y: self.collectionView.contentSize.height - bottomOffset)
+                    snapshotView.removeFromSuperview()
+                }
+            }
+            
+            var config = ToastConfiguration(title: "Jump to present")
+            config.edge = .top
+            config.systemImageName = "arrow.up"
+            config.dismissAutomaticallyAfter = 4
+            config.action = { [unowned self] toast in
+                toast.dismissToast(animated: true)
+                
+                if !applySnapshotBeforeScrolling {
+                    self.dataSource.apply(snapshot, animatingDifferences: false)
+                }
+                
+                self.collectionView.scrollToTop()
+            }
+            self.showToast(configuration: config, animated: true)
+        }
+    }
+    
 }
 
 extension TimelineViewController {
@@ -222,6 +290,7 @@ extension TimelineViewController {
         typealias TimelineItem = String // status ID
         
         case status(id: String, state: StatusState)
+        case gap
         case loadingIndicator
         case confirmLoadMore
         case publicTimelineDescription
@@ -234,6 +303,8 @@ extension TimelineViewController {
             switch (lhs, rhs) {
             case let (.status(id: a, state: _), .status(id: b, state: _)):
                 return a == b
+            case (.gap, .gap):
+                return true
             case (.loadingIndicator, .loadingIndicator):
                 return true
             case (.confirmLoadMore, .confirmLoadMore):
@@ -250,12 +321,14 @@ extension TimelineViewController {
             case .status(id: let id, state: _):
                 hasher.combine(0)
                 hasher.combine(id)
-            case .loadingIndicator:
+            case .gap:
                 hasher.combine(1)
-            case .confirmLoadMore:
+            case .loadingIndicator:
                 hasher.combine(2)
-            case .publicTimelineDescription:
+            case .confirmLoadMore:
                 hasher.combine(3)
+            case .publicTimelineDescription:
+                hasher.combine(4)
             }
         }
         
@@ -270,7 +343,7 @@ extension TimelineViewController {
         
         var isSelectable: Bool {
             switch self {
-            case .publicTimelineDescription, .status(id: _, state: _):
+            case .publicTimelineDescription, .gap, .status(id: _, state: _):
                 return true
             default:
                 return false
@@ -286,50 +359,48 @@ extension TimelineViewController {
     func loadInitial() async throws -> [TimelineItem] {
         try await Task.sleep(nanoseconds: 1 * NSEC_PER_SEC)
         
-        guard let mastodonController else {
-            throw Error.noClient
-        }
-        
         let request = Client.getStatuses(timeline: timeline)
         let (statuses, _) = try await mastodonController.run(request)
         
-        if !statuses.isEmpty {
+        await withCheckedContinuation { continuation in
-            newer = .after(id: statuses.first!.id, count: nil)
-            older = .before(id: statuses.last!.id, count: nil)
-        }
-        
-        return await withCheckedContinuation { continuation in
             mastodonController.persistentContainer.addAll(statuses: statuses) {
-                continuation.resume(returning: statuses.map(\.id))
+                continuation.resume()
             }
         }
+        
+        return statuses.map(\.id)
     }
     
     func loadNewer() async throws -> [TimelineItem] {
-        guard let newer else {
+        let statusesSection = dataSource.snapshot().indexOfSection(.statuses)!
+        guard case .status(id: let id, state: _) = dataSource.itemIdentifier(for: IndexPath(row: 0, section: statusesSection)) else {
             throw Error.noNewer
         }
+        let newer = RequestRange.after(id: id, count: nil)
         
         let request = Client.getStatuses(timeline: timeline, range: newer)
         let (statuses, _) = try await mastodonController.run(request)
         
         guard !statuses.isEmpty else {
-            throw Error.allCaughtUp
+            throw TimelineViewController.Error.allCaughtUp
         }
         
-        self.newer = .after(id: statuses.first!.id, count: nil)
+        await withCheckedContinuation { continuation in
-        
-        return await withCheckedContinuation { continuation in
             mastodonController.persistentContainer.addAll(statuses: statuses) {
-                continuation.resume(returning: statuses.map(\.id))
+                continuation.resume()
             }
         }
+        
+        return statuses.map(\.id)
     }
     
     func loadOlder() async throws -> [TimelineItem] {
-        guard let older else {
+        let snapshot = dataSource.snapshot()
-            throw Error.noOlder
+        let statusesSection = snapshot.indexOfSection(.statuses)!
+        guard case .status(id: let id, state: _) = dataSource.itemIdentifier(for: IndexPath(row: snapshot.numberOfItems(inSection: .statuses) - 1, section: statusesSection)) else {
+            throw Error.noNewer
         }
+        let older = RequestRange.before(id: id, count: nil)
         
         let request = Client.getStatuses(timeline: timeline, range: older)
         let (statuses, _) = try await mastodonController.run(request)
@@ -338,13 +409,170 @@ extension TimelineViewController {
             return []
         }
         
-        self.older = .before(id: statuses.last!.id, count: nil)
+        await withCheckedContinuation { continuation in
-        
-        return await withCheckedContinuation { continuation in
             mastodonController.persistentContainer.addAll(statuses: statuses) {
-                continuation.resume(returning: statuses.map(\.id))
+                continuation.resume()
             }
         }
+        
+        return statuses.map(\.id)
+    }
+    
+    func loadGap(in direction: TimelineGapDirection) async throws -> [TimelineItem] {
+        guard let gapIndexPath = dataSource.indexPath(for: .gap) else {
+            throw Error.noGap
+        }
+        let statusItemsCount = collectionView.numberOfItems(inSection: gapIndexPath.section)
+        let range: RequestRange
+        switch direction {
+        case .above:
+            guard gapIndexPath.row > 0,
+                  case .status(id: let id, state: _) = dataSource.itemIdentifier(for: IndexPath(row: gapIndexPath.row - 1, section: gapIndexPath.section)) else {
+                // not really the right error but w/e
+                throw Error.noGap
+            }
+            range = .before(id: id, count: nil)
+        case .below:
+            guard gapIndexPath.row < statusItemsCount - 1,
+                  case .status(id: let id, state: _) = dataSource.itemIdentifier(for: IndexPath(row: gapIndexPath.row + 1, section: gapIndexPath.section)) else {
+                throw Error.noGap
+            }
+            range = .after(id: id, count: nil)
+        }
+        
+        let request = Client.getStatuses(timeline: timeline, range: range)
+        let (statuses, _) = try await mastodonController.run(request)
+        
+        guard !statuses.isEmpty else {
+            return []
+        }
+        
+        // NOTE: closing the gap (if necessary) happens in handleFillGap
+        
+        await withCheckedContinuation { continuation in
+            mastodonController.persistentContainer.addAll(statuses: statuses) {
+                continuation.resume()
+            }
+        }
+        
+        return statuses.map(\.id)
+    }
+    
+    func handleLoadGapError(_ error: Swift.Error, direction: TimelineGapDirection) async {
+        // TODO: better title, involving direction?
+        let config = ToastConfiguration(from: error, with: "Error Loading", in: self) { [weak self] toast in
+            toast.dismissToast(animated: true)
+            Task {
+                await self?.controller.fillGap(in: direction)
+            }
+        }
+        self.showToast(configuration: config, animated: true)
+    }
+    
+    func handleFillGap(_ timelineItems: [String], direction: TimelineGapDirection) async {
+        var snapshot = dataSource.snapshot()
+        let addedItems: Bool
+        
+        let statusItems = snapshot.itemIdentifiers(inSection: .statuses)
+        let gapIndex = statusItems.firstIndex(of: .gap)!
+        
+        switch direction {
+        case .above:
+            // dropFirst to remove .gap item
+            let afterGap = statusItems[gapIndex...].dropFirst().prefix(20)
+            precondition(!afterGap.contains(.gap))
+            
+            // if there is any overlap, the first overlapping item will be the first item below the gap
+            var indexOfFirstTimelineItemExistingBelowGap: Int?
+            if case .status(id: let id, state: _) = afterGap.first {
+                indexOfFirstTimelineItemExistingBelowGap = timelineItems.firstIndex(of: id)
+            }
+            
+            // the end index of the range of timelineItems that don't yet exist in the data source
+            let endIndex = indexOfFirstTimelineItemExistingBelowGap ?? timelineItems.endIndex
+            let toInsert = timelineItems[..<endIndex].map { Item.status(id: $0, state: .unknown) }
+            if toInsert.isEmpty {
+                addedItems = false
+            } else {
+                snapshot.insertItems(toInsert, beforeItem: .gap)
+                addedItems = true
+            }
+            
+            // if there's any overlap between the items we loaded to insert above the gap
+            // and the items that already exist below the gap, we've completely filled the gap
+            if indexOfFirstTimelineItemExistingBelowGap != nil {
+                snapshot.deleteItems([.gap])
+            }
+        
+            await apply(snapshot, animatingDifferences: !addedItems)
+            
+        case .below:
+            let beforeGap = statusItems[..<gapIndex].suffix(20)
+            precondition(!beforeGap.contains(.gap))
+            
+            // if there's any overlap, last overlapping item will be the last item below the gap
+            var indexOfLastTimelineItemExistingAboveGap: Int?
+            if case .status(id: let id, state: _) = beforeGap.last {
+                indexOfLastTimelineItemExistingAboveGap = timelineItems.lastIndex(of: id)
+            }
+            
+            // the start index of the reange of timeline items that don't yet exist in the data source
+            let startIndex: Int
+            if let indexOfLastTimelineItemExistingAboveGap {
+                // index(after:) because the beginning of the range is inclusive, but we don't want the item at indexOfLastTimelineItemExistingAboveGap
+                startIndex = timelineItems.index(after: indexOfLastTimelineItemExistingAboveGap)
+            } else {
+                startIndex = timelineItems.startIndex
+            }
+            let toInsert = timelineItems[startIndex...].map { Item.status(id: $0, state: .unknown) }
+            if toInsert.isEmpty {
+                addedItems = false
+            } else {
+                snapshot.insertItems(toInsert, afterItem: .gap)
+                addedItems = true
+            }
+            
+            // if there's any overlap between the items we loaded to insert below the gap
+            // and the items that already exist above the gap, we've completely filled the gap
+            if indexOfLastTimelineItemExistingAboveGap != nil {
+                snapshot.deleteItems([.gap])
+            }
+            
+            if addedItems {
+                // use a snapshot of the collection view to hide the flicker as the content offset changes and then changes back
+                let snapshotView = collectionView.snapshotView(afterScreenUpdates: false)!
+                snapshotView.layer.zPosition = 1000
+                snapshotView.frame = view.bounds
+                view.addSubview(snapshotView)
+                
+                // Yes, these are all load bearing. Setting the contentOffset seems to cause the collection view to recalculate the size
+                // of some cells, thus changing the contentSize and the offset necessary to match to match the bottom offset.
+                // Three DispatchQueue.main.async's seems to be the fewest we can reliably get away with.
+                let bottomOffset = collectionView.contentSize.height - collectionView.contentOffset.y
+                dataSource.apply(snapshot, animatingDifferences: false) {
+                    self.collectionView.contentOffset = CGPoint(x: 0, y: self.collectionView.contentSize.height - bottomOffset)
+                    DispatchQueue.main.async {
+                        self.collectionView.contentOffset = CGPoint(x: 0, y: self.collectionView.contentSize.height - bottomOffset)
+                        DispatchQueue.main.async {
+                            self.collectionView.contentOffset = CGPoint(x: 0, y: self.collectionView.contentSize.height - bottomOffset)
+                            DispatchQueue.main.async {
+                                self.collectionView.contentOffset = CGPoint(x: 0, y: self.collectionView.contentSize.height - bottomOffset)
+                                snapshotView.removeFromSuperview()
+                            }
+                        }
+                    }
+                }
+            } else {
+                dataSource.apply(snapshot, animatingDifferences: true) {}
+            }
+        }
+        
+        // if we didn't add any items, that implies the gap was removed, and we want to to make clear what's happening
+        if !addedItems {
+            var config = ToastConfiguration(title: "There's nothing in between!")
+            config.dismissAutomaticallyAfter = 2
+            showToast(configuration: config, animated: true)
+        }
     }
     
     enum Error: TimelineLikeCollectionViewError {
@@ -352,6 +580,7 @@ extension TimelineViewController {
         case noNewer
         case noOlder
         case allCaughtUp
+        case noGap
     }
 }
 
@@ -384,6 +613,13 @@ extension TimelineViewController: UICollectionViewDelegate {
             let status = mastodonController.persistentContainer.status(for: id)!
             // if the status in the timeline is a reblog, show the status that it is a reblog of
             selected(status: status.reblog?.id ?? id, state: state.copy())
+        case .gap:
+            let cell = collectionView.cellForItem(at: indexPath) as! TimelineGapCollectionViewCell
+            cell.showsIndicator = true
+            Task {
+                await controller.fillGap(in: cell.direction)
+                cell.showsIndicator = false
+            }
         case .loadingIndicator, .confirmLoadMore:
             fatalError("unreachable")
         }

Tusker/Screens/Utilities/TimelineLikeCollectionViewController.swift

@@ -19,7 +19,7 @@ protocol TimelineLikeCollectionViewController: UIViewController, TimelineLikeCon
     var controller: TimelineLikeController<TimelineItem>! { get }
     var confirmLoadMore: PassthroughSubject<Void, Never> { get }
     
-    var collectionView: UICollectionView { get }
+    var collectionView: UICollectionView! { get }
     var dataSource: UICollectionViewDiffableDataSource<Section, Item>! { get }
 }
 
@@ -64,7 +64,7 @@ extension TimelineLikeCollectionViewController {
     }
     
     func handleAddLoadingIndicator() async {
-        if case .loadingInitial(_, _) = await controller.state,
+        if case .loadingInitial(_, _) = controller.state,
            let refreshControl = collectionView.refreshControl,
            refreshControl.isRefreshing {
             refreshControl.beginRefreshing()
@@ -85,7 +85,7 @@ extension TimelineLikeCollectionViewController {
     }
     
     func handleRemoveLoadingIndicator() async {
-        if case .loadingInitial(_, _) = await controller.state,
+        if case .loadingInitial(_, _) = controller.state,
            let refreshControl = collectionView.refreshControl,
            refreshControl.isRefreshing {
             refreshControl.endRefreshing()
@@ -179,6 +179,17 @@ extension TimelineLikeCollectionViewController {
         snapshot.appendItems(timelineItems.map { .fromTimelineItem($0) }, toSection: .entries)
         await apply(snapshot, animatingDifferences: false)
     }
+    
+    func loadGap(in direction: TimelineGapDirection) async throws -> [TimelineItem] {
+        fatalError("not supported by \(String(describing: type(of: self)))")
+    }
+    
+    func handleLoadGapError(_ error: Swift.Error, direction: TimelineGapDirection) async {
+    }
+    
+    func handleFillGap(_ timelineItems: [TimelineItem], direction: TimelineGapDirection) async {
+        fatalError("not supported by \(String(describing: type(of: self)))")
+    }
 }
 
 extension TimelineLikeCollectionViewController {
@@ -206,7 +217,7 @@ extension TimelineLikeCollectionViewController {
         let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "confirmLoadMore", for: indexPath) as! ConfirmLoadMoreCollectionViewCell
         cell.confirmLoadMore = self.confirmLoadMore
         Task {
-            if case .loadingOlder(_, _) = await controller.state {
+            if case .loadingOlder(_, _) = controller.state {
                 cell.isLoading = true
             } else {
                 cell.isLoading = false

Tusker/TimelineLikeController.swift

@@ -16,9 +16,11 @@ protocol TimelineLikeControllerDelegate<TimelineItem>: AnyObject {
     
     func loadNewer() async throws -> [TimelineItem]
     
+    func canLoadOlder() async -> Bool
+    
     func loadOlder() async throws -> [TimelineItem]
     
-    func canLoadOlder() async -> Bool
+    func loadGap(in direction: TimelineGapDirection) async throws -> [TimelineItem]
     
     func handleAddLoadingIndicator() async
     func handleRemoveLoadingIndicator() async
@@ -28,13 +30,16 @@ protocol TimelineLikeControllerDelegate<TimelineItem>: AnyObject {
     func handlePrependItems(_ timelineItems: [TimelineItem]) async
     func handleLoadOlderError(_ error: Swift.Error) async
     func handleAppendItems(_ timelineItems: [TimelineItem]) async
+    func handleLoadGapError(_ error: Swift.Error, direction: TimelineGapDirection) async
+    func handleFillGap(_ timelineItems: [TimelineItem], direction: TimelineGapDirection) async
 }
 
 private let logger = Logger(subsystem: Bundle.main.bundleIdentifier!, category: "TimelineLikeController")
 
-actor TimelineLikeController<Item> {
+@MainActor
+class TimelineLikeController<Item> {
     
-    unowned var delegate: any TimelineLikeControllerDelegate<Item>
+    private unowned var delegate: any TimelineLikeControllerDelegate<Item>
     
     private(set) var state = State.notLoadedInitial {
         willSet {
@@ -126,6 +131,27 @@ actor TimelineLikeController<Item> {
         }
     }
     
+    func fillGap(in direction: TimelineGapDirection) async {
+        guard state == .idle else {
+            return
+        }
+        let token = LoadAttemptToken()
+        state = .loadingGap(token, direction)
+        do {
+            let items = try await delegate.loadGap(in: direction)
+            guard case .loadingGap(token, direction) = state else {
+                return
+            }
+            await emit(event: .fillGap(items, direction, token))
+            state = .idle
+        } catch is CancellationError {
+            return
+        } catch {
+            await emit(event: .loadGapError(error, direction, token))
+            state = .idle
+        }
+    }
+    
     private func transition(to newState: State) {
         self.state = newState
     }
@@ -152,6 +178,10 @@ actor TimelineLikeController<Item> {
             await delegate.handleLoadOlderError(error)
         case .appendItems(let items, _):
             await delegate.handleAppendItems(items)
+        case .loadGapError(let error, let direction, _):
+            await delegate.handleLoadGapError(error, direction: direction)
+        case .fillGap(let items, let direction, _):
+            await delegate.handleFillGap(items, direction: direction)
         }
     }
     
@@ -161,6 +191,7 @@ actor TimelineLikeController<Item> {
         case loadingInitial(LoadAttemptToken, hasAddedLoadingIndicator: Bool)
         case loadingNewer(LoadAttemptToken)
         case loadingOlder(LoadAttemptToken, hasAddedLoadingIndicator: Bool)
+        case loadingGap(LoadAttemptToken, TimelineGapDirection)
         
         var debugDescription: String {
             switch self {
@@ -174,6 +205,8 @@ actor TimelineLikeController<Item> {
                 return "loadingNewer(\(ObjectIdentifier(token)))"
             case .loadingOlder(let token, let hasAddedLoadingIndicator):
                 return "loadingOlder(\(ObjectIdentifier(token)), hasAddedLoadingIndicator: \(hasAddedLoadingIndicator))"
+            case .loadingGap(let token, let direction):
+                return "loadingGap(\(ObjectIdentifier(token)), \(direction))"
             }
         }
         
@@ -188,7 +221,7 @@ actor TimelineLikeController<Item> {
                 }
             case .idle:
                 switch to {
-                case .loadingInitial(_, _), .loadingNewer(_), .loadingOlder(_, _):
+                case .loadingInitial(_, _), .loadingNewer(_), .loadingOlder(_, _), .loadingGap(_, _):
                     return true
                 default:
                     return false
@@ -199,6 +232,8 @@ actor TimelineLikeController<Item> {
                 return to == .idle
             case .loadingOlder(let token, let hasAddedLoadingIndicator):
                 return to == .idle || (!hasAddedLoadingIndicator && to == .loadingOlder(token, hasAddedLoadingIndicator: true))
+            case .loadingGap(_, _):
+                return to == .idle
             }
         }
         
@@ -239,6 +274,13 @@ actor TimelineLikeController<Item> {
                 default:
                     return false
                 }
+            case .loadGapError(_, let direction, let token), .fillGap(_, let direction, let token):
+                switch self {
+                case .loadingGap(token, direction):
+                    return true
+                default:
+                    return false
+                }
             }
         }
     }
@@ -252,6 +294,8 @@ actor TimelineLikeController<Item> {
         case prependItems([Item], LoadAttemptToken)
         case loadOlderError(Error, LoadAttemptToken)
         case appendItems([Item], LoadAttemptToken)
+        case loadGapError(Error, TimelineGapDirection, LoadAttemptToken)
+        case fillGap([Item], TimelineGapDirection, LoadAttemptToken)
         
         var debugDescription: String {
             switch self {
@@ -271,6 +315,10 @@ actor TimelineLikeController<Item> {
                 return "loadOlderError(\(error), \(token))"
             case .appendItems(_, let token):
                 return "appendItems(<omitted>, \(token))"
+            case .loadGapError(let error, let direction, let token):
+                return "loadGapError(\(error), \(direction), \(token))"
+            case .fillGap(_, let direction, let token):
+                return "loadGapError(<omitted>, \(direction), \(token))"
             }
         }
     }
@@ -310,3 +358,10 @@ actor TimelineLikeController<Item> {
     }
     
 }
+
+enum TimelineGapDirection {
+    /// Fill in below the gap. I.e., statuses that are immediately newer than the status below the gap.
+    case below
+    /// Fill in above the gap. I.e., statuses that are immediately older than the status above the gap.
+    case above
+}

Tusker/Views/Confirm Load More Cell/ConfirmLoadMoreCollectionViewCell.swift

@@ -28,7 +28,7 @@ class ConfirmLoadMoreCollectionViewCell: UICollectionViewCell {
     override init(frame: CGRect) {
         super.init(frame: frame)
         
-        backgroundColor = .secondarySystemBackground
+        backgroundColor = .systemGroupedBackground
         
         let label = UILabel()
         label.text = "Infinite scrolling is off. Do you want to keep going?"