Sync timeline position using iCloud

Tusker.xcodeproj/project.pbxproj

@@ -219,6 +219,7 @@
 		D6969EA0240C8384002843CE /* EmojiLabel.swift in Sources */ = {isa = PBXBuildFile; fileRef = D6969E9F240C8384002843CE /* EmojiLabel.swift */; };
 		D6A00B1D26379FC900316AD4 /* PollOptionsView.swift in Sources */ = {isa = PBXBuildFile; fileRef = D6A00B1C26379FC900316AD4 /* PollOptionsView.swift */; };
 		D6A3A380295515550036B6EF /* ProfileHeaderButton.swift in Sources */ = {isa = PBXBuildFile; fileRef = D6A3A37F295515550036B6EF /* ProfileHeaderButton.swift */; };
+		D6A3A3822956123A0036B6EF /* TimelinePosition.swift in Sources */ = {isa = PBXBuildFile; fileRef = D6A3A3812956123A0036B6EF /* TimelinePosition.swift */; };
 		D6A3BC7C232195C600FD64D5 /* ActionNotificationGroupTableViewCell.swift in Sources */ = {isa = PBXBuildFile; fileRef = D6A3BC7A232195C600FD64D5 /* ActionNotificationGroupTableViewCell.swift */; };
 		D6A3BC7D232195C600FD64D5 /* ActionNotificationGroupTableViewCell.xib in Resources */ = {isa = PBXBuildFile; fileRef = D6A3BC7B232195C600FD64D5 /* ActionNotificationGroupTableViewCell.xib */; };
 		D6A3BC802321B7E600FD64D5 /* FollowNotificationGroupTableViewCell.swift in Sources */ = {isa = PBXBuildFile; fileRef = D6A3BC7E2321B7E600FD64D5 /* FollowNotificationGroupTableViewCell.swift */; };
@@ -602,6 +603,7 @@
 		D6969E9F240C8384002843CE /* EmojiLabel.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = EmojiLabel.swift; sourceTree = "<group>"; };
 		D6A00B1C26379FC900316AD4 /* PollOptionsView.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = PollOptionsView.swift; sourceTree = "<group>"; };
 		D6A3A37F295515550036B6EF /* ProfileHeaderButton.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ProfileHeaderButton.swift; sourceTree = "<group>"; };
+		D6A3A3812956123A0036B6EF /* TimelinePosition.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = TimelinePosition.swift; sourceTree = "<group>"; };
 		D6A3BC7A232195C600FD64D5 /* ActionNotificationGroupTableViewCell.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = ActionNotificationGroupTableViewCell.swift; sourceTree = "<group>"; };
 		D6A3BC7B232195C600FD64D5 /* ActionNotificationGroupTableViewCell.xib */ = {isa = PBXFileReference; lastKnownFileType = file.xib; path = ActionNotificationGroupTableViewCell.xib; sourceTree = "<group>"; };
 		D6A3BC7E2321B7E600FD64D5 /* FollowNotificationGroupTableViewCell.swift */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.swift; path = FollowNotificationGroupTableViewCell.swift; sourceTree = "<group>"; };
@@ -916,6 +918,7 @@
 				D61F759A29384F9C00C0B37F /* FilterMO.swift */,
 				D61F75AA293AF11400C0B37F /* FilterKeywordMO.swift */,
 				D6D706A62948D4D0000827ED /* TimlineState.swift */,
+				D6A3A3812956123A0036B6EF /* TimelinePosition.swift */,
 				D68A76E229524D2A001DA1B3 /* ListMO.swift */,
 				D68A76D929511CA6001DA1B3 /* AccountPreferences.swift */,
 				D60E2F2D244248BF005F8713 /* MastodonCachePersistentStore.swift */,
@@ -1984,6 +1987,7 @@
 				D61F75B7293C119700C0B37F /* Filterer.swift in Sources */,
 				D64AAE9526C88C5000FC57FB /* ToastableViewController.swift in Sources */,
 				D6895DE928D962C2006341DA /* TimelineLikeController.swift in Sources */,
+				D6A3A3822956123A0036B6EF /* TimelinePosition.swift in Sources */,
 				D6E0DC8E216EDF1E00369478 /* Previewing.swift in Sources */,
 				D6B93667281D937300237D0E /* MainSidebarMyProfileCollectionViewCell.swift in Sources */,
 				D61F75BD293D099600C0B37F /* Lazy.swift in Sources */,

Tusker/CoreData/MastodonCachePersistentStore.swift

@@ -18,6 +18,8 @@ fileprivate let logger = Logger(subsystem: Bundle.main.bundleIdentifier!, catego
 
 class MastodonCachePersistentStore: NSPersistentCloudKitContainer {
     
+    private let accountInfo: LocalData.UserAccountInfo?
+    
     private static let managedObjectModel: NSManagedObjectModel = {
         let url = Bundle.main.url(forResource: "Tusker", withExtension: "momd")!
         return NSManagedObjectModel(contentsOf: url)!
@@ -50,6 +52,8 @@ class MastodonCachePersistentStore: NSPersistentCloudKitContainer {
     let relationshipSubject = PassthroughSubject<String, Never>()
 
     init(for accountInfo: LocalData.UserAccountInfo?, transient: Bool = false) {
+        self.accountInfo = accountInfo
+        
         let group = DispatchGroup()
         var instancesToMigrate: [URL]? = nil
         var hashtagsToMigrate: [Hashtag]? = nil
@@ -153,12 +157,17 @@ class MastodonCachePersistentStore: NSPersistentCloudKitContainer {
             }
 
             if description.configuration == "Cloud" {
-                self.backgroundContext.perform {
+                let context = self.backgroundContext
+                context.perform {
                     instancesToMigrate?.forEach({ url in
+                        if !context.objectExists(for: SavedInstance.fetchRequest(url: url, account: accountInfo!)) {
                             _ = SavedInstance(url: url, account: accountInfo!, context: self.backgroundContext)
+                        }
                     })
                     hashtagsToMigrate?.forEach({ hashtag in
+                        if !context.objectExists(for: SavedHashtag.fetchRequest(name: hashtag.name, account: accountInfo!)) {
                             _ = SavedHashtag(hashtag: hashtag, account: accountInfo!, context: self.backgroundContext)
+                        }
                     })
                     self.save(context: self.backgroundContext)
                 }
@@ -435,9 +444,12 @@ class MastodonCachePersistentStore: NSPersistentCloudKitContainer {
         }
     }
     
-    func getTimelineState(timeline: Timeline) -> TimelineState? {
+    func getTimelinePosition(timeline: Timeline) -> TimelinePosition? {
+        guard let accountInfo else {
+            return  nil
+        }
         do {
-            let req = TimelineState.fetchRequest(timeline: timeline)
+            let req = TimelinePosition.fetchRequest(timeline: timeline, account: accountInfo)
             return try viewContext.fetch(req).first
         } catch {
             return nil
@@ -483,7 +495,6 @@ class MastodonCachePersistentStore: NSPersistentCloudKitContainer {
         return changes
     }
     
-    // the remote change notifications only handle deletes, inserts get handled by the regular managed object did change notifications
     @objc private func remoteChanges(_ notification: Foundation.Notification) {
         guard let token = notification.userInfo?[NSPersistentHistoryTokenKey] as? NSPersistentHistoryToken else {
             return
@@ -497,27 +508,31 @@ class MastodonCachePersistentStore: NSPersistentCloudKitContainer {
                 if let result = try? self.backgroundContext.execute(req) as? NSPersistentHistoryResult,
                    let transactions = result.result as? [NSPersistentHistoryTransaction] {
                     var changes: (hashtags: Bool, instances: Bool) = (false, false)
+                    var changedTimelinePositions: [NSManagedObjectID] = []
                     outer: for transaction in transactions {
                         for change in transaction.changes ?? [] {
                             if change.changedObjectID.entity.name == "SavedHashtag" {
                                 changes.hashtags = true
                             } else if change.changedObjectID.entity.name == "SavedInstance" {
                                 changes.instances = true
-                            }
+                            } else if change.changedObjectID.entity.name == "TimelinePosition" {
-                            if changes.hashtags && changes.instances {
+                                changedTimelinePositions.append(change.changedObjectID)
-                                break outer
                             }
                         }
                     }
-                    if changes.hashtags {
                     DispatchQueue.main.async {
+                        if changes.hashtags {
                             NotificationCenter.default.post(name: .savedHashtagsChanged, object: nil)
                         }
-                    }
                         if changes.instances {
-                        DispatchQueue.main.async {
                             NotificationCenter.default.post(name: .savedInstancesChanged, object: nil)
                         }
+                        for id in changedTimelinePositions {
+                            guard let timelinePosition = try? self.viewContext.existingObject(with: id) as? TimelinePosition else {
+                                continue
+                            }
+                            NotificationCenter.default.post(name: .timelinePositionChanged, object: timelinePosition)
+                        }
                     }
                 }
             }
@@ -525,3 +540,7 @@ class MastodonCachePersistentStore: NSPersistentCloudKitContainer {
     }
     
 }
+
+extension Foundation.Notification.Name {
+    static let timelinePositionChanged = Notification.Name("timelinePositionChanged")
+}

Tusker/CoreData/TimelinePosition.swift

@@ -0,0 +1,83 @@
+//
+//  TimelinePosition.swift
+//  Tusker
+//
+//  Created by Shadowfacts on 12/23/22.
+//  Copyright © 2022 Shadowfacts. All rights reserved.
+//
+
+import Foundation
+import CoreData
+import Pachyderm
+
+@objc(TimelinePosition)
+public final class TimelinePosition: NSManagedObject {
+    
+    @nonobjc class func fetchRequest(timeline: Timeline, account: LocalData.UserAccountInfo) -> NSFetchRequest<TimelinePosition> {
+        let req = NSFetchRequest<TimelinePosition>(entityName: "TimelinePosition")
+        req.predicate = NSPredicate(format: "accountID = %@ AND timelineKind = %@", account.id, toTimelineKind(timeline))
+        return req
+    }
+    
+    @NSManaged public var accountID: String
+    @NSManaged private var timelineKind: String
+    @NSManaged public var centerStatusID: String?
+    @NSManaged private var statusIDsData: Data?
+    
+    @LazilyDecoding(arrayFrom: \TimelinePosition.statusIDsData)
+    var statusIDs: [String]
+    
+    var timeline: Timeline {
+        get { fromTimelineKind(timelineKind) }
+        set { timelineKind = toTimelineKind(newValue) }
+    }
+    
+    convenience init(timeline: Timeline, account: LocalData.UserAccountInfo, context: NSManagedObjectContext) {
+        self.init(context: context)
+        self.timeline = timeline
+        self.accountID = account.id
+    }
+    
+}
+
+// blergh, this is the simplest way of getting the Timeline into a format that A) CoreData can handle and B) is usable in the predicate
+func toTimelineKind(_ timeline: Timeline) -> String {
+    switch timeline {
+    case .home:
+        return "home"
+    case .public(local: true):
+        return "local"
+    case .public(local: false):
+        return "federated"
+    case .direct:
+        return "direct"
+    case .tag(hashtag: let name):
+        return "hashtag:\(name)"
+    case .list(id: let id):
+        return "list:\(id)"
+    }
+}
+
+func fromTimelineKind(_ kind: String) -> Timeline {
+    if kind == "home" {
+        return .home
+    } else if kind == "local" {
+        return .public(local: true)
+    } else if kind == "federated" {
+        return .public(local: false)
+    } else if kind == "direct" {
+        return .direct
+    } else if kind.starts(with: "hashtag:") {
+        return .tag(hashtag: String(trimmingPrefix("hashtag:", of: kind)))
+    } else if kind.starts(with: "list:") {
+        return .list(id: String(trimmingPrefix("list:", of: kind)))
+    } else {
+        fatalError("invalid timeline kind \(kind)")
+    }
+}
+
+// replace with Collection.trimmingPrefix
+@available(iOS, obsoleted: 16.0)
+private func trimmingPrefix(_ prefix: String, of str: String) -> Substring {
+    return str[str.index(str.startIndex, offsetBy: prefix.count)...]
+}

Tusker/CoreData/TimlineState.swift

@@ -13,10 +13,8 @@ import Pachyderm
 @objc(TimelineState)
 public final class TimelineState: NSManagedObject {
     
-    @nonobjc public class func fetchRequest(timeline: Timeline) -> NSFetchRequest<TimelineState> {
+    @nonobjc public class func fetchRequest() -> NSFetchRequest<TimelineState> {
-        let req = NSFetchRequest<TimelineState>(entityName: "TimelineState")
+        return NSFetchRequest<TimelineState>(entityName: "TimelineState")
-        req.predicate = NSPredicate(format: "timelineKind = %@", toTimelineKind(timeline))
-        return req
     }
     
     @NSManaged private var timelineKind: String
@@ -25,65 +23,10 @@ public final class TimelineState: NSManagedObject {
     
     var timeline: Timeline {
         get { fromTimelineKind(timelineKind) }
-        set { timelineKind = toTimelineKind(newValue) }
     }
     
     var statusMOs: [StatusMO] {
         statuses.array as! [StatusMO]
     }
     
-    convenience init(timeline: Timeline, context: NSManagedObjectContext) {
-        self.init(context: context)
-        self.timeline = timeline
-    }
-    
-    func setStatuses(_ statusIDs: [String]) {
-        let context = managedObjectContext!
-        // todo: this feels really inefficient, but I'm not sure if it's better or worse than doing a single "id IN %@" fetch and sorting after
-        let mos = statusIDs.compactMap { try? context.fetch(StatusMO.fetchRequest(id: $0)).first }
-        self.statuses = NSOrderedSet(array: mos)
-    }
-    
-}
-
-// blergh, this is the simplest way of getting the Timeline into a format that A) CoreData can handle and B) is usable in the predicate
-private func toTimelineKind(_ timeline: Timeline) -> String {
-    switch timeline {
-    case .home:
-        return "home"
-    case .public(local: true):
-        return "local"
-    case .public(local: false):
-        return "federated"
-    case .direct:
-        return "direct"
-    case .tag(hashtag: let name):
-        return "hashtag:\(name)"
-    case .list(id: let id):
-        return "list:\(id)"
-    }
-}
-
-private func fromTimelineKind(_ kind: String) -> Timeline {
-    if kind == "home" {
-        return .home
-    } else if kind == "local" {
-        return .public(local: true)
-    } else if kind == "federated" {
-        return .public(local: false)
-    } else if kind == "direct" {
-        return .direct
-    } else if kind.starts(with: "hashtag:") {
-        return .tag(hashtag: String(trimmingPrefix("hashtag:", of: kind)))
-    } else if kind.starts(with: "list:") {
-        return .list(id: String(trimmingPrefix("list:", of: kind)))
-    } else {
-        fatalError("invalid timeline kind \(kind)")
-    }
-}
-
-// replace with Collection.trimmingPrefix
-@available(iOS, obsoleted: 16.0)
-private func trimmingPrefix(_ prefix: String, of str: String) -> Substring {
-    return str[str.index(str.startIndex, offsetBy: prefix.count)...]
 }

Tusker/CoreData/Tusker.xcdatamodeld/Tusker.xcdatamodel/contents

@@ -117,6 +117,12 @@
             </uniquenessConstraint>
         </uniquenessConstraints>
     </entity>
+    <entity name="TimelinePosition" representedClassName="TimelinePosition" syncable="YES">
+        <attribute name="accountID" optional="YES" attributeType="String"/>
+        <attribute name="centerStatusID" optional="YES" attributeType="String"/>
+        <attribute name="statusIDsData" optional="YES" attributeType="Binary" valueTransformerName="TimelinePositionStatusIDsTransformer"/>
+        <attribute name="timelineKind" optional="YES" attributeType="String"/>
+    </entity>
     <entity name="TimelineState" representedClassName="TimelineState" syncable="YES">
         <attribute name="centerStatusID" optional="YES" attributeType="String"/>
         <attribute name="timelineKind" attributeType="String" valueTransformerName="pachydermTimeline" customClassName="Tusker.TimelineContainer"/>
@@ -126,6 +132,7 @@
         <memberEntity name="SavedHashtag"/>
         <memberEntity name="SavedInstance"/>
         <memberEntity name="AccountPreferences"/>
+        <memberEntity name="TimelinePosition"/>
     </configuration>
     <configuration name="Local">
         <memberEntity name="Account"/>

Tusker/Scenes/MainSceneDelegate.swift

@@ -131,7 +131,7 @@ class MainSceneDelegate: UIResponder, UIWindowSceneDelegate, TuskerSceneDelegate
             minDate.addTimeInterval(-7 * 24 * 60 * 60)
             
             let statusReq: NSFetchRequest<NSFetchRequestResult> = StatusMO.fetchRequest()
-            statusReq.predicate = NSPredicate(format: "((lastFetchedAt = nil) OR (lastFetchedAt < %@)) AND (timelines.@count = 0)", minDate as NSDate)
+            statusReq.predicate = NSPredicate(format: "(lastFetchedAt = nil) OR (lastFetchedAt < %@)", minDate as NSDate)
             let deleteStatusReq = NSBatchDeleteRequest(fetchRequest: statusReq)
             deleteStatusReq.resultType = .resultTypeCount
             if let res = try? context.execute(deleteStatusReq) as? NSBatchDeleteResult {

Tusker/Screens/Timeline/TimelineViewController.swift

@@ -25,6 +25,7 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
     private(set) var collectionView: UICollectionView!
     private(set) var dataSource: UICollectionViewDiffableDataSource<Section, Item>!
     
+    private var cancellables = Set<AnyCancellable>()
     private var contentOffsetObservation: NSKeyValueObservation?
     private var activityToRestore: NSUserActivity?
     // the last time this VC disappeared or the scene was backgrounded while it was active, used to decide if we want to check for present when reappearing
@@ -121,6 +122,21 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
         
         NotificationCenter.default.addObserver(self, selector: #selector(sceneWillEnterForeground), name: UIScene.willEnterForegroundNotification, object: nil)
         NotificationCenter.default.addObserver(self, selector: #selector(sceneDidEnterBackground), name: UIScene.didEnterBackgroundNotification, object: nil)
+        NotificationCenter.default.publisher(for: .timelinePositionChanged)
+            .filter { [unowned self] in
+                if let timelinePosition = $0.object as? TimelinePosition,
+                   timelinePosition.accountID == self.mastodonController.accountInfo?.id,
+                   timelinePosition.timeline == self.timeline {
+                    return true
+                } else {
+                    return false
+                }
+            }
+            .debounce(for: .milliseconds(500), scheduler: DispatchQueue.main)
+            .sink { [unowned self] _ in
+                _ = promptToSyncPositionIfNecessary()
+            }
+            .store(in: &cancellables)
     }
     
     // separate method because InstanceTimelineViewController needs to be able to customize it
@@ -208,10 +224,14 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
                     await controller.loadInitial()
                 }
             }
+        } else {
+            if promptToSyncPositionIfNecessary() {
+                // no-op
             } else {
                 checkPresentIfEnoughTimeElapsed()
             }
         }
+    }
     
     override func viewDidAppear(_ animated: Bool) {
         super.viewDidAppear(animated)
@@ -233,19 +253,29 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
         saveState()
     }
     
-    private func saveState() {
+    private func currentCenterVisibleIndexPath(snapshot: NSDiffableDataSourceSnapshot<Section, Item>?) -> IndexPath? {
-        guard isViewLoaded,
+        let snapshot = snapshot ?? dataSource.snapshot()
-              persistsState else {
+        
-            return
-        }
         let visible = collectionView.indexPathsForVisibleItems.sorted()
-        let snapshot = dataSource.snapshot()
         let visibleRect = CGRect(origin: collectionView.contentOffset, size: collectionView.bounds.size)
         let midPoint = CGPoint(x: visibleRect.midX, y: visibleRect.midY)
         guard !visible.isEmpty,
               let statusesSection = snapshot.sectionIdentifiers.firstIndex(of: .statuses),
               let rawCenterVisible = collectionView.indexPathForItem(at: midPoint),
               let centerVisible = visible.first(where: { $0.section == statusesSection && $0 >= rawCenterVisible }) else {
+            return nil
+        }
+        return centerVisible
+    }
+    
+    private func saveState() {
+        guard isViewLoaded,
+              persistsState,
+              let accountInfo = mastodonController.accountInfo else {
+            return
+        }
+        let snapshot = dataSource.snapshot()
+        guard let centerVisible = currentCenterVisibleIndexPath(snapshot: snapshot) else {
             return
         }
         let allItems = snapshot.itemIdentifiers(inSection: .statuses)
@@ -288,10 +318,11 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
         }
         stateRestorationLogger.debug("TimelineViewController: saving state to persistent store with with centerID \(centerVisibleID)")
         
-        let state = mastodonController.persistentContainer.getTimelineState(timeline: timeline) ?? TimelineState(timeline: timeline, context: mastodonController.persistentContainer.viewContext)
+        let context = mastodonController.persistentContainer.viewContext
-        state.setStatuses(ids)
+        let position = mastodonController.persistentContainer.getTimelinePosition(timeline: timeline) ?? TimelinePosition(timeline: timeline, account: accountInfo, context: context)
-        state.centerStatusID = centerVisibleID
+        position.statusIDs = ids
-        mastodonController.persistentContainer.save(context: mastodonController.persistentContainer.viewContext)
+        position.centerStatusID = centerVisibleID
+        mastodonController.persistentContainer.save(context: context)
     }
     
     func stateRestorationActivity() -> NSUserActivity? {
@@ -304,22 +335,47 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
         return activity
     }
     
-    private func restoreState() -> Bool {
+    func restoreState() -> Bool {
         guard persistsState,
               Preferences.shared.timelineStateRestoration,
-              let state = mastodonController.persistentContainer.getTimelineState(timeline: timeline) else {
+              let position = mastodonController.persistentContainer.getTimelinePosition(timeline: timeline) else {
             return false
         }
-        let statusIDs = state.statusMOs.map(\.id)
         loadViewIfNeeded()
-        controller.restoreInitial {
+        Task {
-            var snapshot = dataSource.snapshot()
+            await controller.restoreInitial {
+                await loadStatusesToRestore(position: position)
+                applyItemsToRestore(position: position)
+            }
+        }
+        return true
+    }
+    
+    @MainActor
+    private func loadStatusesToRestore(position: TimelinePosition) async {
+        let unloaded = position.statusIDs.filter({ mastodonController.persistentContainer.status(for: $0) == nil })
+        guard !unloaded.isEmpty else {
+            return
+        }
+        await withTaskGroup(of: Void.self) { group in
+            for id in unloaded {
+                group.addTask { @MainActor in
+                    if let (status, _) = try? await self.mastodonController.run(Client.getStatus(id: id)) {
+                        self.mastodonController.persistentContainer.addOrUpdate(status: status)
+                    }
+                }
+            }
+        }
+    }
+    
+    private func applyItemsToRestore(position: TimelinePosition) {
+        var snapshot = NSDiffableDataSourceSnapshot<Section, Item>()
         snapshot.appendSections([.statuses])
-            let items = statusIDs.map { Item.status(id: $0, collapseState: .unknown, filterState: .unknown) }
+        let items = position.statusIDs.map { Item.status(id: $0, collapseState: .unknown, filterState: .unknown) }
         snapshot.appendItems(items, toSection: .statuses)
         dataSource.apply(snapshot, animatingDifferences: false) {
-                if let centerID = state.centerStatusID,
+            if let centerID = position.centerStatusID,
-                   let index = statusIDs.firstIndex(of: centerID),
+               let index = position.statusIDs.firstIndex(of: centerID),
                let indexPath = self.dataSource.indexPath(for: items[index]) {
                 // it sometimes takes multiple attempts to convert on the right scroll position
                 // since we're dealing with a bunch of unmeasured cells, so just try a few times in a loop
@@ -340,8 +396,6 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
             }
         }
     }
-        return true
-    }
     
     private func removeTimelineDescriptionCell() {
         var snapshot = dataSource.snapshot()
@@ -394,8 +448,12 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
               view.window?.windowScene == scene else {
             return
         }
+        if promptToSyncPositionIfNecessary() {
+            // no-op
+        } else {
             checkPresentIfEnoughTimeElapsed()
         }
+    }
     
     @objc private func sceneDidEnterBackground(_ notification: Foundation.Notification) {
         guard let scene = notification.object as? UIScene,
@@ -407,6 +465,45 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
         saveState()
     }
     
+    private func promptToSyncPositionIfNecessary() -> Bool {
+        guard persistsState,
+              let timelinePosition = mastodonController.persistentContainer.getTimelinePosition(timeline: timeline) else {
+            return false
+        }
+        let snapshot = dataSource.snapshot()
+        guard let centerVisible = currentCenterVisibleIndexPath(snapshot: snapshot),
+              snapshot.sectionIdentifiers.contains(.statuses) else {
+            return false
+        }
+        let statusesSection = snapshot.itemIdentifiers(inSection: .statuses)
+        let centerVisibleStatusID: String
+        switch statusesSection[centerVisible.row] {
+        case .gap:
+            guard case .status(let id, _, _) = statusesSection[centerVisible.row + 1] else {
+                fatalError()
+            }
+            centerVisibleStatusID = id
+        case .status(let id, _, _):
+            centerVisibleStatusID = id
+        default:
+            fatalError()
+        }
+        guard timelinePosition.centerStatusID != centerVisibleStatusID else {
+            return false
+        }
+        var config = ToastConfiguration(title: "Sync Position")
+        config.edge = .top
+        config.dismissAutomaticallyAfter = 5
+        config.systemImageName = "arrow.triangle.2.circlepath"
+        config.action = { [unowned self] toast in
+            toast.dismissToast(animated: true)
+            _ = self.restoreState()
+        }
+        showToast(configuration: config, animated: true)
+        UIAccessibility.post(notification: .announcement, argument: "Synced Position Updated")
+        return true
+    }
+    
     @objc func refresh() {
         Task {
             if case .notLoadedInitial = controller.state {

Tusker/Screens/Timeline/TimelinesPageViewController.swift

@@ -53,7 +53,14 @@ class TimelinesPageViewController: SegmentedPageViewController<TimelinesPageView
                     await vc.checkPresent(jumpImmediately: true)
                 }
                 return true
-            })
+            }),
+            UIAccessibilityCustomAction(name: "Jump to Sync Position", actionHandler: { [unowned self] _ in
+                guard let vc = currentViewController as? TimelineViewController else {
+                    return false
+                }
+                _ = vc.restoreState()
+                return true
+            }),
         ]
         
         pinnedTimelinesObservation = mastodonController.accountPreferences.observe(\.pinnedTimelinesData, changeHandler: { [unowned self] _, _ in

Tusker/TimelineLikeController.swift

@@ -80,12 +80,12 @@ class TimelineLikeController<Item> {
     }
     
     /// Used to indicate to the controller that the initial set of posts have been restored externally.
-    func restoreInitial(doRestore: () -> Void) {
+    func restoreInitial(doRestore: () async -> Void) async {
-        guard state == .notLoadedInitial else {
+        guard state == .notLoadedInitial || state == .idle else {
             return
         }
         state = .restoringInitial
-        doRestore()
+        await doRestore()
         state = .idle
     }
     
@@ -234,7 +234,7 @@ class TimelineLikeController<Item> {
                 }
             case .idle:
                 switch to {
-                case .loadingInitial(_, _), .loadingNewer(_), .loadingOlder(_, _), .loadingGap(_, _):
+                case .restoringInitial, .loadingInitial(_, _), .loadingNewer(_), .loadingOlder(_, _), .loadingGap(_, _):
                     return true
                 default:
                     return false