Transfer timeline position in handoff user activity

Closes #315

Tusker/Scenes/AuxiliarySceneDelegate.swift

@@ -74,7 +74,7 @@ class AuxiliarySceneDelegate: UIResponder, UIWindowSceneDelegate, TuskerSceneDel
     private func viewController(for activity: NSUserActivity, mastodonController: MastodonController) -> UIViewController? {
         switch UserActivityType(rawValue: activity.activityType) {
         case .showTimeline:
-            guard let timeline = UserActivityManager.getTimeline(from: activity) else { return nil }
+            guard let (timeline, _) = UserActivityManager.getTimeline(from: activity) else { return nil }
             return timelineViewController(for: timeline, mastodonController: mastodonController)
             
         case .showConversation:

Tusker/Scenes/MainSceneDelegate.swift

@@ -70,7 +70,7 @@ class MainSceneDelegate: UIResponder, UIWindowSceneDelegate, TuskerSceneDelegate
             stateRestorationLogger.info("MainSceneDelegate cannot resume user activity for different account")
             return
         } else {
-            context = ActiveAccountUserActivityHandlingContext(root: rootViewController!)
+            context = ActiveAccountUserActivityHandlingContext(isHandoff: true, root: rootViewController!)
         }
         _ = userActivity.handleResume(manager: UserActivityManager(scene: scene as! UIWindowScene, context: context))
     }
@@ -184,7 +184,7 @@ class MainSceneDelegate: UIResponder, UIWindowSceneDelegate, TuskerSceneDelegate
                 if activity.isStateRestorationActivity {
                     doRestoreActivity(context: StateRestorationUserActivityHandlingContext(root: rootViewController!))
                 } else if activity.activityType != UserActivityType.mainScene.rawValue {
-                    doRestoreActivity(context: ActiveAccountUserActivityHandlingContext(root: rootViewController!))
+                    doRestoreActivity(context: ActiveAccountUserActivityHandlingContext(isHandoff: false, root: rootViewController!))
                 } else {
                     stateRestorationLogger.fault("MainSceneDelegate launched with non-restorable activity \(activity.activityType, privacy: .public)")
                 }

Tusker/Screens/Timeline/TimelineViewController.swift

@@ -43,6 +43,7 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
     private(set) var dataSource: UICollectionViewDiffableDataSource<Section, Item>!
     
     private var cancellables = Set<AnyCancellable>()
+    private var userActivityNeedsUpdate = PassthroughSubject<Void, Never>()
     private var contentOffsetObservation: NSKeyValueObservation?
     // 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
     private var disappearedAt: Date?
@@ -171,6 +172,18 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
             }
             .store(in: &cancellables)
         NotificationCenter.default.addObserver(self, selector: #selector(handleStatusDeleted), name: .statusDeleted, object: nil)
+        
+        if userActivity != nil {
+            userActivityNeedsUpdate
+                .debounce(for: .seconds(1), scheduler: DispatchQueue.main)
+                .sink { [unowned self] _ in
+                    if let info = self.timelinePositionInfo(),
+                       let userActivity = self.userActivity {
+                        UserActivityManager.addTimelinePositionInfo(to: userActivity, statusIDs: info.statusIDs, centerStatusID: info.centerStatusID)
+                    }
+                }
+                .store(in: &cancellables)
+        }
     }
     
     // separate method because InstanceTimelineViewController needs to be able to customize it
@@ -294,15 +307,13 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
         return centerVisible
     }
     
-    private func saveState() {
-        guard isViewLoaded,
-              persistsState,
-              let accountInfo = mastodonController.accountInfo else {
-            return
+    private func timelinePositionInfo() -> (statusIDs: [String], centerStatusID: String)? {
+        guard isViewLoaded else {
+            return nil
         }
         let snapshot = dataSource.snapshot()
         guard let centerVisible = currentCenterVisibleIndexPath(snapshot: snapshot) else {
-            return
+            return nil
         }
         let allItems = snapshot.itemIdentifiers(inSection: .statuses)
         
@@ -342,6 +353,15 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
         } else {
             fatalError()
         }
+        return (ids, centerVisibleID)
+    }
+    
+    private func saveState() {
+        guard persistsState,
+              let accountInfo = mastodonController.accountInfo,
+              let (ids, centerVisibleID) = timelinePositionInfo() else {
+            return
+        }
         switch Preferences.shared.timelineSyncMode {
         case .icloud:
             stateRestorationLogger.debug("TimelineViewController: saving state to persistent store with with centerID \(centerVisibleID)")
@@ -409,6 +429,22 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
         return loaded
     }
     
+    func restoreStateFromHandoff(statusIDs: [String], centerStatusID: String) async {
+        let crumb = Breadcrumb(level: .debug, category: "TimelineViewController")
+        crumb.message = "Restoring state from handoff activity"
+        SentrySDK.addBreadcrumb(crumb)
+        await controller.restoreInitial { @MainActor in
+            let position = TimelinePosition(context: mastodonController.persistentContainer.viewContext)
+            position.statusIDs = statusIDs
+            position.centerStatusID = centerStatusID
+            let hasStatusesToRestore = await loadStatusesToRestore(position: position)
+            if hasStatusesToRestore {
+                applyItemsToRestore(position: position)
+            }
+            mastodonController.persistentContainer.viewContext.delete(position)
+        }
+    }
+    
     @MainActor
     private func loadStatusesToRestore(position: TimelinePosition) async -> Bool {
         let originalPositionStatusIDs = position.statusIDs
@@ -1293,6 +1329,16 @@ extension TimelineViewController: UICollectionViewDelegate {
             removeTimelineDescriptionCell()
         }
     }
+    
+    func scrollViewDidEndDragging(_ scrollView: UIScrollView, willDecelerate decelerate: Bool) {
+        if !decelerate {
+            userActivityNeedsUpdate.send()
+        }
+    }
+    
+    func scrollViewDidEndDecelerating(_ scrollView: UIScrollView) {
+        userActivityNeedsUpdate.send()
+    }
 }
 
 extension TimelineViewController: UICollectionViewDragDelegate {

Tusker/Shortcuts/UserActivityHandlingContext.swift

@@ -11,6 +11,8 @@ import Duckable
 
 @MainActor
 protocol UserActivityHandlingContext {
+    var isHandoff: Bool { get }
+    
     func select(route: TuskerRoute)
     func present(_ vc: UIViewController)
     
@@ -24,6 +26,7 @@ protocol UserActivityHandlingContext {
 }
 
 struct ActiveAccountUserActivityHandlingContext: UserActivityHandlingContext {
+    let isHandoff: Bool
     let root: TuskerRootViewController
     var navigationDelegate: TuskerNavigationDelegate {
         root.getNavigationDelegate()!
@@ -63,6 +66,8 @@ class StateRestorationUserActivityHandlingContext: UserActivityHandlingContext {
         self.root = root
     }
     
+    var isHandoff: Bool { false }
+    
     func select(route: TuskerRoute) {
         root.select(route: route, animated: false)
         state = .selectedRoute

Tusker/Shortcuts/UserActivityManager.swift

@@ -189,30 +189,53 @@ class UserActivityManager {
         return activity
     }
     
-    static func getTimeline(from activity: NSUserActivity) -> Timeline? {
+    static func addTimelinePositionInfo(to activity: NSUserActivity, statusIDs: [String], centerStatusID: String) {
+        activity.addUserInfoEntries(from: [
+            "statusIDs": statusIDs,
+            "centerStatusID": centerStatusID
+        ])
+    }
+    
+    static func getTimeline(from activity: NSUserActivity) -> (Timeline, (statusIDs: [String], centerStatusID: String)?)? {
         guard activity.activityType == UserActivityType.showTimeline.rawValue,
-              let data = activity.userInfo?["timelineData"] as? Data else {
+              let data = activity.userInfo?["timelineData"] as? Data,
+              let timeline = try? UserActivityManager.decoder.decode(Timeline.self, from: data) else {
             return nil
         }
-        return try? UserActivityManager.decoder.decode(Timeline.self, from: data)
+        var positionInfo: ([String], String)?
+        if let ids = activity.userInfo?["statusIDs"] as? [String],
+           let center = activity.userInfo?["centerStatusID"] as? String {
+            positionInfo = (ids, center)
+        }
+        return (timeline, positionInfo)
     }
     
     func handleShowTimeline(activity: NSUserActivity) {
-        guard let timeline = Self.getTimeline(from: activity) else { return }
+        guard let (timeline, positionInfo) = Self.getTimeline(from: activity) else { return }
         
+        let timelineVC: TimelineViewController
         if let pinned = PinnedTimeline(timeline: timeline),
            mastodonController.accountPreferences.pinnedTimelines.contains(pinned) {
             context.select(route: .timelines)
             context.popToRoot()
-            let rootController = context.topViewController as! TimelinesPageViewController
-            rootController.selectTimeline(pinned, animated: false)
+            let pageController = context.topViewController as! TimelinesPageViewController
+            pageController.selectTimeline(pinned, animated: false)
+            timelineVC = pageController.currentViewController as! TimelineViewController
         } else if case .list(let id) = timeline {
             context.select(route: .list(id: id))
+            timelineVC = context.topViewController! as! TimelineViewController
         } else {
             context.select(route: .explore)
             context.popToRoot()
-            let timeline = TimelineViewController(for: timeline, mastodonController: mastodonController)
-            context.push(timeline)
+            timelineVC = TimelineViewController(for: timeline, mastodonController: mastodonController)
+            context.push(timelineVC)
+        }
+        
+        if let positionInfo,
+           context.isHandoff {
+            Task {
+                await timelineVC.restoreStateFromHandoff(statusIDs: positionInfo.statusIDs, centerStatusID: positionInfo.centerStatusID)
+            }
         }
     }