From 366834e2e45f967305b20243bc1edcb4ab419b61 Mon Sep 17 00:00:00 2001
From: Shadowfacts <me@shadowfacts.net>
Date: Thu, 24 Nov 2022 11:05:56 -0500
Subject: [PATCH] Tweak timeline state restoration to maintain scroll position
 of center item

---
 .../Timeline/TimelineViewController.swift     | 46 ++++++++++---------
 1 file changed, 24 insertions(+), 22 deletions(-)

diff --git a/Tusker/Screens/Timeline/TimelineViewController.swift b/Tusker/Screens/Timeline/TimelineViewController.swift
index 376fab48..1ab2f7fe 100644
--- a/Tusker/Screens/Timeline/TimelineViewController.swift
+++ b/Tusker/Screens/Timeline/TimelineViewController.swift
@@ -186,37 +186,39 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
     func stateRestorationActivity() -> NSUserActivity? {
         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 let currentAccountID = mastodonController.accountInfo?.id,
               !visible.isEmpty,
               let statusesSection = snapshot.sectionIdentifiers.firstIndex(of: .statuses),
-              let firstVisible = visible.first(where: { $0.section == statusesSection }),
-              let lastVisible = visible.last(where: { $0.section == statusesSection }) else {
+              let rawCenterVisible = collectionView.indexPathForItem(at: midPoint),
+              let centerVisible = visible.first(where: { $0.section == statusesSection && $0 >= rawCenterVisible }) else {
             return nil
         }
         let allItems = snapshot.itemIdentifiers(inSection: .statuses)
         
-        let startIndex = max(0, firstVisible.row - 20)
-        let endIndex = min(allItems.count - 1, lastVisible.row + 20)
+        let startIndex = max(0, centerVisible.row - 20)
+        let endIndex = min(allItems.count - 1, centerVisible.row + 20)
         
-        let firstVisibleItem: Item
+        let centerVisibleItem: Item
         var items = allItems[startIndex...endIndex]
         if let gapIndex = items.firstIndex(of: .gap) {
             // if the gap is above the top visible item, we take everything below the gap
             // otherwise, we take everything above the gap
-            if gapIndex <= firstVisible.row {
+            if gapIndex <= centerVisible.row {
                 items = allItems[(gapIndex + 1)...endIndex]
-                if gapIndex == firstVisible.row {
-                    firstVisibleItem = allItems.first!
+                if gapIndex == centerVisible.row {
+                    centerVisibleItem = allItems.first!
                 } else {
-                    assert(items.indices.contains(firstVisible.row))
-                    firstVisibleItem = allItems[firstVisible.row]
+                    assert(items.indices.contains(centerVisible.row))
+                    centerVisibleItem = allItems[centerVisible.row]
                 }
             } else {
                 items = allItems[startIndex..<gapIndex]
-                firstVisibleItem = allItems[firstVisible.row]
+                centerVisibleItem = allItems[centerVisible.row]
             }
         } else {
-            firstVisibleItem = allItems[firstVisible.row]
+            centerVisibleItem = allItems[centerVisible.row]
         }
         let ids = items.map {
             if case .status(id: let id, state: _) = $0 {
@@ -225,18 +227,18 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
                 fatalError()
             }
         }
-        let firstVisibleID: String
-        if case .status(id: let id, state: _) = firstVisibleItem {
-            firstVisibleID = id
+        let centerVisibleID: String
+        if case .status(id: let id, state: _) = centerVisibleItem {
+            centerVisibleID = id
         } else {
             fatalError()
         }
-        stateRestorationLogger.debug("TimelineViewController: creating state restoration activity with topID \(firstVisibleID)")
+        stateRestorationLogger.debug("TimelineViewController: creating state restoration activity with topID \(centerVisibleID)")
         
         let activity = UserActivityManager.showTimelineActivity(timeline: timeline, accountID: currentAccountID)!
         activity.addUserInfoEntries(from: [
             "statusIDs": ids,
-            "topID": firstVisibleID,
+            "centerID": centerVisibleID,
         ])
         activity.isEligibleForPrediction = false
         return activity
@@ -260,8 +262,8 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
             let items = statusIDs.map { Item.status(id: $0, state: .unknown) }
             snapshot.appendItems(items, toSection: .statuses)
             dataSource.apply(snapshot, animatingDifferences: false) {
-                if let topID = activity.userInfo?["topID"] as? String,
-                   let index = statusIDs.firstIndex(of: topID),
+                if let centerID = activity.userInfo?["centerID"] as? String ?? activity.userInfo?["topID"] as? String,
+                   let index = 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
@@ -270,15 +272,15 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
                         count += 1
                         let origOffset = self.collectionView.contentOffset
                         self.collectionView.layoutIfNeeded()
-                        self.collectionView.scrollToItem(at: indexPath, at: .top, animated: false)
+                        self.collectionView.scrollToItem(at: indexPath, at: .centeredVertically, animated: false)
                         let newOffset = self.collectionView.contentOffset
                         if abs(origOffset.y - newOffset.y) <= 1 {
                             break
                         }
                     }
-                    stateRestorationLogger.fault("TimelineViewController: restored statuses with top ID \(topID)")
+                    stateRestorationLogger.fault("TimelineViewController: restored statuses with center ID \(centerID)")
                 } else {
-                    stateRestorationLogger.fault("TimelineViewController: restored statuses, but couldn't find top ID")
+                    stateRestorationLogger.fault("TimelineViewController: restored statuses, but couldn't find center ID")
                 }
             }
         }