Completely replace all items when jumping to present

Tusker/Screens/Timeline/TimelineViewController.swift

@@ -343,21 +343,19 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
     }
     
     private func insertPresentItemsIfNecessary(_ presentItems: [String]) {
-        var snapshot = dataSource.snapshot()
+        let snapshot = dataSource.snapshot()
         guard snapshot.indexOfSection(.statuses) != nil else {
             return
         }
         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
+           // if there's no overlap between presentItems and the existing items in the data source, prompt the user
            !presentItems.contains(firstID) {
             
-            // remove any existing gap, if there is one
-            if let index = currentItems.lastIndex(of: .gap) {
-                snapshot.deleteItems(Array(currentItems[index...]))
-            }
-            snapshot.insertItems([.gap], beforeItem: currentItems.first!)
-            snapshot.insertItems(presentItems.map { .status(id: $0, state: .unknown) }, beforeItem: .gap)
+            // create a new snapshot to reset the timeline to the "present" state
+            var snapshot = NSDiffableDataSourceSnapshot<Section, Item>()
+            snapshot.appendSections([.statuses])
+            snapshot.appendItems(presentItems.map { .status(id: $0, state: .unknown) }, toSection: .statuses)
             
             var config = ToastConfiguration(title: "Jump to present")
             config.edge = .top
@@ -366,12 +364,34 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
             config.action = { [unowned self] toast in
                 toast.dismissToast(animated: true)
                 
+                let origSnapshot = self.dataSource.snapshot()
+                let origItemAtTop: (Item, CGFloat)?
+                if let statusesSection = origSnapshot.indexOfSection(.statuses),
+                   let indexPath = self.collectionView.indexPathsForVisibleItems.sorted().first(where: { $0.section == statusesSection }),
+                   let cell = self.collectionView.cellForItem(at: indexPath),
+                   let item = self.dataSource.itemIdentifier(for: indexPath) {
+                    origItemAtTop = (item, cell.convert(.zero, to: self.view).y - self.view.safeAreaInsets.top)
+                } else {
+                    origItemAtTop = nil
+                }
+                
                 self.dataSource.apply(snapshot, animatingDifferences: true) {
-                    // TODO: we can't set prevScrollOffsetBeforeScrollToTop here to allow undoing the scroll-to-top
-                    // because that would involve scrolling through unmeasured-cell which fucks up the content offset values.
-                    // we probably need a data-source aware implementation of scrollToTop which uses item & offset w/in item
-                    // to track the restore position
                     self.collectionView.scrollToItem(at: IndexPath(row: 0, section: 0), at: .top, animated: true)
+                    
+                    var config = ToastConfiguration(title: "Go back")
+                    config.edge = .top
+                    config.systemImageName = "arrow.down"
+                    config.dismissAutomaticallyAfter = 4
+                    config.action = { [unowned self] toast in
+                        toast.dismissToast(animated: true)
+                        // todo: it would be nice if we could animate this, but that doesn't work with the screen-position-maintaining stuff
+                        if let (item, offset) = origItemAtTop {
+                            self.applySnapshot(snapshot, maintainingScreenPosition: offset, ofItem: item)
+                        } else {
+                            self.dataSource.apply(origSnapshot, animatingDifferences: false)
+                        }
+                    }
+                    self.showToast(configuration: config, animated: true)
                 }
             }
             self.showToast(configuration: config, animated: true)
@@ -380,32 +400,35 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
     
     // NOTE: this only works when items are being inserted ABOVE the item to maintain
     private func applySnapshot(_ snapshot: NSDiffableDataSourceSnapshot<Section, Item>, maintainingBottomRelativeScrollPositionOf itemToMaintain: Item) {
-        // 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)
-        
         var firstItemAfterOriginalGapOffsetFromTop: CGFloat = 0
         if let indexPath = dataSource.indexPath(for: itemToMaintain),
            let cell = collectionView.cellForItem(at: indexPath) {
             // subtract top safe area inset b/c scrollToItem at .top aligns the top of the cell to the top of the safe area
             firstItemAfterOriginalGapOffsetFromTop = cell.convert(.zero, to: view).y - view.safeAreaInsets.top
         }
+        applySnapshot(snapshot, maintainingScreenPosition: firstItemAfterOriginalGapOffsetFromTop, ofItem: itemToMaintain)
+    }
+    
+    private func applySnapshot(_ snapshot: NSDiffableDataSourceSnapshot<Section, Item>, maintainingScreenPosition offsetFromTop: CGFloat, ofItem itemToMaintain: Item) {
+        // 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)
         
         dataSource.apply(snapshot, animatingDifferences: false) {
-            if let indexPathOfItemAfterOriginalGap = self.dataSource.indexPath(for: itemToMaintain) {
+            if let indexPathOfItemToMaintain = self.dataSource.indexPath(for: itemToMaintain) {
                 // scroll up until we've accumulated enough MEASURED height that we can put the
                 // firstItemAfterOriginalGapCell at the top of the screen and then scroll down by
                 // firstItemAfterOriginalGapOffsetFromTop without intruding into unmeasured area
-                var cur = indexPathOfItemAfterOriginalGap
+                var cur = indexPathOfItemToMaintain
                 var amountScrolledUp: CGFloat = 0
                 while true {
                     if cur.row <= 0 {
                         break
                     }
-                    if let cell = self.collectionView.cellForItem(at: indexPathOfItemAfterOriginalGap),
-                       cell.convert(.zero, to: self.view).y - self.view.safeAreaInsets.top > firstItemAfterOriginalGapOffsetFromTop {
+                    if let cell = self.collectionView.cellForItem(at: indexPathOfItemToMaintain),
+                       cell.convert(.zero, to: self.view).y - self.view.safeAreaInsets.top > offsetFromTop {
                         break
                     }
                     cur = IndexPath(row: cur.row - 1, section: cur.section)
@@ -415,7 +438,7 @@ class TimelineViewController: UIViewController, TimelineLikeCollectionViewContro
                     amountScrolledUp += attrs.size.height
                 }
                 self.collectionView.contentOffset.y += amountScrolledUp
-                self.collectionView.contentOffset.y -= firstItemAfterOriginalGapOffsetFromTop
+                self.collectionView.contentOffset.y -= offsetFromTop
             }
             
             snapshotView.removeFromSuperview()