Add a new loading-state for timeline-switches (IOS-235)

Gosh, this took longer than expected. It didn't work for quite some time but the trick is to check for the states in several places, like in HomeTimelineViewModel+Diffable or when the timeline reached the end. Those triggered a reload and we don't want that in case the state is `contextSwitch`.

Mastodon/Scene/HomeTimeline/HomeTimelineViewController+DataSourceProvider.swift

@@ -12,7 +12,7 @@ extension HomeTimelineViewController: DataSourceProvider {
     func item(from source: DataSourceItem.Source) async -> DataSourceItem? {
         var _indexPath = source.indexPath
         if _indexPath == nil, let cell = source.tableViewCell {
-            _indexPath = await self.indexPath(for: cell)
+            _indexPath = self.indexPath(for: cell)
         }
         guard let indexPath = _indexPath else { return nil }
         
@@ -37,8 +37,7 @@ extension HomeTimelineViewController: DataSourceProvider {
         viewModel?.dataController.update(status: status, intent: intent)
     }
 
-    @MainActor
-    private func indexPath(for cell: UITableViewCell) async -> IndexPath? {
+    private func indexPath(for cell: UITableViewCell) -> IndexPath? {
         return tableView.indexPath(for: cell)
     }
 }

Mastodon/Scene/HomeTimeline/HomeTimelineViewController.swift

@@ -109,7 +109,7 @@ final class HomeTimelineViewController: UIViewController, NeedsDependency, Media
             viewModel.timelineContext = .following
             viewModel.dataController.records = []
 
-            viewModel.loadLatestStateMachine.enter(HomeTimelineViewModel.LoadLatestState.LoadingManually.self)
+            viewModel.loadLatestStateMachine.enter(HomeTimelineViewModel.LoadLatestState.ContextSwitch.self)
             timelineSelectorButton.setAttributedTitle(
                 .init(string: "Following", attributes: [
                     .font: UIFontMetrics(forTextStyle: .headline).scaledFont(for: .systemFont(ofSize: 20, weight: .semibold))
@@ -124,9 +124,7 @@ final class HomeTimelineViewController: UIViewController, NeedsDependency, Media
             guard let self, let viewModel = self.viewModel else { return }
 
             viewModel.timelineContext = .localCommunity
-            viewModel.dataController.records = []
-
-            viewModel.loadLatestStateMachine.enter(HomeTimelineViewModel.LoadLatestState.LoadingManually.self)
+            viewModel.loadLatestStateMachine.enter(HomeTimelineViewModel.LoadLatestState.ContextSwitch.self)
             timelineSelectorButton.setAttributedTitle(
                 .init(string: "Local", attributes: [
                     .font: UIFontMetrics(forTextStyle: .headline).scaledFont(for: .systemFont(ofSize: 20, weight: .semibold))
@@ -591,8 +589,12 @@ extension HomeTimelineViewController: UITableViewDelegate, AutoGenerateTableView
     // sourcery:end
     
     func tableView(_ tableView: UITableView, willDisplay cell: UITableViewCell, forRowAt indexPath: IndexPath) {
+        guard let viewModel,
+                let currentState = viewModel.loadLatestStateMachine.currentState as? HomeTimelineViewModel.LoadLatestState,
+              (currentState.self is HomeTimelineViewModel.LoadLatestState.ContextSwitch) == false else { return }
+
         if indexPath.row == tableView.numberOfRows(inSection: indexPath.section) - 1 {
-            viewModel?.timelineDidReachEnd()
+            viewModel.timelineDidReachEnd()
         }
     }
 }

Mastodon/Scene/HomeTimeline/HomeTimelineViewModel+Diffable.swift

@@ -40,6 +40,9 @@ extension HomeTimelineViewModel {
                 guard let self = self else { return }
                 guard let diffableDataSource = self.diffableDataSource else { return }
 
+                guard let currentState = loadLatestStateMachine.currentState as? HomeTimelineViewModel.LoadLatestState,
+                      (currentState.self is HomeTimelineViewModel.LoadLatestState.ContextSwitch) == false else { return }
+
                 Task { @MainActor in
                     let oldSnapshot = diffableDataSource.snapshot()
                     var newSnapshot: NSDiffableDataSourceSnapshot<StatusSection, StatusItem> = {

Mastodon/Scene/HomeTimeline/HomeTimelineViewModel+LoadLatestState.swift

@@ -70,7 +70,33 @@ extension HomeTimelineViewModel.LoadLatestState {
     
     class Idle: HomeTimelineViewModel.LoadLatestState {
         override func isValidNextState(_ stateClass: AnyClass) -> Bool {
-            return stateClass == Loading.self || stateClass == LoadingManually.self
+            return stateClass == Loading.self || stateClass == LoadingManually.self || stateClass == ContextSwitch.self
+        }
+    }
+
+    class ContextSwitch: HomeTimelineViewModel.LoadLatestState {
+        override func isValidNextState(_ stateClass: AnyClass) -> Bool {
+            return stateClass == Loading.self || stateClass == LoadingManually.self  || stateClass == ContextSwitch.self
+        }
+
+        override func didEnter(from previousState: GKState?) {
+            guard let viewModel else { return }
+            guard let diffableDataSource = viewModel.diffableDataSource else {
+                assertionFailure()
+                return
+            }
+
+            OperationQueue.main.addOperation {
+                viewModel.dataController.records = []
+                var snapshot = NSDiffableDataSourceSnapshot<StatusSection, StatusItem>()
+                snapshot.appendSections([.main])
+                snapshot.appendItems([.topLoader], toSection: .main)
+                diffableDataSource.apply(snapshot) { [weak self] in
+                    guard let self else { return }
+
+                    self.stateMachine?.enter(Loading.self)
+                }
+            }
         }
     }
 
@@ -78,7 +104,7 @@ extension HomeTimelineViewModel.LoadLatestState {
         super.didEnter(from: previousState)
 
         guard let viewModel else { return }
-        
+
         let latestFeedRecords = viewModel.dataController.records.prefix(APIService.onceRequestStatusMaxCount)
 
         Task {

Mastodon/Scene/HomeTimeline/HomeTimelineViewModel.swift

@@ -55,6 +55,7 @@ final class HomeTimelineViewModel: NSObject {
             LoadLatestState.LoadingManually(viewModel: self),
             LoadLatestState.Fail(viewModel: self),
             LoadLatestState.Idle(viewModel: self),
+            LoadLatestState.ContextSwitch(viewModel: self),
         ])
         stateMachine.enter(LoadLatestState.Initial.self)
         return stateMachine