Fix back pagination (#432)

* Use pagination and start items for view state.

isBackPaginating and canBackPaginate are updated each time the timeline is rebuilt

* Update some timeline snapshots

The top section has gone, which has altered the layout slightly.

ElementX/Sources/Services/Timeline/TimelineController/MockRoomTimelineController.swift

@@ -42,20 +42,7 @@ class MockRoomTimelineController: RoomTimelineControllerProtocol {
     }
     
     func paginateBackwards(requestSize: UInt, untilNumberOfItems: UInt) async -> Result<Void, RoomTimelineControllerError> {
-        callbacks.send(.startedBackPaginating)
-        
-        guard !backPaginationResponses.isEmpty else {
-            callbacks.send(.finishedBackPaginating)
-            return .failure(.generic)
-        }
-        
-        let newItems = backPaginationResponses.removeFirst()
-        
-        try? await Task.sleep(for: backPaginationDelay)
-        timelineItems.insert(contentsOf: newItems, at: 0)
-        callbacks.send(.updatedTimelineItems)
-        callbacks.send(.finishedBackPaginating)
-        
+        callbacks.send(.canBackPaginate(false))
         return .success(())
     }
     
@@ -125,11 +112,12 @@ class MockRoomTimelineController: RoomTimelineControllerProtocol {
     /// Prepends the next chunk of items to the `timelineItems` array.
     private func simulateBackPagination() async throws {
         guard !backPaginationResponses.isEmpty else { return }
+        callbacks.send(.isBackPaginating(true))
         
         let newItems = backPaginationResponses.removeFirst()
         timelineItems.insert(contentsOf: newItems, at: 0)
         callbacks.send(.updatedTimelineItems)
-        callbacks.send(.finishedBackPaginating)
+        callbacks.send(.isBackPaginating(false))
         
         try? await connection?.send(.success)
     }

ElementX/Sources/Services/Timeline/TimelineController/RoomTimelineController.swift

@@ -61,18 +61,6 @@ class RoomTimelineController: RoomTimelineControllerProtocol {
             }
             .store(in: &cancellables)
         
-        self.timelineProvider
-            .backPaginationPublisher
-            .receive(on: DispatchQueue.main)
-            .sink { [weak self] value in
-                if value {
-                    self?.callbacks.send(.startedBackPaginating)
-                } else {
-                    self?.callbacks.send(.finishedBackPaginating)
-                }
-            }
-            .store(in: &cancellables)
-        
         updateTimelineItems()
         
         NotificationCenter.default.addObserver(self, selector: #selector(contentSizeCategoryDidChange), name: UIContentSizeCategory.didChangeNotification, object: nil)
@@ -230,6 +218,8 @@ class RoomTimelineController: RoomTimelineControllerProtocol {
     // swiftlint:disable:next cyclomatic_complexity
     private func asyncUpdateTimelineItems() async {
         var newTimelineItems = [RoomTimelineItemProtocol]()
+        var canBackPaginate = true
+        var isBackPaginating = false
 
         for (index, itemProxy) in timelineProvider.itemsPublisher.value.enumerated() {
             if Task.isCancelled {
@@ -262,8 +252,10 @@ class RoomTimelineController: RoomTimelineControllerProtocol {
                     }
                 case .loadingIndicator:
                     newTimelineItems.append(PaginationIndicatorRoomTimelineItem())
+                    isBackPaginating = true
                 case .timelineStart:
                     newTimelineItems.append(TimelineStartRoomTimelineItem(name: roomProxy.displayName ?? roomProxy.name))
+                    canBackPaginate = false
                 }
             default:
                 break
@@ -277,6 +269,8 @@ class RoomTimelineController: RoomTimelineControllerProtocol {
         timelineItems = newTimelineItems
         
         callbacks.send(.updatedTimelineItems)
+        callbacks.send(.canBackPaginate(canBackPaginate))
+        callbacks.send(.isBackPaginating(isBackPaginating))
     }
     
     private func computeGroupState(for itemProxy: TimelineItemProxy,

ElementX/Sources/Services/Timeline/TimelineController/RoomTimelineControllerProtocol.swift

@@ -21,8 +21,8 @@ import UIKit
 enum RoomTimelineControllerCallback {
     case updatedTimelineItems
     case updatedTimelineItem(_ itemId: String)
-    case startedBackPaginating
-    case finishedBackPaginating
+    case canBackPaginate(Bool)
+    case isBackPaginating(Bool)
 }
 
 enum RoomTimelineControllerAction {