Stop the timeline from requesting back pagination whilst loading existing items. (#2969)

* Handle Rust error "can't subscribe to the back-pagination status on a focused timeline"

* Fix a bug where the timeline was attempting to paginate before adding the items.

* Stop the timeline from paginating whilst loading existing items.

ElementX/Sources/Screens/RoomScreen/RoomScreenModels.swift

@@ -229,7 +229,7 @@ struct RoomMemberState {
 /// Is also nice to have this as a wrapper for any state that is directly connected to the timeline.
 struct TimelineViewState {
     var isLive = true
-    var paginationState = PaginationState.default
+    var paginationState = PaginationState.initial
     
     /// The room is in the process of loading items from a new timeline (switching to/from a detached timeline).
     var isSwitchingTimelines = false

ElementX/Sources/Screens/RoomScreen/View/Timeline/TimelineTableViewController.swift

@@ -117,7 +117,7 @@ class TimelineTableViewController: UIViewController {
     }
     
     /// The state of pagination (in both directions) of the current timeline.
-    var paginationState: PaginationState = .default {
+    var paginationState: PaginationState = .initial {
         didSet {
             // Paginate again if the threshold hasn't been satisfied.
             paginatePublisher.send(())

ElementX/Sources/Services/Timeline/RoomTimelineProvider.swift

@@ -24,7 +24,7 @@ class RoomTimelineProvider: RoomTimelineProviderProtocol {
     
     private var roomTimelineObservationToken: TaskHandle?
 
-    private let paginationStateSubject = CurrentValueSubject<PaginationState, Never>(.default)
+    private let paginationStateSubject = CurrentValueSubject<PaginationState, Never>(.initial)
     var paginationState: PaginationState {
         paginationStateSubject.value
     }
@@ -74,6 +74,17 @@ class RoomTimelineProvider: RoomTimelineProviderProtocol {
         }
     }
     
+    /// A continuation to signal whether the initial timeline items have been loaded and processed.
+    private var hasLoadedInitialItemsContinuation: CheckedContinuation<Void, Never>?
+    /// A method that allows `await`ing the first update of timeline items from the listener, as the items
+    /// aren't added directly to the provider upon initialisation and may take some time to come in.
+    func waitForInitialItems() async {
+        guard itemProxies.isEmpty else { return }
+        return await withCheckedContinuation { continuation in
+            hasLoadedInitialItemsContinuation = continuation
+        }
+    }
+    
     // MARK: - Private
     
     private func updateItemsWithDiffs(_ diffs: [TimelineDiff]) {
@@ -100,6 +111,11 @@ class RoomTimelineProvider: RoomTimelineProviderProtocol {
         }
         
         MXLog.verbose("Finished applying diffs, current items (\(itemProxies.count)) : \(itemProxies.map(\.debugIdentifier))")
+        
+        if let hasLoadedInitialItemsContinuation {
+            hasLoadedInitialItemsContinuation.resume()
+            self.hasLoadedInitialItemsContinuation = nil
+        }
     }
     
     // swiftlint:disable:next cyclomatic_complexity

ElementX/Sources/Services/Timeline/RoomTimelineProviderProtocol.swift

@@ -18,6 +18,10 @@ import Combine
 import Foundation
 import MatrixRustSDK
 
+enum PaginationDirection: String {
+    case backwards, forwards
+}
+
 enum PaginationStatus {
     case idle
     case timelineEndReached
@@ -25,7 +29,9 @@ enum PaginationStatus {
 }
 
 struct PaginationState: Equatable {
-    static var `default` = PaginationState(backward: .idle, forward: .timelineEndReached)
+    /// An initial state that is used to prevent pagination whilst loading the timeline.
+    /// Once the initial items are loaded the TimelineProxy will publish the correct value.
+    static var initial = PaginationState(backward: .timelineEndReached, forward: .timelineEndReached)
     let backward: PaginationStatus
     let forward: PaginationStatus
 }

ElementX/Sources/Services/Timeline/TimelineProxy.swift

@@ -23,7 +23,8 @@ final class TimelineProxy: TimelineProxyProtocol {
     
     private var backPaginationStatusObservationToken: TaskHandle?
     
-    private let backPaginationSubscriptionSubject = CurrentValueSubject<PaginationStatus, Never>(.idle)
+    // The default values don't matter here, they will be updated when calling subscribeToPagination.
+    private let backPaginationStatusSubject = CurrentValueSubject<PaginationStatus, Never>(.timelineEndReached)
     private let forwardPaginationStatusSubject = CurrentValueSubject<PaginationStatus, Never>(.timelineEndReached)
     
     let isLive: Bool
@@ -48,14 +49,19 @@ final class TimelineProxy: TimelineProxyProtocol {
             return
         }
         
-        let paginationStatePublisher = backPaginationSubscriptionSubject
+        let paginationStatePublisher = backPaginationStatusSubject
             .combineLatest(forwardPaginationStatusSubject)
             .map { PaginationState(backward: $0.0, forward: $0.1) }
             .eraseToAnyPublisher()
         
         await subscribeToPagination()
         
-        innerTimelineProvider = await RoomTimelineProvider(timeline: timeline, isLive: isLive, paginationStatePublisher: paginationStatePublisher)
+        let provider = await RoomTimelineProvider(timeline: timeline, isLive: isLive, paginationStatePublisher: paginationStatePublisher)
+        // Make sure the existing items are built so that we have content in the timeline before
+        // determining whether or not the timeline should paginate to load more items.
+        await provider.waitForInitialItems()
+        
+        innerTimelineProvider = provider
     }
     
     func fetchDetails(for eventID: String) {
@@ -75,6 +81,22 @@ final class TimelineProxy: TimelineProxyProtocol {
     }
     
     func paginateBackwards(requestSize: UInt16) async -> Result<Void, TimelineProxyError> {
+        // We can't subscribe to back pagination on detached timelines and as live timelines
+        // can be shared between multiple instances of the same room on the stack, it is
+        // safer to still use the subscription logic for back pagination when live.
+        await if isLive {
+            paginateBackwardsOnLive(requestSize: requestSize)
+        } else {
+            focussedPaginate(.backwards, requestSize: requestSize)
+        }
+    }
+    
+    func paginateForwards(requestSize: UInt16) async -> Result<Void, TimelineProxyError> {
+        await focussedPaginate(.forwards, requestSize: requestSize)
+    }
+    
+    /// Paginate backwards using the subscription from Rust to drive the pagination state.
+    private func paginateBackwardsOnLive(requestSize: UInt16) async -> Result<Void, TimelineProxyError> {
         MXLog.info("Paginating backwards")
         
         do {
@@ -88,26 +110,36 @@ final class TimelineProxy: TimelineProxyProtocol {
         }
     }
     
-    func paginateForwards(requestSize: UInt16) async -> Result<Void, TimelineProxyError> {
+    /// Paginate forward or backwards using our own logic to drive the pagination state as the
-        // This extra check is necessary as forwards pagination status doesn't support subscribing.
+    /// Rust subscription isn't allowed on focussed/detached timelines.
+    private func focussedPaginate(_ direction: PaginationDirection, requestSize: UInt16) async -> Result<Void, TimelineProxyError> {
+        let subject = switch direction {
+        case .backwards: backPaginationStatusSubject
+        case .forwards: forwardPaginationStatusSubject
+        }
+        
+        // This extra check is necessary as detached timelines don't support subscribing to pagination status.
         // We need it to make sure we send a valid status after a failure.
-        guard forwardPaginationStatusSubject.value == .idle else {
+        guard subject.value == .idle else {
-            MXLog.error("Attempting to paginate forwards when already at the end.")
+            MXLog.error("Attempting to paginate \(direction.rawValue) when already at the end.")
             return .failure(.failedPaginatingEndReached)
         }
-
+        
-        MXLog.info("Paginating forwards")
+        MXLog.info("Paginating \(direction.rawValue)")
-        forwardPaginationStatusSubject.send(.paginating)
+        subject.send(.paginating)
-
+        
         do {
-            let timelineEndReached = try await timeline.focusedPaginateForwards(numEvents: requestSize)
+            let timelineEndReached = try await switch direction {
-            MXLog.info("Finished paginating forwards")
+            case .backwards: timeline.paginateBackwards(numEvents: requestSize)
+            case .forwards: timeline.focusedPaginateForwards(numEvents: requestSize)
+            }
+            MXLog.info("Finished paginating \(direction.rawValue)")
 
-            forwardPaginationStatusSubject.send(timelineEndReached ? .timelineEndReached : .idle)
+            subject.send(timelineEndReached ? .timelineEndReached : .idle)
             return .success(())
         } catch {
-            MXLog.error("Failed paginating forwards with error: \(error)")
+            MXLog.error("Failed paginating \(direction.rawValue) with error: \(error)")
-            forwardPaginationStatusSubject.send(.idle)
+            subject.send(.idle)
             return .failure(.sdkError(error))
         }
     }
@@ -499,26 +531,31 @@ final class TimelineProxy: TimelineProxyProtocol {
     }
     
     private func subscribeToPagination() async {
-        let backPaginationListener = RoomPaginationStatusListener { [weak self] status in
+        if isLive {
-            guard let self else {
+            let backPaginationListener = RoomPaginationStatusListener { [weak self] status in
-                return
+                guard let self else {
+                    return
+                }
+                
+                switch status {
+                case .idle(let hitStartOfTimeline):
+                    backPaginationStatusSubject.send(hitStartOfTimeline ? .timelineEndReached : .idle)
+                case .paginating:
+                    backPaginationStatusSubject.send(.paginating)
+                }
             }
             
-            switch status {
+            do {
-            case .idle(let hitStartOfTimeline):
+                backPaginationStatusObservationToken = try await timeline.subscribeToBackPaginationStatus(listener: backPaginationListener)
-                backPaginationSubscriptionSubject.send(hitStartOfTimeline ? .timelineEndReached : .idle)
+            } catch {
-            case .paginating:
+                MXLog.error("Failed to subscribe to back pagination status with error: \(error)")
-                backPaginationSubscriptionSubject.send(.paginating)
             }
+        } else {
+            // Detached timelines don't support observation, set the initial state ourself.
+            backPaginationStatusSubject.send(.idle)
         }
         
-        do {
+        // Detached timelines don't support observation, set the initial state ourself.
-            backPaginationStatusObservationToken = try await timeline.subscribeToBackPaginationStatus(listener: backPaginationListener)
-        } catch {
-            MXLog.error("Failed to subscribe to back pagination status with error: \(error)")
-        }
-        
-        // Forward pagination doesn't support observation, set the initial state ourself.
         forwardPaginationStatusSubject.send(isLive ? .timelineEndReached : .idle)
     }
 }