Avoid flashing the room list skeletons on launch

On a cold launch the home screen appeared just before the cached rooms
published, briefly showing skeletons that were immediately replaced.

The home screen now starts in a new awaitingCachedRooms mode, keeping
the list blank while a cached room list is about to publish and falling
back to the skeletons after 500ms. Without a cache the skeletons show
straight away as before.

ElementX/Sources/Screens/HomeScreen/HomeScreenModels.swift

@@ -53,12 +53,16 @@ enum HomeScreenViewAction {
 }
 
 enum HomeScreenRoomListMode: CustomStringConvertible {
+    /// Shows nothing briefly on launch as the cached rooms usually publish in time to go straight to them.
+    case awaitingCachedRooms
     case skeletons
     case empty
     case rooms
     
     var description: String {
         switch self {
+        case .awaitingCachedRooms:
+            return "Awaiting cached rooms"
         case .skeletons:
             return "Showing placeholders"
         case .empty:
@@ -96,7 +100,7 @@ struct HomeScreenViewState: BindableState {
     var shouldShowNewSoundBanner = false
     
     var rooms: [HomeScreenRoom] = []
-    var roomListMode: HomeScreenRoomListMode = .skeletons
+    var roomListMode: HomeScreenRoomListMode = .awaitingCachedRooms
     
     var hasPendingInvitations = false
     
@@ -113,7 +117,7 @@ struct HomeScreenViewState: BindableState {
     var isRoomListSearchEnabled = true
     
     var visibleRooms: [HomeScreenRoom] {
-        if roomListMode == .skeletons {
+        if roomListMode == .skeletons || roomListMode == .awaitingCachedRooms {
             return placeholderRooms
         }
         

ElementX/Sources/Screens/HomeScreen/HomeScreenViewModel.swift

@@ -181,6 +181,13 @@ class HomeScreenViewModel: HomeScreenViewModelType, HomeScreenViewModelProtocol
             updateRoomListMode(with: roomSummaryProvider.statePublisher.value,
                                hasRooms: !roomSummaryProvider.roomListPublisher.value.isEmpty)
         }
+        
+        // Fall back to the skeletons if the cached rooms take too long to publish.
+        Task { [weak self] in
+            try? await Task.sleep(for: .milliseconds(500))
+            guard let self, state.roomListMode == .awaitingCachedRooms else { return }
+            state.roomListMode = .skeletons
+        }
     }
     
     // MARK: - Public
@@ -342,8 +349,8 @@ class HomeScreenViewModel: HomeScreenViewModelType, HomeScreenViewModelProtocol
             .empty // Loaded, there are no rooms at all.
         } else if hasRooms || isFiltering {
             .rooms // Loaded and the summaries have published (or filtered down to nothing).
-        } else if state.roomListMode == .skeletons {
+        } else if state.roomListMode == .skeletons || state.roomListMode == .awaitingCachedRooms {
-            .skeletons // Loaded but nothing published yet, flipping to .rooms would flash an empty list.
+            state.roomListMode // Loaded but nothing published yet, flipping to .rooms would flash an empty list.
         } else {
             .rooms
         }
@@ -352,7 +359,7 @@ class HomeScreenViewModel: HomeScreenViewModelType, HomeScreenViewModelProtocol
             return
         }
         
-        if roomListMode == .rooms, state.roomListMode == .skeletons {
+        if roomListMode == .rooms, state.roomListMode == .skeletons || state.roomListMode == .awaitingCachedRooms {
             analyticsService.signpost.finishTransaction(.cachedRoomList)
         }
         

ElementX/Sources/Screens/HomeScreen/View/HomeScreen.swift

@@ -219,7 +219,7 @@ struct HomeScreen_Previews: PreviewProvider, TestablePreview {
         let userID = "@alice:example.com"
         
         let roomSummaryProviderState: RoomSummaryProviderMockConfigurationState = switch mode {
-        case .skeletons:
+        case .awaitingCachedRooms, .skeletons:
             .loading
         case .empty:
             .loaded([])

ElementX/Sources/Screens/HomeScreen/View/HomeScreenContent.swift

@@ -25,7 +25,7 @@ struct HomeScreenContent: View {
         GeometryReader { geometry in
             ScrollView {
                 switch context.viewState.roomListMode {
-                case .skeletons:
+                case .awaitingCachedRooms, .skeletons:
                     LazyVStack(spacing: 0) {
                         ForEach(context.viewState.visibleRooms) { room in
                             HomeScreenRoomCell(room: room, isSelected: false, mediaProvider: context.mediaProvider, action: context.send)
@@ -34,6 +34,8 @@ struct HomeScreenContent: View {
                         }
                     }
                     .disabled(true)
+                    // Laid out but hidden while awaiting so the rooms replace it without a growing animation.
+                    .opacity(context.viewState.roomListMode == .awaitingCachedRooms ? 0 : 1)
                     .accessibilityRepresentation {
                         Text(L10n.commonLoading)
                     }
@@ -103,7 +105,7 @@ struct HomeScreenContent: View {
                 }
             }
             .scrollDismissesKeyboard(.immediately)
-            .scrollDisabled(context.viewState.roomListMode == .skeletons)
+            .scrollDisabled(context.viewState.roomListMode == .skeletons || context.viewState.roomListMode == .awaitingCachedRooms)
             .scrollBounceBehavior(context.viewState.roomListMode == .empty ? .basedOnSize : .automatic)
             .animation(.elementDefault, value: context.viewState.roomListMode)
             .animation(.none, value: context.viewState.visibleRooms)

UnitTests/Sources/HomeScreenViewModelTests.swift

@@ -421,6 +421,25 @@ final class HomeScreenViewModelTests {
         try await deferred.fulfill()
     }
     
+    @Test
+    func roomListModeGoesStraightToTheCachedRooms() async throws {
+        let (roomListSubject, _) = setupViewModelWithManualProvider(state: .loaded(totalNumberOfRooms: 8))
+        #expect(context.viewState.roomListMode == .awaitingCachedRooms)
+        
+        let deferred = deferFulfillment(context.$viewState) { $0.roomListMode == .rooms }
+        roomListSubject.send(.mockRooms)
+        try await deferred.fulfill()
+    }
+    
+    @Test
+    func roomListModeShowsSkeletonsWhenTheCachedRoomsAreSlow() async throws {
+        _ = setupViewModelWithManualProvider(state: .loaded(totalNumberOfRooms: 8))
+        #expect(context.viewState.roomListMode == .awaitingCachedRooms)
+        
+        let deferred = deferFulfillment(context.$viewState) { $0.roomListMode == .skeletons }
+        try await deferred.fulfill()
+    }
+    
     @Test
     func roomListModeDoesntReturnToSkeletonsWhenTheRoomsAreFilteredOut() async throws {
         let (roomListSubject, stateSubject) = setupViewModelWithManualProvider()
@@ -437,9 +456,9 @@ final class HomeScreenViewModelTests {
         try await failure.fulfill()
     }
     
-    private func setupViewModelWithManualProvider() -> (CurrentValueSubject<[RoomSummary], Never>, CurrentValueSubject<RoomSummaryProviderState, Never>) {
+    private func setupViewModelWithManualProvider(state: RoomSummaryProviderState = .notLoaded) -> (CurrentValueSubject<[RoomSummary], Never>, CurrentValueSubject<RoomSummaryProviderState, Never>) {
         let roomListSubject = CurrentValueSubject<[RoomSummary], Never>([])
-        let stateSubject = CurrentValueSubject<RoomSummaryProviderState, Never>(.notLoaded)
+        let stateSubject = CurrentValueSubject<RoomSummaryProviderState, Never>(state)
         
         let provider = RoomSummaryProviderMock()
         provider.roomListPublisher = roomListSubject.asCurrentValuePublisher()