implemented backward pagination for chat history with on-scroll prefetching

lib/view/pages/talk/chat_view.dart

@@ -42,9 +42,15 @@ class ChatView extends StatefulWidget {
 
 class _ChatViewState extends State<ChatView> with RouteAware {
   final ItemScrollController _itemScrollController = ItemScrollController();
+  final ItemPositionsListener _positionsListener =
+      ItemPositionsListener.create();
   final TextEditingController _searchTextController = TextEditingController();
   final Map<int, int> _matchIndices = {};
 
+  // Number of rows currently rendered; kept in sync in build so the scroll
+  // listener can tell when the oldest row (highest index, reverse list) nears.
+  int _itemCount = 0;
+
   bool _searchActive = false;
   String _searchQuery = '';
   List<ChatSearchMatch> _matches = const [];
@@ -63,9 +69,24 @@ class _ChatViewState extends State<ChatView> with RouteAware {
     super.initState();
     _chatBlocRef = context.read<ChatBloc>();
     _chatListBlocRef = context.read<ChatListBloc>();
+    _positionsListener.itemPositions.addListener(_onScrollPositions);
     NotificationTasks.clearNotificationsForChat(widget.room.token);
   }
 
+  /// Loads the next older block once the top of the list comes into view.
+  void _onScrollPositions() {
+    final positions = _positionsListener.itemPositions.value;
+    if (positions.isEmpty) return;
+    // reverse:true → the highest index is the oldest message (top of screen).
+    // Prefetch ~a screenful ahead so the next block is usually already merged
+    // before the user reaches the top — the load stays invisible.
+    final maxIndex = positions.map((p) => p.index).reduce(math.max);
+    if (maxIndex < _itemCount - _kLoadOlderPrefetchRows) return;
+    final data = _chatBlocRef?.state.data;
+    if (data == null || !data.hasMoreOld || data.isLoadingOlder) return;
+    _chatBlocRef?.loadOlder();
+  }
+
   @override
   void didChangeDependencies() {
     super.didChangeDependencies();
@@ -93,6 +114,7 @@ class _ChatViewState extends State<ChatView> with RouteAware {
     if (_subscribedRoute != null) {
       AppRoutes.chatRouteObserver.unsubscribe(this);
     }
+    _positionsListener.itemPositions.removeListener(_onScrollPositions);
     _markAsReadFinal();
     _chatBlocRef?.leaveChat(widget.room.token);
     _searchTextController.dispose();
@@ -281,22 +303,6 @@ class _ChatViewState extends State<ChatView> with RouteAware {
       );
     }
 
-    if (response.data.length >= 200) {
-      messages.insert(
-        0,
-        ChatBubble(
-          isSender: false,
-          bubbleData: GetChatResponseObject.getTextDummy(
-            'Zurzeit können in dieser App nur die letzten 200 vergangenen Nachrichten angezeigt werden. '
-            'Um ältere Nachrichten abzurufen verwende die Webversion unter https://cloud.marianum-fulda.de',
-          ),
-          chatData: widget.room,
-          refetch: ({bool renew = false}) => _refresh(),
-        ),
-      );
-      chronologicalMatchIndex.updateAll((_, v) => v + 1);
-    }
-
     final total = messages.length;
     _matchIndices
       ..clear()
@@ -373,9 +379,27 @@ class _ChatViewState extends State<ChatView> with RouteAware {
                     final items = _buildMessages(
                       state.chatResponse!,
                     ).reversed.toList();
+                    // reverse:true renders index 0 at the bottom, so the top
+                    // marker (spinner / start-of-chat) goes at the end.
+                    if (state.isLoadingOlder) {
+                      items.add(const _LoadingOlderIndicator());
+                    } else if (!state.hasMoreOld) {
+                      items.add(
+                        ChatBubble(
+                          isSender: false,
+                          bubbleData: GetChatResponseObject.getTextDummy(
+                            'Anfang des Chats',
+                          ),
+                          chatData: widget.room,
+                          refetch: ({bool renew = false}) => _refresh(),
+                        ),
+                      );
+                    }
+                    _itemCount = items.length;
                     return ScrollablePositionedList.builder(
                       reverse: true,
                       itemScrollController: _itemScrollController,
+                      itemPositionsListener: _positionsListener,
                       itemCount: items.length,
                       itemBuilder: (ctx, idx) => items[idx],
                     );
@@ -405,3 +429,22 @@ class _ChatViewState extends State<ChatView> with RouteAware {
     );
   }
 }
+
+/// How many rows before the oldest one the older-history prefetch kicks in.
+const _kLoadOlderPrefetchRows = 15;
+
+class _LoadingOlderIndicator extends StatelessWidget {
+  const _LoadingOlderIndicator();
+
+  @override
+  Widget build(BuildContext context) => const Padding(
+    padding: EdgeInsets.symmetric(vertical: 12),
+    child: Center(
+      child: SizedBox(
+        width: 22,
+        height: 22,
+        child: CircularProgressIndicator(strokeWidth: 2),
+      ),
+    ),
+  );
+}