msglist: Add topic list page

This commit creates a new topic list page showing all topics in
current channel with search functionality ordered by recency

Fixes part of zulip#1158.

Author: E-m-i-n-e-n-c-e

lib/widgets/topic_list.dart

@@ -0,0 +1,275 @@
+import 'package:flutter/material.dart';
+
+import '../api/model/model.dart';
+import '../api/route/channels.dart';
+import '../generated/l10n/zulip_localizations.dart';
+import '../model/message_list.dart';
+import '../model/narrow.dart';
+import 'app_bar.dart';
+import 'icons.dart';
+import 'message_list.dart';
+import 'page.dart';
+import 'store.dart';
+import 'theme.dart';
+import 'unread_count_badge.dart';
+
+/// A page that displays a list of topics in a channel.
+class TopicListPage extends StatefulWidget {
+  const TopicListPage({
+    super.key,
+    required this.streamId,
+    required this.messageListView,
+  });
+
+  final int streamId;
+  final MessageListView messageListView;
+  static AccountRoute<void> buildRoute({
+    int? accountId,
+    BuildContext? context,
+    required int streamId,
+    required MessageListView messageListView,
+  }) {
+    return MaterialAccountWidgetRoute(
+      accountId: accountId,
+      context: context,
+      page: TopicListPage(streamId: streamId, messageListView: messageListView),
+    );
+  }
+
+  @override
+  State<TopicListPage> createState() => _TopicListPageState();
+}
+
+class _TopicListPageState extends State<TopicListPage> with PerAccountStoreAwareStateMixin<TopicListPage> {
+  bool _isLoading = true;
+  List<GetStreamTopicsEntry> _topics = [];
+  List<GetStreamTopicsEntry> _filteredTopics = [];
+  MessageListView? _model;
+
+  late TextEditingController _searchController;
+  bool _isSearching = false;
+
+  @override
+  void initState() {
+    super.initState();
+    _searchController = TextEditingController();
+    _searchController.addListener(_filterTopics);
+  }
+
+  @override
+  void onNewStore() {
+    _model = widget.messageListView;
+    _model!.addListener(_onMessageListChanged);
+    _fetchTopics();
+  }
+
+  void _onMessageListChanged() {
+    _fetchTopics();
+  }
+
+  Future<void> _fetchTopics() async {
+      final store = PerAccountStoreWidget.of(context);
+      final result = await getStreamTopics(
+        store.connection,
+        streamId: widget.streamId,
+      );
+
+      setState(() {
+        _topics = result.topics;
+        _filterTopics(); // This will update _filteredTopics
+        _isLoading = false;
+      });
+  }
+
+  void _filterTopics() {
+    setState(() {
+      final query = _searchController.text.trim().toLowerCase();
+      if (query.isEmpty) {
+        _filteredTopics = List.from(_topics);
+      } else {
+        _filteredTopics = _topics
+            .where((topic) => topic.name.displayName.toLowerCase().contains(query))
+            .toList();
+      }
+
+      // Sort filtered topics by recency (most recent first)
+      _filteredTopics.sort((a, b) => b.maxId.compareTo(a.maxId));
+    });
+  }
+
+  void _toggleSearch() {
+    setState(() {
+      _isSearching = !_isSearching;
+      if (!_isSearching) {
+        _searchController.clear();
+      }
+    });
+  }
+
+  @override
+  void dispose() {
+    _searchController.dispose();
+    _model?.removeListener(_onMessageListChanged);
+    super.dispose();
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final store = PerAccountStoreWidget.of(context);
+    final stream = store.streams[widget.streamId];
+    final designVariables = DesignVariables.of(context);
+    final zulipLocalizations = ZulipLocalizations.of(context);
+
+    return Scaffold(
+      appBar: _isSearching
+          ? AppBar(
+              backgroundColor: designVariables.background,
+              title: TextField(
+                controller: _searchController,
+                autofocus: true,
+                decoration: InputDecoration(
+                  hintText: 'Search topic',
+                  border: InputBorder.none,
+                ),
+                style: const TextStyle(fontSize: 16),
+              ),
+              leading: IconButton(
+                icon: const Icon(Icons.arrow_back),
+                onPressed: _toggleSearch,
+              ),
+              actions: [
+                if (_searchController.text.isNotEmpty)
+                  IconButton(
+                    icon: const Icon(Icons.clear),
+                    onPressed: () => _searchController.clear(),
+                  ),
+              ],
+            )
+          : ZulipAppBar(
+              title: Text(stream?.name ?? 'Topics'),
+              backgroundColor: designVariables.background,
+              actions: [
+                IconButton(
+                  icon: const Icon(Icons.search),
+                  onPressed: _toggleSearch,
+                  tooltip: zulipLocalizations.searchTopicsPlaceholder,
+                ),
+              ],
+            ),
+      body: _buildBody(context),
+    );
+  }
+
+  Widget _buildBody(BuildContext context) {
+    if (_isLoading) {
+      return const Center(child: CircularProgressIndicator());
+    }
+
+    if (_topics.isEmpty) {
+      return const Center(
+        child: Text('No topics in this channel'),
+      );
+    }
+
+    if (_filteredTopics.isEmpty && _searchController.text.isNotEmpty) {
+      return Center(
+        child: Text('No topics matching "${_searchController.text}"'),
+      );
+    }
+
+    return ListView.builder(
+      itemCount: _filteredTopics.length,
+      itemBuilder: (context, index) {
+        final topic = _filteredTopics[index];
+        return _TopicItem(
+          streamId: widget.streamId,
+          topic: topic.name,
+        );
+      },
+    );
+  }
+}
+
+class _TopicItem extends StatelessWidget {
+  const _TopicItem({
+    required this.streamId,
+    required this.topic,
+  });
+
+  final int streamId;
+  final TopicName topic;
+
+  @override
+  Widget build(BuildContext context) {
+    final store = PerAccountStoreWidget.of(context);
+    final unreads = store.unreads;
+    final unreadCount = unreads.countInTopicNarrow(streamId, topic);
+    final hasMentions = unreads.mentions.any((id) {
+      final message = store.messages[id];
+      return message is StreamMessage &&
+             message.streamId == streamId &&
+             message.topic == topic;
+    });
+    final isMuted = !store.isTopicVisibleInStream(streamId, topic);
+
+    final designVariables = DesignVariables.of(context);
+    final opacity = isMuted ? 0.55 : 1.0;
+
+    return Material(
+      color: designVariables.background,
+      child: InkWell(
+        onTap: () {
+          Navigator.push(context,
+            MessageListPage.buildRoute(context: context,
+              narrow: TopicNarrow(streamId, topic)));
+        },
+        child: Padding(
+          padding: const EdgeInsets.symmetric(horizontal: 16.0, vertical: 12.0),
+          child: Row(
+            children: [
+              Opacity(
+                opacity: opacity,
+                child: Icon(
+                  ZulipIcons.topic,
+                  size: 18,
+                  color: designVariables.icon,
+                ),
+              ),
+              const SizedBox(width: 8),
+              Expanded(
+                child: Opacity(
+                  opacity: opacity,
+                  child: Text(
+                    topic.displayName,
+                    style: TextStyle(
+                      fontWeight: unreadCount > 0 ? FontWeight.bold : FontWeight.normal,
+                    ),
+                    overflow: TextOverflow.ellipsis,
+                  ),
+                ),
+              ),
+              if (isMuted)
+                Padding(
+                  padding: const EdgeInsets.only(left: 8.0),
+                  child: Icon(
+                    ZulipIcons.mute,
+                    size: 16,
+                    color: designVariables.icon.withValues(alpha: 0.5),
+                  ),
+                ),
+              if (unreadCount > 0)
+                Padding(
+                  padding: const EdgeInsets.only(left: 8.0),
+                  child: UnreadCountBadge(
+                    count: unreadCount,
+                    backgroundColor: null,
+                    bold: hasMentions,
+                  ),
+                ),
+            ],
+          ),
+        ),
+      ),
+    );
+  }
+}