autocomplete: Sort user-mention autocomplete results

Fixes: zulip#228

Author: sm-sayedi

lib/model/autocomplete.dart

@@ -246,7 +246,23 @@ class MentionAutocompleteView extends ChangeNotifier {
 
   Future<List<MentionAutocompleteResult>?> _computeResults(MentionAutocompleteQuery query) async {
     final List<MentionAutocompleteResult> results = [];
-    final Iterable<User> users = store.users.values;
+    final Set<int> userIds;
+
+    switch (narrow) {
+      case AllMessagesNarrow():
+        userIds = {};
+      case StreamNarrow():
+        userIds = store.dmUserIds;
+        userIds.addAll(store.streamUserIds[(narrow as StreamNarrow).streamId] ?? {});
+      case TopicNarrow():
+        userIds = store.dmUserIds;
+        userIds.addAll(store.streamUserIds[(narrow as TopicNarrow).streamId] ?? {});
+      case DmNarrow():
+        userIds = store.dmUserIds;
+    }
+
+    final users = Set.from(userIds.map((id) => store.users[id]!));
+    users.addAll(store.users.values);
 
     final iterator = users.iterator;
     bool isDone = false;

lib/model/message_list.dart

@@ -382,6 +382,7 @@ class MessageListView with ChangeNotifier, _MessageSequence {
       if (_messageVisible(message)) {
         _addMessage(message);
       }
+      store.getMessageUsers(message);
     }
     _fetched = true;
     _haveOldest = result.foundOldest;

lib/model/store.dart

@@ -287,6 +287,12 @@ class PerAccountStore extends ChangeNotifier with StreamStore {
 
   final Map<int, User> users;
 
+  Map<int, Set<int>> get streamUserIds => {..._streamUserIds};
+  final Map<int, Set<int>> _streamUserIds = {};
+
+  Set<int> get dmUserIds => {..._dmUserIds};
+  Set<int> _dmUserIds = {};
+
   ////////////////////////////////
   // Streams, topics, and stuff about them.
 
@@ -353,6 +359,22 @@ class PerAccountStore extends ChangeNotifier with StreamStore {
     super.dispose();
   }
 
+  void getMessageUsers(Message message) {
+    if (message is StreamMessage) {
+      Set<int> userIds = _streamUserIds[message.streamId] ?? {};
+      if (message.senderId == selfUserId) return;
+      userIds = {message.senderId}..addAll(userIds);
+      _streamUserIds[message.streamId] = userIds;
+    } else {
+      final desiredId = message.senderId == selfUserId
+          ? message.recipientId
+          : message.senderId;
+      _dmUserIds = {desiredId}..addAll(_dmUserIds);
+    }
+    debugLog('streamUsers: $_streamUserIds');
+    debugLog('dmUsers: $_dmUserIds');
+  }
+
   void handleEvent(Event event) {
     if (event is HeartbeatEvent) {
       assert(debugLog("server event: heartbeat"));
@@ -436,6 +458,7 @@ class PerAccountStore extends ChangeNotifier with StreamStore {
       notifyListeners();
     } else if (event is MessageEvent) {
       assert(debugLog("server event: message ${jsonEncode(event.message.toJson())}"));
+      getMessageUsers(event.message);
       recentDmConversationsView.handleMessageEvent(event);
       for (final view in _messageListViews) {
         view.maybeAddMessage(event.message);