msglist [nfc]: Migrate most MessageListItem subclasses to use MessageBase

except MessageListMessageItem.

This keeps changes minimal, leaving most of the helpers in
lib/model/message_list.dart untouched, to avoid unnecessary
generalization.

Regarding potential performance impact:
StreamMessage.destination and DmMessage.destination both construct
MessageDestination from scratch, to avoid keeping duplicate info in
memory.  On the flip side, there might be some slight cost of
constructing DmMessage.destination for builds when we convert
DmMessage.allRecipientIds to lists.  Since we are already doing
O(N) traversals on the list anyway (see DmRecipientHeader.build),
this cost should be negligible.

Author: PIG208

lib/model/message_list.dart

@@ -24,13 +24,13 @@ sealed class MessageListItem {
 }
 
 class MessageListRecipientHeaderItem extends MessageListItem {
-  final Message message;
+  final MessageBase message;
 
   MessageListRecipientHeaderItem(this.message);
 }
 
 class MessageListDateSeparatorItem extends MessageListItem {
-  final Message message;
+  final MessageBase message;
 
   MessageListDateSeparatorItem(this.message);
 }
@@ -155,7 +155,7 @@ mixin _MessageSequence {
         }
       case MessageListRecipientHeaderItem(:var message):
       case MessageListDateSeparatorItem(:var message):
-        return (message.id <= messageId) ? -1 : 1;
+        return message.id != null && message.id! <= messageId ? -1 : 1;
       case MessageListMessageItem(:var message): return message.id.compareTo(messageId);
     }
   }

lib/model/narrow.dart

@@ -194,7 +194,9 @@ class DmNarrow extends Narrow implements SendableNarrow {
     );
   }
 
-  factory DmNarrow.ofMessage(DmMessage message, {required int selfUserId}) {
+  factory DmNarrow.ofMessage(MessageBase<DmDestination> message, {
+    required int selfUserId,
+  }) {
     return DmNarrow(
       allRecipientIds: List.unmodifiable(message.destination.userIds),
       selfUserId: selfUserId,

lib/widgets/message_list.dart

@@ -7,6 +7,7 @@ import 'package:flutter_color_models/flutter_color_models.dart';
 import 'package:intl/intl.dart' hide TextDirection;
 
 import '../api/model/model.dart';
+import '../api/route/messages.dart';
 import '../generated/l10n/zulip_localizations.dart';
 import '../model/message_list.dart';
 import '../model/narrow.dart';
@@ -901,23 +902,28 @@ class _MarkAsReadAnimationState extends State<MarkAsReadAnimation> {
 class RecipientHeader extends StatelessWidget {
   const RecipientHeader({super.key, required this.message, required this.narrow});
 
-  final Message message;
+  final MessageBase message;
   final Narrow narrow;
 
   @override
   Widget build(BuildContext context) {
     final message = this.message;
-    return switch (message) {
-      StreamMessage() => StreamMessageRecipientHeader(message: message, narrow: narrow),
-      DmMessage() => DmRecipientHeader(message: message, narrow: narrow),
-    };
+    switch (message) {
+      case MessageBase<StreamDestination>():
+        return StreamMessageRecipientHeader(message: message, narrow: narrow);
+      case MessageBase<DmDestination>():
+        return DmRecipientHeader(message: message, narrow: narrow);
+      case MessageBase<MessageDestination>():
+        assert(false, 'Unexpected concrete subclass of MessageBase<MessageDestination>');
+        return SizedBox.shrink();
+    }
   }
 }
 
 class DateSeparator extends StatelessWidget {
   const DateSeparator({super.key, required this.message});
 
-  final Message message;
+  final MessageBase message;
 
   @override
   Widget build(BuildContext context) {
@@ -1027,7 +1033,7 @@ class StreamMessageRecipientHeader extends StatelessWidget {
     required this.narrow,
   });
 
-  final StreamMessage message;
+  final MessageBase<StreamDestination> message;
   final Narrow narrow;
 
   static bool _containsDifferentChannels(Narrow narrow) {
@@ -1053,11 +1059,11 @@ class StreamMessageRecipientHeader extends StatelessWidget {
     final designVariables = DesignVariables.of(context);
     final zulipLocalizations = ZulipLocalizations.of(context);
 
-    final topic = message.topic;
+    final StreamDestination(:streamId, :topic) = message.destination;
 
     final messageListTheme = MessageListTheme.of(context);
 
-    final subscription = store.subscriptions[message.streamId];
+    final subscription = store.subscriptions[streamId];
     final Color backgroundColor;
     final Color iconColor;
     if (subscription != null) {
@@ -1073,16 +1079,17 @@ class StreamMessageRecipientHeader extends StatelessWidget {
     if (!_containsDifferentChannels(narrow)) {
       streamWidget = const SizedBox(width: 16);
     } else {
-      final stream = store.streams[message.streamId];
+      final stream = store.streams[streamId];
+      final message = this.message;
       final streamName = stream?.name
-        ?? message.displayRecipient
+        ?? (message is StreamMessage ? message.displayRecipient : null)
         ?? zulipLocalizations.unknownChannelName; // TODO(log)
 
       streamWidget = GestureDetector(
         onTap: () => Navigator.push(context,
           MessageListPage.buildRoute(context: context,
-            narrow: ChannelNarrow(message.streamId))),
-        onLongPress: () => showChannelActionSheet(context, channelId: message.streamId),
+            narrow: ChannelNarrow(streamId))),
+        onLongPress: () => showChannelActionSheet(context, channelId: streamId),
         child: Row(
           crossAxisAlignment: CrossAxisAlignment.center,
           children: [
@@ -1130,7 +1137,7 @@ class StreamMessageRecipientHeader extends StatelessWidget {
           Icon(size: 14, color: designVariables.title.withFadedAlpha(0.5),
             // A null [Icon.icon] makes a blank space.
             iconDataForTopicVisibilityPolicy(
-              store.topicVisibilityPolicy(message.streamId, topic))),
+              store.topicVisibilityPolicy(streamId, topic))),
         ]));
 
     return GestureDetector(
@@ -1141,9 +1148,9 @@ class StreamMessageRecipientHeader extends StatelessWidget {
       onTap: narrow is TopicNarrow ? null
         : () => Navigator.push(context,
             MessageListPage.buildRoute(context: context,
-              narrow: TopicNarrow.ofMessage(message))),
+              narrow: TopicNarrow(streamId, topic))),
       onLongPress: () => showTopicActionSheet(context,
-        channelId: message.streamId,
+        channelId: streamId,
         topic: topic,
         someMessageIdInTopic: message.id),
       child: ColoredBox(
@@ -1168,7 +1175,7 @@ class DmRecipientHeader extends StatelessWidget {
     required this.narrow,
   });
 
-  final DmMessage message;
+  final MessageBase<DmDestination> message;
   final Narrow narrow;
 
   @override
@@ -1233,7 +1240,7 @@ TextStyle recipientHeaderTextStyle(BuildContext context, {FontStyle? fontStyle})
 class RecipientHeaderDate extends StatelessWidget {
   const RecipientHeaderDate({super.key, required this.message});
 
-  final Message message;
+  final MessageBase message;
 
   @override
   Widget build(BuildContext context) {

test/model/message_list_test.dart

@@ -1994,11 +1994,11 @@ void checkInvariants(MessageListView model) {
 }
 
 extension MessageListRecipientHeaderItemChecks on Subject<MessageListRecipientHeaderItem> {
-  Subject<Message> get message => has((x) => x.message, 'message');
+  Subject<MessageBase> get message => has((x) => x.message, 'message');
 }
 
 extension MessageListDateSeparatorItemChecks on Subject<MessageListDateSeparatorItem> {
-  Subject<Message> get message => has((x) => x.message, 'message');
+  Subject<MessageBase> get message => has((x) => x.message, 'message');
 }
 
 extension MessageListMessageItemChecks on Subject<MessageListMessageItem> {