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.

This hoists streamId in StreamMessageRecipientHeader.build,
where we used to access streamId via message instead from the
onLongPress callback.  Because Message.streamId only changes on
message moves, we expect the build method to be called again, so
this should be fine.

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 +155,8 @@ mixin _MessageSequence {
         }
       case MessageListRecipientHeaderItem(:var message):
       case MessageListDateSeparatorItem(:var message):
-        return (message.id <= messageId) ? -1 : 1;
+        if (message.id == null)                  return 1;
+        return message.id! <= messageId ? -1 : 1;
       case MessageListMessageItem(:var message): return message.id.compareTo(messageId);
     }
   }

lib/model/narrow.dart

@@ -94,8 +94,8 @@ class ChannelNarrow extends Narrow {
 class TopicNarrow extends Narrow implements SendableNarrow {
   const TopicNarrow(this.streamId, this.topic, {this.with_});
 
-  factory TopicNarrow.ofMessage(StreamMessage message) {
-    return TopicNarrow(message.streamId, message.topic);
+  factory TopicNarrow.ofMessage(MessageBase<StreamConversation> message) {
+    return TopicNarrow(message.conversation.streamId, message.conversation.topic);
   }
 
   final int streamId;
@@ -194,9 +194,11 @@ class DmNarrow extends Narrow implements SendableNarrow {
     );
   }
 
-  factory DmNarrow.ofMessage(DmMessage message, {required int selfUserId}) {
+  factory DmNarrow.ofMessage(MessageBase<DmConversation> message, {
+    required int selfUserId,
+  }) {
     return DmNarrow(
-      allRecipientIds: List.unmodifiable(message.allRecipientIds),
+      allRecipientIds: List.unmodifiable(message.conversation.allRecipientIds),
       selfUserId: selfUserId,
     );
   }

lib/widgets/message_list.dart

@@ -901,23 +901,27 @@ 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),
+      MessageBase<StreamConversation>() =>
+        StreamMessageRecipientHeader(message: message, narrow: narrow),
+      MessageBase<DmConversation>() =>
+        DmRecipientHeader(message: message, narrow: narrow),
+      MessageBase<Conversation>() =>
+        throw StateError('Bad concrete subclass of MessageBase'),
     };
   }
 }
 
 class DateSeparator extends StatelessWidget {
   const DateSeparator({super.key, required this.message});
 
-  final Message message;
+  final MessageBase message;
 
   @override
   Widget build(BuildContext context) {
@@ -1027,7 +1031,7 @@ class StreamMessageRecipientHeader extends StatelessWidget {
     required this.narrow,
   });
 
-  final StreamMessage message;
+  final MessageBase<StreamConversation> message;
   final Narrow narrow;
 
   static bool _containsDifferentChannels(Narrow narrow) {
@@ -1053,11 +1057,11 @@ class StreamMessageRecipientHeader extends StatelessWidget {
     final designVariables = DesignVariables.of(context);
     final zulipLocalizations = ZulipLocalizations.of(context);
 
-    final topic = message.topic;
+    final StreamConversation(:streamId, :topic) = message.conversation;
 
     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 +1077,16 @@ 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 streamName = stream?.name
         ?? message.conversation.displayRecipient
         ?? 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 +1134,7 @@ 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(
@@ -1143,7 +1147,7 @@ class StreamMessageRecipientHeader extends StatelessWidget {
             MessageListPage.buildRoute(context: context,
               narrow: TopicNarrow.ofMessage(message))),
       onLongPress: () => showTopicActionSheet(context,
-        channelId: message.streamId,
+        channelId: streamId,
         topic: topic,
         someMessageIdInTopic: message.id),
       child: ColoredBox(
@@ -1168,20 +1172,21 @@ class DmRecipientHeader extends StatelessWidget {
     required this.narrow,
   });
 
-  final DmMessage message;
+  final MessageBase<DmConversation> message;
   final Narrow narrow;
 
   @override
   Widget build(BuildContext context) {
     final zulipLocalizations = ZulipLocalizations.of(context);
     final store = PerAccountStoreWidget.of(context);
     final String title;
-    if (message.allRecipientIds.length > 1) {
-      title = zulipLocalizations.messageListGroupYouAndOthers(message.allRecipientIds
-        .where((id) => id != store.selfUserId)
-        .map(store.userDisplayName)
-        .sorted()
-        .join(", "));
+    if (message.conversation.allRecipientIds.length > 1) {
+      title = zulipLocalizations.messageListGroupYouAndOthers(
+        message.conversation.allRecipientIds
+          .where((id) => id != store.selfUserId)
+          .map(store.userDisplayName)
+          .sorted()
+          .join(", "));
     } else {
       title = zulipLocalizations.messageListGroupYouWithYourself;
     }
@@ -1233,7 +1238,7 @@ 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> {