api [nfc]: Pull out DisplayableMessage

See also CZO discussion on the design of this, and the drawbacks of
the alternatives:
  https://chat.zulip.org/#narrow/channel/243-mobile-team/topic/A.20new.20variant.20of.20Zulip.20messages.20-.20inheritance.20structure/with/2141288

This will help support locally echoed messages in MessageListView later.

This requiring specifying the element type of `List`s in some tests (or
in some cases, the type of a variable declared).

This is a side effect of making `StreamMessage` and `DmMessage` extend
`Message<T>` with different `T`'s. When both appear in the same `List`,
the upper bound is `Object`, instead of the more specific
`Message<MessageDestination>`.
See "least-upper-bound" tagged issues for reference:
  https://github.com/dart-lang/language/issues?q=state%3Aopen%20label%3A%22least-upper-bound%22

Author: PIG208

lib/api/model/model.dart

@@ -1,5 +1,6 @@
 import 'package:json_annotation/json_annotation.dart';
 
+import '../route/messages.dart';
 import 'events.dart';
 import 'initial_snapshot.dart';
 import 'reaction.dart';
@@ -531,10 +532,29 @@ String? tryParseEmojiCodeToUnicode(String emojiCode) {
   }
 }
 
+/// A common class for messages that can appear in a [MessageList].
+abstract class DisplayableMessage<T extends MessageDestination> {
+  /// The Zulip message ID.
+  ///
+  /// If null, the message doesn't have an ID acknowledged by the server
+  /// (e.g.: a locally-echoed message).
+  int? get id;
+
+  int get senderId;
+  int get timestamp;
+
+  /// The destination this message was sent to.
+  // When implementing this, the return type should be either [StreamDestination]
+  // or [DmDestination]; it should never be [MessageDestination], because we
+  // expect a concrete subclass of [DisplayableMessage] to represent either
+  // channel messages or DM messages, not both.
+  T get destination;
+}
+
 /// As in the get-messages response.
 ///
 /// https://zulip.com/api/get-messages#response
-sealed class Message {
+sealed class Message<T extends MessageDestination> implements DisplayableMessage<T> {
   // final String? avatarUrl; // Use [User.avatarUrl] instead; will live-update
   final String client;
   String content;
@@ -544,6 +564,7 @@ sealed class Message {
   @JsonKey(readValue: MessageEditState._readFromMessage, fromJson: Message._messageEditStateFromJson)
   MessageEditState editState;
 
+  @override
   final int id;
   bool isMeMessage;
   int? lastEditTimestamp;
@@ -554,13 +575,15 @@ sealed class Message {
   final int recipientId;
   final String senderEmail;
   final String senderFullName;
+  @override
   final int senderId;
   final String senderRealmStr;
 
   /// Poll data if "submessages" describe a poll, `null` otherwise.
   @JsonKey(name: 'submessages', readValue: _readPoll, fromJson: Poll.fromJson, toJson: Poll.toJson)
   Poll? poll;
 
+  @override
   final int timestamp;
   String get type;
 
@@ -619,6 +642,8 @@ sealed class Message {
     required this.matchTopic,
   });
 
+  // TODO(dart): This has to be a static method, because factories/constructors
+  //   do not support type parameters: https://github.com/dart-lang/language/issues/647
   static Message fromJson(Map<String, dynamic> json) {
     final type = json['type'] as String;
     if (type == 'stream') return StreamMessage.fromJson(json);
@@ -715,7 +740,7 @@ extension type const TopicName(String _value) {
 }
 
 @JsonSerializable(fieldRename: FieldRename.snake)
-class StreamMessage extends Message {
+class StreamMessage extends Message<StreamDestination> {
   @override
   @JsonKey(includeToJson: true)
   String get type => 'stream';
@@ -735,6 +760,9 @@ class StreamMessage extends Message {
   @JsonKey(name: 'subject')
   TopicName topic;
 
+  @override
+  StreamDestination get destination => StreamDestination(streamId, topic);
+
   StreamMessage({
     required super.client,
     required super.content,
@@ -809,7 +837,7 @@ class DmRecipientListConverter extends JsonConverter<List<DmRecipient>, List<dyn
 }
 
 @JsonSerializable(fieldRename: FieldRename.snake)
-class DmMessage extends Message {
+class DmMessage extends Message<DmDestination> {
   @override
   @JsonKey(includeToJson: true)
   String get type => 'private';
@@ -838,6 +866,9 @@ class DmMessage extends Message {
   /// This is a result of [List.map], so it has an efficient `length`.
   Iterable<int> get allRecipientIds => displayRecipient.map((e) => e.id);
 
+  @override
+  DmDestination get destination => DmDestination(userIds: allRecipientIds.toList());
+
   DmMessage({
     required super.client,
     required super.content,

test/api/model/model_checks.dart

@@ -1,6 +1,7 @@
 import 'package:checks/checks.dart';
 import 'package:zulip/api/model/model.dart';
 import 'package:zulip/api/model/submessage.dart';
+import 'package:zulip/api/route/messages.dart';
 
 extension UserChecks on Subject<User> {
   Subject<int> get userId => has((x) => x.userId, 'userId');
@@ -24,6 +25,13 @@ extension UserChecks on Subject<User> {
 extension ZulipStreamChecks on Subject<ZulipStream> {
 }
 
+extension DisplayableMessageChecks<T extends MessageDestination> on Subject<DisplayableMessage<T>> {
+  Subject<int?> get id => has((e) => e.id, 'id');
+  Subject<int> get senderId => has((e) => e.senderId, 'senderId');
+  Subject<int> get timestamp => has((e) => e.timestamp, 'timestamp');
+  Subject<T> get destination => has((e) => e.destination, 'destination');
+}
+
 extension MessageChecks on Subject<Message> {
   Subject<String> get client => has((e) => e.client, 'client');
   Subject<String> get content => has((e) => e.content, 'content');
@@ -36,10 +44,8 @@ extension MessageChecks on Subject<Message> {
   Subject<int> get recipientId => has((e) => e.recipientId, 'recipientId');
   Subject<String> get senderEmail => has((e) => e.senderEmail, 'senderEmail');
   Subject<String> get senderFullName => has((e) => e.senderFullName, 'senderFullName');
-  Subject<int> get senderId => has((e) => e.senderId, 'senderId');
   Subject<String> get senderRealmStr => has((e) => e.senderRealmStr, 'senderRealmStr');
   Subject<Poll?> get poll => has((e) => e.poll, 'poll');
-  Subject<int> get timestamp => has((e) => e.timestamp, 'timestamp');
   Subject<String> get type => has((e) => e.type, 'type');
   Subject<List<MessageFlag>> get flags => has((e) => e.flags, 'flags');
   Subject<String?> get matchContent => has((e) => e.matchContent, 'matchContent');

test/model/message_list_test.dart

@@ -529,7 +529,7 @@ void main() {
     test('unmute a topic -> refetch from scratch', () => awaitFakeAsync((async) async {
       await prepare(narrow: const CombinedFeedNarrow());
       await prepareMutes(true);
-      final messages = [
+      final messages = <Message>[
         eg.dmMessage(id: 1, from: eg.otherUser, to: [eg.selfUser]),
         eg.streamMessage(id: 2, stream: stream, topic: topic),
       ];

test/model/message_test.dart

@@ -84,7 +84,7 @@ void main() {
       final message1 = eg.streamMessage();
       final message2 = eg.streamMessage();
       final message3 = eg.dmMessage(from: eg.otherUser, to: [eg.selfUser]);
-      final messages = [message1, message2, message3];
+      final messages = <Message>[message1, message2, message3];
       store.reconcileMessages(messages);
       check(messages).deepEquals(
         [message1, message2, message3]
@@ -99,7 +99,7 @@ void main() {
       final message1 = eg.streamMessage();
       final message2 = eg.streamMessage();
       final message3 = eg.dmMessage(from: eg.otherUser, to: [eg.selfUser]);
-      final messages = [message1, message2, message3];
+      final messages = <Message>[message1, message2, message3];
       await addMessages(messages);
       final newMessage = eg.streamMessage();
       store.reconcileMessages([newMessage]);
@@ -139,7 +139,7 @@ void main() {
 
     test('from not-empty', () async {
       await prepare();
-      final messages = [
+      final messages = <Message>[
         eg.streamMessage(),
         eg.streamMessage(),
         eg.dmMessage(from: eg.otherUser, to: [eg.selfUser]),

test/model/unreads_test.dart

@@ -248,7 +248,7 @@ void main() {
     final unreadChannelMessage = eg.streamMessage(flags: []);
     final readChannelMessage = eg.streamMessage(flags: [MessageFlag.read]);
 
-    final allMessages = [
+    final allMessages = <Message>[
       unreadDmMessage, unreadChannelMessage,
       readDmMessage,   readChannelMessage,
     ];
@@ -315,7 +315,7 @@ void main() {
           if (isDirectMentioned)   MessageFlag.mentioned,
           if (isWildcardMentioned) MessageFlag.wildcardMentioned,
         ];
-        final message = isStream
+        final Message message = isStream
           ? eg.streamMessage(flags: flags)
           : eg.dmMessage(from: eg.otherUser, to: [eg.selfUser], flags: flags);
         model.handleMessageEvent(MessageEvent(id: 0, message: message));
@@ -402,7 +402,7 @@ void main() {
       for (final isKnownToModel in [true, false]) {
         for (final isRead in [false, true]) {
           final baseFlags = [if (isRead) MessageFlag.read];
-          for (final (messageDesc, message) in [
+          for (final (messageDesc, message) in <(String, Message)>[
             ('stream', eg.streamMessage(flags: baseFlags)),
             ('1:1 dm', eg.dmMessage(from: eg.otherUser, to: [eg.selfUser], flags: baseFlags)),
           ]) {
@@ -662,7 +662,7 @@ void main() {
     final message13 = eg.streamMessage(id: 13, stream: stream2, topic: 'b', flags: []);
     final message14 = eg.streamMessage(id: 14, stream: stream2, topic: 'b', flags: [MessageFlag.mentioned]);
 
-    final messages = [
+    final messages = <Message>[
       message1, message2, message3, message4, message5,
       message6, message7, message8, message9, message10,
       message11, message12, message13, message14,
@@ -849,7 +849,7 @@ void main() {
       // That case is indistinguishable from an unread that's unknown to
       // the model, so we get coverage for that case too.
       test('add flag: ${mentionFlag.name}', () {
-        final messages = [
+        final messages = <Message>[
           eg.streamMessage(flags: []),
           eg.streamMessage(flags: [MessageFlag.read]),
           eg.dmMessage(from: eg.otherUser, to: [eg.selfUser], flags: []),
@@ -886,7 +886,7 @@ void main() {
       // That case is indistinguishable from an unread that's unknown to
       // the model, so we get coverage for that case too.
       test('remove flag: ${mentionFlag.name}', () {
-        final messages = [
+        final messages = <Message>[
           eg.streamMessage(flags: [mentionFlag]),
           eg.streamMessage(flags: [mentionFlag, MessageFlag.read]),
           eg.dmMessage(from: eg.otherUser, to: [eg.selfUser], flags: [mentionFlag]),
@@ -925,7 +925,7 @@ void main() {
       final message2 = eg.streamMessage(id: 2, flags: [MessageFlag.mentioned]);
       final message3 = eg.dmMessage(id: 3, from: eg.otherUser, to: [eg.selfUser], flags: []);
       final message4 = eg.dmMessage(id: 4, from: eg.otherUser, to: [eg.selfUser], flags: [MessageFlag.wildcardMentioned]);
-      final messages = [message1, message2, message3, message4];
+      final messages = <Message>[message1, message2, message3, message4];
 
       prepare();
       fillWithMessages([message1, message2, message3, message4]);
@@ -974,7 +974,7 @@ void main() {
         final message13 = eg.streamMessage(id: 13, stream: stream2, topic: 'b', flags: []);
         final message14 = eg.streamMessage(id: 14, stream: stream2, topic: 'b', flags: [MessageFlag.mentioned]);
 
-        final messages = [
+        final messages = <Message>[
           message1, message2, message3, message4, message5,
           message6, message7, message8, message9, message10,
           message11, message12, message13, message14,
@@ -1086,7 +1086,7 @@ void main() {
         final message13 = eg.streamMessage(id: 13, stream: stream2, topic: 'b', flags: [MessageFlag.read]);
         final message14 = eg.streamMessage(id: 14, stream: stream2, topic: 'b', flags: [MessageFlag.mentioned, MessageFlag.read]);
 
-        final messages = [
+        final messages = <Message>[
           message1, message2, message3, message4, message5,
           message6, message7, message8, message9, message10,
           message11, message12, message13, message14,

test/widgets/message_list_test.dart

@@ -538,7 +538,7 @@ void main() {
     final streamMessage = eg.streamMessage();
     final topicNarrow = TopicNarrow.ofMessage(streamMessage);
 
-    for (final (description, message, narrow) in [
+    for (final (description, message, narrow) in <(String, Message, SendableNarrow)>[
       ('typing in dm',    dmMessage,      dmNarrow),
       ('typing in topic', streamMessage,  topicNarrow),
     ]) {