WIP model: Add Unreads model, for tracking unread-message counts

TODO tests, stopwatching

For the "TODO assert sorted?" comments in initial_snapshot.dart,
see:
  https://chat.zulip.org/#narrow/stream/412-api-documentation/topic/register.3A.20.60unread_message_ids.60.20sorted.3F/near/1647932
(before merging we should make sure the doc will guarantee sorting)

Author: chrisbobbe

lib/api/model/initial_snapshot.dart

@@ -259,7 +259,7 @@ class UnreadDmSnapshot {
 
   UnreadDmSnapshot({
     required this.otherUserId,
-    required this.unreadMessageIds,
+    required this.unreadMessageIds, // TODO assert sorted?
   });
 
   factory UnreadDmSnapshot.fromJson(Map<String, dynamic> json) =>
@@ -278,7 +278,7 @@ class UnreadStreamSnapshot {
   UnreadStreamSnapshot({
     required this.topic,
     required this.streamId,
-    required this.unreadMessageIds,
+    required this.unreadMessageIds, // TODO assert sorted?
   });
 
   factory UnreadStreamSnapshot.fromJson(Map<String, dynamic> json) =>
@@ -295,7 +295,7 @@ class UnreadHuddleSnapshot {
 
   UnreadHuddleSnapshot({
     required this.userIdsString,
-    required this.unreadMessageIds,
+    required this.unreadMessageIds, // TODO assert sorted?
   });
 
   factory UnreadHuddleSnapshot.fromJson(Map<String, dynamic> json) =>

lib/model/narrow.dart

@@ -1,4 +1,5 @@
 
+import '../api/model/events.dart';
 import '../api/model/initial_snapshot.dart';
 import '../api/model/model.dart';
 import '../api/model/narrow.dart';
@@ -168,6 +169,23 @@ class DmNarrow extends Narrow implements SendableNarrow {
     );
   }
 
+  /// A [DmNarrow] from an [UnreadHuddleSnapshot].
+  factory DmNarrow.ofUnreadHuddleSnapshot(UnreadHuddleSnapshot snapshot, {required int selfUserId}) {
+    final userIds = snapshot.userIdsString.split(',').map((id) => int.parse(id));
+    return DmNarrow(selfUserId: selfUserId,
+      // (already sorted; see API doc)
+      allRecipientIds: userIds.toList(growable: false));
+  }
+
+  factory DmNarrow.ofUpdateMessageFlagsMessageDetail(
+    UpdateMessageFlagsMessageDetail detail, {
+    required int selfUserId,
+  }) {
+    assert(detail.type == MessageType.private);
+    return DmNarrow(selfUserId: selfUserId,
+      allRecipientIds: [...detail.userIds!, selfUserId]..sort());
+  }
+
   /// The user IDs of everyone in the conversation, sorted.
   ///
   /// Each message in the conversation is sent by one of these users

lib/model/store.dart

@@ -17,6 +17,7 @@ import 'autocomplete.dart';
 import 'database.dart';
 import 'message_list.dart';
 import 'recent_dm_conversations.dart';
+import 'unreads.dart';
 
 export 'package:drift/drift.dart' show Value;
 export 'database.dart' show Account, AccountsCompanion;
@@ -154,6 +155,7 @@ class PerAccountStore extends ChangeNotifier {
        realmDefaultExternalAccounts = initialSnapshot.realmDefaultExternalAccounts,
        customProfileFields = _sortCustomProfileFields(initialSnapshot.customProfileFields),
        userSettings = initialSnapshot.userSettings,
+       unreads = Unreads(initial: initialSnapshot.unreadMsgs, selfUserId: account.userId),
        users = Map.fromEntries(
          initialSnapshot.realmUsers
          .followedBy(initialSnapshot.realmNonActiveUsers)
@@ -179,6 +181,7 @@ class PerAccountStore extends ChangeNotifier {
 
   // Data attached to the self-account on the realm.
   final UserSettings? userSettings; // TODO(server-5)
+  final Unreads unreads;
 
   // Users and data about them.
   final Map<int, User> users;
@@ -301,19 +304,23 @@ class PerAccountStore extends ChangeNotifier {
       for (final view in _messageListViews) {
         view.maybeAddMessage(event.message);
       }
+      unreads.handleMessageEvent(event);
     } else if (event is UpdateMessageEvent) {
       assert(debugLog("server event: update_message ${event.messageId}"));
       for (final view in _messageListViews) {
         view.maybeUpdateMessage(event);
       }
+      unreads.handleUpdateMessageEvent(event);
     } else if (event is DeleteMessageEvent) {
       assert(debugLog("server event: delete_message ${event.messageIds}"));
-      // TODO handle
+      // TODO handle in message lists
+      unreads.handleDeleteMessageEvent(event);
     } else if (event is UpdateMessageFlagsEvent) {
       assert(debugLog("server event: update_message_flags/${event.op} ${event.flag.toJson()}"));
       for (final view in _messageListViews) {
         view.maybeUpdateMessageFlags(event);
       }
+      unreads.handleUpdateMessageFlagsEvent(event);
     } else if (event is ReactionEvent) {
       assert(debugLog("server event: reaction/${event.op}"));
       for (final view in _messageListViews) {