msglist: Make mark-as-read button listen to the Unreads model

Instead of just listening to PerAccountStore and (via an ancestor
widget) a MessageListView.

Issue zulip#370 is relevant here -- "Maintain total unread counts
efficiently". The button causes linear scans through Unreads each
time it rebuilds, and ideally we'd avoid that, especially since now
it'll rebuild in more cases than it was before. I think the
additional cases aren't very many, though: I've thought of just one
kind of Zulip event that would cause Unreads but not PerAccountStore
and MessageListView to notify listeners, and I added a test
exercising that.

Author: chrisbobbe

lib/widgets/message_list.dart

@@ -15,6 +15,7 @@ import '../model/message_list.dart';
 import '../model/narrow.dart';
 import '../model/store.dart';
 import '../model/typing_status.dart';
+import '../model/unreads.dart';
 import 'action_sheet.dart';
 import 'actions.dart';
 import 'app_bar.dart';
@@ -1499,9 +1500,30 @@ class MarkAsReadWidget extends StatefulWidget {
   State<MarkAsReadWidget> createState() => _MarkAsReadWidgetState();
 }
 
-class _MarkAsReadWidgetState extends State<MarkAsReadWidget> {
+class _MarkAsReadWidgetState extends State<MarkAsReadWidget> with PerAccountStoreAwareStateMixin {
+  Unreads? unreadsModel;
+
   bool _loading = false;
 
+  void _unreadsModelChanged() {
+    setState(() {
+      // The actual state lives in [unreadsModel].
+    });
+  }
+
+  @override
+  void onNewStore() {
+    final newStore = PerAccountStoreWidget.of(context);
+    unreadsModel?.removeListener(_unreadsModelChanged);
+    unreadsModel = newStore.unreads..addListener(_unreadsModelChanged);
+  }
+
+  @override
+  void dispose() {
+    unreadsModel?.removeListener(_unreadsModelChanged);
+    super.dispose();
+  }
+
   void _handlePress(BuildContext context) async {
     if (!context.mounted) return;
     setState(() => _loading = true);
@@ -1512,8 +1534,7 @@ class _MarkAsReadWidgetState extends State<MarkAsReadWidget> {
   @override
   Widget build(BuildContext context) {
     final zulipLocalizations = ZulipLocalizations.of(context);
-    final store = PerAccountStoreWidget.of(context);
-    final unreadCount = store.unreads.countInNarrow(widget.narrow);
+    final unreadCount = unreadsModel!.countInNarrow(widget.narrow);
     final shouldHide = unreadCount == 0;
 
     final messageListTheme = MessageListTheme.of(context);

test/widgets/message_list_test.dart

@@ -1211,6 +1211,48 @@ void main() {
       check(isMarkAsReadButtonVisible(tester)).isFalse();
     });
 
+    testWidgets('listens to Unreads model, not just PerAccountStore and MessageListView', (tester) async {
+      // Regression test for an edge case where the button wouldn't disappear
+      // when the narrow's unreads were cleared, because the button would only
+      // respond to notifications from PerAccountStore and MessageListView,
+      // not Unreads.
+      //
+      // For this test, there's one unread message in the narrow,
+      // and it's old enough that it hasn't been fetched yet.
+      // We simulate an event that removes the message from the narrow,
+      // thus causing Unreads but not MessageListView or PerAccountStore
+      // to notify listeners.
+      // And we check that the button responds by disappearing.
+
+      final message = eg.streamMessage(id: 100, flags: [MessageFlag.mentioned]);
+      final unreadMsgs = eg.unreadMsgs(
+        channels: [
+          UnreadChannelSnapshot(
+            topic: message.topic,
+            streamId: message.streamId,
+            unreadMessageIds: [message.id],
+          ),
+        ],
+        mentions: [message.id],
+      );
+      await setupMessageListPage(tester,
+        narrow: MentionsNarrow(),
+        unreadMsgs: unreadMsgs,
+        // omit `message`; if present, MessageListView would notify listeners
+        messages: List.generate(300, (i) =>
+          eg.streamMessage(id: 950 + i, sender: eg.selfUser,
+            flags: [MessageFlag.read, MessageFlag.mentioned])),
+        foundOldest: false);
+      check(isMarkAsReadButtonVisible(tester)).isTrue();
+
+      // The message no longer has an @-mention.
+      // It was the only unread message with an @-mention,
+      // so the button should disappear.
+      await store.handleEvent(eg.updateMessageEditEvent(message, flags: []));
+      await tester.pumpAndSettle();
+      check(isMarkAsReadButtonVisible(tester)).isFalse();
+    });
+
     testWidgets("messages don't shift position", (tester) async {
       final message = eg.streamMessage(flags: []);
       final unreadMsgs = eg.unreadMsgs(channels:[