message test [nfc]: Add a MessageListView, optionally

Some of the existing tests of MessageListView in message_list_test.dart
are really almost entirely about acting on individual messages rather
than message lists.  They therefore belong as tests of the overall
message store, to live in this file.

With that "almost", though, they have a few ties to actually testing
the MessageListView itself -- notably, its notification behavior.
So to simplify converting them and moving them into this file,
we add a MessageListView they can use.  This logic is largely
copied from message_list_test.dart.

Author: gnprice

test/model/message_test.dart

@@ -2,24 +2,68 @@ import 'package:checks/checks.dart';
 import 'package:test/scaffolding.dart';
 import 'package:zulip/api/model/events.dart';
 import 'package:zulip/api/model/model.dart';
+import 'package:zulip/model/message_list.dart';
+import 'package:zulip/model/narrow.dart';
 import 'package:zulip/model/store.dart';
 
+import '../api/fake_api.dart';
 import '../example_data.dart' as eg;
+import 'message_list_test.dart';
+import 'test_store.dart';
 
 void main() {
-  // These variables are the common state operated on by each test.
+  // These "late" variables are the common state operated on by each test.
   // Each test case calls [prepare] to initialize them.
+  late Subscription subscription;
   late PerAccountStore store;
+  late FakeApiConnection connection;
+  // [messageList] is here only for the sake of checking when it notifies.
+  // For anything deeper than that, use `message_list_test.dart`.
+  late MessageListView messageList;
+  late int notifiedCount;
+
+  void checkNotified({required int count}) {
+    check(notifiedCount).equals(count);
+    notifiedCount = 0;
+  }
+  void checkNotNotified() => checkNotified(count: 0);
+  void checkNotifiedOnce() => checkNotified(count: 1);
 
   /// Initialize [store] and the rest of the test state.
-  void prepare() {
-    store = eg.store();
+  void prepare({Narrow narrow = const CombinedFeedNarrow()}) {
+    final stream = eg.stream();
+    subscription = eg.subscription(stream);
+    store = eg.store()
+      ..addStream(stream)..addSubscription(subscription);
+    connection = store.connection as FakeApiConnection;
+    notifiedCount = 0;
+    messageList = MessageListView.init(store: store, narrow: narrow)
+      ..addListener(() {
+        notifiedCount++;
+      });
+    check(messageList).fetched.isFalse();
+    checkNotNotified();
+  }
+
+  /// Perform the initial message fetch for [messageList].
+  ///
+  /// The test case must have already called [prepare] to initialize the state.
+  // ignore: unused_element
+  Future<void> prepareMessages(
+    List<Message> messages, {
+    bool foundOldest = false,
+  }) async {
+    connection.prepare(json:
+      newestResult(foundOldest: foundOldest, messages: messages).toJson());
+    await messageList.fetchInitial();
+    checkNotifiedOnce();
   }
 
   Future<void> addMessages(Iterable<Message> messages) async {
     for (final m in messages) {
       await store.handleEvent(MessageEvent(id: 0, message: m));
     }
+    checkNotified(count: messageList.fetched ? messages.length : 0);
   }
 
   group('reconcileMessages', () {