msglist: Add MessageListPageState.narrow

Author: gnprice

lib/widgets/message_list.dart

@@ -29,6 +29,9 @@ import 'theme.dart';
 ///
 /// To obtain one of these, see [MessageListPage.ancestorOf].
 abstract class MessageListPageState {
+  /// The narrow for this page's message list.
+  Narrow get narrow;
+
   /// The controller for this [MessageListPage]'s compose box,
   /// if this [MessageListPage] offers a compose box.
   ComposeBoxController? get composeBoxController;
@@ -65,6 +68,9 @@ class MessageListPage extends StatefulWidget {
 const _kUnsubscribedStreamRecipientHeaderColor = Color(0xfff5f5f5);
 
 class _MessageListPageState extends State<MessageListPage> implements MessageListPageState {
+  @override
+  Narrow get narrow => widget.narrow;
+
   @override
   ComposeBoxController? get composeBoxController => _composeBoxKey.currentState;
 

test/widgets/message_list_test.dart

@@ -105,6 +105,14 @@ void main() {
         .throws<void>();
     });
 
+    testWidgets('MessageListPageState.narrow', (tester) async {
+      final stream = eg.stream();
+      await setupMessageListPage(tester, narrow: StreamNarrow(stream.streamId),
+        messages: [eg.streamMessage(stream: stream, content: "<p>a message</p>")]);
+      final state = MessageListPage.ancestorOf(tester.element(find.text("a message")));
+      check(state.narrow).equals(StreamNarrow(stream.streamId));
+    });
+
     testWidgets('composeBoxController finds compose box', (tester) async {
       final stream = eg.stream();
       await setupMessageListPage(tester, narrow: StreamNarrow(stream.streamId),