msglist: Provide narrow to MessageListPage descendants

lib/widgets/action_sheet.dart

@@ -26,7 +26,8 @@ void showMessageActionSheet({required BuildContext context, required Message mes
   // of the action sheet (we avoid calling composeBoxControllerOf in a build
   // method; see its doc). But currently it will be constant through the life of
   // any message list, so that's fine.
-  final isComposeBoxOffered = MessageListPage.composeBoxControllerOf(context) != null;
+  final messageListPage = MessageListPage.ancestorOf(context);
+  final isComposeBoxOffered = messageListPage.composeBoxController != null;
 
   final hasThumbsUpReactionVote = message.reactions
     ?.aggregated.any((reactionWithVotes) =>
@@ -239,7 +240,7 @@ class QuoteAndReplyButton extends MessageActionSheetMenuItemButton {
     // message action sheet opened, and before "Quote and reply" was pressed.
     // Currently a compose box can't ever disappear, so this is impossible.
     ComposeBoxController composeBoxController =
-      MessageListPage.composeBoxControllerOf(messageListContext)!;
+      MessageListPage.ancestorOf(messageListContext).composeBoxController!;
     final topicController = composeBoxController.topicController;
     if (
       topicController != null
@@ -264,7 +265,8 @@ class QuoteAndReplyButton extends MessageActionSheetMenuItemButton {
     // This will be null only if the compose box disappeared during the
     // quotation request. Currently a compose box can't ever disappear,
     // so this is impossible.
-    composeBoxController = MessageListPage.composeBoxControllerOf(messageListContext)!;
+    composeBoxController =
+      MessageListPage.ancestorOf(messageListContext).composeBoxController!;
     composeBoxController.contentController
       .registerQuoteAndReplyEnd(PerAccountStoreWidget.of(messageListContext), tag,
         message: message,

lib/widgets/message_list.dart

@@ -25,6 +25,18 @@ import 'edit_state_marker.dart';
 import 'text.dart';
 import 'theme.dart';
 
+/// The interface for the state of a [MessageListPage].
+///
+/// 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;
+}
+
 class MessageListPage extends StatefulWidget {
   const MessageListPage({super.key, required this.narrow});
 
@@ -34,14 +46,16 @@ class MessageListPage extends StatefulWidget {
       page: MessageListPage(narrow: narrow));
   }
 
-  /// A [ComposeBoxController], if this [MessageListPage] offers a compose box.
+  /// The [MessageListPageState] above this context in the tree.
   ///
   /// Uses the inefficient [BuildContext.findAncestorStateOfType];
   /// don't call this in a build method.
-  static ComposeBoxController? composeBoxControllerOf(BuildContext context) {
-    final messageListPageState = context.findAncestorStateOfType<_MessageListPageState>();
-    assert(messageListPageState != null, 'No MessageListPage ancestor');
-    return messageListPageState!._composeBoxKey.currentState;
+  // If we do find ourselves wanting this in a build method, it won't be hard
+  // to enable that: we'd just need to add an [InheritedWidget] here.
+  static MessageListPageState ancestorOf(BuildContext context) {
+    final state = context.findAncestorStateOfType<_MessageListPageState>();
+    assert(state != null, 'No MessageListPage ancestor');
+    return state!;
   }
 
   final Narrow narrow;
@@ -53,7 +67,13 @@ class MessageListPage extends StatefulWidget {
 // TODO(design) this seems ad-hoc; is there a better color?
 const _kUnsubscribedStreamRecipientHeaderColor = Color(0xfff5f5f5);
 
-class _MessageListPageState extends State<MessageListPage> {
+class _MessageListPageState extends State<MessageListPage> implements MessageListPageState {
+  @override
+  Narrow get narrow => widget.narrow;
+
+  @override
+  ComposeBoxController? get composeBoxController => _composeBoxKey.currentState;
+
   final GlobalKey<ComposeBoxController> _composeBoxKey = GlobalKey();
 
   @override

test/widgets/message_list_test.dart

@@ -88,6 +88,47 @@ void main() {
     return scrollView.controller;
   }
 
+  group('MessageListPage', () {
+    testWidgets('ancestorOf finds page state from message', (tester) async {
+      await setupMessageListPage(tester,
+        messages: [eg.streamMessage(content: "<p>a message</p>")]);
+      final expectedState = tester.state<State>(find.byType(MessageListPage));
+      check(MessageListPage.ancestorOf(tester.element(find.text("a message"))))
+        .identicalTo(expectedState as MessageListPageState);
+    });
+
+    testWidgets('ancestorOf throws when not a descendant of MessageListPage', (tester) async {
+      await setupMessageListPage(tester,
+        messages: [eg.streamMessage(content: "<p>a message</p>")]);
+      final element = tester.element(find.byType(PerAccountStoreWidget));
+      check(() => MessageListPage.ancestorOf(element))
+        .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),
+        messages: [eg.streamMessage(stream: stream, content: "<p>a message</p>")]);
+      final state = MessageListPage.ancestorOf(tester.element(find.text("a message")));
+      check(state.composeBoxController).isNotNull();
+    });
+
+    testWidgets('composeBoxController null when no compose box', (tester) async {
+      await setupMessageListPage(tester, narrow: const CombinedFeedNarrow(),
+        messages: [eg.streamMessage(content: "<p>a message</p>")]);
+      final state = MessageListPage.ancestorOf(tester.element(find.text("a message")));
+      check(state.composeBoxController).isNull();
+    });
+  });
+
   group('presents message content appropriately', () {
     // regression test for https://github.com/zulip/zulip-flutter/issues/736
     testWidgets('content in "Combined feed" not asked to consume insets (including bottom)', (tester) async {