compose: Support sending to empty topic

This behavior is designed to replace how sending to an empty topic is
effectively sending to "(no topic)". The key difference being that
the `TopicName.apiValue` is actually empty, instead of being converted to
"(no topic)" with `_computeTextNormalized`.

Signed-off-by: Zixuan James Li <[email protected]>

Author: PIG208

lib/widgets/compose_box.dart

@@ -157,15 +157,34 @@ class ComposeTopicController extends ComposeController<TopicValidationError> {
   @override
   String _computeTextNormalized() {
     String trimmed = text.trim();
-    return trimmed.isEmpty ? kNoTopicTopic : trimmed;
+    // TODO(server-10): simplify
+    if (store.connection.zulipFeatureLevel! < 334) {
+      return trimmed.isEmpty ? kNoTopicTopic : trimmed;
+    }
+
+    return trimmed;
   }
 
   /// Whether [textNormalized] would fail a mandatory-topics check
   /// (see [mandatory]).
   ///
   /// The term "Vacuous" draws distinction from [String.isEmpty], in the sense
   /// that certain strings are empty but also indicate the absence of a topic.
-  bool get isTopicVacuous => textNormalized == kNoTopicTopic;
+  bool get isTopicVacuous {
+    bool result = textNormalized.isEmpty
+      // We keep checking for '(no topic)' regardless of the feature level
+      // because it remains equivalent to an empty topic even when FL >= 334.
+      // This can change in the future:
+      //   https://chat.zulip.org/#narrow/channel/412-api-documentation/topic/.28realm_.29mandatory_topics.20behavior/near/2062391
+      || textNormalized == kNoTopicTopic;
+
+    // TODO(server-10): simplify
+    if (store.connection.zulipFeatureLevel! >= 334) {
+      result |= textNormalized == store.realmEmptyTopicDisplayName;
+    }
+
+    return result;
+  }
 
   @override
   List<TopicValidationError> _computeValidationErrors() {

test/widgets/compose_box_test.dart

@@ -716,6 +716,7 @@ void main() {
     Future<void> setupAndTapSend(WidgetTester tester, {
       required String topicInputText,
       required bool mandatoryTopics,
+      int? zulipFeatureLevel,
     }) async {
       TypingNotifier.debugEnable = false;
       addTearDown(TypingNotifier.debugReset);
@@ -724,7 +725,8 @@ void main() {
       final narrow = ChannelNarrow(channel.streamId);
       await prepareComposeBox(tester,
         narrow: narrow, streams: [channel],
-        mandatoryTopics: mandatoryTopics);
+        mandatoryTopics: mandatoryTopics,
+        zulipFeatureLevel: zulipFeatureLevel);
 
       await enterTopic(tester, narrow: narrow, topic: topicInputText);
       await tester.enterText(contentInputFinder, 'test content');
@@ -739,10 +741,21 @@ void main() {
         expectedMessage: 'Topics are required in this organization.');
     }
 
-    testWidgets('empty topic -> "(no topic)"', (tester) async {
+    testWidgets('empty topic -> empty topic', (tester) async {
       await setupAndTapSend(tester,
         topicInputText: '',
         mandatoryTopics: false);
+      check(connection.lastRequest).isA<http.Request>()
+        ..method.equals('POST')
+        ..url.path.equals('/api/v1/messages')
+        ..bodyFields['topic'].equals('');
+    }, skip: true); // null topic names soon to be enabled
+
+    testWidgets('legacy: empty topic -> "(no topic)"', (tester) async {
+      await setupAndTapSend(tester,
+        topicInputText: '',
+        mandatoryTopics: false,
+        zulipFeatureLevel: 333);
       check(connection.lastRequest).isA<http.Request>()
         ..method.equals('POST')
         ..url.path.equals('/api/v1/messages')
@@ -756,12 +769,27 @@ void main() {
       checkMessageNotSent(tester);
     });
 
+    testWidgets('if topics are mandatory, reject `realmEmptyTopicDisplayName`', (tester) async {
+      await setupAndTapSend(tester,
+        topicInputText: eg.defaultRealmEmptyTopicDisplayName,
+        mandatoryTopics: true);
+      checkMessageNotSent(tester);
+    }, skip: true); // null topic names soon to be enabled
+
     testWidgets('if topics are mandatory, reject "(no topic)"', (tester) async {
       await setupAndTapSend(tester,
         topicInputText: '(no topic)',
         mandatoryTopics: true);
       checkMessageNotSent(tester);
     });
+
+    testWidgets('legacy: if topics are mandatory, reject "(no topic)"', (tester) async {
+      await setupAndTapSend(tester,
+        topicInputText: '(no topic)',
+        mandatoryTopics: true,
+        zulipFeatureLevel: 333);
+      checkMessageNotSent(tester);
+    });
   });
 
   group('uploads', () {