compose: Use alternative placeholder if topics are mandatory and missing

Previously, the placeholder reads "#channel name > (no topic)" even if
topics are mandatory for the realm.  Later, we will expand this logic
for "general chat" so that the placeholder is chosen depending on
whether topics are mandatory.

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

Author: PIG208

lib/widgets/compose_box.dart

@@ -588,12 +588,21 @@ class _StreamContentInputState extends State<_StreamContentInput> {
     final streamName = store.streams[widget.narrow.streamId]?.name
       ?? zulipLocalizations.unknownChannelName;
     final topic = TopicName(_topicTextNormalized);
+    final String? topicDisplayName;
+    if (store.realmMandatoryTopics && widget.controller.topic.isTopicVacuous) {
+      topicDisplayName = null;
+    } else {
+      topicDisplayName = topic.displayName;
+    }
+
     return _ContentInput(
       narrow: widget.narrow,
       destination: TopicNarrow(widget.narrow.streamId, topic),
       controller: widget.controller,
-      hintText: zulipLocalizations.composeBoxChannelContentHint(
-        '#$streamName > ${topic.displayName}'));
+      hintText: topicDisplayName == null
+        ? zulipLocalizations.composeBoxChannelContentHint('#$streamName')
+        : zulipLocalizations.composeBoxChannelContentHint(
+            '#$streamName > $topicDisplayName'));
   }
 }
 

test/widgets/compose_box_test.dart

@@ -326,11 +326,13 @@ void main() {
 
     Future<void> prepare(WidgetTester tester, {
       required Narrow narrow,
+      bool? mandatoryTopics,
     }) async {
       await prepareComposeBox(tester,
         narrow: narrow,
         otherUsers: [eg.otherUser, eg.thirdUser],
-        streams: [channel]);
+        streams: [channel],
+        mandatoryTopics: mandatoryTopics);
     }
 
     /// This checks the input's configured hint text without regard to whether
@@ -351,17 +353,40 @@ void main() {
         .decoration.isNotNull().hintText.equals(contentHintText);
     }
 
-    group('to ChannelNarrow', () {
+    group('to ChannelNarrow, topics not mandatory', () {
       testWidgets('with empty topic', (tester) async {
-        await prepare(tester, narrow: ChannelNarrow(channel.streamId));
+        await prepare(tester, narrow: ChannelNarrow(channel.streamId),
+          mandatoryTopics: false);
         checkComposeBoxHintTexts(tester,
           topicHintText: 'Topic',
           contentHintText: 'Message #${channel.name} > (no topic)');
       });
 
       testWidgets('with non-empty topic', (tester) async {
         final narrow = ChannelNarrow(channel.streamId);
-        await prepare(tester, narrow: narrow);
+        await prepare(tester, narrow: narrow,
+          mandatoryTopics: false);
+        await enterTopic(tester, narrow: narrow, topic: 'new topic');
+        await tester.pump();
+        checkComposeBoxHintTexts(tester,
+          topicHintText: 'Topic',
+          contentHintText: 'Message #${channel.name} > new topic');
+      });
+    });
+
+    group('to ChannelNarrow, mandatory topics', () {
+      testWidgets('with empty topic', (tester) async {
+        await prepare(tester, narrow: ChannelNarrow(channel.streamId),
+          mandatoryTopics: true);
+        checkComposeBoxHintTexts(tester,
+          topicHintText: 'Topic',
+          contentHintText: 'Message #${channel.name}');
+      });
+
+      testWidgets('with non-empty topic', (tester) async {
+        final narrow = ChannelNarrow(channel.streamId);
+        await prepare(tester, narrow: narrow,
+          mandatoryTopics: true);
         await enterTopic(tester, narrow: narrow, topic: 'new topic');
         await tester.pump();
         checkComposeBoxHintTexts(tester,