compose: Change content input hint text if topic is empty and mandatory

Previously, "Message #stream > (no topic)" would appear as the hint text
when the topic input is empty but mandatory.  Now, it is shown as
"Message #stream" instead, since the "(no topic)" isn't allowed when
topics are mandatory.

The control flow of `String get hintText` can be simplified. However,
it is structured this way to prepare for a later change to support
showing "general chat" in the hint text, that adds some more advanced
checks.

Signed-off-by: Zixuan James Li <zixuan@zulip.com>

Author: PIG208

lib/widgets/compose_box.dart

@@ -582,23 +582,48 @@ class _StreamContentInputState extends State<_StreamContentInput> {
     super.dispose();
   }
 
-  @override
-  Widget build(BuildContext context) {
+  /// The hint text for [_ContentInput] computed from the topic input.
+  ///
+  /// This is a string referring to the send destination as something like
+  /// "#stream name" when topics are [ComposeTopicController.mandatory] but the
+  /// topic input is empty.
+  ///
+  /// Otherwise, the destination is formatted like "#stream name > topic name".
+  // No i18n of the use of "#" and ">" strings; those are part of how
+  // Zulip expresses channels and topics, not any normal English punctuation,
+  // so don't make sense to translate. See:
+  //   https://github.com/zulip/zulip-flutter/pull/1148#discussion_r1941990585
+  String get hintText {
     final store = PerAccountStoreWidget.of(context);
     final zulipLocalizations = ZulipLocalizations.of(context);
     final streamName = store.streams[widget.narrow.streamId]?.name
       ?? zulipLocalizations.unknownChannelName;
-    final topic = TopicName(widget.controller.topic.textNormalized);
+
+    final textTrimmed = widget.controller.topic.text.trim();
+    if (textTrimmed.isNotEmpty) {
+      return zulipLocalizations.composeBoxChannelContentHint(
+        '#$streamName > $textTrimmed');
+    }
+
+    assert(widget.controller.topic._isTopicVacuous);
+    // Sending to a vacuous topic (see [ComposeTopicController._isTopicVacuous])
+    // is not possible if topics are mandatory.
+    if (widget.controller.topic.mandatory) {
+      return zulipLocalizations.composeBoxChannelContentHint('#$streamName');
+    }
+
+    return zulipLocalizations.composeBoxChannelContentHint(
+      '#$streamName > $kNoTopicTopic');
+  }
+
+  @override
+  Widget build(BuildContext context) {
     return _ContentInput(
       narrow: widget.narrow,
-      destination: TopicNarrow(widget.narrow.streamId, topic),
+      destination: TopicNarrow(widget.narrow.streamId,
+        TopicName(widget.controller.topic.textNormalized)),
       controller: widget.controller,
-      hintText: zulipLocalizations.composeBoxChannelContentHint(
-        // No i18n of this use of "#" and ">" string; those are part of how
-        // Zulip expresses channels and topics, not any normal English punctuation,
-        // so don't make sense to translate. See:
-        //   https://github.com/zulip/zulip-flutter/pull/1148#discussion_r1941990585
-        '#$streamName > ${topic.displayName}'));
+      hintText: hintText);
   }
 }
 

test/widgets/compose_box_test.dart

@@ -353,18 +353,56 @@ void main() {
         .decoration.isNotNull().hintText.equals(contentHintText);
     }
 
-    group('to ChannelNarrow', () {
+    group('to ChannelNarrow, topics not mandatory', () {
       final narrow = ChannelNarrow(channel.streamId);
 
       testWidgets('with empty topic', (tester) async {
-        await prepare(tester, narrow: narrow);
+        await prepare(tester, narrow: narrow, mandatoryTopics: false);
+        checkComposeBoxHintTexts(tester,
+          topicHintText: 'Topic',
+          contentHintText: 'Message #${channel.name} > (no topic)');
+      });
+
+      testWidgets('with non-empty but vacuous topic', (tester) async {
+        await prepare(tester, narrow: narrow, mandatoryTopics: false);
+        await enterTopic(tester, narrow: narrow, topic: '(no topic)');
+        await tester.pump();
+        checkComposeBoxHintTexts(tester,
+          topicHintText: 'Topic',
+          contentHintText: 'Message #${channel.name} > (no topic)');
+      });
+
+      testWidgets('with non-empty topic', (tester) async {
+        await prepare(tester, narrow: narrow, mandatoryTopics: false);
+        await enterTopic(tester, narrow: narrow, topic: 'new topic');
+        await tester.pump();
+        checkComposeBoxHintTexts(tester,
+          topicHintText: 'Topic',
+          contentHintText: 'Message #${channel.name} > new topic');
+      });
+    });
+
+    group('to ChannelNarrow, mandatory topics', () {
+      final narrow = ChannelNarrow(channel.streamId);
+
+      testWidgets('with empty topic', (tester) async {
+        await prepare(tester, narrow: narrow, mandatoryTopics: true);
+        checkComposeBoxHintTexts(tester,
+          topicHintText: 'Topic',
+          contentHintText: 'Message #${channel.name}');
+      });
+
+      testWidgets('with non-empty but vacuous topic', (tester) async {
+        await prepare(tester, narrow: narrow, mandatoryTopics: true);
+        await enterTopic(tester, narrow: narrow, topic: '(no topic)');
+        await tester.pump();
         checkComposeBoxHintTexts(tester,
           topicHintText: 'Topic',
           contentHintText: 'Message #${channel.name} > (no topic)');
       });
 
       testWidgets('with non-empty topic', (tester) async {
-        await prepare(tester, narrow: narrow);
+        await prepare(tester, narrow: narrow, mandatoryTopics: true);
         await enterTopic(tester, narrow: narrow, topic: 'new topic');
         await tester.pump();
         checkComposeBoxHintTexts(tester,