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.

The control flow of `getDestinationString` 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 <[email protected]>

Author: PIG208

lib/widgets/compose_box.dart

@@ -167,6 +167,32 @@ class ComposeTopicController extends ComposeController<TopicValidationError> {
   /// that certain strings are not empty but also indicate the absence of a topic.
   bool get isTopicVacuous => textNormalized == kNoTopicTopic;
 
+  /// The send destination as a string.
+  ///
+  /// This returns a string formatted like "#stream name" when topics are
+  /// [mandatory] but the trimmed input is empty.
+  ///
+  /// Otherwise, returns a string 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 getDestinationString({required String streamName}) {
+    final textTrimmed = text.trim();
+    if (textTrimmed.isNotEmpty) {
+      return '#$streamName > $textTrimmed';
+    }
+
+    assert(isTopicVacuous);
+    // Sending to a vacuous topic (see [isTopicVacuous]) is not possible if
+    // topics are [mandatory].
+    if (mandatory) {
+      return '#$streamName';
+    }
+
+    return '#$streamName > $kNoTopicTopic';
+  }
+
   @override
   List<TopicValidationError> _computeValidationErrors() {
     return [
@@ -585,17 +611,13 @@ class _StreamContentInputState extends State<_StreamContentInput> {
     final zulipLocalizations = ZulipLocalizations.of(context);
     final streamName = store.streams[widget.narrow.streamId]?.name
       ?? zulipLocalizations.unknownChannelName;
-    final topic = TopicName(widget.controller.topic.textNormalized);
     return _ContentInput(
       narrow: widget.narrow,
-      destination: TopicNarrow(widget.narrow.streamId, topic),
+      destination: TopicNarrow(widget.narrow.streamId,
+        TopicName(widget.controller.topic.textNormalized)),
       controller: widget.controller,
       hintText: zulipLocalizations.composeBoxChannelContentHint(
-        // No i18n of this use of "#" and ">" string; those are part of how
-        // Zulip expresses channels and topics, not any normal English punctuation,
-        // so don't make sense to translate. See:
-        //   https://github.com/zulip/zulip-flutter/pull/1148#discussion_r1941990585
-        '#$streamName > ${topic.displayName}'));
+        widget.controller.topic.getDestinationString(streamName: streamName)));
   }
 }
 

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,