api [nfc]: Introduce TopicName.displayName, and use where needed

Each of these call sites will need to be updated for zulip#1250.  Those
sites can now be found by listing the references to this getter.

As a bonus the type-checker will point out most of the places that
need updates, because this getter will become nullable.  (The
exceptions are where it's used in a string interpolation, because
`null.toString()` is valid and returns "null".)

lib/api/model/model.dart

@@ -659,8 +659,20 @@ enum MessageFlag {
 // TODO(#1250): Migrate all implicit uses as String; remove "implements String".
 extension type const TopicName(String _value) implements String {
   /// The string this topic is identified by in the Zulip API.
+  ///
+  /// This should be used in constructing HTTP requests to the server,
+  /// but rarely for other purposes.  See [displayName] and [canonicalize].
   String get apiName => _value;
 
+  /// The string this topic is displayed as to the user in our UI.
+  ///
+  /// At the moment this always equals [apiName].
+  /// In the future this will become null for the "general chat" topic (#1250),
+  /// so that UI code can identify when it needs to represent the topic
+  /// specially in the way prescribed for "general chat".
+  // TODO(#1250) carry out that plan
+  String get displayName => _value;
+
   /// The key to use for "same topic as" comparisons.
   String canonicalize() => apiName.toLowerCase();
 

lib/model/autocomplete.dart

@@ -864,7 +864,8 @@ class TopicAutocompleteQuery extends AutocompleteQuery {
 
   bool testTopic(TopicName topic) {
     // TODO(#881): Sort by match relevance, like web does.
-    return topic != raw && topic.toLowerCase().contains(raw.toLowerCase());
+    return topic.displayName != raw
+      && topic.displayName.toLowerCase().contains(raw.toLowerCase());
   }
 
   @override

lib/model/narrow.dart

@@ -114,7 +114,7 @@ class TopicNarrow extends Narrow implements SendableNarrow {
   StreamDestination get destination => StreamDestination(streamId, topic);
 
   @override
-  String toString() => 'TopicNarrow($streamId, $topic)';
+  String toString() => 'TopicNarrow($streamId, ${topic.displayName})';
 
   @override
   bool operator ==(Object other) {

lib/notifications/display.dart

@@ -264,9 +264,9 @@ class NotificationDisplayManager {
     // the first.
     messagingStyle.conversationTitle = switch (data.recipient) {
       FcmMessageChannelRecipient(:var streamName?, :var topic) =>
-        '#$streamName > $topic',
+        '#$streamName > ${topic.displayName}',
       FcmMessageChannelRecipient(:var topic) =>
-        '#(unknown channel) > $topic', // TODO get stream name from data
+        '#(unknown channel) > ${topic.displayName}', // TODO get stream name from data
       FcmMessageDmRecipient(:var allRecipientIds) when allRecipientIds.length > 2 =>
         zulipLocalizations.notifGroupDmConversationLabel(
           data.senderFullName, allRecipientIds.length - 2), // TODO use others' names, from data

lib/widgets/autocomplete.dart

@@ -334,6 +334,6 @@ class TopicAutocomplete extends AutocompleteField<TopicAutocompleteQuery, TopicA
       },
       child: Padding(
         padding: const EdgeInsets.symmetric(horizontal: 16.0, vertical: 8.0),
-        child: Text(option.topic)));
+        child: Text(option.topic.displayName)));
   }
 }

lib/widgets/compose_box.dart

@@ -91,7 +91,7 @@ class ComposeTopicController extends ComposeController<TopicValidationError> {
   }
 
   void setTopic(TopicName newTopic) {
-    value = TextEditingValue(text: newTopic);
+    value = TextEditingValue(text: newTopic.displayName);
   }
 }
 
@@ -550,7 +550,8 @@ class _FixedDestinationContentInput extends StatelessWidget {
         final store = PerAccountStoreWidget.of(context);
         final streamName = store.streams[streamId]?.name
           ?? zulipLocalizations.composeBoxUnknownChannelName;
-        return zulipLocalizations.composeBoxChannelContentHint(streamName, topic);
+        return zulipLocalizations.composeBoxChannelContentHint(
+          streamName, topic.displayName);
 
       case DmNarrow(otherRecipientIds: []): // The self-1:1 thread.
         return zulipLocalizations.composeBoxSelfDmContentHint;

lib/widgets/inbox.dart

@@ -524,7 +524,7 @@ class _TopicItem extends StatelessWidget {
                 ),
                 maxLines: 2,
                 overflow: TextOverflow.ellipsis,
-                topic))),
+                topic.displayName))),
             const SizedBox(width: 12),
             if (hasMention) const _IconMarker(icon: ZulipIcons.at_sign),
             // TODO(design) copies the "@" marker color; is there a better color?

lib/widgets/message_list.dart

@@ -344,7 +344,7 @@ class MessageListAppBarTitle extends StatelessWidget {
     return Row(
       mainAxisSize: MainAxisSize.min,
       children: [
-        Flexible(child: Text(topic, style: const TextStyle(
+        Flexible(child: Text(topic.displayName, style: const TextStyle(
           fontSize: 13,
         ).merge(weightVariableTextStyle(context)))),
         if (icon != null)
@@ -1091,7 +1091,7 @@ class StreamMessageRecipientHeader extends StatelessWidget {
       child: Row(
         children: [
           Flexible(
-            child: Text(topic,
+            child: Text(topic.displayName,
               // TODO: Give a way to see the whole topic (maybe a
               //   long-press interaction?)
               overflow: TextOverflow.ellipsis,

test/api/model/model_checks.dart

@@ -48,6 +48,7 @@ extension MessageChecks on Subject<Message> {
 
 extension TopicNameChecks on Subject<TopicName> {
   Subject<String> get apiName => has((x) => x.apiName, 'apiName');
+  Subject<String> get displayName => has((x) => x.displayName, 'displayName');
 }
 
 extension StreamMessageChecks on Subject<StreamMessage> {

test/widgets/autocomplete_test.dart

@@ -277,7 +277,7 @@ void main() {
 
   group('TopicAutocomplete', () {
     void checkTopicShown(GetStreamTopicsEntry topic, PerAccountStore store, {required bool expected}) {
-      check(find.text(topic.name).evaluate().length).equals(expected ? 1 : 0);
+      check(find.text(topic.name.displayName).evaluate().length).equals(expected ? 1 : 0);
     }
 
     testWidgets('options appear, disappear, and change correctly', (WidgetTester tester) async {
@@ -302,7 +302,7 @@ void main() {
       await tester.tap(find.text('Topic three'));
       await tester.pumpAndSettle();
       check(tester.widget<TextField>(topicInputFinder).controller!.text)
-        .equals(topic3.name);
+        .equals(topic3.name.displayName);
       checkTopicShown(topic1, store, expected: false);
       checkTopicShown(topic2, store, expected: false);
       checkTopicShown(topic3, store, expected: true); // shown in `_TopicInput` once