action_sheet: Show topic in topic action sheet

Fixes zulip#1533.

Author: chrisbobbe

lib/widgets/action_sheet.dart

@@ -791,7 +791,20 @@ void showTopicActionSheet(BuildContext context, {
     narrow: TopicNarrow(channelId, topic, with_: someMessageIdInTopic),
     pageContext: context));
 
-  _showActionSheet(pageContext, buttonSections: [optionButtons]);
+  final header = BottomSheetHeader(
+    buildTitle: (baseStyle) => Text.rich(
+      style: baseStyle,
+      channelTopicLabelSpan(
+        context: context,
+        channelId: channelId,
+        topic: topic,
+        fontSize: baseStyle.fontSize!,
+        color: baseStyle.color!)));
+
+  _showActionSheet(pageContext,
+    header: header,
+    headerScrollable: false,
+    buttonSections: [optionButtons]);
 }
 
 class UserTopicUpdateButton extends ActionSheetMenuItemButton {

lib/widgets/text.dart

@@ -4,6 +4,7 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/gestures.dart';
 import 'package:flutter/material.dart';
 
+import '../api/model/model.dart';
 import '../generated/l10n/zulip_localizations.dart';
 import 'icons.dart';
 import 'store.dart';
@@ -582,6 +583,11 @@ class InlineIconGeometryData {
       sizeFactor: 0.8,
       alphabeticBaselineFactor: 1 / 16,
       paddingFactor: 1 / 5),
+
+    ZulipIcons.chevron_right: InlineIconGeometryData._(
+      sizeFactor: 1,
+      alphabeticBaselineFactor: 5 / 24,
+      paddingFactor: 0),
  };
 
   static InlineIconGeometryData defaultGeometry = InlineIconGeometryData._(
@@ -637,14 +643,16 @@ WidgetSpan iconWidgetSpan({
     child: child);
 }
 
-/// An [InlineSpan] with a channel privacy icon and channel name.
+/// An [InlineSpan] with a channel privacy icon, channel name,
+/// and optionally a chevron-right icon plus topic.
 ///
 /// Pass this to [Text.rich], which can be styled arbitrarily.
-/// Pass the [fontSize] of surrounding text
-/// so that the icon is sized appropriately.
+/// Pass the [fontSize] and [color] of surrounding text
+/// so that the icons are sized and colored appropriately.
 InlineSpan channelTopicLabelSpan({
   required BuildContext context,
   required int channelId,
+  TopicName? topic,
   required double fontSize,
   required Color color,
 }) {
@@ -671,5 +679,20 @@ InlineSpan channelTopicLabelSpan({
       TextSpan(
         style: TextStyle(fontStyle: FontStyle.italic),
         text: zulipLocalizations.unknownChannelName),
+    if (topic != null) ...[
+      iconWidgetSpan(
+        icon: ZulipIcons.chevron_right,
+        fontSize: fontSize,
+        baselineType: baselineType,
+        color: color,
+        padBefore: true,
+        padAfter: true),
+      if (topic.displayName != null)
+        TextSpan(text: topic.displayName)
+      else
+        TextSpan(
+          style: TextStyle(fontStyle: FontStyle.italic),
+          text: store.realmEmptyTopicDisplayName),
+    ],
   ]);
 }

test/widgets/action_sheet_test.dart

@@ -805,6 +805,38 @@ void main() {
         checkButton('Copy link to topic');
       }
 
+      group('header', () {
+        final findHeader = find.descendant(
+          of: actionSheetFinder,
+          matching: find.byType(BottomSheetHeader));
+
+        Finder findInHeader(Finder finder) =>
+          find.descendant(of: findHeader, matching: finder);
+
+        testWidgets('with topic', (tester) async {
+          await prepare();
+          check(store.streams[someChannel.streamId]).isNotNull()
+            ..inviteOnly.isFalse()..isWebPublic.isFalse();
+          await showFromAppBar(tester);
+          check(findInHeader(find.byIcon(ZulipIcons.hash_sign))).findsOne();
+          check(findInHeader(find.textContaining(someChannel.name))).findsOne();
+          check(findInHeader(find.textContaining(someTopic))).findsOne();
+        });
+
+        testWidgets('without topic (general chat)', (tester) async {
+          await prepare(topic: '');
+          check(store.streams[someChannel.streamId]).isNotNull()
+            ..inviteOnly.isFalse()..isWebPublic.isFalse();
+          final message = eg.streamMessage(
+            stream: someChannel, topic: '', sender: eg.otherUser);
+          await showFromAppBar(tester, messages: [message], topic: eg.t(''));
+          check(findInHeader(find.byIcon(ZulipIcons.hash_sign))).findsOne();
+          check(findInHeader(find.textContaining(someChannel.name))).findsOne();
+          check(findInHeader(find.textContaining(store.realmEmptyTopicDisplayName)))
+            .findsOne();
+        });
+      });
+
       testWidgets('show from inbox; message in Unreads but not in MessageStore', (tester) async {
         await prepare(unreadMsgs: eg.unreadMsgs(count: 1,
           channels: [eg.unreadChannelMsgs(