RecentDmConversationsPage: Add

The screen's content area (so, the list of conversations, but not
the app bar at the top) is built against Vlad's design:
  https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/design.3A.20DM-conversation.20list/near/1594654
except that some features that appear in that design are left
unimplemented for now, since we don't have data structures for them
yet:
- unread counts
- user presence

Fixes: zulip#119

lib/widgets/app.dart

@@ -4,6 +4,7 @@ import '../model/narrow.dart';
 import 'about_zulip.dart';
 import 'login.dart';
 import 'message_list.dart';
+import 'recent_dm_conversations.dart';
 import 'store.dart';
 
 class ZulipApp extends StatelessWidget {
@@ -152,6 +153,11 @@ class HomePage extends StatelessWidget {
               MessageListPage.buildRoute(context: context,
                 narrow: const AllMessagesNarrow())),
             child: const Text("All messages")),
+          const SizedBox(height: 16),
+          ElevatedButton(
+            onPressed: () => Navigator.push(context,
+              RecentDmConversationsPage.buildRoute(context: context)),
+            child: const Text("Direct messages")),
           if (testStreamId != null) ...[
             const SizedBox(height: 16),
             ElevatedButton(

lib/widgets/recent_dm_conversations.dart

@@ -0,0 +1,108 @@
+import 'package:flutter/material.dart';
+
+import '../model/narrow.dart';
+import '../model/recent_dm_conversations.dart';
+import 'content.dart';
+import 'icons.dart';
+import 'message_list.dart';
+import 'page.dart';
+import 'store.dart';
+import 'text.dart';
+
+class RecentDmConversationsPage extends StatefulWidget {
+  const RecentDmConversationsPage({super.key});
+
+  static Route<void> buildRoute({required BuildContext context}) {
+    return MaterialAccountPageRoute(context: context,
+      builder: (context) => const RecentDmConversationsPage());
+  }
+
+  @override
+  State<RecentDmConversationsPage> createState() => _RecentDmConversationsPageState();
+}
+
+class _RecentDmConversationsPageState extends State<RecentDmConversationsPage> with PerAccountStoreAwareStateMixin<RecentDmConversationsPage> {
+  RecentDmConversationsView? model;
+
+  @override
+  void onNewStore() {
+    model?.removeListener(_modelChanged);
+    model = PerAccountStoreWidget.of(context).recentDmConversationsView
+      ..addListener(_modelChanged);
+  }
+
+  void _modelChanged() {
+    setState(() {
+      // The actual state lives in [model].
+      // This method was called because that just changed.
+    });
+  }
+
+  @override
+  Widget build(BuildContext context) {
+    final sorted = model!.sorted;
+    return Scaffold(
+      appBar: AppBar(title: const Text('Direct messages')),
+      body: ListView.builder(
+        itemCount: sorted.length,
+        itemBuilder: (context, index) => RecentDmConversationsItem(narrow: sorted[index])));
+  }
+}
+
+class RecentDmConversationsItem extends StatelessWidget {
+  const RecentDmConversationsItem({super.key, required this.narrow});
+
+  final DmNarrow narrow;
+
+  @override
+  Widget build(BuildContext context) {
+    final store = PerAccountStoreWidget.of(context);
+    final selfUser = store.users[store.account.userId]!;
+
+    final String title;
+    final Widget avatar;
+    switch (narrow.otherRecipientIds) {
+      case []:
+        title = selfUser.fullName;
+        avatar = AvatarImage(userId: selfUser.userId);
+      case [var otherUserId]:
+        final otherUser = store.users[otherUserId];
+        title = otherUser?.fullName ?? '(unknown user)';
+        avatar = AvatarImage(userId: otherUserId);
+      default:
+        // TODO(i18n): List formatting, like you can do in JavaScript:
+        //   new Intl.ListFormat('ja').format(['Chris', 'Greg', 'Alya'])
+        //   // 'Chris、Greg、Alya'
+        title = narrow.otherRecipientIds.map((id) => store.users[id]?.fullName ?? '(unknown user)').join(', ');
+        avatar = ColoredBox(color: const Color(0x33808080),
+          child: Center(
+            child: Icon(ZulipIcons.group_dm, color: Colors.black.withOpacity(0.5))));
+    }
+
+    return InkWell(
+      onTap: () {
+        Navigator.push(context,
+          MessageListPage.buildRoute(context: context, narrow: narrow));
+      },
+      child: ConstrainedBox(constraints: const BoxConstraints(minHeight: 48),
+        child: Row(crossAxisAlignment: CrossAxisAlignment.center, children: [
+          Padding(padding: const EdgeInsets.fromLTRB(12, 8, 0, 8),
+            child: AvatarShape(size: 32, borderRadius: 3, child: avatar)),
+          const SizedBox(width: 8),
+          Expanded(child: Padding(
+            padding: const EdgeInsets.symmetric(vertical: 4),
+            child: Text(
+              style: const TextStyle(
+                fontFamily: 'Source Sans 3',
+                fontSize: 17,
+                height: (20 / 17),
+                color: Color(0xFF222222),
+              ).merge(weightVariableTextStyle(context)),
+              maxLines: 2,
+              overflow: TextOverflow.ellipsis,
+              title))),
+          const SizedBox(width: 8),
+          // TODO(#253): Unread count
+        ])));
+  }
+}

test/test_navigation.dart

@@ -0,0 +1,39 @@
+import 'package:flutter/widgets.dart';
+
+// Inspired by test code in the Flutter tree:
+//   https://github.com/flutter/flutter/blob/53082f65b/packages/flutter/test/widgets/observer_tester.dart
+//   https://github.com/flutter/flutter/blob/53082f65b/packages/flutter/test/widgets/navigator_test.dart
+
+/// A trivial observer for testing the navigator.
+class TestNavigatorObserver extends NavigatorObserver {
+  void Function(Route<dynamic> route, Route<dynamic>? previousRoute)? onPushed;
+  void Function(Route<dynamic> route, Route<dynamic>? previousRoute)? onPopped;
+  void Function(Route<dynamic> route, Route<dynamic>? previousRoute)? onRemoved;
+  void Function(Route<dynamic>? route, Route<dynamic>? previousRoute)? onReplaced;
+  void Function(Route<dynamic> route, Route<dynamic>? previousRoute)? onStartUserGesture;
+
+  @override
+  void didPush(Route<dynamic> route, Route<dynamic>? previousRoute) {
+    onPushed?.call(route, previousRoute);
+  }
+
+  @override
+  void didPop(Route<dynamic> route, Route<dynamic>? previousRoute) {
+    onPopped?.call(route, previousRoute);
+  }
+
+  @override
+  void didRemove(Route<dynamic> route, Route<dynamic>? previousRoute) {
+    onRemoved?.call(route, previousRoute);
+  }
+
+  @override
+  void didReplace({ Route<dynamic>? oldRoute, Route<dynamic>? newRoute }) {
+    onReplaced?.call(newRoute, oldRoute);
+  }
+
+  @override
+  void didStartUserGesture(Route<dynamic> route, Route<dynamic>? previousRoute) {
+    onStartUserGesture?.call(route, previousRoute);
+  }
+}

test/widgets/content_checks.dart

@@ -1,7 +1,19 @@
 import 'package:checks/checks.dart';
+import 'package:flutter/widgets.dart';
+
 import 'package:zulip/widgets/content.dart';
 
 extension RealmContentNetworkImageChecks on Subject<RealmContentNetworkImage> {
   Subject<Uri> get src => has((i) => i.src, 'src');
   // TODO others
 }
+
+extension AvatarImageChecks on Subject<AvatarImage> {
+  Subject<int> get userId => has((i) => i.userId, 'userId');
+}
+
+extension AvatarShapeChecks on Subject<AvatarShape> {
+  Subject<double> get size => has((i) => i.size, 'size');
+  Subject<double> get borderRadius => has((i) => i.borderRadius, 'borderRadius');
+  Subject<Widget> get child => has((i) => i.child, 'child');
+}