lightbox: Prevent hero animation between message lists

lib/widgets/content.dart

@@ -651,6 +651,7 @@ class MessageImage extends StatelessWidget {
         Navigator.of(context).push(getImageLightboxRoute(
           context: context,
           message: message,
+          messageImageContext: context,
           src: resolvedSrcUrl,
           thumbnailUrl: resolvedThumbnailUrl,
           originalWidth: node.originalWidth,
@@ -659,7 +660,7 @@ class MessageImage extends StatelessWidget {
       child: node.loading
         ? const CupertinoActivityIndicator()
         : resolvedSrcUrl == null ? null : LightboxHero(
-            message: message,
+            messageImageContext: context,
             src: resolvedSrcUrl,
             child: RealmContentNetworkImage(
               resolvedThumbnailUrl ?? resolvedSrcUrl,

lib/widgets/lightbox.dart

@@ -15,45 +15,67 @@ import 'dialog.dart';
 import 'page.dart';
 import 'store.dart';
 
-// TODO(#44): Add index of the image preview in the message, to not break if
-//   there are multiple image previews with the same URL in the same
-//   message. Maybe keep `src`, so that on exit the lightbox image doesn't
-//   fly to an image preview with a different URL, following a message edit
-//   while the lightbox was open.
+/// Identifies which [LightboxHero]s should match up with each other
+/// to produce a hero animation.
+///
+/// See [Hero.tag], the field where we use instances of this class.
+///
+/// The intended behavior is that when the user acts on an image
+/// in the message list to have the app expand it in the lightbox,
+/// a hero animation goes from the original view of the image
+/// to the version in the lightbox,
+/// and back to the original upon exiting the lightbox.
 class _LightboxHeroTag {
-  _LightboxHeroTag({required this.messageId, required this.src});
+  _LightboxHeroTag({
+    required this.messageImageContext,
+    required this.src,
+  });
 
-  final int messageId;
+  /// The [BuildContext] for the [MessageImage] being expanded into the lightbox.
+  ///
+  /// In particular this prevents hero animations between
+  /// different message lists that happen to have the same message.
+  /// It also distinguishes different copies of the same image
+  /// in a given message list.
+  // TODO: write a regression test for #44, duplicate images within a message
+  final BuildContext messageImageContext;
+
+  /// The image source URL.
+  ///
+  /// This ensures the animation only occurs between matching images, even if
+  /// the message was edited before navigating back to the message list
+  /// so that the original [MessageImage] has been replaced in the tree
+  /// by a different image.
   final Uri src;
 
   @override
   bool operator ==(Object other) {
     return other is _LightboxHeroTag &&
-      other.messageId == messageId &&
+      other.messageImageContext == messageImageContext &&
       other.src == src;
   }
 
   @override
-  int get hashCode => Object.hash('_LightboxHeroTag', messageId, src);
+  int get hashCode => Object.hash('_LightboxHeroTag', messageImageContext, src);
 }
 
 /// Builds a [Hero] from an image in the message list to the lightbox page.
 class LightboxHero extends StatelessWidget {
   const LightboxHero({
     super.key,
-    required this.message,
+    required this.messageImageContext,
     required this.src,
     required this.child,
   });
 
-  final Message message;
+  final BuildContext messageImageContext;
   final Uri src;
   final Widget child;
 
   @override
   Widget build(BuildContext context) {
     return Hero(
-      tag: _LightboxHeroTag(messageId: message.id, src: src),
+      tag: _LightboxHeroTag(messageImageContext: messageImageContext, src: src),
       flightShuttleBuilder: (
         BuildContext flightContext,
         Animation<double> animation,
@@ -226,6 +248,7 @@ class _ImageLightboxPage extends StatefulWidget {
   const _ImageLightboxPage({
     required this.routeEntranceAnimation,
     required this.message,
+    required this.messageImageContext,
     required this.src,
     required this.thumbnailUrl,
     required this.originalWidth,
@@ -234,6 +257,7 @@ class _ImageLightboxPage extends StatefulWidget {
 
   final Animation<double> routeEntranceAnimation;
   final Message message;
+  final BuildContext messageImageContext;
   final Uri src;
   final Uri? thumbnailUrl;
   final double? originalWidth;
@@ -317,7 +341,7 @@ class _ImageLightboxPageState extends State<_ImageLightboxPage> {
         child: InteractiveViewer(
           child: SafeArea(
             child: LightboxHero(
-              message: widget.message,
+              messageImageContext: widget.messageImageContext,
               src: widget.src,
               child: RealmContentNetworkImage(widget.src,
                 filterQuality: FilterQuality.medium,
@@ -599,6 +623,7 @@ Route<void> getImageLightboxRoute({
   int? accountId,
   BuildContext? context,
   required Message message,
+  required BuildContext messageImageContext,
   required Uri src,
   required Uri? thumbnailUrl,
   required double? originalWidth,
@@ -611,6 +636,7 @@ Route<void> getImageLightboxRoute({
       return _ImageLightboxPage(
         routeEntranceAnimation: animation,
         message: message,
+        messageImageContext: messageImageContext,
         src: src,
         thumbnailUrl: thumbnailUrl,
         originalWidth: originalWidth,

test/widgets/lightbox_test.dart

@@ -1,4 +1,5 @@
 import 'dart:async';
+import 'dart:math';
 
 import 'package:checks/checks.dart';
 import 'package:clock/clock.dart';
@@ -10,12 +11,18 @@ import 'package:video_player_platform_interface/video_player_platform_interface.
 import 'package:video_player/video_player.dart';
 import 'package:zulip/api/model/model.dart';
 import 'package:zulip/model/localizations.dart';
+import 'package:zulip/model/narrow.dart';
+import 'package:zulip/model/store.dart';
 import 'package:zulip/widgets/app.dart';
 import 'package:zulip/widgets/content.dart';
 import 'package:zulip/widgets/lightbox.dart';
+import 'package:zulip/widgets/message_list.dart';
 
+import '../api/fake_api.dart';
 import '../example_data.dart' as eg;
 import '../model/binding.dart';
+import '../model/content_test.dart';
+import '../model/test_store.dart';
 import '../test_images.dart';
 import 'dialog_checks.dart';
 import 'test_app.dart';
@@ -197,6 +204,113 @@ class FakeVideoPlayerPlatform extends Fake
 void main() {
   TestZulipBinding.ensureInitialized();
 
+  group('LightboxHero', () {
+    late PerAccountStore store;
+    late FakeApiConnection connection;
+
+    final channel = eg.stream();
+    final message = eg.streamMessage(stream: channel,
+      topic: 'test topic', contentMarkdown: ContentExample.imageSingle.html);
+
+    // From ContentExample.imageSingle.
+    final imageSrcUrlStr = 'https://chat.example/user_uploads/thumbnail/2/ce/nvoNL2LaZOciwGZ-FYagddtK/image.jpg/840x560.webp';
+    final imageSrcUrl = Uri.parse(imageSrcUrlStr);
+    final imageFinder = find.byWidgetPredicate(
+      (widget) => widget is RealmContentNetworkImage && widget.src == imageSrcUrl);
+
+    Future<void> setupMessageListPage(WidgetTester tester) async {
+      addTearDown(testBinding.reset);
+      final subscription = eg.subscription(channel);
+      await testBinding.globalStore.add(eg.selfAccount, eg.initialSnapshot(
+        streams: [channel], subscriptions: [subscription]));
+      store = await testBinding.globalStore.perAccount(eg.selfAccount.id);
+      connection = store.connection as FakeApiConnection;
+      await store.addUser(eg.selfUser);
+
+      connection.prepare(json:
+        eg.newestGetMessagesResult(foundOldest: true, messages: [message]).toJson());
+      await tester.pumpWidget(TestZulipApp(accountId: eg.selfAccount.id,
+        child: MessageListPage(initNarrow: const CombinedFeedNarrow())));
+      await tester.pumpAndSettle();
+    }
+
+    testWidgets('Hero animation occurs smoothly when opening lightbox from message list', (tester) async {
+      double dist(Rect a, Rect b) =>
+        sqrt(pow(a.top - b.top, 2) + pow(a.left - b.left, 2));
+
+      prepareBoringImageHttpClient();
+
+      await setupMessageListPage(tester);
+
+      final initialImagePosition = tester.getRect(imageFinder);
+      await tester.tap(imageFinder);
+      await tester.pump();
+      // pump to start hero animation
+      await tester.pump();
+
+      const heroAnimationDuration = Duration(milliseconds: 300);
+      const steps = 150;
+      final stepDuration = heroAnimationDuration ~/ steps;
+      final animatedPositions = <Rect>[];
+      for (int i = 1; i <= steps; i++) {
+        await tester.pump(stepDuration);
+        animatedPositions.add(tester.getRect(imageFinder));
+      }
+
+      final totalDistance = dist(initialImagePosition, animatedPositions.last);
+      Rect previousPosition = initialImagePosition;
+      double maxStepDistance = 0.0;
+      for (final position in animatedPositions) {
+        final stepDistance = dist(previousPosition, position);
+        maxStepDistance = max(maxStepDistance, stepDistance);
+        check(position).not((pos) => pos.equals(previousPosition));
+
+        previousPosition = position;
+      }
+      check(maxStepDistance).isLessThan(0.03 * totalDistance);
+
+      debugNetworkImageHttpClientProvider = null;
+    });
+
+    testWidgets('no hero animation occurs between different message list pages for same image', (tester) async {
+      Rect getElementRect(Element element) =>
+        tester.getRect(find.byElementPredicate((e) => e == element));
+
+      prepareBoringImageHttpClient();
+
+      await setupMessageListPage(tester);
+
+      final firstElement = tester.element(imageFinder);
+      final firstImagePosition = getElementRect(firstElement);
+
+      connection.prepare(json:
+        eg.newestGetMessagesResult(foundOldest: true, messages: [message]).toJson());
+      await tester.tap(find.descendant(
+        of: find.byType(StreamMessageRecipientHeader),
+        matching: find.text('test topic')));
+      await tester.pumpAndSettle();
+
+      final secondElement = tester.element(imageFinder);
+      final secondImagePosition = getElementRect(secondElement);
+
+      await tester.tap(find.byType(BackButton));
+      await tester.pump();
+
+      const heroAnimationDuration = Duration(milliseconds: 300);
+      const steps = 150;
+      final stepDuration = heroAnimationDuration ~/ steps;
+      for (int i = 0; i < steps; i++) {
+        await tester.pump(stepDuration);
+        check(tester.elementList(imageFinder))
+          .unorderedEquals([firstElement, secondElement]);
+        check(getElementRect(firstElement)).equals(firstImagePosition);
+        check(getElementRect(secondElement)).equals(secondImagePosition);
+      }
+
+      debugNetworkImageHttpClientProvider = null;
+    });
+  });
+
   group('_ImageLightboxPage', () {
     final src = Uri.parse('https://chat.example/lightbox-image.png');
 
@@ -216,6 +330,7 @@ void main() {
       unawaited(navigator.push(getImageLightboxRoute(
         accountId: eg.selfAccount.id,
         message: message ?? eg.streamMessage(),
+        messageImageContext: navigator.context,
         src: src,
         thumbnailUrl: thumbnailUrl,
         originalHeight: null,