[web] Support flutterViewId in platform view messages (flutter/engine#46891)

- Accept a new `flutterViewId` field in platform view messages.
- Keep transitory support for legacy platform view messages that don't contain `flutterViewId`.
- Default view factories set `width:100%` and `height:100%`.

engine/src/flutter/lib/web_ui/lib/src/engine/platform_dispatcher.dart

@@ -21,6 +21,9 @@ ui.VoidCallback? scheduleFrameCallback;
 typedef HighContrastListener = void Function(bool enabled);
 typedef _KeyDataResponseCallback = void Function(bool handled);
 
+const StandardMethodCodec standardCodec = StandardMethodCodec();
+const JSONMethodCodec jsonCodec = JSONMethodCodec();
+
 /// Determines if high contrast is enabled using media query 'forced-colors: active' for Windows
 class HighContrastSupport {
   static HighContrastSupport instance = HighContrastSupport();
@@ -129,13 +132,13 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
 
   /// The current list of windows.
   @override
-  Iterable<ui.FlutterView> get views => viewData.values;
-  final Map<int, ui.FlutterView> viewData = <int, ui.FlutterView>{};
+  Iterable<EngineFlutterView> get views => viewData.values;
+  final Map<int, EngineFlutterView> viewData = <int, EngineFlutterView>{};
 
   /// Returns the [FlutterView] with the provided ID if one exists, or null
   /// otherwise.
   @override
-  ui.FlutterView? view({required int id}) => viewData[id];
+  EngineFlutterView? view({required int id}) => viewData[id];
 
   /// A map of opaque platform window identifiers to window configurations.
   ///
@@ -470,8 +473,7 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
 
       /// This should be in sync with shell/common/shell.cc
       case 'flutter/skia':
-        const MethodCodec codec = JSONMethodCodec();
-        final MethodCall decoded = codec.decodeMethodCall(data);
+        final MethodCall decoded = jsonCodec.decodeMethodCall(data);
         switch (decoded.method) {
           case 'Skia.setResourceCacheMaxBytes':
             if (renderer is CanvasKitRenderer) {
@@ -486,7 +488,7 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
             // Also respond in HTML mode. Otherwise, apps would have to detect
             // CanvasKit vs HTML before invoking this method.
             replyToPlatformMessage(
-                callback, codec.encodeSuccessEnvelope(<bool>[true]));
+                callback, jsonCodec.encodeSuccessEnvelope(<bool>[true]));
         }
         return;
 
@@ -496,8 +498,7 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
         return;
 
       case 'flutter/platform':
-        const MethodCodec codec = JSONMethodCodec();
-        final MethodCall decoded = codec.decodeMethodCall(data);
+        final MethodCall decoded = jsonCodec.decodeMethodCall(data);
         switch (decoded.method) {
           case 'SystemNavigator.pop':
             // TODO(a-wallen): As multi-window support expands, the pop call
@@ -505,13 +506,13 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
             // supported.
             implicitView!.browserHistory.exit().then((_) {
               replyToPlatformMessage(
-                  callback, codec.encodeSuccessEnvelope(true));
+                  callback, jsonCodec.encodeSuccessEnvelope(true));
             });
             return;
           case 'HapticFeedback.vibrate':
             final String? type = decoded.arguments as String?;
             vibrate(_getHapticFeedbackDuration(type));
-            replyToPlatformMessage(callback, codec.encodeSuccessEnvelope(true));
+            replyToPlatformMessage(callback, jsonCodec.encodeSuccessEnvelope(true));
             return;
           case 'SystemChrome.setApplicationSwitcherDescription':
             final Map<String, Object?> arguments = decoded.arguments as Map<String, Object?>;
@@ -520,24 +521,24 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
             final int primaryColor = arguments['primaryColor'] as int? ?? 0xFF000000;
             domDocument.title = label;
             setThemeColor(ui.Color(primaryColor));
-            replyToPlatformMessage(callback, codec.encodeSuccessEnvelope(true));
+            replyToPlatformMessage(callback, jsonCodec.encodeSuccessEnvelope(true));
             return;
           case 'SystemChrome.setSystemUIOverlayStyle':
             final Map<String, Object?> arguments = decoded.arguments as Map<String, Object?>;
             final int? statusBarColor = arguments['statusBarColor'] as int?;
             setThemeColor(statusBarColor == null ? null : ui.Color(statusBarColor));
-            replyToPlatformMessage(callback, codec.encodeSuccessEnvelope(true));
+            replyToPlatformMessage(callback, jsonCodec.encodeSuccessEnvelope(true));
             return;
           case 'SystemChrome.setPreferredOrientations':
             final List<dynamic> arguments = decoded.arguments as List<dynamic>;
             ScreenOrientation.instance.setPreferredOrientation(arguments).then((bool success) {
               replyToPlatformMessage(
-                  callback, codec.encodeSuccessEnvelope(success));
+                  callback, jsonCodec.encodeSuccessEnvelope(success));
             });
             return;
           case 'SystemSound.play':
             // There are no default system sounds on web.
-            replyToPlatformMessage(callback, codec.encodeSuccessEnvelope(true));
+            replyToPlatformMessage(callback, jsonCodec.encodeSuccessEnvelope(true));
             return;
           case 'Clipboard.setData':
             ClipboardMessageHandler().setDataMethodCall(decoded, callback);
@@ -560,23 +561,21 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
         return;
 
       case 'flutter/contextmenu':
-        const MethodCodec codec = JSONMethodCodec();
-        final MethodCall decoded = codec.decodeMethodCall(data);
+        final MethodCall decoded = jsonCodec.decodeMethodCall(data);
         switch (decoded.method) {
           case 'enableContextMenu':
             implicitView!.contextMenu.enable();
-            replyToPlatformMessage(callback, codec.encodeSuccessEnvelope(true));
+            replyToPlatformMessage(callback, jsonCodec.encodeSuccessEnvelope(true));
             return;
           case 'disableContextMenu':
             implicitView!.contextMenu.disable();
-            replyToPlatformMessage(callback, codec.encodeSuccessEnvelope(true));
+            replyToPlatformMessage(callback, jsonCodec.encodeSuccessEnvelope(true));
             return;
         }
         return;
 
       case 'flutter/mousecursor':
-        const MethodCodec codec = StandardMethodCodec();
-        final MethodCall decoded = codec.decodeMethodCall(data);
+        final MethodCall decoded = standardCodec.decodeMethodCall(data);
         final Map<dynamic, dynamic> arguments = decoded.arguments as Map<dynamic, dynamic>;
         switch (decoded.method) {
           case 'activateSystemCursor':
@@ -585,15 +584,21 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
         return;
 
       case 'flutter/web_test_e2e':
-        const MethodCodec codec = JSONMethodCodec();
         replyToPlatformMessage(
             callback,
-            codec.encodeSuccessEnvelope(
-                _handleWebTestEnd2EndMessage(codec, data)));
+            jsonCodec.encodeSuccessEnvelope(
+                _handleWebTestEnd2EndMessage(jsonCodec, data)));
         return;
 
       case 'flutter/platform_views':
-        implicitView!.platformViewMessageHandler.handlePlatformViewCall(data, callback!);
+        final MethodCall(:String method, :dynamic arguments) = standardCodec.decodeMethodCall(data);
+        final int? flutterViewId = tryViewId(arguments);
+        if (flutterViewId == null) {
+          implicitView!.platformViewMessageHandler.handleLegacyPlatformViewCall(method, arguments, callback!);
+          return;
+        }
+        arguments as Map<dynamic, dynamic>;
+        viewData[flutterViewId]!.platformViewMessageHandler.handlePlatformViewCall(method, arguments, callback!);
         return;
 
       case 'flutter/accessibility':
@@ -609,8 +614,7 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
         // supported.
         implicitView!.handleNavigationMessage(data).then((bool handled) {
           if (handled) {
-            const MethodCodec codec = JSONMethodCodec();
-            replyToPlatformMessage(callback, codec.encodeSuccessEnvelope(true));
+            replyToPlatformMessage(callback, jsonCodec.encodeSuccessEnvelope(true));
           } else {
             callback?.call(null);
           }
@@ -1350,7 +1354,7 @@ class ViewConfiguration {
   });
 
   ViewConfiguration copyWith({
-    ui.FlutterView? view,
+    EngineFlutterView? view,
     double? devicePixelRatio,
     ui.Rect? geometry,
     bool? visible,
@@ -1375,7 +1379,7 @@ class ViewConfiguration {
     );
   }
 
-  final ui.FlutterView? view;
+  final EngineFlutterView? view;
   final double devicePixelRatio;
   final ui.Rect geometry;
   final bool visible;

engine/src/flutter/lib/web_ui/lib/src/engine/platform_views/content_manager.dart

@@ -249,5 +249,7 @@ DomElement _defaultFactory(
 }) {
   params!;
   params as Map<Object?, Object?>;
-  return domDocument.createElement(params.readString('tagName'));
+  return domDocument.createElement(params.readString('tagName'))
+    ..style.width = '100%'
+    ..style.height = '100%';
 }

engine/src/flutter/lib/web_ui/lib/src/engine/platform_views/message_handler.dart

@@ -53,7 +53,7 @@ class PlatformViewMessageHandler {
 
   /// Handle a `create` Platform View message.
   ///
-  /// This will attempt to render the `contents` and of a Platform View, if its
+  /// This will attempt to render the `contents` of a Platform View, if its
   /// `viewType` has been registered previously.
   ///
   /// (See [PlatformViewManager.registerFactory] for more details.)
@@ -63,37 +63,34 @@ class PlatformViewMessageHandler {
   /// If all goes well, this function will `callback` with an empty success envelope.
   /// In case of error, this will `callback` with an error envelope describing the error.
   void _createPlatformView(
-    MethodCall methodCall,
-    _PlatformMessageResponseCallback callback,
-  ) {
-    final Map<dynamic, dynamic> args = methodCall.arguments as Map<dynamic, dynamic>;
-    final int viewId = args.readInt('id');
-    final String viewType = args.readString('viewType');
-    final Object? params = args['params'];
-
-    if (!_contentManager.knowsViewType(viewType)) {
+    _PlatformMessageResponseCallback callback, {
+    required int platformViewId,
+    required String platformViewType,
+    required Object? params,
+  }) {
+    if (!_contentManager.knowsViewType(platformViewType)) {
       callback(_codec.encodeErrorEnvelope(
         code: 'unregistered_view_type',
         message: 'A HtmlElementView widget is trying to create a platform view '
-            'with an unregistered type: <$viewType>.',
+            'with an unregistered type: <$platformViewType>.',
         details: 'If you are the author of the PlatformView, make sure '
             '`registerViewFactory` is invoked.',
       ));
       return;
     }
 
-    if (_contentManager.knowsViewId(viewId)) {
+    if (_contentManager.knowsViewId(platformViewId)) {
       callback(_codec.encodeErrorEnvelope(
         code: 'recreating_view',
         message: 'trying to create an already created view',
-        details: 'view id: $viewId',
+        details: 'view id: $platformViewId',
       ));
       return;
     }
 
     final DomElement content = _contentManager.renderContent(
-      viewType,
-      viewId,
+      platformViewType,
+      platformViewId,
       params,
     );
 
@@ -106,7 +103,7 @@ class PlatformViewMessageHandler {
   /// Handle a `dispose` Platform View message.
   ///
   /// This will clear the cached information that the framework has about a given
-  /// `viewId`, through the [_contentManager].
+  /// `platformViewId`, through the [_contentManager].
   ///
   /// Once that's done, the dispose call is delegated to the [_disposeHandler]
   /// function, so the active rendering backend can dispose of whatever resources
@@ -114,34 +111,67 @@ class PlatformViewMessageHandler {
   ///
   /// This function should always `callback` with an empty success envelope.
   void _disposePlatformView(
-    MethodCall methodCall,
-    _PlatformMessageResponseCallback callback,
-  ) {
-    final int viewId = methodCall.arguments as int;
-
+    _PlatformMessageResponseCallback callback, {
+    required int platformViewId,
+  }) {
     // The contentManager removes the slot and the contents from its internal
     // cache, and the DOM.
-    _contentManager.clearPlatformView(viewId);
+    _contentManager.clearPlatformView(platformViewId);
 
     callback(_codec.encodeSuccessEnvelope(null));
   }
 
+  /// Handles legacy PlatformViewCalls that don't contain a Flutter View ID.
+  ///
+  /// This is transitional code to support the old platform view channel. As
+  /// soon as the framework code is updated to send the Flutter View ID, this
+  /// method can be removed.
+  void handleLegacyPlatformViewCall(
+    String method,
+    dynamic arguments,
+    _PlatformMessageResponseCallback callback,
+  ) {
+    switch (method) {
+      case 'create':
+        arguments as Map<dynamic, dynamic>;
+        _createPlatformView(
+          callback,
+          platformViewId: arguments.readInt('id'),
+          platformViewType: arguments.readString('viewType'),
+          params: arguments['params'],
+        );
+        return;
+      case 'dispose':
+        _disposePlatformView(callback, platformViewId: arguments as int);
+        return;
+    }
+    callback(null);
+  }
+
   /// Handles a PlatformViewCall to the `flutter/platform_views` channel.
   ///
   /// This method handles two possible messages:
   /// * `create`: See [_createPlatformView]
   /// * `dispose`: See [_disposePlatformView]
   void handlePlatformViewCall(
-    ByteData? data,
+    String method,
+    Map<dynamic, dynamic> arguments,
     _PlatformMessageResponseCallback callback,
   ) {
-    final MethodCall decoded = _codec.decodeMethodCall(data);
-    switch (decoded.method) {
+    switch (method) {
       case 'create':
-        _createPlatformView(decoded, callback);
+        _createPlatformView(
+          callback,
+          platformViewId: arguments.readInt('platformViewId'),
+          platformViewType: arguments.readString('platformViewType'),
+          params: arguments['params'],
+        );
         return;
       case 'dispose':
-        _disposePlatformView(decoded, callback);
+        _disposePlatformView(
+          callback,
+          platformViewId: arguments.readInt('platformViewId'),
+        );
         return;
     }
     callback(null);

engine/src/flutter/lib/web_ui/lib/src/engine/util.dart

@@ -14,6 +14,7 @@ import 'package:ui/ui.dart' as ui;
 import 'browser_detection.dart';
 import 'dom.dart';
 import 'safe_browser_api.dart';
+import 'services.dart';
 import 'vector_math.dart';
 
 /// Generic callback signature, used by [_futurize].
@@ -627,6 +628,27 @@ extension JsonExtensions on Map<dynamic, dynamic> {
   }
 }
 
+/// Extracts view ID from the [MethodCall.arguments] map.
+///
+/// Throws if the view ID is not present or if [arguments] is not a map.
+int readViewId(Object? arguments) {
+  final int? viewId = tryViewId(arguments);
+  if (viewId == null) {
+    throw Exception('Could not find a `viewId` in the arguments: $arguments');
+  }
+  return viewId;
+}
+
+/// Extracts view ID from the [MethodCall.arguments] map.
+///
+/// Returns null if the view ID is not present or if [arguments] is not a map.
+int? tryViewId(Object? arguments) {
+  if (arguments is Map) {
+    return arguments.tryInt('viewId');
+  }
+  return null;
+}
+
 /// Prints a list of bytes in hex format.
 ///
 /// Bytes are separated by one space and are padded on the left to always show

engine/src/flutter/lib/web_ui/test/engine/platform_views/legacy_message_handler_test.dart

@@ -0,0 +1,265 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'dart:async';
+import 'dart:typed_data';
+
+import 'package:test/bootstrap/browser.dart';
+import 'package:test/test.dart';
+import 'package:ui/src/engine.dart';
+
+void main() {
+  internalBootstrapBrowserTest(() => testMain);
+}
+
+const MethodCodec codec = StandardMethodCodec();
+
+typedef PlatformViewFactoryCall = ({int viewId, Object? params});
+
+void testMain() {
+  group('PlatformViewMessageHandler', () {
+    group('handlePlatformViewCall', () {
+      const String viewType = 'forTest';
+      const int viewId = 6;
+      late PlatformViewManager contentManager;
+      late Completer<ByteData?> completer;
+
+      setUp(() {
+        contentManager = PlatformViewManager();
+        completer = Completer<ByteData?>();
+      });
+
+      group('"create" message', () {
+        test('unregistered viewType, fails with descriptive exception',
+            () async {
+          final PlatformViewMessageHandler messageHandler = PlatformViewMessageHandler(
+            platformViewsContainer: createDomElement('div'),
+            contentManager: contentManager,
+          );
+          final Map<dynamic, dynamic> arguments = _getCreateArguments(viewType, viewId);
+
+          messageHandler.handleLegacyPlatformViewCall('create', arguments, completer.complete);
+
+          final ByteData? response = await completer.future;
+          try {
+            codec.decodeEnvelope(response!);
+          } on PlatformException catch (e) {
+            expect(e.code, 'unregistered_view_type');
+            expect(e.message, contains(viewType));
+            expect(e.details, contains('registerViewFactory'));
+          }
+        });
+
+        test('duplicate viewId, fails with descriptive exception', () async {
+          contentManager.registerFactory(
+              viewType, (int id) => createDomHTMLDivElement());
+          contentManager.renderContent(viewType, viewId, null);
+          final PlatformViewMessageHandler messageHandler = PlatformViewMessageHandler(
+            platformViewsContainer: createDomElement('div'),
+            contentManager: contentManager,
+          );
+          final Map<dynamic, dynamic> arguments = _getCreateArguments(viewType, viewId);
+
+          messageHandler.handleLegacyPlatformViewCall('create', arguments, completer.complete);
+
+          final ByteData? response = await completer.future;
+          try {
+            codec.decodeEnvelope(response!);
+          } on PlatformException catch (e) {
+            expect(e.code, 'recreating_view');
+            expect(e.details, contains('$viewId'));
+          }
+        });
+
+        test('returns a successEnvelope when the view is created normally',
+            () async {
+          contentManager.registerFactory(
+              viewType, (int id) => createDomHTMLDivElement()..id = 'success');
+          final PlatformViewMessageHandler messageHandler = PlatformViewMessageHandler(
+            platformViewsContainer: createDomElement('div'),
+            contentManager: contentManager,
+          );
+          final Map<dynamic, dynamic> arguments = _getCreateArguments(viewType, viewId);
+
+          messageHandler.handleLegacyPlatformViewCall('create', arguments, completer.complete);
+
+          final ByteData? response = await completer.future;
+          expect(codec.decodeEnvelope(response!), isNull,
+              reason:
+                  'The response should be a success envelope, with null in it.');
+        });
+
+        test('inserts the created view into the platformViewsContainer',
+            () async {
+          final DomElement platformViewsContainer = createDomElement('pv-container');
+          contentManager.registerFactory(
+              viewType, (int id) => createDomHTMLDivElement()..id = 'success');
+          final PlatformViewMessageHandler messageHandler = PlatformViewMessageHandler(
+            platformViewsContainer: platformViewsContainer,
+            contentManager: contentManager,
+          );
+          final Map<dynamic, dynamic> arguments = _getCreateArguments(viewType, viewId);
+
+          messageHandler.handleLegacyPlatformViewCall('create', arguments, completer.complete);
+
+          final ByteData? response = await completer.future;
+
+          expect(
+            platformViewsContainer.children.single,
+            isNotNull,
+            reason: 'The container has a single child, the created view.',
+          );
+          final DomElement platformView = platformViewsContainer.children.single;
+          expect(
+            platformView.querySelector('div#success'),
+            isNotNull,
+            reason: 'The element created by the factory should be present in the created view.',
+          );
+          expect(
+            codec.decodeEnvelope(response!),
+            isNull,
+            reason: 'The response should be a success envelope, with null in it.',
+          );
+        });
+
+        test('passes creation params to the factory', () async {
+          final List<PlatformViewFactoryCall> factoryCalls = <PlatformViewFactoryCall>[];
+          contentManager.registerFactory(viewType, (int viewId, {Object? params}) {
+            factoryCalls.add((viewId: viewId, params: params));
+            return createDomHTMLDivElement();
+          });
+          final PlatformViewMessageHandler messageHandler = PlatformViewMessageHandler(
+            platformViewsContainer: createDomElement('div'),
+            contentManager: contentManager,
+          );
+
+          final List<Completer<ByteData?>> completers = <Completer<ByteData?>>[];
+
+          completers.add(Completer<ByteData?>());
+          messageHandler.handleLegacyPlatformViewCall(
+            'create',
+            _getCreateArguments(viewType, 111),
+            completers.last.complete,
+          );
+
+          completers.add(Completer<ByteData?>());
+          messageHandler.handleLegacyPlatformViewCall(
+            'create',
+            _getCreateArguments(viewType, 222, <dynamic, dynamic>{'foo': 'bar'}),
+            completers.last.complete,
+          );
+
+          completers.add(Completer<ByteData?>());
+          messageHandler.handleLegacyPlatformViewCall(
+            'create',
+            _getCreateArguments(viewType, 333, 'foobar'),
+            completers.last.complete,
+          );
+
+          completers.add(Completer<ByteData?>());
+          messageHandler.handleLegacyPlatformViewCall(
+            'create',
+            _getCreateArguments(viewType, 444, <dynamic>[1, null, 'str']),
+            completers.last.complete,
+          );
+
+          final List<ByteData?> responses = await Future.wait(
+            completers.map((Completer<ByteData?> c) => c.future),
+          );
+
+          for (final ByteData? response in responses) {
+            expect(
+              codec.decodeEnvelope(response!),
+              isNull,
+              reason: 'The response should be a success envelope, with null in it.',
+            );
+          }
+
+          expect(factoryCalls, hasLength(4));
+          expect(factoryCalls[0].viewId, 111);
+          expect(factoryCalls[0].params, isNull);
+          expect(factoryCalls[1].viewId, 222);
+          expect(factoryCalls[1].params, <dynamic, dynamic>{'foo': 'bar'});
+          expect(factoryCalls[2].viewId, 333);
+          expect(factoryCalls[2].params, 'foobar');
+          expect(factoryCalls[3].viewId, 444);
+          expect(factoryCalls[3].params, <dynamic>[1, null, 'str']);
+        });
+
+        test('fails if the factory returns a non-DOM object', () async {
+          contentManager.registerFactory(viewType, (int viewId) {
+            // Return an object that's not a DOM element.
+            return Object();
+          });
+
+          final PlatformViewMessageHandler messageHandler = PlatformViewMessageHandler(
+            platformViewsContainer: createDomElement('div'),
+            contentManager: contentManager,
+          );
+          final Map<dynamic, dynamic> arguments = _getCreateArguments(viewType, viewId);
+
+          expect(() {
+            messageHandler.handleLegacyPlatformViewCall('create', arguments, (_) {});
+          }, throwsA(isA<TypeError>()));
+        });
+      });
+
+      group('"dispose" message', () {
+        late Completer<int> viewIdCompleter;
+
+        setUp(() {
+          viewIdCompleter = Completer<int>();
+        });
+
+        test('never fails, even for unknown viewIds', () async {
+          final PlatformViewMessageHandler messageHandler = PlatformViewMessageHandler(
+            platformViewsContainer: createDomElement('div'),
+            contentManager: contentManager,
+          );
+
+          messageHandler.handleLegacyPlatformViewCall('dispose', viewId, completer.complete);
+
+          final ByteData? response = await completer.future;
+          expect(codec.decodeEnvelope(response!), isNull,
+              reason:
+                  'The response should be a success envelope, with null in it.');
+        });
+
+        test('never fails, even for unknown viewIds', () async {
+          final PlatformViewMessageHandler messageHandler = PlatformViewMessageHandler(
+            platformViewsContainer: createDomElement('div'),
+            contentManager: _FakePlatformViewManager(viewIdCompleter.complete),
+          );
+
+          messageHandler.handleLegacyPlatformViewCall('dispose', viewId, completer.complete);
+
+          final int disposedViewId = await viewIdCompleter.future;
+          expect(disposedViewId, viewId,
+              reason:
+                  'The viewId to dispose should be passed to the contentManager');
+        });
+      });
+    });
+  });
+}
+
+class _FakePlatformViewManager extends PlatformViewManager {
+  _FakePlatformViewManager(void Function(int) clearFunction)
+      : _clearPlatformView = clearFunction;
+
+  final void Function(int) _clearPlatformView;
+
+  @override
+  void clearPlatformView(int viewId) {
+    return _clearPlatformView(viewId);
+  }
+}
+
+Map<dynamic, dynamic> _getCreateArguments(String viewType, int viewId, [Object? params]) {
+  return <String, dynamic>{
+    'id': viewId,
+    'viewType': viewType,
+    if (params != null) 'params': params,
+  };
+}

engine/src/flutter/lib/web_ui/test/engine/platform_views/message_handler_test.dart

@@ -20,8 +20,8 @@ typedef PlatformViewFactoryCall = ({int viewId, Object? params});
 void testMain() {
   group('PlatformViewMessageHandler', () {
     group('handlePlatformViewCall', () {
-      const String viewType = 'forTest';
-      const int viewId = 6;
+      const String platformViewType = 'forTest';
+      const int platformViewId = 6;
       late PlatformViewManager contentManager;
       late Completer<ByteData?> completer;
 
@@ -37,52 +37,64 @@ void testMain() {
             platformViewsContainer: createDomElement('div'),
             contentManager: contentManager,
           );
-          final ByteData? message = _getCreateMessage(viewType, viewId);
+          final Map<dynamic, dynamic> arguments = _getCreateArguments(
+            platformViewType: platformViewType,
+            platformViewId: platformViewId,
+            viewId: kImplicitViewId,
+          );
 
-          messageHandler.handlePlatformViewCall(message, completer.complete);
+          messageHandler.handlePlatformViewCall('create', arguments, completer.complete);
 
           final ByteData? response = await completer.future;
           try {
             codec.decodeEnvelope(response!);
           } on PlatformException catch (e) {
             expect(e.code, 'unregistered_view_type');
-            expect(e.message, contains(viewType));
+            expect(e.message, contains(platformViewType));
             expect(e.details, contains('registerViewFactory'));
           }
         });
 
         test('duplicate viewId, fails with descriptive exception', () async {
           contentManager.registerFactory(
-              viewType, (int id) => createDomHTMLDivElement());
-          contentManager.renderContent(viewType, viewId, null);
+              platformViewType, (int id) => createDomHTMLDivElement());
+          contentManager.renderContent(platformViewType, platformViewId, null);
           final PlatformViewMessageHandler messageHandler = PlatformViewMessageHandler(
             platformViewsContainer: createDomElement('div'),
             contentManager: contentManager,
           );
-          final ByteData? message = _getCreateMessage(viewType, viewId);
+          final Map<dynamic, dynamic> arguments = _getCreateArguments(
+            platformViewType: platformViewType,
+            platformViewId: platformViewId,
+            viewId: kImplicitViewId,
+          );
 
-          messageHandler.handlePlatformViewCall(message, completer.complete);
+          messageHandler.handlePlatformViewCall('create', arguments, completer.complete);
 
           final ByteData? response = await completer.future;
           try {
             codec.decodeEnvelope(response!);
           } on PlatformException catch (e) {
             expect(e.code, 'recreating_view');
-            expect(e.details, contains('$viewId'));
+            expect(e.details, contains('$platformViewId'));
           }
         });
 
         test('returns a successEnvelope when the view is created normally',
             () async {
           contentManager.registerFactory(
-              viewType, (int id) => createDomHTMLDivElement()..id = 'success');
+              platformViewType, (int id) => createDomHTMLDivElement()..id = 'success');
           final PlatformViewMessageHandler messageHandler = PlatformViewMessageHandler(
             platformViewsContainer: createDomElement('div'),
             contentManager: contentManager,
           );
-          final ByteData? message = _getCreateMessage(viewType, viewId);
+          final Map<dynamic, dynamic> arguments = _getCreateArguments(
+            platformViewType: platformViewType,
+            platformViewId: platformViewId,
+            viewId: kImplicitViewId,
+          );
 
-          messageHandler.handlePlatformViewCall(message, completer.complete);
+          messageHandler.handlePlatformViewCall('create', arguments, completer.complete);
 
           final ByteData? response = await completer.future;
           expect(codec.decodeEnvelope(response!), isNull,
@@ -94,14 +106,18 @@ void testMain() {
             () async {
           final DomElement platformViewsContainer = createDomElement('pv-container');
           contentManager.registerFactory(
-              viewType, (int id) => createDomHTMLDivElement()..id = 'success');
+              platformViewType, (int id) => createDomHTMLDivElement()..id = 'success');
           final PlatformViewMessageHandler messageHandler = PlatformViewMessageHandler(
             platformViewsContainer: platformViewsContainer,
             contentManager: contentManager,
           );
-          final ByteData? message = _getCreateMessage(viewType, viewId);
+          final Map<dynamic, dynamic> arguments = _getCreateArguments(
+            platformViewType: platformViewType,
+            platformViewId: platformViewId,
+            viewId: kImplicitViewId,
+          );
 
-          messageHandler.handlePlatformViewCall(message, completer.complete);
+          messageHandler.handlePlatformViewCall('create', arguments, completer.complete);
 
           final ByteData? response = await completer.future;
 
@@ -125,7 +141,7 @@ void testMain() {
 
         test('passes creation params to the factory', () async {
           final List<PlatformViewFactoryCall> factoryCalls = <PlatformViewFactoryCall>[];
-          contentManager.registerFactory(viewType, (int viewId, {Object? params}) {
+          contentManager.registerFactory(platformViewType, (int viewId, {Object? params}) {
             factoryCalls.add((viewId: viewId, params: params));
             return createDomHTMLDivElement();
           });
@@ -138,25 +154,48 @@ void testMain() {
 
           completers.add(Completer<ByteData?>());
           messageHandler.handlePlatformViewCall(
-            _getCreateMessage(viewType, 111),
+            'create',
+            _getCreateArguments(
+              platformViewType: platformViewType,
+              platformViewId: 111,
+              viewId: kImplicitViewId,
+            ),
             completers.last.complete,
           );
 
           completers.add(Completer<ByteData?>());
           messageHandler.handlePlatformViewCall(
-            _getCreateMessage(viewType, 222, <dynamic, dynamic>{'foo': 'bar'}),
+            'create',
+            _getCreateArguments(
+              platformViewType: platformViewType,
+              platformViewId: 222,
+              viewId: kImplicitViewId,
+              params: <dynamic, dynamic>{'foo': 'bar'},
+            ),
             completers.last.complete,
           );
 
           completers.add(Completer<ByteData?>());
           messageHandler.handlePlatformViewCall(
-            _getCreateMessage(viewType, 333, 'foobar'),
+            'create',
+            _getCreateArguments(
+              platformViewType: platformViewType,
+              platformViewId: 333,
+              viewId: kImplicitViewId,
+              params: 'foobar',
+            ),
             completers.last.complete,
           );
 
           completers.add(Completer<ByteData?>());
           messageHandler.handlePlatformViewCall(
-            _getCreateMessage(viewType, 444, <dynamic>[1, null, 'str']),
+            'create',
+            _getCreateArguments(
+              platformViewType: platformViewType,
+              platformViewId: 444,
+              viewId: kImplicitViewId,
+              params: <dynamic>[1, null, 'str'],
+            ),
             completers.last.complete,
           );
 
@@ -184,7 +223,7 @@ void testMain() {
         });
 
         test('fails if the factory returns a non-DOM object', () async {
-          contentManager.registerFactory(viewType, (int viewId) {
+          contentManager.registerFactory(platformViewType, (int viewId) {
             // Return an object that's not a DOM element.
             return Object();
           });
@@ -193,10 +232,14 @@ void testMain() {
             platformViewsContainer: createDomElement('div'),
             contentManager: contentManager,
           );
-          final ByteData? message = _getCreateMessage(viewType, viewId);
+          final Map<dynamic, dynamic> arguments = _getCreateArguments(
+            platformViewType: platformViewType,
+            platformViewId: platformViewId,
+            viewId: kImplicitViewId,
+          );
 
           expect(() {
-            messageHandler.handlePlatformViewCall(message, (_) {});
+            messageHandler.handlePlatformViewCall('create', arguments, (_) {});
           }, throwsA(isA<TypeError>()));
         });
       });
@@ -213,9 +256,12 @@ void testMain() {
             platformViewsContainer: createDomElement('div'),
             contentManager: contentManager,
           );
-          final ByteData? message = _getDisposeMessage(viewId);
+          final Map<dynamic, dynamic> arguments = _getDisposeArguments(
+            platformViewId: platformViewId,
+            viewId: kImplicitViewId,
+          );
 
-          messageHandler.handlePlatformViewCall(message, completer.complete);
+          messageHandler.handlePlatformViewCall('dispose', arguments, completer.complete);
 
           final ByteData? response = await completer.future;
           expect(codec.decodeEnvelope(response!), isNull,
@@ -228,12 +274,15 @@ void testMain() {
             platformViewsContainer: createDomElement('div'),
             contentManager: _FakePlatformViewManager(viewIdCompleter.complete),
           );
-          final ByteData? message = _getDisposeMessage(viewId);
+          final Map<dynamic, dynamic> arguments = _getDisposeArguments(
+            platformViewId: platformViewId,
+            viewId: kImplicitViewId,
+          );
 
-          messageHandler.handlePlatformViewCall(message, completer.complete);
+          messageHandler.handlePlatformViewCall('dispose', arguments, completer.complete);
 
           final int disposedViewId = await viewIdCompleter.future;
-          expect(disposedViewId, viewId,
+          expect(disposedViewId, platformViewId,
               reason:
                   'The viewId to dispose should be passed to the contentManager');
         });
@@ -254,20 +303,26 @@ class _FakePlatformViewManager extends PlatformViewManager {
   }
 }
 
-ByteData? _getCreateMessage(String viewType, int viewId, [Object? params]) {
-  return codec.encodeMethodCall(MethodCall(
-    'create',
-    <String, dynamic>{
-      'id': viewId,
-      'viewType': viewType,
-      if (params != null) 'params': params,
-    },
-  ));
+Map<dynamic, dynamic> _getCreateArguments({
+  required String platformViewType,
+  required int platformViewId,
+  required int viewId,
+  Object? params,
+}) {
+  return <String, dynamic>{
+    'platformViewId': platformViewId,
+    'platformViewType': platformViewType,
+    if (params != null) 'params': params,
+    'viewId': viewId,
+  };
 }
 
-ByteData? _getDisposeMessage(int viewId) {
-  return codec.encodeMethodCall(MethodCall(
-    'dispose',
-    viewId,
-  ));
+Map<dynamic, dynamic> _getDisposeArguments({
+  required int platformViewId,
+  required int viewId,
+}) {
+  return <dynamic, dynamic>{
+    'platformViewId': platformViewId,
+    'viewId': viewId,
+  };
 }