Support connecting to VM service via MessagePort / postMessage

packages/devtools_app/lib/src/framework/framework_core.dart

@@ -18,6 +18,7 @@ import 'package:vm_service/vm_service.dart';
 
 import '../extensions/extension_service.dart';
 import '../screens/debugger/breakpoint_manager.dart';
+import '../service/post_message_service.dart';
 import '../service/service_manager.dart';
 import '../service/vm_service_wrapper.dart';
 import '../shared/analytics/analytics.dart' as ga;
@@ -140,29 +141,34 @@ extension FrameworkCore on Never {
       final finishedCompleter = Completer<void>();
 
       try {
-        final service = await connect<VmServiceWrapper>(
-          uri: uri,
-          finishedCompleter: finishedCompleter,
-          serviceFactory:
-              ({
-                // ignore: avoid-dynamic, mirrors types of [VmServiceFactory].
-                required Stream<dynamic> /*String|List<int>*/ inStream,
-                required void Function(String message) writeMessage,
-                Log? log,
-                DisposeHandler? disposeHandler,
-                Future? streamClosed,
-                String? wsUri,
-                bool trackFutures = false,
-              }) => VmServiceWrapper.defaultFactory(
-                inStream: inStream,
-                writeMessage: writeMessage,
-                log: log,
-                disposeHandler: disposeHandler,
-                streamClosed: streamClosed,
-                wsUri: wsUri,
-                trackFutures: integrationTestMode,
-              ),
-        );
+        final service = isPostMessageUri(uri)
+            ? await connectWithPostMessage(
+                uri: uri,
+                finishedCompleter: finishedCompleter,
+              )
+            : await connect<VmServiceWrapper>(
+                uri: uri,
+                finishedCompleter: finishedCompleter,
+                serviceFactory:
+                    ({
+                      // ignore: avoid-dynamic, mirrors types of [VmServiceFactory].
+                      required Stream<dynamic> /*String|List<int>*/ inStream,
+                      required void Function(String message) writeMessage,
+                      Log? log,
+                      DisposeHandler? disposeHandler,
+                      Future? streamClosed,
+                      String? wsUri,
+                      bool trackFutures = false,
+                    }) => VmServiceWrapper.defaultFactory(
+                      inStream: inStream,
+                      writeMessage: writeMessage,
+                      log: log,
+                      disposeHandler: disposeHandler,
+                      streamClosed: streamClosed,
+                      wsUri: wsUri,
+                      trackFutures: integrationTestMode,
+                    ),
+              );
 
         await serviceConnection.serviceManager.vmServiceOpened(
           service,

packages/devtools_app/lib/src/service/_post_message_service_stub.dart

@@ -0,0 +1,13 @@
+// Copyright 2026 The Flutter Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file or at https://developers.google.com/open-source/licenses/bsd.
+
+import 'dart:async';
+
+import 'vm_service_wrapper.dart';
+
+/// Connects to a VM Service using a `MessagePort` transferred via postMessage.
+Future<VmServiceWrapper> connectWithPostMessage({
+  required Uri uri,
+  required Completer<void> finishedCompleter,
+}) => throw UnsupportedError('unsupported platform');

packages/devtools_app/lib/src/service/_post_message_service_web.dart

@@ -0,0 +1,88 @@
+// Copyright 2026 The Flutter Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file or at https://developers.google.com/open-source/licenses/bsd.
+
+import 'dart:async';
+import 'dart:convert';
+import 'dart:js_interop';
+
+import 'package:web/web.dart' as web;
+
+import 'vm_service_wrapper.dart';
+
+/// Extracts the target origin for window.postMessage from [uri].
+///
+/// The URI is expected in the form `post-message:<origin>` (e.g.
+/// `post-message:https://dartpad.dev` or `post-message:*`).
+String _extractTargetOrigin(Uri uri) {
+  if (uri.path.isEmpty || uri.path == '*') {
+    return '*';
+  }
+  final target = Uri.tryParse(uri.path);
+  if (target != null && target.hasScheme && target.hasAuthority) {
+    return target.origin;
+  }
+  return uri.path;
+}
+
+/// Connects to a Dart VM service via a browser `MessagePort`.
+///
+/// Creates a [web.MessageChannel], transfers `port1` to `window.parent`
+/// via a postMessage with payload `{'vmServicePort': port1}`, and binds
+/// `port2` to a [VmServiceWrapper].
+Future<VmServiceWrapper> connectWithPostMessage({
+  required Uri uri,
+  required Completer<void> finishedCompleter,
+}) async {
+  final targetOrigin = _extractTargetOrigin(uri);
+  final channel = web.MessageChannel();
+  // ignore: avoid-dynamic, mirrors types of [Stream<dynamic> inStream] in [VmService].
+  final inStreamController = StreamController<dynamic>();
+
+  final message = {'vmServicePort': channel.port1}.jsify();
+  web.window.parent?.postMessage(
+    message,
+    web.WindowPostMessageOptions(
+      targetOrigin: targetOrigin,
+      transfer: [channel.port1].toJS,
+    ),
+  );
+
+  channel.port2.onmessage = ((web.MessageEvent event) {
+    final data = event.data;
+    if (data != null) {
+      if (data.isA<JSString>()) {
+        inStreamController.add((data as JSString).toDart);
+      } else if (data.isA<JSUint8Array>()) {
+        inStreamController.add((data as JSUint8Array).toDart);
+      } else if (data.isA<JSArrayBuffer>()) {
+        inStreamController.add((data as JSArrayBuffer).toDart.asUint8List());
+      } else {
+        inStreamController.add(jsonEncode(data.dartify()));
+      }
+    }
+  }).toJS;
+  channel.port2.start();
+
+  final service = VmServiceWrapper.defaultFactory(
+    inStream: inStreamController.stream,
+    writeMessage: (String msg) {
+      channel.port2.postMessage(msg.toJS);
+    },
+    streamClosed: finishedCompleter.future,
+    wsUri: uri.toString(),
+  );
+
+  unawaited(
+    finishedCompleter.future.then((_) async {
+      channel.port2.close();
+      await inStreamController.close();
+      await service.dispose();
+    }),
+  );
+
+  // Validate the connection with a getVersion call.
+  await service.getVersion();
+
+  return service;
+}

packages/devtools_app/lib/src/service/post_message_service.dart

@@ -0,0 +1,14 @@
+// Copyright 2026 The Flutter Authors
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file or at https://developers.google.com/open-source/licenses/bsd.
+
+export '_post_message_service_stub.dart'
+    if (dart.library.js_interop) '_post_message_service_web.dart';
+
+/// Returns whether [uri] uses the `post-message:` or `postmessage:` scheme.
+///
+/// This scheme is used when DevTools is embedded in an `<iframe>` and
+/// communicates with the target Dart/Flutter application via a transferred
+/// `MessagePort` instead of a WebSocket or SSE connection.
+bool isPostMessageUri(Uri uri) =>
+    uri.scheme == 'post-message' || uri.scheme == 'postmessage';