feat(remote_config, web): add web support for onConfigUpdated

packages/firebase_remote_config/firebase_remote_config/lib/src/firebase_remote_config.dart

@@ -160,7 +160,8 @@ class FirebaseRemoteConfig extends FirebasePluginPlatform {
   /// Starts listening for real-time config updates from the Remote Config backend and automatically
   /// fetches updates from the RC backend when they are available.
   ///
-  /// This feature is not supported on Web.
+  /// On web, you must call [fetchAndActivate] before listening to this stream. Events will only be
+  /// received after an initial call to [fetchAndActivate].
   ///
   /// If a connection to the Remote Config backend is not already open, calling this method will
   /// open it. Multiple listeners can be added by calling this method again, but subsequent calls

packages/firebase_remote_config/firebase_remote_config_web/lib/firebase_remote_config_web.dart

@@ -184,7 +184,8 @@ class FirebaseRemoteConfigWeb extends FirebaseRemoteConfigPlatform {
 
   @override
   Stream<RemoteConfigUpdate> get onConfigUpdated {
-    throw UnsupportedError('onConfigUpdated is not supported for web');
+    return _delegate.onConfigUpdated
+        .map((event) => RemoteConfigUpdate(event.updatedKeys));
   }
 
   @override

packages/firebase_remote_config/firebase_remote_config_web/lib/src/interop/firebase_remote_config.dart

@@ -2,6 +2,7 @@
 // for details. 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:convert' show utf8;
 import 'dart:js_interop';
 
@@ -158,6 +159,32 @@ class RemoteConfig
         .setCustomSignals(jsObject, customSignals.jsify()! as JSObject)
         .toDart;
   }
+
+  StreamController<RemoteConfigUpdatePayload>? _onConfigUpdatedController;
+
+  Stream<RemoteConfigUpdatePayload> get onConfigUpdated {
+    if (_onConfigUpdatedController == null) {
+      _onConfigUpdatedController =
+          StreamController<RemoteConfigUpdatePayload>.broadcast(sync: true);
+      final errorWrapper = (JSObject error) {
+        _onConfigUpdatedController?.addError(error);
+      };
+      final nextWrapper =
+          (remote_config_interop.ConfigUpdateJsImpl configUpdate) {
+        _onConfigUpdatedController
+            ?.add(RemoteConfigUpdatePayload._fromJsObject(configUpdate));
+      };
+      remote_config_interop.ConfigUpdateObserver observer =
+          remote_config_interop.ConfigUpdateObserver(
+        error: errorWrapper.toJS,
+        next: nextWrapper.toJS,
+      );
+
+      remote_config_interop.onConfigUpdate(jsObject, observer);
+    }
+
+    return _onConfigUpdatedController!.stream;
+  }
 }
 
 ValueSource getSource(String source) {
@@ -220,3 +247,19 @@ enum RemoteConfigLogLevel {
   error,
   silent,
 }
+
+class RemoteConfigUpdatePayload
+    extends JsObjectWrapper<remote_config_interop.ConfigUpdateJsImpl> {
+  RemoteConfigUpdatePayload._fromJsObject(
+    remote_config_interop.ConfigUpdateJsImpl jsObject,
+  ) : super.fromJsObject(jsObject);
+
+  Set<String> get updatedKeys {
+    final updatedKeysSet = <String>{};
+    final callback = (JSAny key, JSString value, JSAny set) {
+      updatedKeysSet.add(value.toDart);
+    };
+    jsObject.getUpdatedKeys().forEach(callback.toJS);
+    return updatedKeysSet;
+  }
+}

packages/firebase_remote_config/firebase_remote_config_web/lib/src/interop/firebase_remote_config_interop.dart

@@ -103,3 +103,41 @@ extension SettingsJsImplExtension on SettingsJsImpl {
   external JSNumber get fetchTimeoutMillis;
   external set fetchTimeoutMillis(JSNumber value);
 }
+
+@JS()
+@staticInterop
+@anonymous
+abstract class ConfigUpdateObserver {
+  external factory ConfigUpdateObserver({
+    JSAny complete,
+    JSAny error,
+    JSAny next,
+  });
+}
+
+extension ConfigUpdateObserverJsImpl on ConfigUpdateObserver {
+  external JSAny get next;
+  external JSAny get error;
+  external JSAny get complete;
+}
+
+@JS()
+@staticInterop
+@anonymous
+abstract class ConfigUpdateJsImpl {}
+
+extension ConfigUpdateJsImplExtension on ConfigUpdateJsImpl {
+  external JSSet getUpdatedKeys();
+}
+
+@JS()
+@staticInterop
+external JSFunction onConfigUpdate(
+  RemoteConfigJsImpl remoteConfig,
+  ConfigUpdateObserver observer,
+);
+
+@JS('Set')
+extension type JSSet._(JSObject _) implements JSObject {
+  external void forEach(JSAny callback);
+}

tests/integration_test/firebase_remote_config/firebase_remote_config_e2e_test.dart

@@ -138,8 +138,6 @@ void main() {
 
           await configSubscription.cancel();
         },
-        // This feature is not supported on Web
-        skip: kIsWeb,
       );
 
       test('default values', () async {