Expose participantIdentity on DataReceivedEvent

.changes/data-received-participant-identity

@@ -0,0 +1 @@
+patch type="added" "`DataReceivedEvent.participantIdentity` reports the sender even when `participant` is null, for example when data arrives before the sender's join is announced or after the sender has left"

lib/src/core/room.dart

@@ -1046,6 +1046,7 @@ class Room extends DisposableChangeNotifier with EventsEmittable<RoomEvent> {
 
     final event = DataReceivedEvent(
       participant: senderParticipant,
+      participantIdentity: dataPacketEvent.identity,
       data: dataPacketEvent.packet.payload,
       topic: dataPacketEvent.packet.topic,
     );

lib/src/events.dart

@@ -446,22 +446,32 @@ class ParticipantConnectionQualityUpdatedEvent with RoomEvent, ParticipantEvent
 /// Data received from  [RemoteParticipant].
 /// Data packets provides the ability to use LiveKit to send/receive arbitrary
 /// payloads.
+/// When [participant] is null, use [participantIdentity] to tell who sent the
+/// data. It is empty for data sent from Server API or by a hidden participant.
 /// Emitted by [Room] and [RemoteParticipant].
 class DataReceivedEvent with RoomEvent, ParticipantEvent {
   /// Sender of the data. This may be null if data is sent from Server API.
   final RemoteParticipant? participant;
+
+  /// Identity of the sender. Set even when [participant] is null, for example
+  /// when the data arrives before the sender's join is announced or after the
+  /// sender has left. Empty for data sent from Server API or by a hidden
+  /// participant.
+  final String participantIdentity;
   final List<int> data;
   final String? topic;
+  // TODO: In the next major release, mark this constructor @internal and make participantIdentity required.
   const DataReceivedEvent({
     required this.participant,
     required this.data,
     required this.topic,
+    this.participantIdentity = '',
   });
 
   @override
   String toString() =>
       '${runtimeType}'
-      '(participant: ${participant}, topic: ${topic}, data: ${data})';
+      '(participant: ${participant}, participantIdentity: ${participantIdentity}, topic: ${topic}, data: ${data})';
 }
 
 /// The participant's isSpeaking property has changed

test/core/data_received_identity_test.dart

@@ -0,0 +1,78 @@
+// Copyright 2026 LiveKit, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+@Timeout(Duration(seconds: 10))
+library;
+
+import 'dart:async';
+import 'dart:convert';
+
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:livekit_client/livekit_client.dart';
+import 'package:livekit_client/src/proto/livekit_models.pb.dart' as lk_models;
+import '../mock/e2e_container.dart';
+
+void main() {
+  late E2EContainer container;
+  late Room room;
+
+  setUpAll(() async {
+    container = E2EContainer();
+    room = container.room;
+    await container.connectRoom();
+    await container.simulateRemoteParticipantJoin('remote-a');
+  });
+
+  tearDownAll(() async {
+    await container.dispose();
+  });
+
+  Future<DataReceivedEvent> deliverFrom(String identity) async {
+    final completer = Completer<DataReceivedEvent>();
+    final cancel = room.events.on<DataReceivedEvent>(completer.complete);
+
+    container.deliverInboundDataPacket(
+      lk_models.DataPacket(
+        kind: lk_models.DataPacket_Kind.RELIABLE,
+        participantIdentity: identity,
+        user: lk_models.UserPacket(
+          payload: utf8.encode('hello'),
+          participantIdentity: identity,
+        ),
+      ),
+    );
+
+    final event = await completer.future.timeout(const Duration(seconds: 3));
+    await cancel();
+    return event;
+  }
+
+  test('reports the sender identity when the participant is not in the room', () async {
+    // Data sent by the Server API, or by a participant that has already left,
+    // has no RemoteParticipant to attach to. Without the identity the receiver
+    // cannot tell who sent it at all.
+    final event = await deliverFrom('server-api-sender');
+
+    expect(event.participant, isNull);
+    expect(event.participantIdentity, 'server-api-sender');
+  });
+
+  test('reports the sender identity for a known participant too', () async {
+    final event = await deliverFrom('remote-a');
+
+    expect(event.participant?.identity, 'remote-a');
+    expect(event.participantIdentity, 'remote-a');
+  });
+}