Ensure that platform messages without response handles can be dispatched.

shell/platform/embedder/embedder.cc

@@ -823,7 +823,7 @@ FlutterEngineResult FlutterEngineSendPlatformMessage(
       SAFE_ACCESS(flutter_message, response_handle, nullptr);
 
   fml::RefPtr<flutter::PlatformMessageResponse> response;
-  if (response_handle->message) {
+  if (response_handle && response_handle->message) {
     response = response_handle->message->response();
   }
 

shell/platform/embedder/fixtures/main.dart

@@ -1,6 +1,7 @@
 import 'dart:async';
 import 'dart:typed_data';
 import 'dart:ui';
+import 'dart:convert';
 
 void main() {}
 
@@ -45,6 +46,7 @@ Float64List kTestTransform = () {
 }();
 
 void signalNativeTest() native 'SignalNativeTest';
+void signalNativeMessage(String message) native 'SignalNativeMessage';
 void notifySemanticsEnabled(bool enabled) native 'NotifySemanticsEnabled';
 void notifyAccessibilityFeatures(bool reduceMotion) native 'NotifyAccessibilityFeatures';
 void notifySemanticsAction(int nodeId, int action, List<int> data) native 'NotifySemanticsAction';
@@ -153,3 +155,15 @@ void platform_messages_response() {
   };
   signalNativeTest();
 }
+
+@pragma('vm:entry-point')
+void platform_messages_no_response() {
+  window.onPlatformMessage = (String name, ByteData data, PlatformMessageResponseCallback callback) {
+    var list = data.buffer.asUint8List(data.offsetInBytes, data.lengthInBytes);
+    signalNativeMessage(utf8.decode(list));
+    // This does nothing because no one is listening on the other side. But complete the loop anyway
+    // to make sure all null checking on response handles in the engine is in place.
+    callback(data);
+  };
+  signalNativeTest();
+}

shell/platform/embedder/tests/embedder_unittests.cc

@@ -16,6 +16,7 @@
 #include "flutter/shell/platform/embedder/tests/embedder_config_builder.h"
 #include "flutter/shell/platform/embedder/tests/embedder_test.h"
 #include "flutter/testing/testing.h"
+#include "third_party/tonic/converter/dart_converter.h"
 
 namespace flutter {
 namespace testing {
@@ -352,5 +353,52 @@ TEST_F(EmbedderTest, PlatformMessagesCanReceiveResponse) {
   captures.latch.Wait();
 }
 
+//------------------------------------------------------------------------------
+/// Tests that a platform message can be sent with no response handle. Instead
+/// of the platform message integrity checked via a response handle, a native
+/// callback with the response is invoked to assert integrity.
+///
+TEST_F(EmbedderTest, PlatformMessagesCanBeSentWithoutResponseHandles) {
+  auto& context = GetEmbedderContext();
+  EmbedderConfigBuilder builder(context);
+
+  builder.SetDartEntrypoint("platform_messages_no_response");
+
+  const std::string message_data = "Hello but don't call me back.";
+
+  fml::AutoResetWaitableEvent ready, message;
+  context.AddNativeCallback(
+      "SignalNativeTest",
+      CREATE_NATIVE_ENTRY(
+          [&ready](Dart_NativeArguments args) { ready.Signal(); }));
+  context.AddNativeCallback(
+      "SignalNativeMessage",
+      CREATE_NATIVE_ENTRY(
+          ([&message, &message_data](Dart_NativeArguments args) {
+            auto received_message = tonic::DartConverter<std::string>::FromDart(
+                Dart_GetNativeArgument(args, 0));
+            ASSERT_EQ(received_message, message_data);
+            message.Signal();
+          })));
+
+  auto engine = builder.LaunchEngine();
+
+  ASSERT_TRUE(engine.is_valid());
+  ready.Wait();
+
+  FlutterPlatformMessage platform_message = {};
+  platform_message.struct_size = sizeof(FlutterPlatformMessage);
+  platform_message.channel = "test_channel";
+  platform_message.message =
+      reinterpret_cast<const uint8_t*>(message_data.data());
+  platform_message.message_size = message_data.size();
+  platform_message.response_handle = nullptr;  // No response needed.
+
+  auto result =
+      FlutterEngineSendPlatformMessage(engine.get(), &platform_message);
+  ASSERT_EQ(result, kSuccess);
+  message.Wait();
+}
+
 }  // namespace testing
 }  // namespace flutter