C++

• Copy Dart SDK (${FLUTTER_DIR}/bin/cache/dart-sdk/include) to your project, e.g. ios/Classes/
• Add your own code like the following fragment

#include "dart_api_dl.h"

// Receives NativePort ID from Flutter code
static Dart_Port_DL dart_port = 0;

void set_dart_port(Dart_Port_DL port)
{
    dart_port = port;
}

// Sample usage of Dart_PostCObject_DL to post message to Flutter side
void debug_print(const char *message)
{
    if (!dart_port)
        return;
    Dart_CObject msg;
    msg.type = Dart_CObject_kString;
    msg.value.as_string = (char *)message;
    // The function is thread-safe; you can call it anywhere on your C++ code
    Dart_PostCObject_DL(dart_port, &msg);
}

Flutter

• Call Dart_InitializeApiDL via ffi with NativeApi.initializeApiDLData as a parameter
• Create ReceivePort and call listen
• listen returns a StreamSubscription and pass the StreamSubscription.sendPort.nativePort value to C++ using the function set_dart_port defined above

import 'package:ffi/ffi.dart';

class FfiNativePort {
  static final mozJpegLib =
      Platform.isAndroid ? DynamicLibrary.open("libffi_native_port.so") : DynamicLibrary.process();

  // Dart_InitializeApiDL defined in Dart SDK (dart_api_dl.c)
  static final Dart_InitializeApiDLFunc _Dart_InitializeApiDL =
      mozJpegLib.lookup<NativeFunction<Dart_InitializeApiDLFunc>>("Dart_InitializeApiDL").asFunction();

  // The set_dart_port function defined on the C++ code above
  static final _SetDartPortFunc _setDartPort =
      mozJpegLib.lookup<NativeFunction<Void Function(Int64)>>("set_dart_port").asFunction();

  static Pointer<Void>? cookie;

  // initialization
  static void initialize() {
    if (cookie != null) {
      return;
    }

    // Call Dart_InitializeApiDL with NativeApi.initializeApiDLData
    cookie = _Dart_InitializeApiDL(NativeApi.initializeApiDLData);

     final pub = ReceivePort()
      ..listen((message) {
        // TODO: processing messages from C++ code
      });

    // Pass NativePort value (int) to C++ code
    _setDartPort(pub.sendPort.nativePort);
  }
}

Reference(s)

• flutter/flutter #63255 How to use async callback between C++ and Dart with FFI?
• mraleph/go_dart_ffi_example - GitHub