C++ 层

DART_EXPORT void RegisterMyCallbackBlocking(Dart_Port send_port,
                                            intptr_t (*callback1)(intptr_t)) {
  my_callback_blocking_fp_ = callback1;
  my_callback_blocking_send_port_ = send_port;
}

DART_EXPORT void RegisterMyCallbackNonBlocking(Dart_Port send_port,
                                               void (*callback)(intptr_t)) {
  my_callback_non_blocking_fp_ = callback;
  my_callback_non_blocking_send_port_ = send_port;
}

intptr_t (*my_callback_blocking_fp_)(intptr_t);
Dart_Port my_callback_blocking_send_port_;

void (*my_callback_non_blocking_fp_)(intptr_t);
Dart_Port my_callback_non_blocking_send_port_;

Dart 层

int globalResult = 0;
int numCallbacks1 = 0;
int numCallbacks2 = 0;

final dl = dlopenPlatformSpecific("ffi_test_functions");

final registerCallback1 = dl.lookupFunction<
        Void Function(Int64 sendPort,
            Pointer<NativeFunction<IntPtr Function(IntPtr)>> functionPointer),
        void Function(int sendPort,
            Pointer<NativeFunction<IntPtr Function(IntPtr)>> functionPointer)>(
    'RegisterMyCallbackBlocking');

final registerCallback2 = dl.lookupFunction<
        Void Function(Int64 sendPort,
            Pointer<NativeFunction<Void Function(IntPtr)>> functionPointer),
        void Function(int sendPort,
            Pointer<NativeFunction<Void Function(IntPtr)>> functionPointer)>(
    'RegisterMyCallbackNonBlocking');

final callback1FP = Pointer.fromFunction<IntPtr Function(IntPtr)>(callback1, 0);

final callback2FP = Pointer.fromFunction<Void Function(IntPtr)>(callback2);

int callback1(int a) {
  print("Dart:     callback1($a).");
  numCallbacks1++;
  return a + 3;
}

void callback2(int a) {
  print("Dart:     callback2($a).");
  globalResult += a;
  numCallbacks2++;
}

调用

print("Dart: Setup.");
  Expect.isTrue(NativeApi.majorVersion == 2);
  Expect.isTrue(NativeApi.minorVersion >= 0);
  final initializeApi = dl.lookupFunction<IntPtr Function(Pointer<Void>),
      int Function(Pointer<Void>)>("InitDartApiDL");
  Expect.isTrue(initializeApi(NativeApi.initializeApiDLData) == 0);

  final interactiveCppRequests = ReceivePort()..listen(requestExecuteCallback);
  final int nativePort = interactiveCppRequests.sendPort.nativePort;
  registerCallback1(nativePort, callback1FP);
  registerCallback2(nativePort, callback2FP);

https://github.com/dart-lang/sdk/blob/da008fb3fc5a61b39583fc2ef1477471b45fa4ac/samples_2/ffi/async/sample_async_callback.dart