"Cannot invoke a native callback outside of an isolate" Flutter FFI

[sanjaygholap]

"Cannot invoke a native callback outside of an isolate".Within the C++ DLL file. A separate thread with a callback was started internally. we also check for NativeCallable.listener but issue is still persistent

[dcharkes]

we also check for NativeCallable.listener but issue is still persistent

Are you sure? This error message can only occur with NativeCallable.isolateLocal.

Could you provide a minimal reproduction on a GitHub repo?

[sanjaygholap]

Thank you for your response.

We call CPP dll in Flutter using Flutter ffi. This is our CPP function in dll.

extern "C" __declspec(dllexport) void StartServerSession (const char* Name,const char* js,int& sessionID,int& ERESULT,void(eventhandler)(const char,int))
export "C"__declspec(dllexport) void InitializeServer(void(eventhandler)(const char,int)){ PtrObj->Initialize(eventhandler); }

final DynamicLibrary DllPath = DynamicLibrary.open('path to dll');
typedef InitializeFunc = Void Function(Pointer<Void> eventhandler);
typedef InitializeFun = void Function(Pointer<Void> eventhandler);

typedef StartServerSessionC = Void Function(Pointer<Utf8> Name, Pointer<Utf8> js, Pointer<Pointer<Int32>> sessionID, Pointer<Pointer<Int32>> ERESULT,Pointer<Void> eventhandler);
typedef StartServerSession = void Function(Pointer<Utf8> Name, Pointer<Utf8> js, Pointer<Pointer<Int32>> sessionID, Pointer<Pointer<Int32>> ERESULT,Pointer<Void> eventhandler);

void eventHandler(Pointer<Utf8> response, int length) {
  print('Response: ${response.cast<Utf8>.toDartString()}');
}


final initialize = DllPath.lookupFunction<InitializeFunc , InitializeFun>('Initialize');

final startServerSession = DllPath
    .lookupFunction<StartServerSessionC, StartServerSession>('StartServerSession');


void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: Text('FFI Demo')),
        body: Center(
          child: ElevatedButton(
            onPressed: () async {
              // Example usage of the FFI functions
                await callStartServerIsolate();
            },
            child: Text('Call DLL Function'),
          ),
        ),
      ),
    );
  }
}

void StartServerFunction(SendPort sendPort) 
{
   Pointer<Utf8> name = ('path to second dll'.toNativeUtf8());
   Pointer<Utf8> js = {json value}.toNativeUtf8();
           
   final sessionID = calloc<Int32>();
   final ERESULT = calloc<Int32>();
   initialize (Pointer.fromFunction<Void Function(Pointer<Utf8>, Int32)>(eventHandler)as Pointer<Void>);
   startServerSession (
							   name,
                               js,
                               sessionID as Pointer<Pointer<Int32>>,
                               ERESULT as Pointer<Pointer<Int32>>, 
                               Pointer.fromFunction<Void Function(Pointer<Utf8>, Int32)>(eventHandler)as Pointer<Void>);
    
    sendPort.send("task Complete");
}
Future<Void> callStartServerIsolate()async{
final receivePort = ReceivePort();
final sendPort = receivePort.sendPort;
await Isolate.spawn(StartServerFunction,sendPort)
}

[liamappelbe]

Pointer.fromFunction is a sync callback. You're using it in two places in StartServerFunction. Use NativeCallable.listener instead.

[sanjaygholap]

@liamappelbe sir and @dcharkes sir thank you issue is resloved