Unhandled Exception: Invalid argument(s): Illegal argument in isolate message: (object extends NativeWrapper – Library:'dart:ui' Class: Path)

Issue

Can anyone tell me whats wrong in this code?

void onPressed() async {
    //Navigator.pushNamed(context, "/screen2", arguments: []);
    var receivePort = ReceivePort();
    await Isolate.spawn(gotoNext, [receivePort.sendPort]);
    final msg = await receivePort.first;
    print(msg);
  }

  void gotoNext(List<dynamic> args) {
    SendPort sendPort = args[0];
    log(args.toString());
    Isolate.exit(sendPort, "OK");
  }

E/flutter (12062): [ERROR:flutter/lib/ui/ui_dart_state.cc(209)] Unhandled Exception: Invalid argument(s): Illegal argument in isolate message: (object extends NativeWrapper – Library:’dart:ui’ Class: Path)

Solution

Hii There is a mistake in your code. As far as I know from the official documentation The callback method for an Isolate should be a top level function or a static method.
So there are two solution for this problem.

Solution 1. declare callback function as top level function.

class MyHomePage extends StatelessWidget {
  final String title;

  const MyHomePage({Key? key, required this.title}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(child: TextButton(
        child: const Text("Run Isolate"),
        onPressed: _onPressed,
      ));
    );
  }

  // Callback function for Text Button Event this should be a class member
  void _onPressed() async {
    var receivePort = ReceivePort();
    // Here runMyIsolate methos should be a top level function
    await Isolate.spawn(runMyIsolate, [receivePort.sendPort, "My Custom Message"]);
    print(await receivePort.first);
  }
}


// We declare a top level function here for an isolated callback function
void runMyIsolate(List<dynamic> args) {
  var sendPort = args[0] as SendPort;
  print("In runMyIsolate ");
  Isolate.exit(sendPort, args);
}

Solution 2. instead this top level function we can declare this
function as a static function for the same class, consider below
example.

    class MyHomePage extends StatelessWidget {
  final String title;

  const MyHomePage({Key? key, required this.title}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(child: TextButton(
        child: const Text("Run Isolate"),
        onPressed: _onPressed,
      ));
    );
  }

  // Callback function for Text Button Event this should be a class member
  void _onPressed() async {
    var receivePort = ReceivePort();
    // Here runMyIsolate methos should be a top level function
    await Isolate.spawn(runMyIsolate, [receivePort.sendPort, "My Custom Message"]);
    print(await receivePort.first);
  }
  
  // We declare a static function here for an isolated callback function
  static void runMyIsolate(List<dynamic> args) {
    var sendPort = args[0] as SendPort;
    print("In runMyIsolate ");
    Isolate.exit(sendPort, args);
  }
}

Answered By – Varun Rajput

Answer Checked By – Terry (FlutterFixes Volunteer)