_send is somehow destroying additional field in Computer

[Ben1980]

I'm currently working on BLE support. Because of that, I needed to extend Computer with the additional field BluetoothDevice? device. The problem starts when calling download via download, afterward the object computer no longer contains a valid BluetoothDevice object. All other fields are fine.

@override
  Future<List<Dive>> download(
    Computer computer,
    ComputerTransport transport, [
    String? lastFingerprint,
  ]) async {
    await _send((
      DiveComputerMethod.download,
      [computer, transport, lastFingerprint],
    ));
    return (_downloadedDives = Completer()).future;
  }

class Computer {
  Computer(
    this.vendor,
    this.product, {
    this.transports = const [],
  });

  final String vendor, product;
  final List<ComputerTransport> transports;
  BluetoothDevice? device;

  void addBleDevice(BluetoothDevice device) {
    this.device = device;
  }

  @override
  String toString() =>
      '$vendor $product ${device ?? ''} [${transports.map((t) => t.name).join(', ')}]';

  @override
  bool operator ==(Object other) =>
      identical(this, other) ||
      other is Computer &&
          runtimeType == other.runtimeType &&
          vendor == other.vendor &&
          product == other.product;

  @override
  int get hashCode {
    // hash codes are not equal across isolates, so we need to generate a
    // consistent hash code for each computer
    return '$vendor $product'.codeUnits.fold(1, (a, b) => a * b);
  }
}

[Sese-Schneider]

@Ben1980 I think its an issue on how Isolates transfer objects.

You either have to make the BluetoothDevice final or make sure it can be serialized.

If this doesn't work I suggest caching the object on the isolate site and only passing an identifier, and retrieving from cached objects when the identifier is passed back.

---

Isolates in Dart can transfer objects between them, but with some limitations:

1. Primitive data types and built-in objects: These types (e.g., integers, strings, lists, maps) are transferred directly without any modifications.
2. Custom objects:
   • Final fields: These fields are serialized and transferred to the receiving isolate, maintaining their values.
   • Non-final fields: Due to serialization and deserialization during the transfer process, their values of final fields might not be preserved if they:
     • Contain references to mutable objects: The references themselves are transferred, but the actual objects might not be, potentially leading to unexpected behavior.
     • Implement custom serialization logic: If the class defines its own toJson or other serialization methods, those methods will be used, and the resulting representation might not include all non-final fields.

There are a few more limitations, read here.

[Ben1980]

You're right, i think as well this is the issue. BluetoothDevice is a foreign class from flutter_blue_plus unfortunately so I will need to got the caching way. Do you have any hint or suggestion how to do it? Otherwise I would utilize shared_preferences.

[Sese-Schneider]

@Ben1980 first you should try making the field final in Computer.
But if the object contains more non-final children this will not work.

I wouldn't go with shared preferences or any persistent caches, but instead a normal memory cache.
First make sure the objects are comparable (hashCode and equality implemented), then just create a HashMap with the objects hashCode as key.
Forward the hashcode to the non-isolate layer, and when getting it back try to read the object back from the memory cache.

[Ben1980]

How would you do that for a foreign class of another plugin like BluetoothDevice? As far as I can see it is not providing a proper hashCode nor equality function. I've never used a memory cache, do you have any examples?

[Sese-Schneider]

@Ben1980 I think it does implement the required equality functions!

https://github.com/boskokg/flutter_blue_plus/blob/9381858018ae448b7c9e1ac6f1b621d2db240439/lib/src/bluetooth_device.dart#L667-L673

So what you can do now is the following:

final bluetoothDeviceCache = Map<int, BluetoothDevice>();

/// Function returns a hashCode which can be used to retrieve the BluetoothDevice
int storeBluetoothDevice(BluetoothDevice device) {
  bluetoothDeviceCache[device.hashCode] = device;
  return device.hashCode;
}

/// Function returns a BluetoothDevice based on its hashCode.
BluetoothDevice? getBluetoothDevice(int hashCode) {
  return bluetoothDeviceCache[hashCode];
}

now just pass the hashCode int receved from storeBluetoothDevice to the main layer and back to the isolate.

[Ben1980]

Ok currently i don't know how to solve thisproblem. I tried passing BluetoothDevice to the isolate but tht seems not compatible because i don't get a proper object. I also tried the caching idea but also there i'm not sure how to go on because the isolate doesn't share memory. I think a call would help, you know more about flutter/dart so you probably know what to do.