DroppableDisposedException error encountered when using RustOpaque and Struct in 2.0

[OfficialBoyfriend]

Rust code:

pub struct Mqtt {
    pub client_id: String,
    // TODO
    pub test: AtomicBool,
}

impl Mqtt {
    #[frb(sync)]
    pub fn new(client_id: String) -> Mqtt {
        // ...
    }

    pub async fn send(&self, payload: String) -> Result<()> {
        // ...
    }

    pub fn connect(&self, host: String, sink: StreamSink<String>) {
        // ...
    }
}

I use it in Dart like this:

void main() {
    Mqtt.newMqtt(clientId: 'one').connect(host: "x.x.x.x").listen((e) {
        setState(() => msg = e);
        // TODO
        clientOne?.send(payload: 'one: $event');
    });
}

Whenever I receive data and call the send method, I get the following error:

[ERROR:flutter/runtime/dart_vm_initializer.cc(41)] Unhandled Exception: DroppableDisposedException: Try to use `RustArc<dynamic>` after it has been disposed
#0      Droppable.dangerousReadInternalPtr (package:flutter_rust_bridge/src/droppable/_common.dart:24:16)
#1      RustArc._ptr (package:flutter_rust_bridge/src/rust_arc/_common.dart:13:42)
#2      RustArc.intoRaw (package:flutter_rust_bridge/src/rust_arc/_common.dart:44:17)
#3      RustOpaque.cstEncode (package:flutter_rust_bridge/src/misc/rust_opaque.dart:52:19)
#4      RustOpaque.sseEncode (package:flutter_rust_bridge/src/misc/rust_opaque.dart:57:34)
#5      RustLibApiImpl.sse_encode_Auto_Owned_RustOpaque_flutter_rust_bridgefor_generatedrust_asyncRwLockAtomicBool (package:class_card_util/frb_generated.dart:4600:27)
#6      RustLibApiImpl.sse_encode_mqtt (package:class_card_util/frb_generated.dart:5406:5)
#7      RustLibApiImpl.sse_encode_box_autoadd_mqtt (package:class_card_util/frb_generated.dart:4746:5)
#8      RustLibApiImpl.mqttSend.<anonymous closure> (package:class_card_util/frb_generated.dart:1807:9)
#9      BaseHandler.executeNormal (package:flutter_rust_bridge/src/main_components/handler.dart:18:17)
#10     RustLibApiImpl.mqttSend (package:class_card_util/frb_generated.dart:1804:20)
#11     Mqtt.send (package:class_card_util/api/mqtt.dart:85:28)
#12     _HomePageState.connectMqtt.<anonymous closure>.<anonymous closure> (package:class_card_util_example/main.dart:129:20)
#13     _RootZone.runUnaryGuarded (dart:async/zone.dart:1594:10)
#14     _BufferingStreamSubscription._sendData (dart:async/stream_impl.dart:339:11)
#15     _BufferingStreamSubscription._add (dart:async/stream_impl.dart:271:7)
#16     _SyncStreamControllerDispatch._sendData (dart:async/stream_controller.dart:784:19)
#17     _StreamController._add (dart:async/stream_controller.dart:658:7)
#18     _StreamController.add (dart:async/stream_controller.dart:606:5)
#19     _AsyncStarStreamController.add (dart:async-patch/async_patch.dart:76:16)
#20     BaseHandler._executeStreamInner (package:flutter_rust_bridge/src/main_components/handler.dart)
<asynchronous suspension>
#21     _HomePageState.connectMqtt.<anonymous closure>.<anonymous closure> (package:class_card_util_example/main.dart:127:19)
<asynchronous suspension>

When I remove the test field in the Mqtt structure, everything goes back to normal. The relevant documentation I found is https://cjycode.com/flutter_rust_bridge/guides/contributing/submodules/rust-opaque.

Is this error expected? Or is there something wrong with the way I'm using it?

[fzyzcjy]

When I remove the test field in the Mqtt structure, everything goes back to normal.

I guess it is because Mqtt auto becomes opaque when you have a non-translatable field.

Could you please provide a minimal reproducible sample (would be great to be a separate github repo s.t. I can clone and try it)? More specifically, I want to know more about clientOne - suspect this may be source of problem.

[OfficialBoyfriend]

https://github.com/OfficialBoyfriend/flutter_demo_code

This is a minimal demo with no actual functionality and is intended only to reproduce the issues I encountered.

The actual application scenario is that after MQTT receives the message, it needs to send a reply.

[fzyzcjy]

Hmm, I suspect this is a bug. I will look at it hopefully within a day.

[fzyzcjy]

Ok I see: It is not a bug, and just add #[frb(opaque)] struct Mqtt { ... }. (Otherwise, as can be seen from the generated dart code, frb automatically thinks Mqtt is a translatable struct, while the AtomicBool is an owned opaque.)

Feel free to ask questions if you find any problems!

[OfficialBoyfriend]

I'm a little curious, is it possible for flutter_rust_bridge_codegen to be able to give a certain prompt when encountering this kind of code instead of a runtime error?

[fzyzcjy]

This may not be very trivial. The main problem is that, this may be something that the user really wants, instead of something wrong! I can surely forbid such scenario, but then those users will not be able to use it.

[OfficialBoyfriend]

Ok, I see😊