Can't use async callback

[WonyoungChoi]

Due to the thread problem, asynchronous callbacks are currently unavailable.

example of a problem situation

  Pointer<NativeFunction<device_changed_cb>> _batteryChangedCallback =
      Pointer.fromFunction<device_changed_cb>(_batteryChanged);

   int ret = tizen.device_add_callback(
        device_callback_e.DEVICE_CALLBACK_BATTERY_CHARGING,
        _batteryChangedCallback,
        Pointer.fromAddress(0));
    if (ret != device_error_e.DEVICE_ERROR_NONE) {
      throw Exception('Failed to register battery callback');
    }
  }

  static void _batteryChanged(
      int type, Pointer<Void> value, Pointer<Void> userData) {
    print('batteryChanged');
  }

If the charging status is changed, following error will be raised.
error: Cannot invoke native callback outside an isolate.

sample code

https://github.com/WonyoungChoi/tizen_interop_async_test

repro with sample code

1. git clone https://github.com/WonyoungChoi/tizen_interop_async_test
2. cd tizen_interop_async_test/example
3. flutter_tizen run (on emulator)
4. open "Control Panel" of the emulator and choose "Batter".
5. Change "Charger" state to "on "or "off".
6. Application will be terminated with following error:

[E] ../../third_party/dart/runtime/vm/runtime_entry.cc: 3621: error: Cannot invoke native callback outside an isolate.
version=2.16.1 (stable) (Tue Feb 8 12:02:33 2022 +0100) on "linux_ia32"
pid=29992, thread=29992, isolate_group=(nil)((nil)), isolate=(nil)((nil))
isolate_instructions=0, vm_instructions=a65d5620
  pc 0xa672441e fp 0xbfbe4c38 /proc/self/fd/24/lib/libflutter_engine.so+0x9c641e
[E]   pc 0xa6a15cd1 fp 0xbfbe4c58 /proc/self/fd/24/lib/libflutter_engine.so+0xcb7cd1
  pc 0xa65d57d6 fp 0xbfbe4c88 /proc/self/fd/24/lib/libflutter_engine.so+0x8777d6
  pc 0xa67730be fp 0xbfbe4cc8 /proc/self/fd/24/lib/libflutter_engine.so+0xa150be
  pc 0xa67730f0 fp 0xbfbe4ce8 /proc/self/fd/24/lib/libflutter_engine.so+0xa150f0

reason

The above code is executed on the Flutter UI thread, but most of the tizen native callbacks come from the main-loop of the main thread. Dart does not support calls between isolates.
See more: dart-lang/sdk#37022

solution?

• Adding some C wrapper codes for every functions that handle async callbacks using ReceivePort?
  • https://api.dart.dev/stable/2.10.5/dart-isolate/ReceivePort-class.html
  • https://unsuitable001.medium.com/package-cronet-an-http-dart-flutter-package-with-dart-ffi-84f9b69c8a24

[swift-kim]

It seems not all callback-based APIs have the same issue. e.g. path_provider_tizen and shared_preferences_tizen use callback-based APIs (maybe they are just synchronous callbacks).

[roc2539]

Have you solved your problem?

[swift-kim]

@WonyoungChoi has started working on the solution but it may take quite a while to finish.

[WonyoungChoi]

@roc2539 It's still going on. We are currently checking the feasibility with several solutions internally.
As soon as it goes further, you will be able to check the work situation.

[swift-kim]

A related functionality (dart-lang/sdk@13ec074) will be added in Dart 3.1 but it has the following limitation:

The return type of the function passed to 'NativeCallable.listener' must be 'void'

NativeCallable.listener creates a native callable that can be invoked
from any thread. The native code that invokes the callable sends a message
back to the isolate that created the callable, and doesn't wait for a
response. So it isn't possible to return a result from the callable.

https://github.com/dart-lang/sdk/blob/af8fb2cd73c3b5ec96a39c59e85f470efe78d8a4/pkg/analyzer/tool/diagnostics/diagnostics.md#L13440-L13453