`midiInOpen` takes a Pointer to a callback but `Cannot invoke native callback outside an isolate`

[akvus]

I'm trying to connect my MIDI device to a Flutter app running on Windows. I have the following:

final Pointer<HMIDIIN> hMidiDevice = malloc();

Pointer<NativeFunction<MidiInProc>> callbackPointer =
	Pointer.fromFunction(midiInCallback);

final result = midiInOpen(
  hMidiDevice,
  0,
  callbackPointer.address,
  0,
  CALLBACK_FUNCTION,
);
midiInStart(hMidiDevice.value);

midiInOpen takes a pointer to a function as 3rd argument. Here is my callback method:

static void midiInCallback(
	int hMidiIn,
	int wMsg,
	int dwInstance,
	int dwParam1,
	int dwParam2,
) {
	print('Message: $wMsg dwParam1: $dwParam1');
}

This compiles and works with a connected USB MIDI device. However, when I press a key on my MIDI device, then I get the following error:

../../third_party/dart/runtime/vm/runtime_entry.cc: 3657: error: Cannot invoke native callback outside an isolate.
pid=11004, thread=21860, isolate_group=(nil)(0000000000000000), isolate=(nil)(0000000000000000)
isolate_instructions=0, vm_instructions=7ffef50837c0
  pc 0x00007ffef51a3732 fp 0x00000057468ff990 angle::PlatformMethods::operator=+0x322d8a
-- End of DumpStackTrace

What am I doing wrong?

[dcharkes]

The callback is invoked on a different thread, which is currently not (yet) supported in dart:ffi: https://github.com/dart-lang/sdk/issues/37022.

The work around is to use the dart_api with its native ports, see the linked issue.

[halildurmus]

Closing this as Dart 3.1 introduced NativeCallable.listener, which can be used to create callbacks that allow native code to call into Dart code from any thread.