How to pass a callback to a win32 function with dart ffi?

Question

I'm trying to connect to my MIDI device to a Flutter app running on Windows. I'm using win32 and dart ffi. 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 does it mean and what can I do so that my callback is called with MIDI data?

Answer 1

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. Only void functions are supported.

Here's a revised version of your example, now incorporating the NativeCallable.listener:

final Pointer<HMIDIIN> hMidiDevice = malloc();

final nativeCallable = NativeCallable<MidiInProc>.listener(midiInCallback);

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

// Don't forget to close the callback when it is no longer needed.
// Otherwise, the Isolate will be kept alive indefinitely.
nativeCallable.close();