dart:ffi only supports calling static Dart functions from native code

[azan-n]

dart --version
Dart SDK version: 2.15.0-116.0.dev (dev) (Thu Sep 16 09:47:01 2021 -0700) on "linux_x64"

I've been looking up examples for callbacks and I have tried to get callbacks working for me in FFI.

My current situation

I have a function in my library which expects a pointer to a function. The bindings for the same generated by ffigen seem correct to me.

int SetCallback(
    CallbackType callback,
  ) {
    return _SetCallback(
      callback,
    );
  }

  late final _SetCallbackPtr =
      _lookup<NativeFunction<Int32 Function(CallbackType)>>(
          'SetCallback');
  late final _SetCallback =
      _SetCallbackPtr.asFunction<int Function(CallbackType)>();

where, typedef CallbackType = Pointer<NativeFunction<Void Function(Uint32)>>;.

What I want to do here is to setup this callback in Dart, pass it to the FFI, essentially using it as my callback as I would have in C. In my API which abstracts away from FFI code (which means I have a class MyLibrary full of static functions that the user will call directly, which in turn calls functions from an object _nativeLibrary of the class MyNativeLibrary I have created), I have:

  static int SetCallback({required CallbackFuncDart callback}) {
    Pointer<NativeFunction<CallbackFunc>> pointer = Pointer.fromFunction(callback);

    int status = _nativeLibrary.SetCallback(
      pointer,
    );
    if (STATUS_OK != status) {
      throw LibLexemeException(status);
    }
    return status;
  }

typedef CallbackFunc = Void Function(Uint32);
typedef CallbackFuncDart = void Function(int);

While the sqlite ffi example states here that

Features which dart:ffi does not support yet:

Callbacks from C back into Dart.

I believe the docs haven't been updated to reflect the changes at the samples here. The samples haven't been very clear due to them not having any C/C++ files, or an idea of how the C functions work. I do believe that the ffi documentation is a bit lacking and I'd like to contribute if it is of any use. Even so, I think this example contains a segment(last code block) where a Dart function is being passed as a callback which I have replicated in my program. It is not clear to me how this will work but upon trying to compile my program I get:

ERROR: ../lib/library_lexeme.dart:180:74: Error: fromFunction expects a static function as parameter. dart:ffi only supports calling static Dart functions from native code. Closures and tear-offs are not supported because they can capture context.
ERROR:     Pointer<NativeFunction<CallbackFunc>> pointer = Pointer.fromFunction(callback);

[mattetti]

Based on #35903 I think that this limitation was added on purpose. As the error explains: dart:ffi only supports calling static Dart functions from native code. Closures and tear-offs are not supported because they can capture context. The only way I understand you can use callbacks is to define them outside of your method/class. Which is exactly what is done in the example you pointed to: https://github.com/dart-lang/sdk/blob/master/samples/ffi/sample_ffi_functions_callbacks.dart#L28

int myPlus(int a, int b) {}

Is a static function, the author then gets a native function pointer to it and pass it as a callback:

Pointer<NativeFunction<NativeIntptrBinOp>> pointer =
        Pointer.fromFunction(myPlus, 0);
// [...]
int result = applyTo42And74(pointer);

Where applyTo42And74() is the C function taking the callback.

[dcharkes]

NativeCallables can now take closures.

https://api.dart.dev/dev/3.3.0-214.0.dev/dart-ffi/NativeCallable-class.html

We're not intending to add this support to Pointer.fromFunction.