Dart/Flutter ffi (Foreig Function Interface) native callbacks eg: sqlite3_exec

Question

Hello I am using dart:ffi to build an interface with my native c/c++ library. and I needed a way to get a callback from c to dart as an example in sqlite:

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Answer 1

I got an example to work. Hopefully you can adapt this to your case.

Example C function

EXTERNC int32_t foo(
                    int32_t bar,
                    int32_t (*callback)(void*, int32_t)
                    ) {
    return callback(nullptr, bar);
}

Dart code

First the typedefs. We need two for the native function foo and one for the Dart callback.

typedef example_foo = Int32 Function(
    Int32 bar, Pointer<NativeFunction<example_callback>>);
typedef ExampleFoo = int Function(
    int bar, Pointer<NativeFunction<example_callback>>);

typedef example_callback = Int32 Function(Pointer<Void>, Int32);

and the code for the callback

  static int callback(Pointer<Void> ptr, int i) {
    print('in callback i=$i');
    return i + 1;
  }

and the lookup

  ExampleFoo nativeFoo =
    nativeLib.lookup<NativeFunction<example_foo>>('foo').asFunction();

and, finally, use it like this:

  int foo(int i) {
    return nativeFoo(
      i,
      Pointer.fromFunction<example_callback>(callback, except),
    );
  }

as expected, foo(123) prints flutter: in callback i=123 and returns 124