lib/native_opencv.dart

import 'dart:ffi' as ffi;
import 'dart:io';
import 'package:ffi/ffi.dart';

// C function signatures
typedef _CVersionFunc = ffi.Pointer<Utf8> Function();
typedef _CProcessImageFunc = ffi.Void Function(
  ffi.Pointer<Utf8>,
  ffi.Pointer<Utf8>,
);

// Dart function signatures
typedef _VersionFunc = ffi.Pointer<Utf8> Function();
typedef _ProcessImageFunc = void Function(ffi.Pointer<Utf8>, ffi.Pointer<Utf8>);

// Getting a library that holds needed symbols
ffi.DynamicLibrary _openDynamicLibrary() {
  if (Platform.isAndroid) {
    return ffi.DynamicLibrary.open('libnative_opencv.so');
  } else if (Platform.isWindows) {
    return ffi.DynamicLibrary.open("native_opencv_windows_plugin.dll");
  }

  return ffi.DynamicLibrary.process();
}

ffi.DynamicLibrary _lib = _openDynamicLibrary();

// Looking for the functions
final _VersionFunc _version =
    _lib.lookup<ffi.NativeFunction<_CVersionFunc>>('version').asFunction();
final _ProcessImageFunc _processImage = _lib
    .lookup<ffi.NativeFunction<_CProcessImageFunc>>('process_image')
    .asFunction();

String opencvVersion() {
  return _version().toDartString();
}

void processImage(ProcessImageArguments args) {
  _processImage(args.inputPath.toNativeUtf8(), args.outputPath.toNativeUtf8());
}

class ProcessImageArguments {
  final String inputPath;
  final String outputPath;

  ProcessImageArguments(this.inputPath, this.outputPath);
}