simple sample to build c/c++ to ..so file then called by ffi dart

https://github.com/badpaybad/AndroidNdkBuildWithCmakelist

some words

U can use Method Channel to invoke lib native in java. But look inside native java. they also call native to c. c/c++ So why we need use method channel if we can direct call to c/c++ with shared lib (.so) build from source code. Let try .....

dartffi_dlib_opencv

Base on idea and build libdlib.so for android with java https://github.com/Luca96/dlib-for-android/tree/master and https://github.com/tzutalin/dlib-android

I try to build and use libdlib.so with FFI dart. You dont need to build .so cause I prebuild in android/app/src/main/cppLibs/dlib/lib

Just try to write your c/c++ to wrap dlib and dart ffi

dlib http://dlib.net/books.html

android/app/src/main/cppLibs/CMakeLists.txt will link wrap dlib to you flutter

            var libraryPath = "libDlibOpencvFfi.so";
            print("ffi.DynamicLibrary file: $libraryPath");
            _dylib_dlib_opencv = ffi.DynamicLibrary.open(libraryPath);

code c/c++ in folder: android/app/src/main/cppLibs
code dart ffi in folder: lib/interop 

should down load manual flutter sdk dont use snap

https://dart.dev/get-dart#install-using-apt-get

https://docs.flutter.dev/get-started/install/linux

why dlib ? http://dlib.net/books.html

Cause it write in c/c++ version 11 so mostly work cross platform

Some challenger about AI eg: face detect, get vector from face ( can use to recognize face with cosin compare ...) , can train your own data for object detection...

https://opencv.org/about/

Opencv got alot of famous functions to work with images

After porting use dart ffi we will have strong lib or package to process image with ML DL AI - the computer vision :D

/work/flutter-dart-ffi-dlib-opencv/android/app/build.gradle

android {
....
...
 buildTypes {
        release {
            // TODO: Add your own signing config for the release build.
            // Signing with the debug keys for now, so `flutter run --release` works.
            signingConfig signingConfigs.debug
        }
    }
    externalNativeBuild {
        cmake {
            path "CMakeLists.txt"
        }
    }
    defaultConfig {
        externalNativeBuild {
            cmake {
                cppFlags '-frtti -fexceptions -std=c++11'
                arguments "-DANDROID_STL=c++_shared"
            }
        }
    }
}

build .so file

prebuild also in this git at NATIVE_DIR="$PROJECT_PATH/app/src/main/cppLibs" , if want build new version dlib opencv goto bellow

folder download or git clone

Assume root prj folder: /work/flutter-dart-ffi-dlib-opencv/dlibopencvbuild

|--android
|--dlib.19.16  # download from http://dlib.net/files/  extract http://dlib.net/files/dlib-19.16.zip
|----dlib
|----docs
|----...
|--opencv-4.0.1-android-sdk  # https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&cad=rja&uact=8&ved=2ahUKEwiLgOzW9JCCAxWAh1YBHcaDDjIQFnoECBUQAQ&url=https%3A%2F%2Fsourceforge.net%2Fprojects%2Fopencvlibrary%2Ffiles%2F4.0.1%2Fopencv-4.0.1-android-sdk.zip%2Fdownload&usg=AOvVaw1PepXxKcB-OPWMXKLrCvB7&opi=89978449
|----OpenCV-android-sdk # https://docs.opencv.org/4.x/da/d2a/tutorial_O4A_SDK.html
|------apk
|------sdk
|------....

goto setupdlibopencv.sh change value to your folder

Will declare in grade.build Code c/c++ will build by NDK and link to our prj

.../android/app/src/main/cppLibs/CMakeLists.txt

# Android project path: REPLACE WITH YOUR PROJECT PATH!
PROJECT_PATH='/work/flutter-dart-ffi-dlib-opencv/android'
# Directory for storing native libraries
NATIVE_DIR="$PROJECT_PATH/app/src/main/cppLibs"
#NATIVE_DIR="/work/dlib4android.so"
# Dlib library path: REPLACE WITH YOUR DLIB PATH!
DLIB_PATH='/work/flutter-dart-ffi-dlib-opencv/dlib.19.16'
# OpenCV library path: REPLACE WITH YOUR OPENCV PATH!
OPENCV_PATH='/work/flutter-dart-ffi-dlib-opencv/opencv-4.0.1-android-sdk/OpenCV-android-sdk/sdk/native'
# Android-cmake path: REPLACE WITH YOUR CMAKE PATH!
AndroidCmake='/home/dunp/Android/Sdk/cmake/3.22.1/bin/cmake'
# Android-ndk path: REPLACE WITH YOUR NDK PATH!
#NDK="${ANDROID_NDK:/home/dunp/Android/Sdk/ndk/26.1.10909125}"
ANDROID_NDK="/home/dunp/Android/Sdk/ndk/21.0.6113669"
NDK="/home/dunp/Android/Sdk/ndk/21.0.6113669"
TOOLCHAIN="$NDK/build/cmake/android.toolchain.cmake"
# path to strip tool: REPLACE WITH YOURS, ACCORDING TO OS!!
STRIP_PATH="$NDK/toolchains/llvm/prebuilt/linux-x86_64/bin"

complete will copy to NATIVE_DIR="$PROJECT_PATH/app/src/main/cppLibs"

            sudo chmod 777 setupdlibopencv.sh 
            ./setupdlibopencv.sh

result will NATIVE_DIR="$PROJECT_PATH/app/src/main/cppLibs"

.../app/src/main/cppLibs/
|--dlib
|----include # c file
|----lib # .so files
|--opencv
|----arm64-v8a
|----...

code PoC

c/c++ usage dlib

extern "C" __attribute__((visibility("default"))) __attribute__((used))
char *detect_face_cpu(char *file_path) {
    array2d<unsigned char> img;
    ....

dart ffi call

Future<void> detect_face_cpu(
    DynamicLibrary dylib, String file_path_img) async {
  DartStringFunc dunp_func = dylib
      .lookup<NativeFunction<FfiStringFunc>>('detect_face_cpu')
      .asFunction();
...

test in flutter app check file: main.dart