Flutter, use DynamicLibrary.open() on prebuild GO .so library without the need to write Native Code(Java/Swift)

Question

I made a GO library and build it as an .so library. Is working loading the library in Java with System.loadLibrary() but I can not manage it to load directly from flutter/dart using DynamicLibrary.open().

I want to skip the process of calling native code, and load Shared Libraries directly in flutter.

DynamicLibrary.open is only available on dev channel v1.10.14.

Documentation with examples using Cmake: https://flutter.dev/docs/development/platform-integration/c-interop

My code:

static final DynamicLibrary nativeAddLib = DynamicLibrary.open("lib-mylib.so");
final int Function (String ifName, int tunFd, String settings) addStuff = nativeAddLib.lookup<NativeFunction<Int32 Function(String, Int32, String)>>("addStuff").asFunction();

Error:

Compiler message:
lib/vpn_connection/vpn_connection_bloc.dart:20:164: Error: Expected type 'NativeFunction<Int32 Function(String, Int32, String)>' to be a valid   and instantiated subtype of 'NativeType'.
- 'NativeFunction' is from 'dart:ffi'.
- 'Int32' is from 'dart:ffi'.
final int Function (String ifName, int tunFd, String settings) addStuff = nativeAddLib.lookup<NativeFunction<Int32 Function(String, Int32, String)>>("addStuff").asFunction();
                                                                                                                                                                 ^
Exception: Errors during snapshot creation: null
#0      KernelSnapshot.build (package:flutter_tools/src/build_system  /targets/dart.dart:226:7)
<asynchronous suspension>
#1      _BuildInstance._invokeInternal (package:flutter_tools/src/build_system/build_system.dart:526:25)
<asynchronous suspension>
#2      _BuildInstance.invokeTarget.<anonymous closure>   (package:flutter_tools/src/build_system/build_system.dart:481:35)
#3      new Future.sync (dart:async/future.dart:224:31)
#4      AsyncMemoizer.runOnce (package:async/src/async_memoizer.dart:43:45)
#5      _BuildInstance.invokeTarget (package:flutter_tools/src/build_system/build_system.dart:481:21)

It looks like is not finding the file/function.

My gradle:

android {
compileSdkVersion 28

lintOptions {
    disable 'InvalidPackage'
}

sourceSets.main {
    jniLibs.srcDirs += files(extraJniDirectory)
}

defaultConfig {
    applicationId "com.custom.android"
    minSdkVersion 21
    targetSdkVersion 28
    versionCode flutterVersionCode.toInteger()
    versionName flutterVersionName
    testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"

    ndk {
        abiFilters "armeabi", "x86", "armeabi-v7a", "arm64-v8a", "x86_64"
    }
}

buildTypes {
    release {
        signingConfig signingConfigs.debug

        ndk {
            if (project.hasProperty('target-platform') &&
                    project.property('target-platform') == 'android-arm64') {
                abiFilters 'arm64-v8a'
            } else {
                abiFilters 'armeabi-v7a'
            }
        }
    }
}
}

Update:

I created a .so file with CMake as in the flutter example, I extract it from the apk and put it in the same folder with my go build .so file, and is working, but I can't find why my first .so file is not working from flutter but, is working from android.

Update2:

DynamicLibrary.open("lib-mylib.so") is loaded, nativeAddLib.lookup<NativeFunction<Int32 Function(String, Int32, String)>>("addStuff") is returning a pointer, that means the function is found, but when calling .asFunction() it breaks.

By simplifying the code:

var addStuff = nativeAddLib.lookup("addStuff").asFunction();

I get the error:

Error: Expected type 'NativeType' to be a valid and instantiated subtype of 'NativeType'.
- 'NativeType' is from 'dart:ffi'.

Solution

I think the problem might be that Dart's FFI doesn't directly support Strings. Have a look at this sample for a way to marshal strings using the separate ffi package: https://github.com/dart-lang/samples/blob/master/ffi/system-command/linux.dart

Comments

[jalanga]

I will give it a try, I don't think is from String support, because I tried with different methods which are accepting Int32 and not returning(void).

[eug]

If you paste the code & error messages from the simpler Int32 example that might help. void is actually similar to String - one needs to use a special ffi.Void type in the native typedef.

[eug]

Glad that helped! I sure hope they improve the error messages soon..