I've managed to make it work nicely on iOS but I'm not able to build CxxModule for Android (using Android Studio) because of following error:
Build command failed.
Error while executing process /Users/sebastian/Library/Android/sdk/ndk-bundle/ndk-build with arguments {NDK_PROJECT_PATH=null APP_BUILD_SCRIPT=/Users/sebastian/Dev/cxxtest/react-native-HelloCxxModule/android/app/src/main/jni/Android.mk NDK_APPLICATION_MK=/Users/sebastian/Dev/cxxtest/react-native-HelloCxxModule/android/app/src/main/jni/Application.mk APP_ABI=x86_64 NDK_ALL_ABIS=x86_64 NDK_DEBUG=1 APP_PLATFORM=android-16 NDK_OUT=/Users/sebastian/Dev/cxxtest/react-native-HelloCxxModule/android/app/build/intermediates/ndkBuild/debug/obj NDK_LIBS_OUT=/Users/sebastian/Dev/cxxtest/react-native-HelloCxxModule/android/app/build/intermediates/ndkBuild/debug/lib NDK_DEBUG=0 /Users/sebastian/Dev/cxxtest/react-native-HelloCxxModule/android/app/build/intermediates/ndkBuild/debug/obj/local/x86_64/librnpackage-hellocxx.so}
[x86_64] Compile++ : rnpackage-hellocxx <= HelloCxxModule.cpp
[x86_64] Prebuilt : libfolly_json.so <= /Users/sebastian/Dev/cxxtest/react-native-HelloCxxModule/android/app/src/main/jni/../../../../../node_modules/react-native/ReactAndroid/build/react-ndk/exported//x86_64/
[x86_64] Prebuilt : libfb.so <= /Users/sebastian/Dev/cxxtest/react-native-HelloCxxModule/android/app/src/main/jni/../../../../../node_modules/react-native/ReactAndroid/build/react-ndk/exported//x86_64/
[x86_64] Prebuilt : libreactnativejni.so <= /Users/sebastian/Dev/cxxtest/react-native-HelloCxxModule/android/app/src/main/jni/../../../../../node_modules/react-native/ReactAndroid/build/react-ndk/exported//x86_64/
[x86_64] Prebuilt : libc++_shared.so <= <NDK>/sources/cxx-stl/llvm-libc++/libs/x86_64/
[x86_64] SharedLibrary : librnpackage-hellocxx.so
/Users/sebastian/Dev/cxxtest/react-native-HelloCxxModule/android/app/src/main/jni/../../../../../node_modules/react-native/ReactAndroid/build/third-party-ndk/folly/folly/Conv.h:703: error: undefined reference to 'google::LogMessageFatal::LogMessageFatal(char const*, int)'
/Users/sebastian/Dev/cxxtest/react-native-HelloCxxModule/android/app/src/main/jni/../../../../../node_modules/react-native/ReactAndroid/build/third-party-ndk/folly/folly/Conv.h:703: error: undefined reference to 'google::LogMessage::stream()'
/Users/sebastian/Dev/cxxtest/react-native-HelloCxxModule/android/app/src/main/jni/../../../../../node_modules/react-native/ReactAndroid/build/third-party-ndk/folly/folly/Conv.h:703: error: undefined reference to 'google::LogMessageFatal::~LogMessageFatal()'
/Users/sebastian/Dev/cxxtest/react-native-HelloCxxModule/android/app/src/main/jni/../../../../../node_modules/react-native/ReactAndroid/build/third-party-ndk/folly/folly/Conv.h:703: error: undefined reference to 'google::LogMessageFatal::~LogMessageFatal()'
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [/Users/sebastian/Dev/cxxtest/react-native-HelloCxxModule/android/app/build/intermediates/ndkBuild/debug/obj/local/x86_64/librnpackage-hellocxx.so] Error 1
I'm starting to be a little bit hopeless and I'm wondering if you could help me. Thank you so much for any help!