Registering a Pure C++ Native Module within a library

[Zach-Dean-Attractions-io]

Hi,

I've successfully followed the documentation on pure C++ modules and I am trying to create a library, which exposes a pure C++ module. The reason for needing a pure C++ module is that I want to use the jsi::Runtime object so that I can create a jsi::HostObject type and tie the lifecycle of a JavaScript object to an underlying native object (side note - maybe I should use jsi::NativeObject instead). The jsi::Runtime object isn't exposed in the Java/Objective-C implementations.

However, I'm struggling when trying to migrate this to a library. The library project create-react-native-library doesn't support pure C++ modules rather the C++ template is implemented in Kotlin/Objective-C and then calls into shared C++ code and they have recently dropped this template callstack/react-native-builder-bob#818.

iOS
Since 0.79.0 of React Native I can register the pure C++ module within the codegen spec using ios.modulesProvider.

I did somewhat get that working, it called into my pure C++ implementation but then crashed with the error [runtime not ready]: ReferenceError: Property 'window' doesn't yet exist, js engine: hermes

I have a sample project that should reproduce the error using this commit d9a8cbe.

create-react-native-library doesn't yet support 0.79.+ of React Native so it might be something I did wrong in upgrading.

As a side question, does the consuming app also need to be running 0.79.+ of React Native for this change to work? I think it does otherwise when I try to build the example app in Xcode I get an error in the module provider that RCTModuleProvider could not be found.

I did get iOS working by registering the C++ turbo module in the consuming app's AppDelegate file, well using an Objective-C++ bridge file to implement the registration, since the AppDelegate is now a Swift file. This commit 84cc053 demonstrates it, in particular RCTAppDelegate+TurboModule.h & RCTAppDelegate+TurboModule.mm

However, ideally I don't want the consumer of my library to need to concern themselves with this code, the library should be able to register the module for them.

Android
The main issue I am facing here is how to register the C++ turbo module. In the documentation it is done using the OnLoad.cpp file.

However, I've been unable to utilise this approach from a library. I have tried including the OnLoad.cpp file in my library project but I can't get the app to build because it complains #include <ReactCommon/TurboModule.h> can not be found. This commit a6ea11c shows my attempt. I imagine I am missing something in my CMake file. I note that I am not using include(${REACT_ANDROID_DIR}/cmake-utils/ReactNative-application.cmake) because I think ${REACT_ANDROID_DIR} isn't populated in the library project and so it fails to find the file. I am also getting issues at times with the generated PackageList.java file looking for a Java implementation of my module, which doesn't exist.

Is it just a case of the consuming app needing to register my module for me? Or how is best to approach this situation?

[Zach-Dean-Attractions-io]

I came across this video https://www.youtube.com/watch?v=KURIqiCPbco and in there they used an Objective-C file with a + (void) load function to register the C++ module. This seems to work and removes the need for the consuming app to implement any logic in their AppDelegate file. See this commit: e1240cf. Is this a valid solution, (it also seems to work fine with the Swift AppDelegate)?

[Zach-Dean-Attractions-io]

Ah I have also implemented it on Android using the same author's video for Android: https://www.youtube.com/watch?v=ibgHObp8YwA&t=8s. This commit 79aa951 demonstrates it.

In particular not registering the module within the <name>Package.kt file but instead adding to the react-native.config.js file the values for cxxModuleCMakeListsModuleName, cxxModuleCMakeListsPath & cxxModuleHeaderName, which I think is documented in the autolinking docs.

module.exports = {
  dependency: {
    platforms: {
      android: {
        cmakeListsPath: 'generated/jni/CMakeLists.txt',
+        cxxModuleCMakeListsModuleName: 'pure-cpp-turbo-module-library',
+        cxxModuleCMakeListsPath: 'CMakeLists.txt',
+        cxxModuleHeaderName: 'PureCppImpl',
      },
    },
  },
};

So I think I have everything now!