react-native: Build fails due to header files missing when using -fcxx-modules flag (AppDelegate.mm)

I managed to temporarily fix this by copying the files (which are symbolic links anyway) from ios/Pods/Headers/Private/Yoga/yoga to ios/Pods/Headers/Public/Yoga/yoga, started with BitUtils.h then ended up dragging a few more until it built successfully.

For me this started happening (maybe, still have to confirm but 99% sure) after I installed expo.

@cortinico @pedpess In the migration steps here, there is an info box saying exactly that! 😄

Aha! It’s indeed there @cipolleschi 😄 tks. This page should be marketed more because I was not even aware it existed. I guess most of us are using the RNUpgradeHelper and reading what are the warnings there that we should be aware of before migrating to newer versions

but XCode didn’t pick the new name even after a fresh build cleaning everything the build failed 😕

Ah I see. Thanks for explaining. @cipolleschi what’s your take here. Perhaps we can clearly mention in the doc that the file should be renamed inside Xcode or there is a way to regenerate the xcodeproj so that the .mm file is picked up.

I’ve created an issue here: https://github.com/Salt-PepperEngineering/react-native-nordic-dfu/issues/14

We could, but right now React Native minimum support is for C++ 17 and Cxx modules are a feature of C++ 20, so we can’t enforce it right now. As soon as Meta decides to move the minimum support to C++ 20, we could plan the work to do so.

In this half I’m doing a bunch of work to ensure that use_frameworks! will work. This could bring compatibility with C++ modules also (but I’m not sure about that) or at least move us in the right direction. Unfortunately, the framework is quite big and intricated, so it’s not trivial to support them. This effort requires planned work.

You can use the flag on v69+. I’m on latest react native now and it works.

I needed to enable the -fcxx-modules flag not because of expo though, because of the react-native-nordic-dfu library which uses @import syntax. I can try to create a repo for you guys, with the -fcxx-modules flag set I guess?

Yep, ideally something like we can clone, install dependencies, build and see it failing! 👍

Given that the expo is highly related to this, we could also ask @Kudo if he has any insight on this.

I was able to solve this by creating an Objective C class with static methods that called to my problematic module (in this case, Iterable), and then importing and calling that from the Objective C++ file.

First, my iOS build was not recognising AppDelegate.mm file, like the file name was never changed during the React Native upgrade.

I had then to go to XCode and change it there so it would reflect in project.pbxproj because XCode was still referencing .m over .mm. Probably a thing to add as a tip for the community @cortinico?

Now I have the same issue as @WNemencha has with @import Firebase, which I changed to #import <Firebase/Firebase.h> and it worked! thanks for sharing 🙏🏻

Hey! Not much help there but, I also have this issue with a different library. I tried adding -fcxx-modules flag and yoga was broken as well in the same file. The library in question is quite popular one – Segment Analytics. I was trying to call Segment Track in AppDelegate.mm I think it would fail if I tried to initialise it as well since it depends on the same Header file.

I guess this will break other things in the future (maybe the new arch) but at least I can work with this now.

Yup that’s correct, you won’t be able to use the New Architecture with .m

FYI I managed to get the build working again by removing all cpp flags and renaming AppDelegate.mm back to AppDelegate.m. I guess this will break other things in the future (maybe the new arch) but at least I can work with this now.