Summary:
Note: PR to react-native-macos here https://github.com/microsoft/react-native-macos/pull/733
Internally in Microsoft code, we ran into a deadlock where the main queue and the UIManager queue were both trying to access `[RCTI18nUtil sharedInstance]`, and were blocked on each other. This is similar to an earlier issue with RCTScreenScale decsribed [here](https://github.com/facebook/react-native/issues/18096).
To summarize:
1- RCTShadowView (on the UIManager queue) and RCTView (on the main queue) both try to access `[RCTI18nUtil sharedInstance]`
2- The UIManager thread gets there first, and lazily initializes the sharedInstance. Meanwhile, the main thread is waiting on a lock possessed by the UIManager thread
3- As part of the initialization, we set an NSUserDefault, which seems to require the (blocked) main thread.
4- Deadlock.
For whatever reason, this only happens on debug. I did not figure out why, but I do know based on [this comment](https://github.com/facebook/react-native/issues/18096#issuecomment-368718081), that the UIManagerQueue should never block the main queue.
The fix is to not use NSUserDefaults, and simpy use atomic properties instead. We get the thread safety for free, and it also simplifies the code somewhat without changing the public API. The downside is values aren't persisted anymore, but I do not think that was necessary / intended.
## Changelog
<!-- Help reviewers and the release process by writing your own changelog entry. For an example, see:
https://github.com/facebook/react-native/wiki/Changelog
-->
[iOS] [Fixed] - Fix deadlock on RCTi18nUtil
Pull Request resolved: https://github.com/facebook/react-native/pull/31032
Test Plan:
Ran the RTLExample in RNTester, and ensured switching to RTL still worked, and that setting forceRTL would still work after reloading the bundle.
https://user-images.githubusercontent.com/6722175/108775429-aefdae80-7526-11eb-9a89-3114f7ddc2af.mov
Reviewed By: javache
Differential Revision: D29522152
Pulled By: RSNara
fbshipit-source-id: 160840f63a7b1d6721b0fd8294fb11990a4509fa
fcead14b0e