[zoontek]

Summary:

This is a proposal to simplify / cleanup I18nManager. It contains multiple breaking changes, and it's obviously open to discussion.

I started working on it as the current API is messy / hard to grasp:

• RTL support has two switches on Android (android:supportsRtl in the manifest and allowRTL at runtime) but only one on iOS (allowRTL), so the platforms do not match.
• allowRTL, forceRTL and swapLeftAndRightInRTL are saved to disk and only take effect after a restart. They act as app configuration written from JS.
• The left / right swap is on by default and has no equivalent on the web, where margin-left stays on the left in RTL and only logical properties (margin-inline-start) follow the direction.

This PR:

• Makes forceRTL apply at runtime: surfaces relayout, and isRTL updates immediately (bridgeless only). The value is no longer persisted.
• Removes the left / right swap. left / right are always physical, like on the web. Use start / end for direction-aware styles (already supported for a while).
• Reads RTL support from app configuration: new RCTSupportsRTL in Info.plist on iOS, android:supportsRtl on Android (unchanged). true by default.
• Deprecates allowRTL, swapLeftAndRightInRTL and doLeftAndRightSwapInRTL (no-ops with a warning).
• Returns localeIdentifier on both platforms, in BCP 47 format (en-US) - same format as navigator.language on web.

Changelog:

[GENERAL] [BREAKING] - I18nManager: forceRTL applies at runtime and is no longer persisted, left / right styles are no longer swapped in RTL, allowRTL and swapLeftAndRightInRTL are deprecated
[IOS] [BREAKING] - RTL requires RCTSupportsRTL set to true in Info.plist
[ANDROID] [CHANGED] - I18nManager.getConstants().localeIdentifier uses BCP 47 format (en-US instead of en_US)

Test Plan:

Commands I ran, and their results:

• yarn flow-check: 0 errors
• yarn build-types && yarn test-generated-typescript: passed
• yarn test packages/react-native/Libraries packages/jest-preset: passed
• python -m scripts.cxx-api.parser --validate: all snapshots match
• yarn format-check-cpp, yarn format-check-kotlin: clean

Videos:

RNTester's RTL example toggles forceRTL without a reload:

RTL.-.Android.mp4

RTL.-.iOS.mp4

[github-actions]

Warning

JavaScript API change detected

This PR commits an update to ReactNativeApi.d.ts, indicating a change to React Native's public JavaScript API.

• Please include a clear changelog message.
• This change will be subject to additional review.

This change was flagged as: POTENTIALLY_BREAKING