fix: keep device sync free of expo by registering device locales from i18n #75

src/i18n/index.ts

@@ -7,6 +7,7 @@
 import { I18nManager } from 'react-native';
 import { getLocales } from 'expo-localization';
 import { formatMessage, type MessageParams } from './format';
+import { setDeviceLocales } from '../lib/intl-locale';
 
 // English is the fallback for every lookup, so it is loaded eagerly. The other
 // catalogs are loaded on first use; see CATALOG_LOADERS below.
@@ -212,6 +213,20 @@ export function detectDeviceLocale(): LocaleCode {
   return 'en';
 }
 
+/**
+ * Hand the device locales to the date formatters (lib/intl-locale), so an
+ * en-AU phone formats dates as en-AU while reading the English catalog.
+ */
+export function registerDeviceLocales(): void {
+  try {
+    setDeviceLocales(getLocales().map((l) => ({ appLocale: resolveLocaleTag(l), languageTag: l.languageTag })));
+  } catch {
+    setDeviceLocales([]);
+  }
+}
+
+registerDeviceLocales();
+
 function lookup(dict: Dictionary, key: string): string | undefined {
   let current: unknown = dict;
   for (const part of key.split('.')) {

src/lib/__tests__/intl-locale.test.ts

@@ -7,13 +7,14 @@ vi.mock('expo-localization', () => ({
   getCalendars: () => [{ timeZone: 'UTC', firstWeekday: 1 }],
 }));
 
-import { intlLocale, resetIntlLocaleCache } from '../intl-locale';
+import { intlLocale } from '../intl-locale';
+import { registerDeviceLocales } from '../../i18n';
 import { formatCalendarDate, formatListDate } from '../date-format';
 import { formatHeaderDate } from '../email-date';
 
 function setDevice(...tags: string[]) {
   device.locales = tags.map((t) => ({ languageTag: t, languageCode: t.split('-')[0] }));
-  resetIntlLocaleCache();
+  registerDeviceLocales();
 }
 
 describe('intlLocale', () => {

src/lib/intl-locale.ts

@@ -3,11 +3,25 @@
 // order: en-AU writes 15/10/2026, en-US 10/15/2026. intlLocale() hands the
 // date formatters the device's full tag when its language matches the app
 // language, so an Australian or British phone gets its own dates (#75).
-import { getLocales } from 'expo-localization';
-import { resolveLocaleTag } from '../i18n';
+//
+// This module stays free of react-native and expo because the device sync
+// engine reaches it through quote-header (see device-sync purity test). The
+// i18n module reads the device locales and registers them here on load.
 
+export interface DeviceLocale {
+  /** The app language this device locale selects, from resolveLocaleTag. */
+  appLocale: string | null;
+  languageTag: string;
+}
+
+let deviceLocales: DeviceLocale[] = [];
 const cache = new Map<string, string>();
 
+export function setDeviceLocales(locales: DeviceLocale[]): void {
+  deviceLocales = locales;
+  cache.clear();
+}
+
 // Android 14 appends regional preferences such as `-u-fw-mon-mu-celsius`.
 // The app has its own time-format and week-start settings, so keep only the
 // language, script and region.
@@ -25,14 +39,8 @@ function isUsable(tag: string): boolean {
 }
 
 function deviceTagFor(locale: string): string | null {
-  let devices: ReturnType<typeof getLocales>;
-  try {
-    devices = getLocales();
-  } catch {
-    return null;
-  }
-  for (const l of devices) {
-    if (!l.languageTag || resolveLocaleTag(l) !== locale) continue;
+  for (const l of deviceLocales) {
+    if (!l.languageTag || l.appLocale !== locale) continue;
     const tag = baseTag(l.languageTag);
     // A tag without a region adds nothing over the language subtag.
     if (tag.includes('-') && isUsable(tag)) return tag;
@@ -52,8 +60,3 @@ export function intlLocale(locale: string | null | undefined): string {
   cache.set(lang, resolved);
   return resolved;
 }
-
-/** Test hook: forget the resolved tags so a changed device locale is seen. */
-export function resetIntlLocaleCache(): void {
-  cache.clear();
-}