fix: lazily evaluate Platform in Settings to resolve circular dependency (#56967)

Author: Siddesh Gawande

packages/react-native/Libraries/Settings/Settings.js

@@ -4,24 +4,49 @@
  * This source code is licensed under the MIT license found in the
  * LICENSE file in the root directory of this source tree.
  *
- * @flow
+ * @flow strict-local
  * @format
  */
 
-import Platform from '../Utilities/Platform';
-
-let Settings: {
+type SettingsStatic = {
   get(key: string): any,
   set(settings: Object): void,
   watchKeys(keys: string | Array<string>, callback: () => void): number,
   clearWatch(watchId: number): void,
   ...
 };
 
-if (Platform.OS === 'ios') {
-  Settings = require('./Settings').default;
-} else {
-  Settings = require('./SettingsFallback').default;
+let SettingsImpl: ?SettingsStatic = null;
+
+function getSettings(): SettingsStatic {
+  if (SettingsImpl != null) {
+    return SettingsImpl;
+  }
+  const Platform = require('../Utilities/Platform').default;
+  if (Platform.OS === 'ios') {
+    SettingsImpl = require('./Settings').default;
+  } else {
+    SettingsImpl = require('./SettingsFallback').default;
+  }
+  return (SettingsImpl: SettingsStatic);
 }
 
+const Settings: SettingsStatic = {
+  get(key: string): any {
+    return getSettings().get(key);
+  },
+
+  set(settings: Object): void {
+    getSettings().set(settings);
+  },
+
+  watchKeys(keys: string | Array<string>, callback: () => void): number {
+    return getSettings().watchKeys(keys, callback);
+  },
+
+  clearWatch(watchId: number): void {
+    getSettings().clearWatch(watchId);
+  },
+};
+
 export default Settings;

packages/react-native/Libraries/Settings/__tests__/Settings-test.js

@@ -0,0 +1,107 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict-local
+ * @format
+ */
+
+describe('Settings', () => {
+  beforeEach(() => {
+    jest.resetModules();
+  });
+
+  describe('lazy Platform initialization', () => {
+    it('does not access Platform when the module is first loaded', () => {
+      let platformAccessCount = 0;
+      jest.doMock('../../Utilities/Platform', () => ({
+        __esModule: true,
+        get default() {
+          platformAccessCount++;
+          return {OS: 'android'};
+        },
+      }));
+
+      require('../Settings.js');
+
+      expect(platformAccessCount).toBe(0);
+
+      const warnSpy = jest.spyOn(console, 'warn').mockImplementation(() => {});
+      const Settings = require('../Settings.js').default;
+      Settings.get('any');
+      expect(platformAccessCount).toBeGreaterThan(0);
+      warnSpy.mockRestore();
+    });
+
+    it('can be required when Platform is undefined during module load', () => {
+      jest.doMock('../../Utilities/Platform', () => ({
+        __esModule: true,
+        default: undefined,
+      }));
+
+      expect(() => {
+        require('../Settings.js');
+      }).not.toThrow();
+    });
+  });
+
+  describe('platform-specific implementation', () => {
+    it('uses fallback implementation on Android', () => {
+      jest.doMock('../../Utilities/Platform', () => ({
+        __esModule: true,
+        default: {OS: 'android'},
+      }));
+
+      const warnSpy = jest.spyOn(console, 'warn').mockImplementation(() => {});
+      const Settings = require('../Settings.js').default;
+
+      expect(Settings.get('foo')).toBeNull();
+      expect(Settings.watchKeys('foo', () => {})).toBe(-1);
+      expect(warnSpy).toHaveBeenCalled();
+      warnSpy.mockRestore();
+    });
+
+    it('delegates get and set to the iOS implementation', () => {
+      const setValues = jest.fn();
+      jest.doMock('../../Utilities/Platform', () => ({
+        __esModule: true,
+        default: {OS: 'ios'},
+      }));
+      jest.doMock('../NativeSettingsManager', () => ({
+        __esModule: true,
+        default: {
+          getConstants: () => ({settings: {myKey: 'initial'}}),
+          setValues,
+        },
+      }));
+
+      const Settings = require('../Settings.js').default;
+
+      expect(Settings.get('myKey')).toBe('initial');
+      Settings.set({myKey: 'updated'});
+      expect(Settings.get('myKey')).toBe('updated');
+      expect(setValues).toHaveBeenCalledWith({myKey: 'updated'});
+    });
+
+    it('caches the platform implementation across calls', () => {
+      let platformAccessCount = 0;
+      jest.doMock('../../Utilities/Platform', () => ({
+        __esModule: true,
+        get default() {
+          platformAccessCount++;
+          return {OS: 'android'};
+        },
+      }));
+
+      const warnSpy = jest.spyOn(console, 'warn').mockImplementation(() => {});
+      const Settings = require('../Settings.js').default;
+
+      Settings.get('a');
+      Settings.get('b');
+      expect(platformAccessCount).toBe(1);
+      warnSpy.mockRestore();
+    });
+  });
+});