Preserve lazy ViewManager initialization in UIManager

packages/react-native/Libraries/ReactNative/UIManager.js

@@ -23,9 +23,7 @@ const UIManagerImpl: UIManagerJSInterface =
     ? require('./BridgelessUIManager').default
     : require('./PaperUIManager').default;
 
-// $FlowFixMe[cannot-spread-interface]
-const UIManager: UIManagerJSInterface = {
-  ...UIManagerImpl,
+const UIManagerOverrides = {
   /**
    * Determines the location on screen, width, and height of the given view and
    * returns the values via an async callback. If successful, the callback will
@@ -243,4 +241,19 @@ const UIManager: UIManagerJSInterface = {
   },
 };
 
+// Copy property descriptors instead of spreading UIManagerImpl. In Bridgeless
+// mode, UIManagerImpl may define enumerable lazy ViewManager getters. Spreading
+// the object would invoke every getter during module initialization.
+// $FlowFixMe[incompatible-type] Flow cannot infer properties from descriptors.
+const UIManager: UIManagerJSInterface = Object.create(
+  Object.getPrototypeOf(UIManagerImpl),
+  {
+    ...Object.getOwnPropertyDescriptors(
+      // $FlowFixMe[class-object-subtyping] UIManagerImpl is a runtime object.
+      UIManagerImpl,
+    ),
+    ...Object.getOwnPropertyDescriptors(UIManagerOverrides),
+  },
+);
+
 export default UIManager;

packages/react-native/Libraries/ReactNative/__tests__/UIManager-test.js

@@ -0,0 +1,74 @@
+/**
+ * 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
+ */
+
+'use strict';
+
+jest.mock('../../Utilities/Platform', () => ({
+  OS: 'android',
+  select: spec => spec.android ?? spec.native ?? spec.default,
+}));
+jest.unmock('../UIManager');
+
+describe('UIManager', () => {
+  it('preserves lazy ViewManager getters when composing the Bridgeless implementation', () => {
+    const originalBridgelessValue = global.RN$Bridgeless;
+    const originalGetConstants = global.RN$LegacyInterop_UIManager_getConstants;
+    const originalGetConstantsForViewManager =
+      global.RN$LegacyInterop_UIManager_getConstantsForViewManager;
+    const originalGetDefaultEventTypes =
+      global.RN$LegacyInterop_UIManager_getDefaultEventTypes;
+    const viewManagerConfig = {Commands: {focus: 1}};
+    const getConstants = jest.fn(() => ({
+      LazyViewManagersEnabled: true,
+      ViewManagerNames: ['RCTTestView'],
+    }));
+    const getConstantsForViewManager = jest.fn(() => viewManagerConfig);
+
+    jest.resetModules();
+    try {
+      // $FlowExpectedError[cannot-write]
+      global.RN$Bridgeless = true;
+      // $FlowExpectedError[cannot-write]
+      global.RN$LegacyInterop_UIManager_getConstants = getConstants;
+      // $FlowExpectedError[cannot-write]
+      global.RN$LegacyInterop_UIManager_getConstantsForViewManager =
+        getConstantsForViewManager;
+      // $FlowExpectedError[cannot-write]
+      global.RN$LegacyInterop_UIManager_getDefaultEventTypes = jest.fn(
+        () => ({}),
+      );
+
+      const UIManager = require('../UIManager').default;
+
+      expect(getConstants).toHaveBeenCalledTimes(1);
+      expect(getConstantsForViewManager).not.toHaveBeenCalled();
+      expect(
+        Object.getOwnPropertyDescriptor(UIManager, 'RCTTestView'),
+      ).toBeDefined();
+      expect(Object.keys(UIManager)).toContain('RCTTestView');
+
+      expect(Reflect.get(UIManager, 'RCTTestView')).toBe(viewManagerConfig);
+      expect(Reflect.get(UIManager, 'RCTTestView')).toBe(viewManagerConfig);
+      expect(getConstantsForViewManager).toHaveBeenCalledTimes(1);
+    } finally {
+      // $FlowExpectedError[cannot-write]
+      global.RN$Bridgeless = originalBridgelessValue;
+      // $FlowExpectedError[cannot-write]
+      global.RN$LegacyInterop_UIManager_getConstants = originalGetConstants;
+      // $FlowExpectedError[cannot-write]
+      global.RN$LegacyInterop_UIManager_getConstantsForViewManager =
+        originalGetConstantsForViewManager;
+      // $FlowExpectedError[cannot-write]
+      global.RN$LegacyInterop_UIManager_getDefaultEventTypes =
+        originalGetDefaultEventTypes;
+      jest.resetModules();
+    }
+  });
+});