Refactor test setup to avoid importing the Fantom module before the body of the test

packages/react-native-fantom/runner/entrypoint-template.js

@@ -40,7 +40,8 @@ module.exports = function entrypointTemplate({
  */
 
 import {registerTest} from '${setupModulePath}';
-import {setConstants} from '@react-native/fantom';
+import {setConstants} from '@react-native/fantom/src/Constants';
+
 ${
   Object.keys(testConfig.flags.jsOnly).length > 0
     ? `import * as ReactNativeFeatureFlags from '${featureFlagsModulePath}';

packages/react-native-fantom/runtime/patchWeakRef.js

@@ -8,8 +8,6 @@
  * @format
  */
 
-import * as Fantom from '@react-native/fantom';
-
 let initialized = false;
 
 /**
@@ -31,6 +29,9 @@ export default function patchWeakRef(): void {
 
   // $FlowExpectedError[cannot-write]
   WeakRef.prototype.deref = function patchedDeref<T>(this: WeakRef<T>): T {
+    // Lazily require the module to avoid loading it before the test logic.
+    const Fantom = require('@react-native/fantom');
+
     if (!Fantom.isInWorkLoop()) {
       throw new Error(
         'Unexpected call to `WeakRef.deref()` outside of the Event Loop. Please use this method within `Fantom.runTask()`.',
@@ -43,6 +44,9 @@ export default function patchWeakRef(): void {
   const OriginalWeakRef = WeakRef;
 
   global.WeakRef = function WeakRef(...args) {
+    // Lazily require the module to avoid loading it before the test logic.
+    const Fantom = require('@react-native/fantom');
+
     if (!Fantom.isInWorkLoop()) {
       throw new Error(
         'Unexpected instantiation of `WeakRef` outside of the Event Loop. Please create the instance within `Fantom.runTask()`.',

packages/react-native-fantom/runtime/setup.js

@@ -14,7 +14,6 @@ import expect from './expect';
 import {createMockFunction} from './mocks';
 import patchWeakRef from './patchWeakRef';
 import {setupSnapshotConfig, snapshotContext} from './snapshotContext';
-import NativeFantom from 'react-native/src/private/testing/fantom/specs/NativeFantom';
 
 export type TestCaseResult = {
   ancestorTitles: Array<string>,
@@ -380,6 +379,10 @@ function runSuite(suite: Suite): TestCaseResult[] {
 }
 
 function reportTestSuiteResult(testSuiteResult: TestSuiteResult): void {
+  // Force the import of the native module to be lazy
+  const NativeFantom =
+    require('react-native/src/private/testing/fantom/specs/NativeFantom').default;
+
   NativeFantom.reportTestSuiteResultsJSON(
     JSON.stringify({
       type: 'test-result',
@@ -389,6 +392,10 @@ function reportTestSuiteResult(testSuiteResult: TestSuiteResult): void {
 }
 
 function validateEmptyMessageQueue(): void {
+  // Force the import of the native module to be lazy
+  const NativeFantom =
+    require('react-native/src/private/testing/fantom/specs/NativeFantom').default;
+
   NativeFantom.validateEmptyMessageQueue();
 }
 

packages/react-native-fantom/src/Constants.js

@@ -0,0 +1,27 @@
+/**
+ * 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
+ */
+
+type FantomConstants = $ReadOnly<{
+  isRunningFromCI: boolean,
+  fantomConfigSummary: string,
+}>;
+
+let constants: FantomConstants = {
+  isRunningFromCI: false,
+  fantomConfigSummary: '',
+};
+
+export function getConstants(): FantomConstants {
+  return constants;
+}
+
+export function setConstants(newConstants: FantomConstants): void {
+  constants = newConstants;
+}

packages/react-native-fantom/src/__tests__/FantomModuleInit-itest.js

@@ -0,0 +1,21 @@
+/**
+ * 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('FantomModuleInit', () => {
+  it('should not load the Fantom module eagerly', () => {
+    expect(global.__FANTOM_PACKAGE_LOADED__).toBeUndefined();
+  });
+
+  it('should load the Fantom module lazily', () => {
+    require('@react-native/fantom');
+
+    expect(global.__FANTOM_PACKAGE_LOADED__).toBe(true);
+  });
+});

packages/react-native-fantom/src/index.js

@@ -18,6 +18,7 @@ import type ReactNativeDocument from 'react-native/src/private/webapis/dom/nodes
 
 import ReactNativeElement from '../../react-native/src/private/webapis/dom/nodes/ReadOnlyNode';
 import * as Benchmark from './Benchmark';
+import {getConstants} from './Constants';
 import getFantomRenderedOutput from './getFantomRenderedOutput';
 import {LogBox} from 'react-native';
 import {createRootTag} from 'react-native/Libraries/ReactNative/RootTag';
@@ -38,6 +39,8 @@ export type RootConfig = {
   devicePixelRatio?: number,
 };
 
+export {getConstants} from './Constants';
+
 // Defaults use iPhone 14 values (very common device).
 const DEFAULT_VIEWPORT_WIDTH = 390;
 const DEFAULT_VIEWPORT_HEIGHT = 844;
@@ -520,24 +523,6 @@ export function enqueueModalSizeUpdate(
 
 export const unstable_benchmark = Benchmark;
 
-type FantomConstants = $ReadOnly<{
-  isRunningFromCI: boolean,
-  fantomConfigSummary: string,
-}>;
-
-let constants: FantomConstants = {
-  isRunningFromCI: false,
-  fantomConfigSummary: '',
-};
-
-export function getConstants(): FantomConstants {
-  return constants;
-}
-
-export function setConstants(newConstants: FantomConstants): void {
-  constants = newConstants;
-}
-
 /**
  * Quick and dirty polyfills required by tinybench.
  */
@@ -662,3 +647,5 @@ function runLogBoxCheck() {
     throw new Error(message);
   }
 }
+
+global.__FANTOM_PACKAGE_LOADED__ = true;