Refactor test setup to avoid importing the Fantom module before the body of the test (#51696)

Summary:
Pull Request resolved: #51696

Changelog: [internal]

I realized that the Fantom module was being initialized before the execution of the test module itself (as part of the code generated by the runner), which could have problems if the test sets up the global environment that Fantom consumes. For example, if Fantom needs to use the `EventTarget` type from the global scope, it needs to wait until the initialization of the runtime so it can use it safely.

This refactors all the code calling into Fantom as part of our infra to always initialize it lazily, so the first thing that runs as part of the test execution is the test itself (apart from our test setup, which is supposed to be side-effect free).

Reviewed By: sammy-SC

Differential Revision: D75681181

fbshipit-source-id: 91e4b903a49fcee59c5875e73db314cde0adea03

Author: rubennorte

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;