refactor: inline Metro integration into bundler

packages/bundler-metro/babel-transformer.cjs

@@ -0,0 +1,24 @@
+const { rnHarnessPlugins } = require('@react-native-harness/babel-preset');
+
+const transform = (args) => {
+  const { plugins } = args;
+  const upstreamTransformerPath =
+    process.env.RN_HARNESS_UPSTREAM_TRANSFORMER_PATH;
+
+  if (!upstreamTransformerPath || typeof upstreamTransformerPath !== 'string') {
+    throw new Error('Upstream transformer path is not a string');
+  }
+
+  const upstreamTransformer = require(upstreamTransformerPath);
+  const pluginsWithHarness = [
+    ...((plugins ?? [])),
+    ...rnHarnessPlugins,
+  ];
+
+  return upstreamTransformer.transform({
+    ...args,
+    plugins: pluginsWithHarness,
+  });
+};
+
+module.exports = { transform };

packages/bundler-metro/eslint.config.mjs

@@ -9,6 +9,7 @@ export default [
         'error',
         {
           ignoredFiles: ['{projectRoot}/eslint.config.{js,cjs,mjs,ts,cts,mts}'],
+          ignoredDependencies: ['@react-native-harness/babel-preset', 'vitest'],
         },
       ],
     },

packages/bundler-metro/package.json

@@ -16,21 +16,27 @@
     }
   },
   "dependencies": {
-    "@react-native-harness/metro": "workspace:*",
+    "@react-native/metro-config": "*",
+    "@react-native-harness/babel-preset": "workspace:*",
     "@react-native-harness/tools": "workspace:*",
     "@react-native-harness/config": "workspace:*",
+    "@react-native-harness/runtime": "workspace:*",
     "connect": "^3.7.0",
     "nocache": "^4.0.0",
     "tslib": "^2.3.0"
   },
   "peerDependencies": {
     "metro": "*",
-    "metro-config": "*"
+    "metro-cache": "*",
+    "metro-config": "*",
+    "metro-resolver": "*"
   },
   "devDependencies": {
     "@types/connect": "^3.4.38",
     "metro": "*",
-    "metro-config": "*"
+    "metro-cache": "*",
+    "metro-config": "*",
+    "metro-resolver": "*"
   },
   "license": "MIT"
 }

packages/metro/src/__tests__/paths.test.ts → packages/bundler-metro/src/__tests__/paths.test.ts

@@ -12,7 +12,9 @@ import {
 const tempDirs: string[] = [];
 
 const createTempProjectRoot = (): string => {
-  const tempDir = fs.mkdtempSync(path.join(os.tmpdir(), 'rn-harness-metro-'));
+  const tempDir = fs.mkdtempSync(
+    path.join(os.tmpdir(), 'rn-harness-bundler-metro-')
+  );
   tempDirs.push(tempDir);
   return tempDir;
 };
@@ -23,7 +25,7 @@ afterEach(() => {
   }
 });
 
-describe('metro paths', () => {
+describe('bundler metro paths', () => {
   it('resolves the harness root under the project root', () => {
     const projectRoot = createTempProjectRoot();
 

packages/bundler-metro/src/babel-transformer.ts

@@ -0,0 +1,16 @@
+import { fileURLToPath } from 'node:url';
+import { MetroConfig } from '@react-native/metro-config';
+
+export const getHarnessBabelTransformerPath = (
+  metroConfig: MetroConfig
+): string => {
+  const upstreamTransformerPath = metroConfig.transformer?.babelTransformerPath;
+
+  if (!upstreamTransformerPath || typeof upstreamTransformerPath !== 'string') {
+    throw new Error('Upstream transformer path is not a string');
+  }
+
+  process.env.RN_HARNESS_UPSTREAM_TRANSFORMER_PATH = upstreamTransformerPath;
+
+  return fileURLToPath(new URL('../babel-transformer.cjs', import.meta.url));
+};

packages/bundler-metro/src/factory.ts

@@ -1,4 +1,3 @@
-import { withRnHarness } from '@react-native-harness/metro';
 import { logger } from '@react-native-harness/tools';
 import type { Server as HttpServer } from 'node:http';
 import type { Server as HttpsServer } from 'node:https';
@@ -15,6 +14,7 @@ import {
 } from './reporter.js';
 import { getExpoMiddleware } from './middlewares/expo-middleware.js';
 import { getStatusMiddleware } from './middlewares/status-middleware.js';
+import { withRnHarness } from './withRnHarness.js';
 
 const waitForBundler = async (
   reporter: Reporter,

packages/metro/src/getHarnessSerializer.ts → packages/bundler-metro/src/getHarnessSerializer.ts

@@ -1,11 +1,16 @@
+import { createRequire } from 'node:module';
 import type { MetroConfig } from 'metro-config';
 
+const require = createRequire(import.meta.url);
+
 export type Serializer = NonNullable<
   NonNullable<MetroConfig['serializer']>['customSerializer']
 >;
 
 const getBaseSerializer = (): Serializer => {
-  const baseJSBundle = require('metro/private/DeltaBundler/Serializers/baseJSBundle');
+  const baseJSBundle = require(
+    'metro/private/DeltaBundler/Serializers/baseJSBundle'
+  );
   const bundleToString = require('metro/private/lib/bundleToString');
 
   return (entryPoint, prepend, graph, bundleOptions) =>
@@ -20,19 +25,16 @@ export const getHarnessSerializer = (): Serializer => {
 
   return async (entryPoint, preModules, graph, options) => {
     if (options.modulesOnly) {
-      // This is most likely a test file
       return baseSerializer(entryPoint, preModules, graph, {
         ...options,
         processModuleFilter: (mod) => {
           if (
             options.processModuleFilter &&
             !options.processModuleFilter(mod)
           ) {
-            // If the module is not allowed by the processModuleFilter, skip it
             return false;
           }
 
-          // If the module is in the main entry point, skip it
           return !mainEntryPointModules.has(mod.path);
         },
       });

packages/bundler-metro/src/index.ts

@@ -2,3 +2,4 @@ export { getMetroInstance } from './factory.js';
 export type { MetroInstance, MetroFactory, MetroOptions } from './types.js';
 export { prewarmMetroBundle } from './prewarm.js';
 export type { Reporter, ReportableEvent } from './reporter.js';
+export { isMetroCacheReusable } from './paths.js';

packages/metro/src/jest-globals-mock.ts → packages/bundler-metro/src/jest-globals-mock.ts

@@ -1,5 +1,5 @@
 // Mock module for @jest/globals imports
-// This module throws immediately when imported to warn users about using Jest APIs
+// This module throws immediately to explain the supported import path.
 
 throw new Error(
   "Importing '@jest/globals' is not supported in Harness tests. Import from 'react-native-harness' instead."

packages/metro/src/manifest.ts → packages/bundler-metro/src/manifest.ts

@@ -1,7 +1,7 @@
-import path from 'node:path';
 import fs from 'node:fs';
+import path from 'node:path';
 import { Config as HarnessConfig } from '@react-native-harness/config';
-import { getHarnessManifestPath } from './paths';
+import { getHarnessManifestPath } from './paths.js';
 
 const getManifestContent = (harnessConfig: HarnessConfig): string => {
   return `global.RN_HARNESS = { 

packages/metro/src/metro-cache.ts → packages/bundler-metro/src/metro-cache.ts

@@ -1,8 +1,8 @@
-import { CacheStore, MetroCache } from 'metro-cache';
-import type { MixedOutput, TransformResult } from 'metro';
 import fs from 'node:fs';
 import type { CacheStoresConfigT } from 'metro-config';
-import { getHarnessMetroCachePath } from './paths';
+import { CacheStore, MetroCache } from 'metro-cache';
+import type { MixedOutput, TransformResult } from 'metro';
+import { getHarnessMetroCachePath } from './paths.js';
 
 export const getHarnessCacheStores = (): ((
   metroCache: MetroCache

packages/metro/src/resolvers/composite-resolver.ts → packages/bundler-metro/src/resolvers/composite-resolver.ts

@@ -1,4 +1,4 @@
-import type { HarnessResolver, MetroResolver } from './types';
+import type { HarnessResolver, MetroResolver } from './types.js';
 
 export const createHarnessResolver = (
   resolvers: HarnessResolver[]

packages/metro/src/resolvers/resolver.ts → packages/bundler-metro/src/resolvers/resolver.ts

@@ -1,36 +1,47 @@
+import { createRequire } from 'node:module';
+import path from 'node:path';
 import type { MetroConfig } from '@react-native/metro-config';
 import type { Config as HarnessConfig } from '@react-native-harness/config';
-import path from 'node:path';
-import { createHarnessResolver } from './composite-resolver';
-import { createTsConfigResolver } from './tsconfig-resolver';
-import type { HarnessResolver, MetroResolver } from './types';
+import { createHarnessResolver } from './composite-resolver.js';
+import { createTsConfigResolver } from './tsconfig-resolver.js';
+import type { HarnessResolver, MetroResolver } from './types.js';
 
-// Safely resolves a path and strips its extension
-const getExtensionlessAbsolutePath = (basePath: string, relativePath = ''): string => {
+const require = createRequire(import.meta.url);
+
+const getExtensionlessAbsolutePath = (
+  basePath: string,
+  relativePath = ''
+): string => {
   const fullPath = path.resolve(basePath, relativePath);
   const parsed = path.parse(fullPath);
   return path.join(parsed.dir, parsed.name);
-}
+};
 
-export const createHarnessEntryPointResolver = (harnessConfig: HarnessConfig): HarnessResolver => {
+export const createHarnessEntryPointResolver = (
+  harnessConfig: HarnessConfig
+): HarnessResolver => {
   const rootPath = path.resolve(process.cwd());
-  const expectedEntryPoint = getExtensionlessAbsolutePath(rootPath, harnessConfig.entryPoint);
-  const resolvedHarnessPath = require.resolve('@react-native-harness/runtime/entry-point');
+  const expectedEntryPoint = getExtensionlessAbsolutePath(
+    rootPath,
+    harnessConfig.entryPoint
+  );
+  const resolvedHarnessPath = require.resolve(
+    '@react-native-harness/runtime/entry-point'
+  );
 
-  return (context, moduleName, _platform) => {
-    // 1. Resolve the origin path of the file making the import
+  return (context, moduleName, platform) => {
+    void platform;
     const currentOrigin = path.resolve(context.originModulePath);
 
-    // Fast Fail: If the import isn't happening from the root directory, skip it immediately
     if (currentOrigin !== rootPath) {
       return null;
     }
 
-    // 2. Resolve the module being imported and strip its extension
-    // This safely normalizes './index', './index.js', 'index.js', etc.
-    const requestedModule = getExtensionlessAbsolutePath(currentOrigin, moduleName);
+    const requestedModule = getExtensionlessAbsolutePath(
+      currentOrigin,
+      moduleName
+    );
 
-    // 3. String comparison
     if (requestedModule === expectedEntryPoint) {
       return {
         type: 'sourceFile',
@@ -43,12 +54,12 @@ export const createHarnessEntryPointResolver = (harnessConfig: HarnessConfig): H
 };
 
 export const createJestGlobalsResolver = (): HarnessResolver => {
-  return (_context, moduleName, _platform) => {
-    // Intercept @jest/globals imports and redirect to mock module
+  return (_context, moduleName, platform) => {
+    void platform;
     if (moduleName === '@jest/globals') {
       return {
         type: 'sourceFile',
-        filePath: require.resolve('../jest-globals-mock'),
+        filePath: require.resolve('../jest-globals-mock.js'),
       };
     }
 
@@ -64,7 +75,8 @@ export const createJsxRuntimeResolver = (): HarnessResolver => {
     '@react-native-harness/runtime/jsx-dev-runtime'
   );
 
-  return (_context, moduleName, _platform) => {
+  return (_context, moduleName, platform) => {
+    void platform;
     if (moduleName === '@react-native-harness/runtime/jsx-runtime') {
       return {
         type: 'sourceFile',