feat: implement Metro caching (#23)

This pull request adds support for Metro's transformation cache. It may help in cases when Metro struggles with re-transforming the same files over and over.

Author: V3RON

.github/workflows/e2e-tests.yml

@@ -66,6 +66,14 @@ jobs:
           node-version: '24.10.0'
           cache: 'pnpm'
 
+      - name: Metro cache
+        uses: actions/cache@v4
+        with:
+          path: apps/${{ matrix.app }}/node_modules/.cache/rn-harness/metro-cache
+          key: metro-cache-${{ hashFiles('apps/${{ matrix.app }}/node_modules/.cache/rn-harness/metro-cache/**/*') }}
+          restore-keys: |
+            metro-cache
+
       - name: Install dependencies
         run: |
           pnpm install
@@ -169,6 +177,14 @@ jobs:
           node-version: '24.10.0'
           cache: 'pnpm'
 
+      - name: Metro cache
+        uses: actions/cache@v4
+        with:
+          path: apps/${{ matrix.app }}/node_modules/.cache/rn-harness/metro-cache
+          key: metro-cache-${{ hashFiles('apps/${{ matrix.app }}/node_modules/.cache/rn-harness/metro-cache/**/*') }}
+          restore-keys: |
+            metro-cache
+
       - name: Install Watchman
         run: brew install watchman
 

packages/config/src/types.ts

@@ -15,6 +15,7 @@ export const ConfigSchema = z
 
     resetEnvironmentBetweenTestFiles: z.boolean().optional().default(true),
     unstable__skipAlreadyIncludedModules: z.boolean().optional().default(false),
+    unstable__enableMetroCache: z.boolean().optional().default(false),
 
     // Deprecated property - used for migration detection
     include: z.array(z.string()).optional(),

packages/metro/src/metro-cache.ts

@@ -0,0 +1,23 @@
+import { CacheStore, MetroCache } from 'metro-cache';
+import type { MixedOutput, TransformResult } from 'metro';
+import fs from 'node:fs';
+import path from 'node:path';
+
+const CACHE_ROOT = path.resolve(
+  process.cwd(),
+  'node_modules/.cache/rn-harness/metro-cache'
+);
+
+export const getHarnessCacheStores = (): ((
+  metroCache: MetroCache
+) => CacheStore<TransformResult<MixedOutput>>[]) => {
+  return ({ FileStore }) => {
+    fs.mkdirSync(CACHE_ROOT, { recursive: true });
+
+    return [
+      new FileStore({ root: CACHE_ROOT }) as CacheStore<
+        TransformResult<MixedOutput>
+      >,
+    ];
+  };
+};

packages/metro/src/utils.ts

@@ -0,0 +1,3 @@
+export type NotReadOnly<T> = {
+  -readonly [K in keyof T]: T[K];
+};

packages/metro/src/withRnHarness.ts

@@ -1,9 +1,11 @@
-import { MetroConfig } from '@react-native/metro-config';
+import type { MetroConfig } from '@react-native/metro-config';
 import { getConfig } from '@react-native-harness/config';
 import { patchModuleSystem } from './moduleSystem';
 import { getHarnessResolver } from './resolver';
 import { getHarnessManifest } from './manifest';
 import { getHarnessBabelTransformerPath } from './babel-transformer';
+import { getHarnessCacheStores } from './metro-cache';
+import type { NotReadOnly } from './utils';
 
 const INTERNAL_CALLSITES_REGEX =
   /(^|[\\/])(node_modules[/\\]@react-native-harness)([\\/]|$)/;
@@ -29,7 +31,7 @@ export const withRnHarness = (
     const harnessBabelTransformerPath =
       getHarnessBabelTransformerPath(metroConfig);
 
-    const patchedConfig: MetroConfig = {
+    const patchedConfig: NotReadOnly<MetroConfig> = {
       ...metroConfig,
       cacheVersion: 'react-native-harness',
       serializer: {
@@ -83,6 +85,10 @@ export const withRnHarness = (
       },
     };
 
+    if (harnessConfig.unstable__enableMetroCache) {
+      patchedConfig.cacheStores = getHarnessCacheStores();
+    }
+
     if (harnessConfig.unstable__skipAlreadyIncludedModules) {
       patchedConfig.serializer!.customSerializer =
         require('./getHarnessSerializer').getHarnessSerializer();