Add an internal Expo-project marker to the Uniwind Metro config and use it to lazily choose the correct transform worker.

CONTEXT.md

@@ -113,6 +113,7 @@ Metro integration:
 - `withUniwindConfig(config, uniwindConfig)` patches Metro graph support for uncached modules.
 - Metro adds `css` as source extension and removes it from asset extensions.
 - Metro transformer handles the configured CSS entry file specially.
+- Metro transformer worker selection is lazy, cached per Expo/non-Expo config type, and follows Expo transformer paths or Expo-specific config markers.
 - Native platform CSS transforms into a JS module that calls `Uniwind.__reinit(...)`.
 - Web platform CSS transforms into CSS plus web runtime setup.
 - Resolver swaps React Native component imports to Uniwind-aware implementations where needed.

packages/uniwind/src/bundler/adapters/metro/metro.ts

@@ -9,6 +9,29 @@ import { nativeResolver, webResolver } from './resolvers'
 
 const isUniwindRequest = (moduleName: string) => moduleName === 'uniwind' || moduleName.startsWith('uniwind/')
 
+type ExpoTransformerConfig = NonNullable<MetroConfig['transformer']> & {
+    _expoRelativeProjectRoot?: string
+    _expoRouterPath?: string
+    expo_customTransformerPath?: string | false
+    postcssHash?: string | null
+}
+
+const isExpoMetroConfig = (config: MetroConfig) => {
+    const transformerPath = config.transformerPath
+    const transformer = config.transformer as ExpoTransformerConfig | undefined
+    const hasExpoTransformerField = transformer
+        ? '_expoRelativeProjectRoot' in transformer
+            || '_expoRouterPath' in transformer
+            || 'expo_customTransformerPath' in transformer
+            || 'postcssHash' in transformer
+        : false
+
+    return Boolean(
+        transformerPath?.includes('@expo/metro-config')
+            || hasExpoTransformerField,
+    )
+}
+
 export const withUniwindConfig = <T extends MetroConfig>(
     config: T,
     uniwindConfig: UniwindConfig,
@@ -24,7 +47,10 @@ export const withUniwindConfig = <T extends MetroConfig>(
         transformerPath: require.resolve('./transformer.cjs'),
         transformer: {
             ...config.transformer,
-            uniwind: bundlerConfig.toMetroConfig(),
+            uniwind: {
+                ...bundlerConfig.toMetroConfig(),
+                isExpoProject: isExpoMetroConfig(config),
+            },
         },
         resolver: {
             ...config.resolver,

packages/uniwind/src/bundler/adapters/metro/transformer.ts

@@ -9,18 +9,31 @@ import path from 'path'
 
 const cssArtifactPath = path.resolve(__dirname, '../../uniwind.css')
 
-let worker: typeof MetroTransformWorker
+const workerCache = new Map<boolean, typeof MetroTransformWorker>()
 
-try {
-    try {
-        const { unstable_transformerPath } = require('@expo/metro-config') as typeof ExpoMetroConfig
+const getTransformWorker = (isExpoProject?: boolean): typeof MetroTransformWorker => {
+    const cacheKey = Boolean(isExpoProject)
+    const cachedWorker = workerCache.get(cacheKey)
 
-        worker = require(unstable_transformerPath)
-    } catch {
-        worker = require('@expo/metro-config/build/transform-worker/transform-worker.js')
+    if (cachedWorker) {
+        return cachedWorker
     }
-} catch {
-    worker = require('metro-transform-worker')
+
+    const resolvedWorker: typeof MetroTransformWorker = cacheKey
+        ? (() => {
+            try {
+                const { unstable_transformerPath } = require('@expo/metro-config') as typeof ExpoMetroConfig
+
+                return require(unstable_transformerPath)
+            } catch {
+                return require('@expo/metro-config/build/transform-worker/transform-worker.js')
+            }
+        })()
+        : require('metro-transform-worker')
+
+    workerCache.set(cacheKey, resolvedWorker)
+
+    return resolvedWorker
 }
 
 export const transform = async (
@@ -32,6 +45,7 @@ export const transform = async (
     data: Buffer,
     options: JsTransformOptions,
 ) => {
+    const worker = getTransformWorker(config.uniwind.isExpoProject)
     const isCss = options.type !== 'asset' && path.join(process.cwd(), config.uniwind.cssEntryFile) === path.join(projectRoot, filePath)
 
     if (filePath.endsWith('/components/web/metro-injected.js')) {

packages/uniwind/src/bundler/types.ts

@@ -11,5 +11,6 @@ export type Polyfills = {
 export type UniwindMetroConfig = UniwindConfig & {
     polyfills?: Polyfills
     debug?: boolean
+    isExpoProject?: boolean
     isTV?: boolean
 }