feat: add Metro bundle pre-warming

packages/bundler-metro/src/index.ts

@@ -1,3 +1,4 @@
 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';

packages/bundler-metro/src/prewarm.ts

@@ -0,0 +1,36 @@
+import { METRO_PORT } from './constants.js';
+import { getResolvedEntryPointWithoutExtension } from './entry-point-utils.js';
+
+type PrewarmOptions = {
+  projectRoot: string;
+  entryPoint: string;
+  platform: string;
+  dev: boolean;
+  minify: boolean;
+  signal: AbortSignal;
+};
+
+export const prewarmMetroBundle = async (
+  options: PrewarmOptions
+): Promise<void> => {
+  const { projectRoot, entryPoint, platform, dev, minify, signal } = options;
+  const resolvedEntryPoint = getResolvedEntryPointWithoutExtension(
+    projectRoot,
+    entryPoint
+  );
+  const searchParams = new URLSearchParams({
+    platform,
+    dev: String(dev),
+    minify: String(minify),
+  });
+  const url = `http://localhost:${METRO_PORT}/${resolvedEntryPoint}.bundle?${searchParams.toString()}`;
+
+  const response = await fetch(url, { signal });
+
+  if (!response.ok) {
+    const snippet = (await response.text()).trim();
+    throw new Error(
+      `Metro pre-warm failed (${response.status} ${response.statusText}). ${snippet}`
+    );
+  }
+};

packages/config/src/types.ts

@@ -10,6 +10,7 @@ const RunnerSchema = z.object({
     ),
   config: z.record(z.any()),
   runner: z.string(),
+  platformId: z.string(),
 });
 
 export const ConfigSchema = z

packages/jest/src/harness.ts

@@ -13,13 +13,15 @@ import {
 } from '@react-native-harness/platforms';
 import {
   getMetroInstance,
+  prewarmMetroBundle,
   Reporter,
   ReportableEvent,
 } from '@react-native-harness/bundler-metro';
 import { InitializationTimeoutError, MaxAppRestartsError } from './errors.js';
 import { Config as HarnessConfig } from '@react-native-harness/config';
 import { createCrashMonitor, CrashMonitor } from './crash-monitor.js';
 import { createClientLogListener } from './client-log-handler.js';
+import { logMetroPrewarmCompleted } from './logs.js';
 
 export type HarnessRunTestsOptions = Exclude<TestExecutionOptions, 'platform'>;
 
@@ -176,6 +178,16 @@ const getHarnessInternal = async (
   }
 
   try {
+    await prewarmMetroBundle({
+      projectRoot,
+      entryPoint: config.entryPoint,
+      platform: platform.platformId,
+      dev: true,
+      minify: false,
+      signal,
+    });
+    logMetroPrewarmCompleted(platform);
+
     await waitForAppReady({
       metroEvents: metroInstance.events,
       serverBridge,

packages/jest/src/logs.ts

@@ -25,6 +25,12 @@ export const logTestEnvironmentReady = (runner: HarnessPlatform): void => {
   log(`${TAG} Runner ${chalk.bold(runner.name)} ready\n`);
 };
 
+export const logMetroPrewarmCompleted = (runner: HarnessPlatform): void => {
+  log(
+    `${TAG} Metro pre-warm for ${chalk.bold(runner.name)} completed\n`
+  );
+};
+
 export const getErrorMessage = (error: HarnessError): string => {
   return `${ERROR_TAG} ${error.message}\n`;
 };

packages/platform-android/src/factory.ts

@@ -30,4 +30,5 @@ export const androidPlatform = (
   name: config.name,
   config,
   runner: import.meta.resolve('./runner.js'),
+  platformId: 'android',
 });

packages/platform-ios/src/factory.ts

@@ -25,4 +25,5 @@ export const applePlatform = (
   name: config.name,
   config,
   runner: import.meta.resolve('./runner.js'),
+  platformId: 'ios',
 });

packages/platform-vega/src/factory.ts

@@ -12,4 +12,5 @@ export const vegaPlatform = (
   name: config.name,
   config,
   runner: import.meta.resolve('./runner.js'),
+  platformId: 'vega',
 });

packages/platform-web/src/factory.ts

@@ -7,6 +7,7 @@ export const webPlatform = (
   name: config.name,
   config,
   runner: import.meta.resolve('./runner.js'),
+  platformId: 'web',
 });
 
 export const chromium = (

packages/platforms/src/types.ts

@@ -10,6 +10,7 @@ export type HarnessPlatform<TConfig = Record<string, unknown>> = {
   name: string;
   config: TConfig;
   runner: string;
+  platformId: string;
 };
 
 export type RunTarget = {