Add waitForBundler option to API

Summary:
Adds the `waitForBundler` option to `runServer()` and `runMetro()`. If set to `true`, this option causes Metro to fully initialise the bundler (crawling the filesystem, creating the transformer, etc) before setting up the HTTP server. If `false` (the default), the HTTP server and bundler are initialised in parallel, which means the first bundle request might need to wait for initialisation to finish.

This option is primarily useful for "prewarming" the bundler (in particular the dependency graph / Haste map) by programmatically creating a short-lived server instance.

Reviewed By: feedthejim

Differential Revision: D31354827

fbshipit-source-id: c24ae44c7724a5849eac9a92dab592915a99c4d3

Author: motiz88

docs/GettingStarted.md

@@ -87,6 +87,7 @@ We recommend using `runMetro` instead of `runServer`, `runMetro` calls this func
 * `secureKey (string)`: **DEPRECATED** The key to use for `https` when `secure` is on.
 * `secureCert (string)`: **DEPRECATED** The cert to use for `https` when `secure` is on.
 * `secureServerOptions (Object)`: The options object to pass to the Metro's https server. The presence of this object will make Metro's server run on `https`. Refer to the [nodejs docs](https://nodejs.org/api/https.html#https_https_createserver_options_requestlistener) for valid options.
+* `waitForBundler (boolean)`: Whether to wait for the bundler to finish initializing before returning the server instance.
 
 ```js
 const config = await Metro.loadConfig();

packages/metro/src/index.js

@@ -47,6 +47,11 @@ type MetroMiddleWare = {|
   middleware: Middleware,
 |};
 
+export type RunMetroOptions = {
+  ...ServerOptions,
+  waitForBundler?: boolean,
+};
+
 export type RunServerOptions = {|
   hasReducedPerformance?: boolean,
   host?: string,
@@ -57,6 +62,7 @@ export type RunServerOptions = {|
   secure?: boolean, // deprecated
   secureCert?: string, // deprecated
   secureKey?: string, // deprecated
+  waitForBundler?: boolean,
   websocketEndpoints?: {
     [path: string]: typeof ws.Server,
   },
@@ -115,7 +121,7 @@ async function getConfig(config: InputConfigT): Promise<ConfigT> {
 
 async function runMetro(
   config: InputConfigT,
-  options?: ServerOptions,
+  options?: RunMetroOptions,
 ): Promise<MetroServer> {
   const mergedConfig = await getConfig(config);
   const {
@@ -131,9 +137,10 @@ async function runMetro(
     type: 'initialize_started',
   });
 
-  const server = new MetroServer(mergedConfig, options);
+  const {waitForBundler = false, ...serverOptions} = options ?? {};
+  const server = new MetroServer(mergedConfig, serverOptions);
 
-  server
+  const readyPromise = server
     .ready()
     .then(() => {
       reporter.update({
@@ -148,6 +155,9 @@ async function runMetro(
         error,
       });
     });
+  if (waitForBundler) {
+    await readyPromise;
+  }
 
   return server;
 }
@@ -157,7 +167,7 @@ exports.loadConfig = loadConfig;
 
 exports.createConnectMiddleware = async function(
   config: ConfigT,
-  options?: ServerOptions,
+  options?: RunMetroOptions,
 ): Promise<MetroMiddleWare> {
   const metroServer = await runMetro(config, options);
 
@@ -210,6 +220,7 @@ exports.runServer = async (
     secure, //deprecated
     secureCert, // deprecated
     secureKey, // deprecated
+    waitForBundler = false,
     websocketEndpoints = {},
   }: RunServerOptions,
 ): Promise<HttpServer | HttpsServer> => {
@@ -231,6 +242,7 @@ exports.runServer = async (
     config,
     {
       hasReducedPerformance,
+      waitForBundler,
     },
   );