perf(dev): warm the fork pool on first file change instead of on ready

Author: danielroe

packages/nuxt-cli/src/commands/dev.ts

@@ -153,7 +153,7 @@ const command = defineCommand({
     }
 
     // Start the initial dev server in-process with listener
-    const { listener, close, reload, onRestart, onReady } = await initialize({ cwd, args: ctx.args }, {
+    const { listener, close, reload, onRestart, onReady, onFileChange } = await initialize({ cwd, args: ctx.args }, {
       data: ctx.data,
       listenOverrides,
       showBanner: true,
@@ -176,14 +176,18 @@ const command = defineCommand({
       inspect,
     })
 
-    // When ready, start warming up the fork pool
     onReady((_address) => {
-      pool.startWarming()
       if (startTime) {
         debug(`Dev server ready for connections in ${Date.now() - startTime}ms`)
       }
     })
 
+    // Warming costs a whole extra Node process, so wait until the user shows
+    // signs of editing the project rather than paying for it on every `nuxt dev`.
+    onFileChange(() => {
+      pool.startWarming()
+    })
+
     // On hard restart, use a fork from the pool
     let cleanupCurrentFork: (() => Promise<void>) | undefined
 

packages/nuxt-cli/src/dev/index.ts

@@ -63,6 +63,8 @@ interface InitializeReturn {
   /** Reload Nuxt in place, keeping the current listener. */
   reload: (reason?: string) => Promise<void>
   onReady: (callback: (address: string) => void) => void
+  /** Called the first time a watched file changes, before Nuxt reloads. */
+  onFileChange: (callback: () => void) => void
   onRestart: (callback: (devServer: NuxtDevServer) => void) => void
 }
 
@@ -171,6 +173,9 @@ export async function initialize(devContext: NuxtDevContext, ctx: InitializeOpti
         devServer.once('ready', payload => callback(payload))
       }
     },
+    onFileChange: (callback: () => void) => {
+      devServer.once('change', callback)
+    },
     onRestart: (callback: (devServer: NuxtDevServer) => void) => {
       let restarted = false
       function restart() {

packages/nuxt-cli/src/dev/utils.ts

@@ -153,6 +153,7 @@ interface DevServerEventMap {
   'loading': [loadingMessage: string]
   'ready': [address: string]
   'restart': []
+  'change': []
 }
 
 export class NuxtDevServer extends EventEmitter<DevServerEventMap> {
@@ -390,6 +391,10 @@ export class NuxtDevServer extends EventEmitter<DevServerEventMap> {
       throw new Error('Nuxt must be loaded before configuration')
     }
 
+    this.#currentNuxt.hooks.hook('builder:watch', () => {
+      this.emit('change')
+    })
+
     // Connect Vite HMR
     if (!process.env.NUXI_DISABLE_VITE_HMR) {
       this.#currentNuxt.hooks.hook('vite:extend', ({ config }) => {
@@ -613,7 +618,10 @@ export class NuxtDevServer extends EventEmitter<DevServerEventMap> {
       this.#cwd,
       this.options.dotenv.fileName,
       () => this.emit('restart'),
-      file => this.loadDebounced(true, `${file} updated`),
+      (file) => {
+        this.emit('change')
+        this.loadDebounced(true, `${file} updated`)
+      },
       getLocalLayerDirs(this.#currentNuxt?.options._layers ?? [], this.#cwd),
     )
   }