feat: new setupOnce utils

packages/vitest/src/index.ts

@@ -3,8 +3,9 @@ import type { MatcherState, MatchersObject } from './integrations/chai/types'
 import type { Constructable, InlineConfig } from './types'
 
 export { suite, test, describe, it } from './runtime/suite'
-
 export * from './runtime/hooks'
+
+export { runOnce } from './integrations/run-once'
 export * from './integrations/chai'
 export * from './integrations/jest-mock'
 export * from './integrations/vi'

packages/vitest/src/integrations/run-once.ts

@@ -0,0 +1,34 @@
+import type { Awaitable } from '../types'
+import { getWorkerState } from '../utils'
+
+const filesCount = new Map<string, number>()
+const cache = new Map<string, any>()
+
+/**
+ * This utils allows computational intensive tasks to only be ran once
+ * across test reruns to improve the watch mode performance.
+ *
+ * Currently only works with `isolate: false`
+ *
+ * @experimental
+ */
+export async function runOnce<T>(fn: (() => Awaitable<T>), key?: string): Promise<T> {
+  if (!key) {
+    const filepath = getWorkerState().filepath || '__unknown_files__'
+    filesCount.set(filepath, (filesCount.get(filepath) || 0) + 1)
+    const count = filesCount.get(filepath)!
+    key = `${filepath}:${count}`
+  }
+
+  if (!cache.has(key))
+    cache.set(key, fn())
+
+  return await cache.get(key)
+}
+
+/**
+ * @internal
+ */
+export function resetRunOnceCounter() {
+  filesCount.clear()
+}

packages/vitest/src/runtime/setup.ts

@@ -4,10 +4,13 @@ import { environments } from '../integrations/env'
 import type { ResolvedConfig } from '../types'
 import { getWorkerState, toArray } from '../utils'
 import * as VitestIndex from '../index'
+import { resetRunOnceCounter } from '../integrations/run-once'
 import { rpc } from './rpc'
 
 let globalSetup = false
 export async function setupGlobalEnv(config: ResolvedConfig) {
+  resetRunOnceCounter()
+
   Object.defineProperty(globalThis, '__vitest_index__', {
     value: VitestIndex,
     enumerable: false,

test/core/vitest.config.ts

@@ -38,7 +38,6 @@ export default defineConfig({
   },
   test: {
     testTimeout: 2000,
-    // threads: false,
     setupFiles: [
       './test/setup.ts',
     ],

test/run-once/package.json

@@ -0,0 +1,11 @@
+{
+  "name": "@vitest/test-run-once",
+  "private": true,
+  "scripts": {
+    "test": "vitest",
+    "coverage": "vitest run --coverage"
+  },
+  "devDependencies": {
+    "vitest": "workspace:*"
+  }
+}

test/run-once/test/run-once.test.ts

@@ -0,0 +1,21 @@
+import { expect, it, runOnce } from 'vitest'
+
+let dummy = 0
+
+const one = await runOnce(() => {
+  dummy += 1
+  return 1
+})
+
+const two = await runOnce(async() => {
+  dummy += 1
+  return 2
+})
+
+it('run once', async() => {
+  expect(one).toBe(1)
+  expect(two).toBe(2)
+
+  // edit the file to trigger the hmr and dummy should be 0
+  expect(dummy).toBe(2)
+})

test/run-once/vitest.config.ts

@@ -0,0 +1,7 @@
+import { defineConfig } from 'vite'
+
+export default defineConfig({
+  test: {
+    isolate: false,
+  },
+})