Inject HMR code in Worker entry file (#10001)

* Inject HMR code into Worker entry file

* Add changeset

* Add test

* Skip HMR test for builds

* Update changeset

Author: jamesopstad

.changeset/orange-donkeys-check.md

@@ -0,0 +1,13 @@
+---
+"@cloudflare/vite-plugin": minor
+---
+
+We now automatically inject the following HMR code into your Worker entry file:
+
+```ts
+if (import.meta.hot) {
+	import.meta.hot.accept();
+}
+```
+
+This prevents file changes from invalidating the full module graph and improves HMR performance in development.

packages/vite-plugin-cloudflare/playground/worker/__tests__/worker.spec.ts

@@ -1,5 +1,13 @@
-import { expect, test } from "vitest";
-import { getTextResponse, serverLogs, viteTestUrl } from "../../__test-utils__";
+import * as fs from "node:fs";
+import * as path from "node:path";
+import { expect, test, vi } from "vitest";
+import {
+	getTextResponse,
+	isBuild,
+	rootDir,
+	serverLogs,
+	viteTestUrl,
+} from "../../__test-utils__";
 
 test("basic hello-world functionality", async () => {
 	expect(await getTextResponse()).toEqual("Hello World!");
@@ -20,3 +28,16 @@ test("receives the original host as the `X-Forwarded-Host` header", async () =>
 test("does not cause unhandled rejection", async () => {
 	expect(serverLogs.errors.join()).not.toContain("__unhandled rejection__");
 });
+
+test.runIf(!isBuild)(
+	"updates using HMR code in Worker entry file",
+	async () => {
+		const workerEntryPath = path.join(rootDir, "src", "index.ts");
+		const originalContent = fs.readFileSync(workerEntryPath, "utf-8");
+		fs.writeFileSync(workerEntryPath, originalContent);
+
+		await vi.waitFor(() => {
+			expect(serverLogs.info.join()).toContain("[vite] hot updated");
+		});
+	}
+);

packages/vite-plugin-cloudflare/src/index.ts

@@ -167,10 +167,6 @@ export function cloudflare(pluginConfig: PluginConfig = {}): vite.Plugin[] {
 					},
 				};
 			},
-			buildStart() {
-				// This resets the value when the dev server restarts
-				workersConfigsWarningShown = false;
-			},
 			configResolved(config) {
 				resolvedViteConfig = config;
 
@@ -181,6 +177,34 @@ export function cloudflare(pluginConfig: PluginConfig = {}): vite.Plugin[] {
 					);
 				}
 			},
+			buildStart() {
+				// This resets the value when the dev server restarts
+				workersConfigsWarningShown = false;
+			},
+			async transform(code, id) {
+				const workerConfig = getWorkerConfig(this.environment.name);
+
+				if (!workerConfig) {
+					return;
+				}
+
+				const resolvedWorkerEntry = await this.resolve(workerConfig.main);
+
+				if (id === resolvedWorkerEntry?.id) {
+					const modified = new MagicString(code);
+					const hmrCode = `
+if (import.meta.hot) {
+  import.meta.hot.accept();
+}
+						`;
+					modified.append(hmrCode);
+
+					return {
+						code: modified.toString(),
+						map: modified.generateMap({ hires: "boundary", source: id }),
+					};
+				}
+			},
 			generateBundle(_, bundle) {
 				assertIsNotPreview(resolvedPluginConfig);