feat(@angular/build): support dev server direct component style serving

The Vite-based development server now provides support for serving individual
component stylesheets both with and without emulated view encapsulation. This
capability is not yet used by the Angular runtime code. The ability to use
external stylesheets instead of bundling the style content is an enabling
capability primarily for automatic component style HMR features. Additionally,
it has potential future benefits for development mode deferred style processing
which may reduce the initial build time when using the development server. The
application build itself also does not yet generate external stylesheets.

Author: clydin

packages/angular/build/BUILD.bazel

@@ -59,6 +59,7 @@ ts_library(
         "//packages/angular_devkit/architect",
         "@npm//@ampproject/remapping",
         "@npm//@angular/common",
+        "@npm//@angular/compiler",
         "@npm//@angular/compiler-cli",
         "@npm//@angular/core",
         "@npm//@angular/localize",

packages/angular/build/package.json

@@ -45,6 +45,7 @@
     "watchpack": "2.4.2"
   },
   "peerDependencies": {
+    "@angular/compiler": "^19.0.0-next.0",
     "@angular/compiler-cli": "^19.0.0-next.0",
     "@angular/localize": "^19.0.0-next.0",
     "@angular/platform-server": "^19.0.0-next.0",

packages/angular/build/src/builders/dev-server/vite-server.ts

@@ -143,6 +143,7 @@ export async function* serveWithVite(
     implicitServer: [],
     explicit: [],
   };
+  const usedComponentStyles = new Map<string, string[]>();
 
   // Add cleanup logic via a builder teardown.
   let deferred: () => void;
@@ -255,7 +256,14 @@ export async function* serveWithVite(
         ...new Set([...server.config.server.fs.allow, ...assetFiles.values()]),
       ];
 
-      handleUpdate(normalizePath, generatedFiles, server, serverOptions, context.logger);
+      handleUpdate(
+        normalizePath,
+        generatedFiles,
+        server,
+        serverOptions,
+        context.logger,
+        usedComponentStyles,
+      );
 
       if (requiresServerRestart) {
         // Restart the server to force SSR dep re-optimization when a dependency has been added.
@@ -300,6 +308,7 @@ export async function* serveWithVite(
         prebundleTransformer,
         target,
         isZonelessApp(polyfills),
+        usedComponentStyles,
         browserOptions.loader as EsbuildLoaderOption | undefined,
         extensions?.middleware,
         transformers?.indexHtml,
@@ -357,6 +366,7 @@ function handleUpdate(
   server: ViteDevServer,
   serverOptions: NormalizedDevServerOptions,
   logger: BuilderContext['logger'],
+  usedComponentStyles: Map<string, string[]>,
 ): void {
   const updatedFiles: string[] = [];
 
@@ -380,7 +390,17 @@ function handleUpdate(
       const timestamp = Date.now();
       server.hot.send({
         type: 'update',
-        updates: updatedFiles.map((filePath) => {
+        updates: updatedFiles.flatMap((filePath) => {
+          const componentIds = usedComponentStyles.get(filePath);
+          if (componentIds) {
+            return componentIds.map((id) => ({
+              type: 'css-update',
+              timestamp,
+              path: `${filePath}?component` + (id ? `=${id}` : ''),
+              acceptedPath: filePath,
+            }));
+          }
+
           return {
             type: 'css-update',
             timestamp,
@@ -483,6 +503,7 @@ export async function setupServer(
   prebundleTransformer: JavaScriptTransformer,
   target: string[],
   zoneless: boolean,
+  usedComponentStyles: Map<string, string[]>,
   prebundleLoaderExtensions: EsbuildLoaderOption | undefined,
   extensionMiddleware?: Connect.NextHandleFunction[],
   indexHtmlTransformer?: (content: string) => Promise<string>,
@@ -591,6 +612,7 @@ export async function setupServer(
         indexHtmlTransformer,
         extensionMiddleware,
         normalizePath,
+        usedComponentStyles,
       }),
       createRemoveIdPrefixPlugin(externalMetadata.explicit),
     ],

packages/angular/build/src/tools/vite/angular-memory-plugin.ts

@@ -29,6 +29,7 @@ export interface AngularMemoryPluginOptions {
   extensionMiddleware?: Connect.NextHandleFunction[];
   indexHtmlTransformer?: (content: string) => Promise<string>;
   normalizePath: (path: string) => string;
+  usedComponentStyles: Map<string, string[]>;
 }
 
 export function createAngularMemoryPlugin(options: AngularMemoryPluginOptions): Plugin {
@@ -42,6 +43,7 @@ export function createAngularMemoryPlugin(options: AngularMemoryPluginOptions):
     extensionMiddleware,
     indexHtmlTransformer,
     normalizePath,
+    usedComponentStyles,
   } = options;
 
   return {
@@ -113,7 +115,9 @@ export function createAngularMemoryPlugin(options: AngularMemoryPluginOptions):
       };
 
       // Assets and resources get handled first
-      server.middlewares.use(createAngularAssetsMiddleware(server, assets, outputFiles));
+      server.middlewares.use(
+        createAngularAssetsMiddleware(server, assets, outputFiles, usedComponentStyles),
+      );
 
       if (extensionMiddleware?.length) {
         extensionMiddleware.forEach((middleware) => server.middlewares.use(middleware));

packages/angular/build/src/tools/vite/middlewares/assets-middleware.ts

@@ -9,16 +9,20 @@
 import { lookup as lookupMimeType } from 'mrmime';
 import { extname } from 'node:path';
 import type { Connect, ViteDevServer } from 'vite';
+import { loadEsmModule } from '../../../utils/load-esm';
 import {
   AngularMemoryOutputFiles,
   appendServerConfiguredHeaders,
   pathnameWithoutBasePath,
 } from '../utils';
 
+const COMPONENT_REGEX = /%COMP%/g;
+
 export function createAngularAssetsMiddleware(
   server: ViteDevServer,
   assets: Map<string, string>,
   outputFiles: AngularMemoryOutputFiles,
+  usedComponentStyles: Map<string, string[]>,
 ): Connect.NextHandleFunction {
   return function (req, res, next) {
     if (req.url === undefined || res.writableEnded) {
@@ -69,13 +73,52 @@ export function createAngularAssetsMiddleware(
     if (extension !== '.js' && extension !== '.html') {
       const outputFile = outputFiles.get(pathname);
       if (outputFile?.servable) {
+        const data = outputFile.contents;
+        if (extension === '.css') {
+          // Inject component ID for view encapsulation if requested
+          const componentId = new URL(req.url, 'http://localhost').searchParams.get('component');
+          if (componentId !== null) {
+            // Record the component style usage for HMR updates
+            const usedIds = usedComponentStyles.get(pathname);
+            if (usedIds === undefined) {
+              usedComponentStyles.set(pathname, [componentId]);
+            } else {
+              usedIds.push(componentId);
+            }
+            // Shim the stylesheet if a component ID is provided
+            if (componentId.length > 0) {
+              // Validate component ID
+              // TODO: Determine if this regex is too strict for the APP_ID segment
+              if (/[.-A-Za-z0-9]+-c\d{9}$/.test(componentId)) {
+                loadEsmModule<typeof import('@angular/compiler')>('@angular/compiler')
+                  .then((compilerModule) => {
+                    const encapsulatedData = compilerModule
+                      .encapsulateStyle(new TextDecoder().decode(data))
+                      .replaceAll(COMPONENT_REGEX, componentId);
+
+                    res.setHeader('Content-Type', 'text/css');
+                    res.setHeader('Cache-Control', 'no-cache');
+                    appendServerConfiguredHeaders(server, res);
+                    res.end(encapsulatedData);
+                  })
+                  .catch((e) => next(e));
+
+                return;
+              } else {
+                // eslint-disable-next-line no-console
+                console.error('Invalid component stylesheet ID request: ' + componentId);
+              }
+            }
+          }
+        }
+
         const mimeType = lookupMimeType(extension);
         if (mimeType) {
           res.setHeader('Content-Type', mimeType);
         }
         res.setHeader('Cache-Control', 'no-cache');
         appendServerConfiguredHeaders(server, res);
-        res.end(outputFile.contents);
+        res.end(data);
 
         return;
       }

yarn.lock

@@ -407,6 +407,7 @@ __metadata:
     vite: "npm:5.4.0"
     watchpack: "npm:2.4.2"
   peerDependencies:
+    "@angular/compiler": ^19.0.0-next.0
     "@angular/compiler-cli": ^19.0.0-next.0
     "@angular/localize": ^19.0.0-next.0
     "@angular/platform-server": ^19.0.0-next.0