perf(@angular/build): avoid reading external component stylesheets twice

- Skip calling `this.readFile(filename)` in `host.readResource(filename)` when `!hasTemplateExtension(filename)` so external component stylesheets (`styleUrl` / `styleUrls`) are not read from disk on the Angular compiler worker thread and serialized over the worker `MessagePort` only to be ignored by `transformStylesheet`.
- Only short-circuit empty `data` in `host.transformResource(data, context)` for inline styles (`!context.resourceFile`) so external stylesheets still proceed to `transformStylesheet`.

Author: alan-agius4

packages/angular/build/src/tools/angular/angular-host.ts

@@ -182,6 +182,12 @@ export function createAngularCompilerHost(
   // The AOT compiler currently requires this hook to allow for a transformResource hook.
   // Once the AOT compiler allows only a transformResource hook, this can be reevaluated.
   host.readResource = async function (filename) {
+    // Only template resources need to be read by the compiler host.
+    // External stylesheets are read and bundled by the stylesheet bundler during `transformResource`.
+    if (!hasTemplateExtension(filename)) {
+      return '';
+    }
+
     return this.readFile(filename) ?? '';
   };
 
@@ -197,8 +203,8 @@ export function createAngularCompilerHost(
       'External runtime stylesheets should not be transformed: ' + context.resourceFile,
     );
 
-    // No transformation required if the resource is empty
-    if (data.trim().length === 0) {
+    // No transformation required if an inline resource is empty
+    if (!context.resourceFile && data.trim().length === 0) {
       return { content: '' };
     }
 

packages/angular/build/src/tools/angular/angular-host_spec.ts

@@ -0,0 +1,97 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.dev/license
+ */
+
+import ts from 'typescript';
+import { type AngularHostOptions, createAngularCompilerHost } from './angular-host';
+
+describe('createAngularCompilerHost', () => {
+  function createHost() {
+    const transformStylesheet = jasmine
+      .createSpy<AngularHostOptions['transformStylesheet']>('transformStylesheet')
+      .and.resolveTo('.compiled {}');
+    const processWebWorker = jasmine
+      .createSpy<AngularHostOptions['processWebWorker']>('processWebWorker')
+      .and.returnValue('worker.js');
+
+    const host = createAngularCompilerHost(
+      ts,
+      {},
+      {
+        transformStylesheet,
+        processWebWorker,
+      },
+      undefined,
+    );
+
+    return { host, transformStylesheet };
+  }
+
+  describe('readResource', () => {
+    it('should not call readFile for .css and .scss stylesheet files', async () => {
+      const { host } = createHost();
+      const readFileSpy = spyOn(host, 'readFile').and.returnValue('.btn { color: red; }');
+
+      expect(await host.readResource?.('/app/src/app.component.css')).toBe('');
+      expect(await host.readResource?.('/app/src/app.component.scss')).toBe('');
+      expect(readFileSpy).not.toHaveBeenCalled();
+    });
+
+    it('should call readFile for .html and .svg template files', async () => {
+      const { host } = createHost();
+      const readFileSpy = spyOn(host, 'readFile').and.callFake(
+        (fileName) => `<content of ${fileName}>`,
+      );
+
+      expect(await host.readResource?.('/app/src/app.component.html')).toBe(
+        '<content of /app/src/app.component.html>',
+      );
+      expect(await host.readResource?.('/app/src/icon.component.svg')).toBe(
+        '<content of /app/src/icon.component.svg>',
+      );
+      expect(readFileSpy).toHaveBeenCalledTimes(2);
+    });
+  });
+
+  describe('transformResource', () => {
+    it('should invoke transformStylesheet when context.resourceFile is set even if data is empty', async () => {
+      const { host, transformStylesheet } = createHost();
+
+      const result = await host.transformResource?.('', {
+        type: 'style',
+        containingFile: '/app/src/app.component.ts',
+        resourceFile: '/app/src/app.component.scss',
+        order: 0,
+        className: 'AppComponent',
+      });
+
+      expect(result).toEqual({ content: '.compiled {}' });
+      expect(transformStylesheet).toHaveBeenCalledOnceWith(
+        '',
+        '/app/src/app.component.ts',
+        '/app/src/app.component.scss',
+        0,
+        'AppComponent',
+      );
+    });
+
+    it('should short-circuit empty inline styles without calling transformStylesheet', async () => {
+      const { host, transformStylesheet } = createHost();
+
+      const result = await host.transformResource?.('   ', {
+        type: 'style',
+        containingFile: '/app/src/app.component.ts',
+        resourceFile: null,
+        order: 0,
+        className: 'AppComponent',
+      });
+
+      expect(result).toEqual({ content: '' });
+      expect(transformStylesheet).not.toHaveBeenCalled();
+    });
+  });
+});