feat(@angular/build): enable component template hot replacement by default

When using the `application` builder (default for new projects) with the
development server, component template only changes will now automatically
replace the template within the running application without a full reload
of the page. No application code changes are necessary and both file-based
(`templateUrl`) and inline (`template`) component templates are supported.
Additionally, changing a components styles in combination with a template
change is also supported for hot replacement. This includes both inline and
file-based changes.

If any issues are encountered or it is preferred to not hot replace component
templates, the `NG_HMR_TEMPLATES=0` environment variable can be used to disable
the feature. Setting the `liveReload` option or `hmr` option to false will
also disable all updates.

Author: clydin

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

@@ -138,17 +138,14 @@ export async function* serveWithVite(
     process.setSourceMapsEnabled(true);
   }
 
-  // Enable to support component style hot reloading (`NG_HMR_CSTYLES=0` can be used to disable selectively)
+  // Enable to support link-based component style hot reloading (`NG_HMR_CSTYLES=0` can be used to disable selectively)
   browserOptions.externalRuntimeStyles =
     serverOptions.liveReload && serverOptions.hmr && useComponentStyleHmr;
 
-  // Enable to support component template hot replacement (`NG_HMR_TEMPLATE=1` can be used to enable)
-  browserOptions.templateUpdates = !!serverOptions.liveReload && useComponentTemplateHmr;
-  if (browserOptions.templateUpdates) {
-    context.logger.warn(
-      'Experimental support for component template hot replacement has been enabled via the "NG_HMR_TEMPLATE" environment variable.',
-    );
-  }
+  // Enable to support component template hot replacement (`NG_HMR_TEMPLATE=0` can be used to disable selectively)
+  // This will also replace file-based/inline styles as code if external runtime styles are not enabled.
+  browserOptions.templateUpdates =
+    serverOptions.liveReload && serverOptions.hmr && useComponentTemplateHmr;
 
   // Setup the prebundling transformer that will be shared across Vite prebundling requests
   const prebundleTransformer = new JavaScriptTransformer(

packages/angular/build/src/utils/environment-options.ts

@@ -107,7 +107,7 @@ export const useComponentStyleHmr =
 
 const hmrComponentTemplateVariable = process.env['NG_HMR_TEMPLATES'];
 export const useComponentTemplateHmr =
-  isPresent(hmrComponentTemplateVariable) && isEnabled(hmrComponentTemplateVariable);
+  !isPresent(hmrComponentTemplateVariable) || !isDisabled(hmrComponentTemplateVariable);
 
 const partialSsrBuildVariable = process.env['NG_BUILD_PARTIAL_SSR'];
 export const usePartialSsrBuild =

tests/legacy-cli/e2e/tests/basic/rebuild.ts

@@ -9,13 +9,8 @@ export default async function () {
   const validBundleRegEx = esbuild ? /sent to client/ : /Compiled successfully\./;
   const lazyBundleRegEx = esbuild ? /chunk-/ : /src_app_lazy_lazy_component_ts\.js/;
 
-  // Disable component stylesheet HMR to support page reload based rebuild testing.
-  // Ideally this environment variable would be passed directly to the new serve process
-  // but this would require signficant test changes due to the existing `ngServe` signature.
-  const oldHMRValue = process.env['NG_HMR_CSTYLES'];
-  process.env['NG_HMR_CSTYLES'] = '0';
-  const port = await ngServe();
-  process.env['NG_HMR_CSTYLES'] = oldHMRValue;
+  // Disable HMR to support page reload based rebuild testing.
+  const port = await ngServe('--no-hmr');
 
   // Add a lazy route.
   await silentNg('generate', 'component', 'lazy');

tests/legacy-cli/e2e/tests/vite/ssr-entry-express.ts

@@ -90,6 +90,7 @@ export default async function () {
     'src/app/home/home.component.html',
     'home works',
     'yay home works!!!',
+    true,
   );
   await validateResponse('/api/test', /foo/);
   await validateResponse('/home', /yay home works/);
@@ -111,9 +112,12 @@ async function modifyFileAndWaitUntilUpdated(
   filePath: string,
   searchValue: string,
   replaceValue: string,
+  hmr = false,
 ): Promise<void> {
   await Promise.all([
-    waitForAnyProcessOutputToMatch(/Page reload sent to client/),
+    waitForAnyProcessOutputToMatch(
+      hmr ? /Component update sent to client/ : /Page reload sent to client/,
+    ),
     setTimeout(100).then(() => replaceInFile(filePath, searchValue, replaceValue)),
   ]);
 }

tests/legacy-cli/e2e/tests/vite/ssr-entry-fastify.ts

@@ -90,6 +90,7 @@ export default async function () {
     'src/app/home/home.component.html',
     'home works',
     'yay home works!!!',
+    true,
   );
   await validateResponse('/api/test', /foo/);
   await validateResponse('/home', /yay home works/);
@@ -111,9 +112,12 @@ async function modifyFileAndWaitUntilUpdated(
   filePath: string,
   searchValue: string,
   replaceValue: string,
+  hmr = false,
 ): Promise<void> {
   await Promise.all([
-    waitForAnyProcessOutputToMatch(/Page reload sent to client/),
+    waitForAnyProcessOutputToMatch(
+      hmr ? /Component update sent to client/ : /Page reload sent to client/,
+    ),
     setTimeout(100).then(() => replaceInFile(filePath, searchValue, replaceValue)),
   ]);
 }

tests/legacy-cli/e2e/tests/vite/ssr-entry-h3.ts

@@ -81,6 +81,7 @@ export default async function () {
     'src/app/home/home.component.html',
     'home works',
     'yay home works!!!',
+    true,
   );
   await validateResponse('/api/test', /foo/);
   await validateResponse('/home', /yay home works/);
@@ -102,9 +103,12 @@ async function modifyFileAndWaitUntilUpdated(
   filePath: string,
   searchValue: string,
   replaceValue: string,
+  hmr = false,
 ): Promise<void> {
   await Promise.all([
-    waitForAnyProcessOutputToMatch(/Page reload sent to client/),
+    waitForAnyProcessOutputToMatch(
+      hmr ? /Component update sent to client/ : /Page reload sent to client/,
+    ),
     setTimeout(100).then(() => replaceInFile(filePath, searchValue, replaceValue)),
   ]);
 }

tests/legacy-cli/e2e/tests/vite/ssr-entry-hono.ts

@@ -3,7 +3,7 @@ import { setTimeout } from 'node:timers/promises';
 import { replaceInFile, writeMultipleFiles } from '../../utils/fs';
 import { ng, silentNg, waitForAnyProcessOutputToMatch } from '../../utils/process';
 import { installPackage, installWorkspacePackages, uninstallPackage } from '../../utils/packages';
-import { ngServe, updateJsonFile, useSha } from '../../utils/project';
+import { ngServe, useSha } from '../../utils/project';
 import { getGlobalVariable } from '../../utils/env';
 
 export default async function () {
@@ -73,6 +73,7 @@ export default async function () {
     'src/app/home/home.component.html',
     'home works',
     'yay home works!!!',
+    true,
   );
   await validateResponse('/api/test', /foo/);
   await validateResponse('/home', /yay home works/);
@@ -94,9 +95,12 @@ async function modifyFileAndWaitUntilUpdated(
   filePath: string,
   searchValue: string,
   replaceValue: string,
+  hmr = false,
 ): Promise<void> {
   await Promise.all([
-    waitForAnyProcessOutputToMatch(/Page reload sent to client/),
+    waitForAnyProcessOutputToMatch(
+      hmr ? /Component update sent to client/ : /Page reload sent to client/,
+    ),
     setTimeout(100).then(() => replaceInFile(filePath, searchValue, replaceValue)),
   ]);
 }