refactor(platform-server): expose internal render method for extensibility (angular#60416)

The `renderApplication` and `renderModule` methods currently encapsulate the entire rendering process, making it difficult to intercept key phases from a non-Angular context. This change exports the internal `render` method, allowing us to perform operations such as:

- Flushing headers before hydration preparation
- Handling non static redirects (e.g., 302 responses)
- Intercepting router events for additional processing

This refactor serves as an experimental step toward improving the API for better customization and integration in the future.

PR Close angular#60416

Author: alan-agius4

packages/platform-server/src/private_export.ts

@@ -10,5 +10,5 @@ export {
   INTERNAL_SERVER_PLATFORM_PROVIDERS as ɵINTERNAL_SERVER_PLATFORM_PROVIDERS,
   SERVER_RENDER_PROVIDERS as ɵSERVER_RENDER_PROVIDERS,
 } from './server';
-export {SERVER_CONTEXT as ɵSERVER_CONTEXT} from './utils';
+export {SERVER_CONTEXT as ɵSERVER_CONTEXT, renderInternal as ɵrenderInternal} from './utils';
 export {ENABLE_DOM_EMULATION as ɵENABLE_DOM_EMULATION} from './tokens';

packages/platform-server/src/utils.ts

@@ -174,17 +174,22 @@ function insertEventRecordScript(
   stopMeasuring(measuringLabel);
 }
 
-async function _render(platformRef: PlatformRef, applicationRef: ApplicationRef): Promise<string> {
-  const measuringLabel = 'whenStable';
-  startMeasuring(measuringLabel);
-
-  // Block until application is stable.
-  await applicationRef.whenStable();
-
-  stopMeasuring(measuringLabel);
-
+/**
+ * Renders an Angular application to a string.
+ *
+ * @private
+ *
+ * @param platformRef - Reference to the Angular platform.
+ * @param applicationRef - Reference to the Angular application.
+ * @returns A promise that resolves to the rendered string.
+ */
+export async function renderInternal(
+  platformRef: PlatformRef,
+  applicationRef: ApplicationRef,
+): Promise<string> {
   const platformState = platformRef.injector.get(PlatformState);
   prepareForHydration(platformState, applicationRef);
+  appendServerContextInfo(applicationRef);
 
   // Run any BEFORE_APP_SERIALIZED callbacks just before rendering to string.
   const environmentInjector = applicationRef.injector;
@@ -212,8 +217,6 @@ async function _render(platformRef: PlatformRef, applicationRef: ApplicationRef)
     }
   }
 
-  appendServerContextInfo(applicationRef);
-
   return platformState.renderToString();
 }
 
@@ -273,7 +276,14 @@ export async function renderModule<T>(
   try {
     const moduleRef = await platformRef.bootstrapModule(moduleType);
     const applicationRef = moduleRef.injector.get(ApplicationRef);
-    return await _render(platformRef, applicationRef);
+
+    const measuringLabel = 'whenStable';
+    startMeasuring(measuringLabel);
+    // Block until application is stable.
+    await applicationRef.whenStable();
+    stopMeasuring(measuringLabel);
+
+    return await renderInternal(platformRef, applicationRef);
   } finally {
     await asyncDestroyPlatform(platformRef);
   }
@@ -315,7 +325,14 @@ export async function renderApplication<T>(
     stopMeasuring(bootstrapLabel);
 
     startMeasuring(_renderLabel);
-    const rendered = await _render(platformRef, applicationRef);
+
+    const measuringLabel = 'whenStable';
+    startMeasuring(measuringLabel);
+    // Block until application is stable.
+    await applicationRef.whenStable();
+    stopMeasuring(measuringLabel);
+
+    const rendered = await renderInternal(platformRef, applicationRef);
     stopMeasuring(_renderLabel);
     return rendered;
   } finally {