refactor(http): expose withXhr to prepare for fetch by default

This commit sets up the necessary changes that would allow us to safely migrate G3 before switch to the `FetchBackend` by default.

For now the `HttpXhrBackend` is still the default backend for the `HttpClient`.

goldens/public-api/common/http/index.api.md

@@ -2680,7 +2680,9 @@ export enum HttpFeatureKind {
     // (undocumented)
     NoXsrfProtection = 3,
     // (undocumented)
-    RequestsMadeViaParent = 5
+    RequestsMadeViaParent = 5,
+    // (undocumented)
+    Xhr = 7
 }
 
 // @public
@@ -3347,6 +3349,9 @@ export function withNoXsrfProtection(): HttpFeature<HttpFeatureKind.NoXsrfProtec
 // @public
 export function withRequestsMadeViaParent(): HttpFeature<HttpFeatureKind.RequestsMadeViaParent>;
 
+// @public
+export function withXhr(): HttpFeature<HttpFeatureKind.Xhr>;
+
 // @public
 export function withXsrfConfiguration({ cookieName, headerName, }: {
     cookieName?: string;

packages/common/http/public_api.ts

@@ -35,6 +35,7 @@ export {
   HttpFeatureKind,
   provideHttpClient,
   withFetch,
+  withXhr,
   withInterceptors,
   withInterceptorsFromDi,
   withJsonpSupport,

packages/common/http/src/backend-default-value.ts

@@ -0,0 +1,15 @@
+/**
+ * @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.io/license
+ */
+
+import {HttpXhrBackend} from './xhr';
+
+/**
+ * A constant defining the default the default Http Backend.
+ * Extracted to a separate file to facilitate G3 patches.
+ */
+export const NG_DEFAULT_HTTP_BACKEND = HttpXhrBackend;

packages/common/http/src/provider.ts

@@ -15,6 +15,7 @@ import {
 } from '@angular/core';
 
 import {HttpBackend, HttpHandler, HttpInterceptorHandler} from './backend';
+import {NG_DEFAULT_HTTP_BACKEND} from './backend-default-value';
 import {HttpClient} from './client';
 import {FETCH_BACKEND, FetchBackend} from './fetch';
 import {HTTP_INTERCEPTOR_FNS, HttpInterceptorFn, legacyInterceptorFnFactory} from './interceptor';
@@ -40,6 +41,7 @@ export enum HttpFeatureKind {
   JsonpSupport,
   RequestsMadeViaParent,
   Fetch,
+  Xhr,
 }
 
 /**
@@ -89,7 +91,7 @@ function makeHttpFeature<KindT extends HttpFeatureKind>(
  * @see {@link withNoXsrfProtection}
  * @see {@link withJsonpSupport}
  * @see {@link withRequestsMadeViaParent}
- * @see {@link withFetch}
+ * @see {@link withXhr}
  */
 export function provideHttpClient(
   ...features: HttpFeature<HttpFeatureKind>[]
@@ -110,12 +112,13 @@ export function provideHttpClient(
 
   const providers: Provider[] = [
     HttpClient,
+    NG_DEFAULT_HTTP_BACKEND,
     HttpInterceptorHandler,
     {provide: HttpHandler, useExisting: HttpInterceptorHandler},
     {
       provide: HttpBackend,
       useFactory: () => {
-        return inject(FETCH_BACKEND, {optional: true}) ?? inject(HttpXhrBackend);
+        return inject(FETCH_BACKEND, {optional: true}) ?? inject(NG_DEFAULT_HTTP_BACKEND);
       },
     },
     {
@@ -297,3 +300,18 @@ export function withFetch(): HttpFeature<HttpFeatureKind.Fetch> {
     {provide: HttpBackend, useExisting: FetchBackend},
   ]);
 }
+
+/**
+ * Configures the current `HttpClient` instance to make requests using the Xhr API.
+ *
+ * Use this feature if you want to report progress on uploads as the Xhr API supports it.
+ *
+ * @see {@link provideHttpClient}
+ * @publicApi
+ */
+export function withXhr(): HttpFeature<HttpFeatureKind.Xhr> {
+  return makeHttpFeature(HttpFeatureKind.Xhr, [
+    HttpXhrBackend,
+    {provide: HttpBackend, useExisting: HttpXhrBackend},
+  ]);
+}