refactor(cdk/a11y): add ID generator service

Adds a service to generate unique IDs.

Author: crisbeto

src/cdk/a11y/id-generator.ts

@@ -0,0 +1,40 @@
+/**
+ * @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 {APP_ID, inject, Injectable} from '@angular/core';
+
+/**
+ * Keeps track of the ID count per prefix. This helps us make the IDs a bit more deterministic
+ * like they were before the service was introduced. Note that ideally we wouldn't have to do
+ * this, but there are some internal tests that rely on the IDs.
+ */
+const counters: Record<string, number> = {};
+
+/** Service that generates unique IDs for DOM nodes. */
+@Injectable({providedIn: 'root'})
+export class _IdGenerator {
+  private _appId = inject(APP_ID);
+
+  /**
+   * Generates a unique ID with a specific prefix.
+   * @param prefix Prefix to add to the ID.
+   */
+  getId(prefix: string): string {
+    // Omit the app ID if it's the default `ng`. Since the vast majority of pages have one
+    // Angular app on them, we can reduce the amount of breakages by not adding it.
+    if (this._appId !== 'ng') {
+      prefix += this._appId;
+    }
+
+    if (!counters.hasOwnProperty(prefix)) {
+      counters[prefix] = 0;
+    }
+
+    return `${prefix}${counters[prefix]++}`;
+  }
+}

src/cdk/a11y/public-api.ts

@@ -36,3 +36,4 @@ export {
   HighContrastModeDetector,
   HighContrastMode,
 } from './high-contrast-mode/high-contrast-mode-detector';
+export {_IdGenerator} from './id-generator';

tools/public_api_guard/cdk/a11y.md

@@ -291,6 +291,15 @@ export interface Highlightable extends ListKeyManagerOption {
     setInactiveStyles(): void;
 }
 
+// @public
+export class _IdGenerator {
+    getId(prefix: string): string;
+    // (undocumented)
+    static ɵfac: i0.ɵɵFactoryDeclaration<_IdGenerator, never>;
+    // (undocumented)
+    static ɵprov: i0.ɵɵInjectableDeclaration<_IdGenerator>;
+}
+
 // @public
 export const INPUT_MODALITY_DETECTOR_DEFAULT_OPTIONS: InputModalityDetectorOptions;