fix(cdk/a11y): re-apply the high contrast mode class when the forced-colors media query changes (#25088)

Adds some logic to reapply the high contrast mode class when the `forced-colors` media query changes so that the user doesn't have to refresh the page.

Also reworks the testing setup for the `HighContrastMode` detector, because it was constructing the detector manually.

(cherry picked from commit bc81e7d)

Author: crisbeto

src/cdk/a11y/BUILD.bazel

@@ -20,6 +20,7 @@ ng_module(
         "//src:dev_mode_types",
         "//src/cdk/coercion",
         "//src/cdk/keycodes",
+        "//src/cdk/layout",
         "//src/cdk/observers",
         "//src/cdk/platform",
         "@npm//@angular/core",

src/cdk/a11y/high-contrast-mode/high-contrast-mode-detector.spec.ts

@@ -6,76 +6,80 @@ import {
   WHITE_ON_BLACK_CSS_CLASS,
 } from './high-contrast-mode-detector';
 import {Platform} from '@angular/cdk/platform';
-import {inject} from '@angular/core/testing';
+import {TestBed} from '@angular/core/testing';
+import {Provider} from '@angular/core';
+import {A11yModule} from '../a11y-module';
+import {DOCUMENT} from '@angular/common';
 
 describe('HighContrastModeDetector', () => {
-  let fakePlatform: Platform;
+  function getDetector(document: unknown, platform?: Platform) {
+    const providers: Provider[] = [{provide: DOCUMENT, useValue: document}];
 
-  beforeEach(inject([Platform], (p: Platform) => {
-    fakePlatform = p;
-  }));
+    if (platform) {
+      providers.push({provide: Platform, useValue: platform});
+    }
+
+    TestBed.configureTestingModule({imports: [A11yModule], providers});
+    return TestBed.inject(HighContrastModeDetector);
+  }
 
   it('should detect NONE for non-browser platforms', () => {
-    fakePlatform.isBrowser = false;
-    const detector = new HighContrastModeDetector(fakePlatform, {});
+    const detector = getDetector(getFakeDocument(''), {isBrowser: false} as Platform);
+
     expect(detector.getHighContrastMode())
       .withContext('Expected high-contrast mode `NONE` on non-browser platforms')
       .toBe(HighContrastMode.NONE);
   });
 
   it('should not apply any css classes for non-browser platforms', () => {
-    fakePlatform.isBrowser = false;
     const fakeDocument = getFakeDocument('');
-    const detector = new HighContrastModeDetector(fakePlatform, fakeDocument);
+    const detector = getDetector(fakeDocument, {isBrowser: false} as Platform);
     detector._applyBodyHighContrastModeCssClasses();
     expect(fakeDocument.body.className)
       .withContext('Expected body not to have any CSS classes in non-browser platforms')
       .toBe('');
   });
 
   it('should detect WHITE_ON_BLACK when backgrounds are coerced to black', () => {
-    const detector = new HighContrastModeDetector(fakePlatform, getFakeDocument('rgb(0,0,0)'));
+    const detector = getDetector(getFakeDocument('rgb(0,0,0)'));
     expect(detector.getHighContrastMode())
       .withContext('Expected high-contrast mode `WHITE_ON_BLACK`')
       .toBe(HighContrastMode.WHITE_ON_BLACK);
   });
 
   it('should detect BLACK_ON_WHITE when backgrounds are coerced to white ', () => {
-    const detector = new HighContrastModeDetector(
-      fakePlatform,
-      getFakeDocument('rgb(255,255,255)'),
-    );
+    const detector = getDetector(getFakeDocument('rgb(255,255,255)'));
     expect(detector.getHighContrastMode())
       .withContext('Expected high-contrast mode `BLACK_ON_WHITE`')
       .toBe(HighContrastMode.BLACK_ON_WHITE);
   });
 
   it('should detect NONE when backgrounds are not coerced ', () => {
-    const detector = new HighContrastModeDetector(fakePlatform, getFakeDocument('rgb(1,2,3)'));
+    const detector = getDetector(getFakeDocument('rgb(1,2,3)'));
     expect(detector.getHighContrastMode())
       .withContext('Expected high-contrast mode `NONE`')
       .toBe(HighContrastMode.NONE);
   });
 
   it('should apply css classes for BLACK_ON_WHITE high-contrast mode', () => {
     const fakeDocument = getFakeDocument('rgb(255,255,255)');
-    const detector = new HighContrastModeDetector(fakePlatform, fakeDocument);
+    const detector = getDetector(fakeDocument);
     detector._applyBodyHighContrastModeCssClasses();
     expect(fakeDocument.body.classList).toContain(HIGH_CONTRAST_MODE_ACTIVE_CSS_CLASS);
     expect(fakeDocument.body.classList).toContain(BLACK_ON_WHITE_CSS_CLASS);
   });
 
   it('should apply css classes for WHITE_ON_BLACK high-contrast mode', () => {
     const fakeDocument = getFakeDocument('rgb(0,0,0)');
-    const detector = new HighContrastModeDetector(fakePlatform, fakeDocument);
+    const detector = getDetector(fakeDocument);
     detector._applyBodyHighContrastModeCssClasses();
     expect(fakeDocument.body.classList).toContain(HIGH_CONTRAST_MODE_ACTIVE_CSS_CLASS);
     expect(fakeDocument.body.classList).toContain(WHITE_ON_BLACK_CSS_CLASS);
   });
 
   it('should not apply any css classes when backgrounds are not coerced', () => {
     const fakeDocument = getFakeDocument('');
-    const detector = new HighContrastModeDetector(fakePlatform, fakeDocument);
+    const detector = getDetector(fakeDocument);
     detector._applyBodyHighContrastModeCssClasses();
     expect(fakeDocument.body.className)
       .withContext('Expected body not to have any CSS classes in non-browser platforms')
@@ -88,6 +92,7 @@ function getFakeDocument(fakeComputedBackgroundColor: string) {
   return {
     body: document.createElement('body'),
     createElement: (tag: string) => document.createElement(tag),
+    querySelectorAll: (selector: string) => document.querySelectorAll(selector),
     defaultView: {
       getComputedStyle: () => ({backgroundColor: fakeComputedBackgroundColor}),
     },

src/cdk/a11y/high-contrast-mode/high-contrast-mode-detector.ts

@@ -6,9 +6,11 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
+import {inject, Inject, Injectable, OnDestroy} from '@angular/core';
+import {BreakpointObserver} from '@angular/cdk/layout';
 import {Platform} from '@angular/cdk/platform';
 import {DOCUMENT} from '@angular/common';
-import {Inject, Injectable} from '@angular/core';
+import {Subscription} from 'rxjs';
 
 /** Set of possible high-contrast mode backgrounds. */
 export const enum HighContrastMode {
@@ -38,16 +40,26 @@ export const HIGH_CONTRAST_MODE_ACTIVE_CSS_CLASS = 'cdk-high-contrast-active';
  * browser extension.
  */
 @Injectable({providedIn: 'root'})
-export class HighContrastModeDetector {
+export class HighContrastModeDetector implements OnDestroy {
   /**
    * Figuring out the high contrast mode and adding the body classes can cause
    * some expensive layouts. This flag is used to ensure that we only do it once.
    */
   private _hasCheckedHighContrastMode: boolean;
   private _document: Document;
+  private _breakpointSubscription: Subscription;
 
   constructor(private _platform: Platform, @Inject(DOCUMENT) document: any) {
     this._document = document;
+
+    this._breakpointSubscription = inject(BreakpointObserver)
+      .observe('(forced-colors: active)')
+      .subscribe(() => {
+        if (this._hasCheckedHighContrastMode) {
+          this._hasCheckedHighContrastMode = false;
+          this._applyBodyHighContrastModeCssClasses();
+        }
+      });
   }
 
   /** Gets the current high-contrast-mode for the page. */
@@ -88,6 +100,10 @@ export class HighContrastModeDetector {
     return HighContrastMode.NONE;
   }
 
+  ngOnDestroy(): void {
+    this._breakpointSubscription.unsubscribe();
+  }
+
   /** Applies CSS classes indicating high-contrast mode to document body (browser-only). */
   _applyBodyHighContrastModeCssClasses(): void {
     if (!this._hasCheckedHighContrastMode && this._platform.isBrowser && this._document.body) {

tools/public_api_guard/cdk/a11y.md

@@ -264,11 +264,13 @@ export const enum HighContrastMode {
 }
 
 // @public
-export class HighContrastModeDetector {
+export class HighContrastModeDetector implements OnDestroy {
     constructor(_platform: Platform, document: any);
     _applyBodyHighContrastModeCssClasses(): void;
     getHighContrastMode(): HighContrastMode;
     // (undocumented)
+    ngOnDestroy(): void;
+    // (undocumented)
     static ɵfac: i0.ɵɵFactoryDeclaration<HighContrastModeDetector, never>;
     // (undocumented)
     static ɵprov: i0.ɵɵInjectableDeclaration<HighContrastModeDetector>;