fix(snack-bar): set appropriate role based on passed in politeness (#13864)

Currently we always have a `role` of `alert` on the snack bar container which has an implicit politeness of `assertive` and which can override the message announced by the live announcer. These changes set the role to `status` if it's set to `polite` or remove the role completely if the politeness is turned off.

Fixes #13493.

Author: crisbeto

src/lib/snack-bar/snack-bar-container.ts

@@ -44,7 +44,7 @@ import {MatSnackBarConfig} from './snack-bar-config';
   encapsulation: ViewEncapsulation.None,
   animations: [matSnackBarAnimations.snackBarState],
   host: {
-    'role': 'alert',
+    '[attr.role]': '_role',
     'class': 'mat-snack-bar-container',
     '[@state]': '_animationState',
     '(@state.done)': 'onAnimationEnd($event)'
@@ -66,6 +66,9 @@ export class MatSnackBarContainer extends BasePortalOutlet implements OnDestroy
   /** The state of the snack bar animations. */
   _animationState = 'void';
 
+  /** ARIA role for the snack bar container. */
+  _role: 'alert' | 'status' | null;
+
   constructor(
     private _ngZone: NgZone,
     private _elementRef: ElementRef<HTMLElement>,
@@ -74,6 +77,16 @@ export class MatSnackBarContainer extends BasePortalOutlet implements OnDestroy
     public snackBarConfig: MatSnackBarConfig) {
 
     super();
+
+    // Based on the ARIA spec, `alert` and `status` roles have an
+    // implicit `assertive` and `polite` politeness respectively.
+    if (snackBarConfig.politeness === 'assertive') {
+      this._role = 'alert';
+    } else if (snackBarConfig.politeness === 'polite') {
+      this._role = 'status';
+    } else {
+      this._role = null;
+    }
   }
 
   /** Attach a component portal as content to this snack bar container. */

src/lib/snack-bar/snack-bar.spec.ts

@@ -67,13 +67,31 @@ describe('MatSnackBar', () => {
     testViewContainerRef = viewContainerFixture.componentInstance.childViewContainer;
   });
 
-  it('should have the role of alert', () => {
-    snackBar.open(simpleMessage, simpleActionLabel);
+  it('should have the role of `alert` with an `assertive` politeness', () => {
+    snackBar.open(simpleMessage, simpleActionLabel, {politeness: 'assertive'});
+    viewContainerFixture.detectChanges();
 
-    let containerElement = overlayContainerElement.querySelector('snack-bar-container')!;
+    const containerElement = overlayContainerElement.querySelector('snack-bar-container')!;
     expect(containerElement.getAttribute('role'))
         .toBe('alert', 'Expected snack bar container to have role="alert"');
-   });
+  });
+
+  it('should have the role of `status` with a `polite` politeness', () => {
+    snackBar.open(simpleMessage, simpleActionLabel, {politeness: 'polite'});
+    viewContainerFixture.detectChanges();
+
+    const containerElement = overlayContainerElement.querySelector('snack-bar-container')!;
+    expect(containerElement.getAttribute('role'))
+        .toBe('status', 'Expected snack bar container to have role="status"');
+  });
+
+  it('should remove the role if the politeness is turned off', () => {
+    snackBar.open(simpleMessage, simpleActionLabel, {politeness: 'off'});
+    viewContainerFixture.detectChanges();
+
+    const containerElement = overlayContainerElement.querySelector('snack-bar-container')!;
+    expect(containerElement.getAttribute('role')).toBeFalsy('Expected role to be removed');
+  });
 
   it('should open and close a snackbar without a ViewContainerRef', fakeAsync(() => {
     let snackBarRef = snackBar.open('Snack time!', 'Chew');