feat(a11y): add input to control the duration of the aria live directive

Adds an input that allows the consumer to control how long it takes before the messages that are announced by `CdkAriaLive` to be cleared from the DOM.

Author: crisbeto

src/cdk/a11y/live-announcer/live-announcer.spec.ts

@@ -1,5 +1,5 @@
 import {MutationObserverFactory} from '@angular/cdk/observers';
-import {Component, Input} from '@angular/core';
+import {Component} from '@angular/core';
 import {ComponentFixture, fakeAsync, flush, inject, TestBed, tick} from '@angular/core/testing';
 import {By} from '@angular/platform-browser';
 import {A11yModule} from '../index';
@@ -284,7 +284,7 @@ describe('CdkAriaLive', () => {
     invokeMutationCallbacks();
     flush();
 
-    expect(announcer.announce).toHaveBeenCalledWith('New content', 'polite');
+    expect(announcer.announce).toHaveBeenCalledWith('New content', 'polite', undefined);
 
     announcerSpy.calls.reset();
     fixture.componentInstance.politeness = 'off';
@@ -302,7 +302,7 @@ describe('CdkAriaLive', () => {
     invokeMutationCallbacks();
     flush();
 
-    expect(announcer.announce).toHaveBeenCalledWith('Newest content', 'assertive');
+    expect(announcer.announce).toHaveBeenCalledWith('Newest content', 'assertive', undefined);
   }));
 
   it('should not announce the same text multiple times', fakeAsync(() => {
@@ -320,6 +320,16 @@ describe('CdkAriaLive', () => {
     expect(announcer.announce).toHaveBeenCalledTimes(1);
   }));
 
+  it('should be able to pass in a duration', fakeAsync(() => {
+    fixture.componentInstance.content = 'New content';
+    fixture.componentInstance.duration = 1337;
+    fixture.detectChanges();
+    invokeMutationCallbacks();
+    flush();
+
+    expect(announcer.announce).toHaveBeenCalledWith('New content', 'polite', 1337);
+  }));
+
 });
 
 
@@ -336,8 +346,11 @@ class TestApp {
   }
 }
 
-@Component({template: `<div [cdkAriaLive]="politeness">{{content}}</div>`})
+@Component({
+  template: `<div [cdkAriaLive]="politeness" [cdkAriaLiveDuration]="duration">{{content}}</div>`
+})
 class DivWithCdkAriaLive {
-  @Input() politeness = 'polite';
-  @Input() content = 'Initial content';
+  politeness = 'polite';
+  content = 'Initial content';
+  duration: number;
 }

src/cdk/a11y/live-announcer/live-announcer.ts

@@ -205,7 +205,7 @@ export class CdkAriaLive implements OnDestroy {
             // The `MutationObserver` fires also for attribute
             // changes which we don't want to announce.
             if (elementText !== this._previousAnnouncedText) {
-              this._liveAnnouncer.announce(elementText, this._politeness);
+              this._liveAnnouncer.announce(elementText, this._politeness, this.duration);
               this._previousAnnouncedText = elementText;
             }
           });
@@ -214,6 +214,9 @@ export class CdkAriaLive implements OnDestroy {
   }
   private _politeness: AriaLivePoliteness = 'off';
 
+  /** Time in milliseconds after which to clear out the announcer element. */
+  @Input('cdkAriaLiveDuration') duration: number;
+
   private _previousAnnouncedText?: string;
   private _subscription: Subscription | null;
 

tools/public_api_guard/cdk/a11y.d.ts

@@ -28,6 +28,7 @@ export declare const CDK_DESCRIBEDBY_HOST_ATTRIBUTE = "cdk-describedby-host";
 export declare const CDK_DESCRIBEDBY_ID_PREFIX = "cdk-describedby-message";
 
 export declare class CdkAriaLive implements OnDestroy {
+    duration: number;
     politeness: AriaLivePoliteness;
     constructor(_elementRef: ElementRef, _liveAnnouncer: LiveAnnouncer, _contentObserver: ContentObserver, _ngZone: NgZone);
     ngOnDestroy(): void;