address comments

Author: mmalerba

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

@@ -128,7 +128,7 @@ describe('CdkAriaLive', () => {
       providers: [{
         provide: MutationObserverFactory,
         useValue: {
-          create: function(callback: Function) {
+          create: (callback: Function) => {
             mutationCallbacks.push(callback);
 
             return {

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

@@ -95,6 +95,7 @@ export class LiveAnnouncer implements OnDestroy {
   selector: '[cdkAriaLive]'
 })
 export class CdkAriaLive implements OnDestroy {
+  /** The aria-live politeness level to use when announcing messages. */
   @Input('cdkAriaLive')
   get politeness(): AriaLivePoliteness { return this._politeness; }
   set politeness(value: AriaLivePoliteness) {
@@ -105,10 +106,12 @@ export class CdkAriaLive implements OnDestroy {
         this._subscription = null;
       }
     } else {
-      this._subscription = this._ngZone.runOutsideAngular(
-          () => this._contentObserver.observe(this._elementRef.nativeElement).subscribe(
-              () => this._liveAnnouncer.announce(
-                  this._elementRef.nativeElement.innerText, this._politeness)));
+      if (!this._subscription) {
+        this._subscription = this._ngZone.runOutsideAngular(
+            () => this._contentObserver.observe(this._elementRef.nativeElement).subscribe(
+                () => this._liveAnnouncer.announce(
+                    this._elementRef.nativeElement.innerText, this._politeness)));
+      }
     }
   }
   private _politeness: AriaLivePoliteness = 'off';