fix(cdk/observers): don't observe content of comments (#28858)

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

@@ -7,8 +7,8 @@ import {By} from '@angular/platform-browser';
 import {A11yModule} from '../index';
 import {LiveAnnouncer} from './live-announcer';
 import {
-  LIVE_ANNOUNCER_ELEMENT_TOKEN,
   LIVE_ANNOUNCER_DEFAULT_OPTIONS,
+  LIVE_ANNOUNCER_ELEMENT_TOKEN,
   LiveAnnouncerDefaultOptions,
 } from './live-announcer-tokens';
 
@@ -291,7 +291,7 @@ describe('CdkAriaLive', () => {
   let announcerSpy: jasmine.Spy;
   let fixture: ComponentFixture<DivWithCdkAriaLive>;
 
-  const invokeMutationCallbacks = () => mutationCallbacks.forEach(cb => cb());
+  const invokeMutationCallbacks = () => mutationCallbacks.forEach(cb => cb([{type: 'fake'}]));
 
   beforeEach(fakeAsync(() => {
     TestBed.configureTestingModule({

src/cdk/observers/observe-content.spec.ts

@@ -1,11 +1,11 @@
 import {Component, ElementRef, ViewChild} from '@angular/core';
 import {
-  waitForAsync,
   ComponentFixture,
+  TestBed,
   fakeAsync,
   inject,
-  TestBed,
   tick,
+  waitForAsync,
 } from '@angular/core/testing';
 import {ContentObserver, MutationObserverFactory, ObserversModule} from './observe-content';
 
@@ -112,9 +112,9 @@ describe('Observe content directive', () => {
     }));
 
     it('should debounce the content changes', fakeAsync(() => {
-      invokeCallbacks();
-      invokeCallbacks();
-      invokeCallbacks();
+      invokeCallbacks([{type: 'fake'}]);
+      invokeCallbacks([{type: 'fake'}]);
+      invokeCallbacks([{type: 'fake'}]);
 
       tick(500);
       expect(fixture.componentInstance.spy).toHaveBeenCalledTimes(1);
@@ -166,7 +166,7 @@ describe('ContentObserver injectable', () => {
       expect(spy).not.toHaveBeenCalled();
 
       fixture.componentInstance.text = 'text';
-      invokeCallbacks();
+      invokeCallbacks([{type: 'fake'}]);
 
       expect(spy).toHaveBeenCalled();
     }));
@@ -186,19 +186,90 @@ describe('ContentObserver injectable', () => {
         expect(mof.create).toHaveBeenCalledTimes(1);
 
         fixture.componentInstance.text = 'text';
-        invokeCallbacks();
+        invokeCallbacks([{type: 'fake'}]);
 
         expect(spy).toHaveBeenCalledTimes(2);
 
         spy.calls.reset();
         sub1.unsubscribe();
         fixture.componentInstance.text = 'text text';
-        invokeCallbacks();
+        invokeCallbacks([{type: 'fake'}]);
 
         expect(spy).toHaveBeenCalledTimes(1);
       }),
     ));
   });
+
+  describe('real behavior', () => {
+    let spy: jasmine.Spy;
+    let contentEl: HTMLElement;
+    let contentObserver: ContentObserver;
+
+    beforeEach(waitForAsync(() => {
+      TestBed.configureTestingModule({
+        imports: [ObserversModule, UnobservedComponentWithTextContent],
+      });
+
+      TestBed.compileComponents();
+      const fixture = TestBed.createComponent(UnobservedComponentWithTextContent);
+      fixture.autoDetectChanges();
+      spy = jasmine.createSpy('content observer');
+      contentObserver = TestBed.inject(ContentObserver);
+      contentEl = fixture.componentInstance.contentEl.nativeElement;
+      contentObserver.observe(contentEl).subscribe(spy);
+    }));
+
+    it('should ignore addition or removal of comments', waitForAsync(async () => {
+      const comment = document.createComment('cool');
+      await new Promise(r => setTimeout(r));
+
+      spy.calls.reset();
+      contentEl.appendChild(comment);
+      await new Promise(r => setTimeout(r));
+      expect(spy).not.toHaveBeenCalled();
+
+      comment.remove();
+      await new Promise(r => setTimeout(r));
+      expect(spy).not.toHaveBeenCalled();
+    }));
+
+    it('should not ignore addition or removal of text', waitForAsync(async () => {
+      const text = document.createTextNode('cool');
+      await new Promise(r => setTimeout(r));
+
+      spy.calls.reset();
+      contentEl.appendChild(text);
+      await new Promise(r => setTimeout(r));
+      expect(spy).toHaveBeenCalled();
+
+      spy.calls.reset();
+      text.remove();
+      await new Promise(r => setTimeout(r));
+      expect(spy).toHaveBeenCalled();
+    }));
+
+    it('should ignore comment content change', waitForAsync(async () => {
+      const comment = document.createComment('cool');
+      contentEl.appendChild(comment);
+      await new Promise(r => setTimeout(r));
+
+      spy.calls.reset();
+      comment.textContent = 'beans';
+      await new Promise(r => setTimeout(r));
+      expect(spy).not.toHaveBeenCalled();
+    }));
+
+    it('should not ignore text content change', waitForAsync(async () => {
+      const text = document.createTextNode('cool');
+      contentEl.appendChild(text);
+      await new Promise(r => setTimeout(r));
+
+      spy.calls.reset();
+      text.textContent = 'beans';
+      await new Promise(r => setTimeout(r));
+      expect(spy).toHaveBeenCalled();
+    }));
+  });
 });
 
 @Component({

src/cdk/observers/observe-content.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {coerceNumberProperty, coerceElement, NumberInput} from '@angular/cdk/coercion';
+import {NumberInput, coerceElement, coerceNumberProperty} from '@angular/cdk/coercion';
 import {
   AfterContentInit,
   Directive,
@@ -20,8 +20,35 @@ import {
   Output,
   booleanAttribute,
 } from '@angular/core';
-import {Observable, Subject, Subscription, Observer} from 'rxjs';
-import {debounceTime} from 'rxjs/operators';
+import {Observable, Observer, Subject, Subscription} from 'rxjs';
+import {debounceTime, filter, map} from 'rxjs/operators';
+
+// Angular may add, remove, or edit comment nodes during change detection. We don't care about
+// these changes because they don't affect the user-preceived content, and worse it can cause
+// infinite change detection cycles where the change detection updates a comment, triggering the
+// MutationObserver, triggering another change detection and kicking the cycle off again.
+function shouldIgnoreRecord(record: MutationRecord) {
+  // Ignore changes to comment text.
+  if (record.type === 'characterData' && record.target instanceof Comment) {
+    return true;
+  }
+  // Ignore addition / removal of comments.
+  if (record.type === 'childList') {
+    for (let i = 0; i < record.addedNodes.length; i++) {
+      if (!(record.addedNodes[i] instanceof Comment)) {
+        return false;
+      }
+    }
+    for (let i = 0; i < record.removedNodes.length; i++) {
+      if (!(record.removedNodes[i] instanceof Comment)) {
+        return false;
+      }
+    }
+    return true;
+  }
+  // Observe everything else.
+  return false;
+}
 
 /**
  * Factory that creates a new MutationObserver and allows us to stub it out in unit tests.
@@ -70,7 +97,12 @@ export class ContentObserver implements OnDestroy {
 
     return new Observable((observer: Observer<MutationRecord[]>) => {
       const stream = this._observeElement(element);
-      const subscription = stream.subscribe(observer);
+      const subscription = stream
+        .pipe(
+          map(records => records.filter(record => !shouldIgnoreRecord(record))),
+          filter(records => !!records.length),
+        )
+        .subscribe(observer);
 
       return () => {
         subscription.unsubscribe();

src/material/tabs/tab-header.spec.ts

@@ -715,7 +715,7 @@ describe('MDC-based MatTabHeader', () => {
         label.textContent += extraText;
       });
 
-      mutationCallbacks.forEach(callback => callback());
+      mutationCallbacks.forEach(callback => callback([{type: 'fake'}]));
       fixture.detectChanges();
 
       expect(tabHeaderElement.classList).toContain(enabledClass);