feat(observe-content): allow for the MutationObserver to be disabled

Adds the ability for users to disable the underlying `MutationObserver` inside the `CdkObserveContent` directive. This can be useful in the cases where it might be expensive to continue observing an element while it is invisible (e.g. an item inside of a closed dropdown).

Author: crisbeto

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

@@ -22,11 +22,11 @@ describe('Observe content', () => {
 
       // If the hint label is empty, expect no label.
       const spy = spyOn(fixture.componentInstance, 'doSomething').and.callFake(() => {
-        expect(spy.calls.any()).toBe(true);
+        expect(spy).toHaveBeenCalled();
         done();
       });
 
-      expect(spy.calls.any()).toBe(false);
+      expect(spy).not.toHaveBeenCalled();
 
       fixture.componentInstance.text = 'text';
       fixture.detectChanges();
@@ -38,15 +38,43 @@ describe('Observe content', () => {
 
       // If the hint label is empty, expect no label.
       const spy = spyOn(fixture.componentInstance, 'doSomething').and.callFake(() => {
-        expect(spy.calls.any()).toBe(true);
+        expect(spy).toHaveBeenCalled();
         done();
       });
 
-      expect(spy.calls.any()).toBe(false);
+      expect(spy).not.toHaveBeenCalled();
 
       fixture.componentInstance.text = 'text';
       fixture.detectChanges();
     });
+
+    it('should disconnect the MutationObserver when the directive is disabled', () => {
+      const observeSpy = jasmine.createSpy('observe spy');
+      const disconnectSpy = jasmine.createSpy('disconnect spy');
+
+      // Note: since we can't know exactly when the native MutationObserver will emit, we can't
+      // test this scenario reliably without risking flaky tests, which is why we supply a mock
+      // MutationObserver and check that the methods are called at the right time.
+      TestBed.overrideProvider(MutationObserverFactory, {
+        deps: [],
+        useFactory: () => ({
+          create: () => ({observe: observeSpy, disconnect: disconnectSpy})
+        })
+      });
+
+      const fixture = TestBed.createComponent(ComponentWithTextContent);
+      fixture.detectChanges();
+
+      expect(observeSpy).toHaveBeenCalledTimes(1);
+      expect(disconnectSpy).not.toHaveBeenCalled();
+
+      fixture.componentInstance.disabled = true;
+      fixture.detectChanges();
+
+      expect(observeSpy).toHaveBeenCalledTimes(1);
+      expect(disconnectSpy).toHaveBeenCalledTimes(1);
+    });
+
   });
 
   describe('debounced', () => {
@@ -93,9 +121,16 @@ describe('Observe content', () => {
 });
 
 
-@Component({ template: `<div (cdkObserveContent)="doSomething()">{{text}}</div>` })
+@Component({
+  template: `
+    <div
+      (cdkObserveContent)="doSomething()"
+      [cdkObserveContentDisabled]="disabled">{{text}}</div>
+  `
+})
 class ComponentWithTextContent {
   text = '';
+  disabled = false;
   doSomething() {}
 }
 

src/cdk/observers/observe-content.ts

@@ -17,7 +17,10 @@ import {
   AfterContentInit,
   Injectable,
   NgZone,
+  OnChanges,
+  SimpleChanges,
 } from '@angular/core';
+import {coerceBooleanProperty} from '@angular/cdk/coercion';
 import {Subject} from 'rxjs/Subject';
 import {debounceTime} from 'rxjs/operators/debounceTime';
 
@@ -40,12 +43,23 @@ export class MutationObserverFactory {
   selector: '[cdkObserveContent]',
   exportAs: 'cdkObserveContent',
 })
-export class CdkObserveContent implements AfterContentInit, OnDestroy {
+export class CdkObserveContent implements AfterContentInit, OnChanges, OnDestroy {
   private _observer: MutationObserver | null;
+  private _disabled = false;
 
   /** Event emitted for each change in the element's content. */
   @Output('cdkObserveContent') event = new EventEmitter<MutationRecord[]>();
 
+  /**
+   * Whether observing content is disabled. This option can be used
+   * to disconnect the underlying MutationObserver until it is needed.
+   */
+  @Input('cdkObserveContentDisabled')
+  get disabled() { return this._disabled; }
+  set disabled(value: any) {
+    this._disabled = coerceBooleanProperty(value);
+  }
+
   /** Used for debouncing the emitted values to the observeContent event. */
   private _debouncer = new Subject<MutationRecord[]>();
 
@@ -73,21 +87,36 @@ export class CdkObserveContent implements AfterContentInit, OnDestroy {
       });
     });
 
-    if (this._observer) {
-      this._observer.observe(this._elementRef.nativeElement, {
-        'characterData': true,
-        'childList': true,
-        'subtree': true
-      });
+    if (!this.disabled) {
+      this._enable();
+    }
+  }
+
+  ngOnChanges(changes: SimpleChanges) {
+    if (changes['disabled']) {
+      changes['disabled'].currentValue ? this._disable() : this._enable();
     }
   }
 
   ngOnDestroy() {
+    this._disable();
+    this._debouncer.complete();
+  }
+
+  private _disable() {
     if (this._observer) {
       this._observer.disconnect();
     }
+  }
 
-    this._debouncer.complete();
+  private _enable() {
+    if (this._observer) {
+      this._observer.observe(this._elementRef.nativeElement, {
+        characterData: true,
+        childList: true,
+        subtree: true
+      });
+    }
   }
 }