fix(cdk/testing): fix behavior in fakeAsync tests

Author: mmalerba

src/cdk/testing/testbed/testbed-harness-environment.ts

@@ -67,13 +67,22 @@ function uninstallAutoChangeDetectionStatusHandler(fixture: ComponentFixture<unk
   }
 }
 
+/** Whether we are currently in the fake async zone. */
+function isInFakeAsyncZone() {
+  return Zone?.current.get('FakeAsyncTestZoneSpec') != null;
+}
+
 /**
  * Triggers change detection for a specific fixture.
  * @param fixture The fixture to trigger change detection for.
  */
 async function detectChanges(fixture: ComponentFixture<unknown>) {
   fixture.detectChanges();
-  await fixture.whenStable();
+  if (isInFakeAsyncZone()) {
+    flush();
+  } else {
+    await fixture.whenStable();
+  }
 }
 
 /** A `HarnessEnvironment` implementation for Angular's Testbed. */
@@ -153,7 +162,7 @@ export class TestbedHarnessEnvironment extends HarnessEnvironment<Element> {
     // cannot just rely on the task state observable to become stable because the state will
     // never change. This is because the task queue will be only drained if the fake async
     // zone is being flushed.
-    if (Zone!.current.get('FakeAsyncTestZoneSpec')) {
+    if (isInFakeAsyncZone()) {
       flush();
     }
 

src/cdk/testing/tests/testbed.spec.ts

@@ -83,6 +83,13 @@ describe('TestbedHarnessEnvironment', () => {
         const element = TestbedHarnessEnvironment.getNativeElement(await harness.host());
         expect(element.id).toContain('root');
       });
+
+      it('should wait for async operation to complete in fakeAsync test', fakeAsync(async () => {
+        const asyncCounter = await harness.asyncCounter();
+        expect(await asyncCounter.text()).toBe('5');
+        await harness.increaseCounter(3);
+        expect(await asyncCounter.text()).toBe('8');
+      }));
     });
 
     describe('change detection behavior', () => {