feat(material/snackbar): add isDismissed harness method

src/material/snack-bar/snack-bar-container.ts

@@ -152,6 +152,12 @@ export class MatSnackBarContainer extends BasePortalOutlet implements OnDestroy
     // where multiple snack bars are opened in quick succession (e.g. two consecutive calls to
     // `MatSnackBar.open`).
     this._animationState = 'hidden';
+
+    // Mark this element with an 'exit' attribute to indicate that the snackbar has
+    // been dismissed and will soon be removed from the DOM. This is used by the snackbar
+    // test harness.
+    this._elementRef.nativeElement.setAttribute('mat-exit', '');
+
     return this._onExit;
   }
 

src/material/snack-bar/snack-bar.spec.ts

@@ -177,6 +177,10 @@ describe('MatSnackBar', () => {
     snackBarRef.afterDismissed().subscribe({complete: dismissCompleteSpy});
 
     snackBarRef.dismiss();
+    const messageElement = overlayContainerElement.querySelector('snack-bar-container')!;
+    expect (messageElement.hasAttribute('mat-exit'))
+        .toBe(true, 'Expected the snackbar container to have the "exit" attribute upon dismiss');
+
     viewContainerFixture.detectChanges();  // Run through animations for dismissal
     flush();
 

src/material/snack-bar/testing/shared.spec.ts

@@ -128,8 +128,13 @@ export function runHarnessTests(
     let actionCount = 0;
     snackBarRef.onAction().subscribe(() => actionCount++);
 
+    expect(await snackBar.isDismissed())
+        .toBe(false, 'The snackbar should be present in the DOM before dismiss');
+
     await snackBar.dismissWithAction();
     expect(actionCount).toBe(1);
+    expect(await snackBar.isDismissed())
+        .toBe(true, 'The snackbar should be absent from the DOM after dismiss');
 
     fixture.componentInstance.openSimple('No action');
     snackBar = await loader.getHarness(snackBarHarness);

src/material/snack-bar/testing/snack-bar-harness.ts

@@ -74,13 +74,29 @@ export class MatSnackBarHarness extends ComponentHarness {
     return (await this._simpleSnackBarMessage()).text();
   }
 
+  /** Gets whether the snack-bar has been dismissed. */
+  async isDismissed(): Promise<boolean> {
+    // We consider the snackbar dismissed if it's not in the DOM. We can assert that the
+    // element isn't in the DOM by seeing that its width and height are zero.
+
+    const host = await this.host();
+    const [exit, dimensions] = await Promise.all([
+      // The snackbar container is marked with the "exit" attribute after it has been dismissed
+      // but before the animation has finished (after which it's removed from the DOM).
+      host.getAttribute('mat-exit'),
+      host.getDimensions(),
+    ]);
+
+    return exit != null || (!!dimensions && dimensions.height === 0 && dimensions.width === 0);
+  }
+
   /**
    * Asserts that the current snack-bar does not use custom content. Promise rejects if
    * custom content is used.
    */
   private async _assertSimpleSnackBar(): Promise<void> {
     if (!await this._isSimpleSnackBar()) {
-      throw new Error('Method cannot be used for snack-bar with custom content.');
+      throw Error('Method cannot be used for snack-bar with custom content.');
     }
   }
 
@@ -91,7 +107,7 @@ export class MatSnackBarHarness extends ComponentHarness {
   private async _assertSimpleSnackBarWithAction(): Promise<void> {
     await this._assertSimpleSnackBar();
     if (!await this.hasAction()) {
-      throw new Error('Method cannot be used for standard snack-bar without action.');
+      throw Error('Method cannot be used for standard snack-bar without action.');
     }
   }
 

tools/public_api_guard/material/snack-bar/testing.d.ts

@@ -4,6 +4,7 @@ export declare class MatSnackBarHarness extends ComponentHarness {
     getMessage(): Promise<string>;
     getRole(): Promise<'alert' | 'status' | null>;
     hasAction(): Promise<boolean>;
+    isDismissed(): Promise<boolean>;
     static hostSelector: string;
     static with(options?: SnackBarHarnessFilters): HarnessPredicate<MatSnackBarHarness>;
 }