fix(snack-bar): dismiss snack bar on destroy (#13042)

Dismisses the currently-opened snack bar in `ngOnDestroy`.

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

@@ -452,6 +452,18 @@ describe('MatSnackBar', () => {
         .toContain('custom-class', 'Expected class applied through the defaults to be applied.');
   }));
 
+  it('should dismiss the open snack bar on destroy', fakeAsync(() => {
+    snackBar.open(simpleMessage);
+    viewContainerFixture.detectChanges();
+    expect(overlayContainerElement.childElementCount).toBeGreaterThan(0);
+
+    snackBar.ngOnDestroy();
+    viewContainerFixture.detectChanges();
+    flush();
+
+    expect(overlayContainerElement.childElementCount).toBe(0);
+  }));
+
   describe('with custom component', () => {
     it('should open a custom component', () => {
       const snackBarRef = snackBar.openFromComponent(BurritosNotification);
@@ -603,6 +615,18 @@ describe('MatSnackBar with parent MatSnackBar', () => {
     expect(overlayContainerElement.textContent)
         .toContain('Taco', 'Expected child snackbar msg to be dismissed by opening from parent');
   }));
+
+  it('should not dismiss parent snack bar if child is destroyed', fakeAsync(() => {
+    parentSnackBar.open('Pizza');
+    fixture.detectChanges();
+    expect(overlayContainerElement.childElementCount).toBeGreaterThan(0);
+
+    childSnackBar.ngOnDestroy();
+    fixture.detectChanges();
+    flush();
+
+    expect(overlayContainerElement.childElementCount).toBeGreaterThan(0);
+  }));
 });
 
 describe('MatSnackBar Positioning', () => {

src/lib/snack-bar/snack-bar.ts

@@ -20,6 +20,7 @@ import {
   Optional,
   SkipSelf,
   TemplateRef,
+  OnDestroy,
 } from '@angular/core';
 import {take, takeUntil} from 'rxjs/operators';
 import {SimpleSnackBar} from './simple-snack-bar';
@@ -45,7 +46,7 @@ export function MAT_SNACK_BAR_DEFAULT_OPTIONS_FACTORY(): MatSnackBarConfig {
  * Service to dispatch Material Design snack bar messages.
  */
 @Injectable({providedIn: MatSnackBarModule})
-export class MatSnackBar {
+export class MatSnackBar implements OnDestroy {
   /**
    * Reference to the current snack bar in the view *at this level* (in the Angular injector tree).
    * If there is a parent snack-bar service, all operations should delegate to that parent
@@ -129,6 +130,13 @@ export class MatSnackBar {
     }
   }
 
+  ngOnDestroy() {
+    // Only dismiss the snack bar at the current level on destroy.
+    if (this._snackBarRefAtThisLevel) {
+      this._snackBarRefAtThisLevel.dismiss();
+    }
+  }
+
   /**
    * Attaches the snack bar container component to the overlay.
    */