fix(snack-bar): prevent content from overriding configured aria-live message (#12294)

* Currently the `announcementMessage` config option is always being overwritten when opening a simple snackbar.

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

@@ -26,7 +26,10 @@ export class MatSnackBarConfig<D = any> {
   /** The politeness level for the MatAriaLiveAnnouncer announcement. */
   politeness?: AriaLivePoliteness = 'assertive';
 
-  /** Message to be announced by the MatAriaLiveAnnouncer */
+  /**
+   * Message to be announced by the LiveAnnouncer. When opening a snackbar without a custom
+   * component or template, the announcement message will default to the specified message.
+   */
   announcementMessage?: string = '';
 
   /** The view container to place the overlay for the snack bar into. */

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

@@ -75,21 +75,21 @@ describe('MatSnackBar', () => {
         .toBe('alert', 'Expected snack bar container to have role="alert"');
    });
 
-   it('should open and close a snackbar without a ViewContainerRef', fakeAsync(() => {
-      let snackBarRef = snackBar.open('Snack time!', 'Chew');
-      viewContainerFixture.detectChanges();
+  it('should open and close a snackbar without a ViewContainerRef', fakeAsync(() => {
+    let snackBarRef = snackBar.open('Snack time!', 'Chew');
+    viewContainerFixture.detectChanges();
 
-      let messageElement = overlayContainerElement.querySelector('snack-bar-container')!;
-      expect(messageElement.textContent).toContain('Snack time!',
-         'Expected snack bar to show a message without a ViewContainerRef');
+    let messageElement = overlayContainerElement.querySelector('snack-bar-container')!;
+    expect(messageElement.textContent).toContain('Snack time!',
+       'Expected snack bar to show a message without a ViewContainerRef');
 
-      snackBarRef.dismiss();
-      viewContainerFixture.detectChanges();
-      flush();
+    snackBarRef.dismiss();
+    viewContainerFixture.detectChanges();
+    flush();
 
-      expect(overlayContainerElement.childNodes.length)
-          .toBe(0, 'Expected snack bar to be dismissed without a ViewContainerRef');
-   }));
+    expect(overlayContainerElement.childNodes.length)
+        .toBe(0, 'Expected snack bar to be dismissed without a ViewContainerRef');
+  }));
 
   it('should open a simple message with a button', () => {
     let config: MatSnackBarConfig = {viewContainerRef: testViewContainerRef};
@@ -153,6 +153,36 @@ describe('MatSnackBar', () => {
         .toBe(0, 'Expected the overlay container element to have no child elements');
   }));
 
+
+  it('should default to the passed message for the announcement message', fakeAsync(() => {
+    spyOn(liveAnnouncer, 'announce');
+
+    snackBar.open(simpleMessage);
+    viewContainerFixture.detectChanges();
+
+    expect(overlayContainerElement.childElementCount)
+      .toBe(1, 'Expected the overlay with the default announcement message to be added');
+
+    // Expect the live announcer to have been called with the display message and some
+    // string for the politeness. We do not want to test for the default politeness here.
+    expect(liveAnnouncer.announce).toHaveBeenCalledWith(simpleMessage, jasmine.any(String));
+  }));
+
+  it('should be able to specify a custom announcement message', fakeAsync(() => {
+    spyOn(liveAnnouncer, 'announce');
+
+    snackBar.open(simpleMessage, '', {
+      announcementMessage: 'Custom announcement',
+      politeness: 'assertive'
+    });
+    viewContainerFixture.detectChanges();
+
+    expect(overlayContainerElement.childElementCount)
+      .toBe(1, 'Expected the overlay with a custom `announcementMessage` to be added');
+
+    expect(liveAnnouncer.announce).toHaveBeenCalledWith('Custom announcement', 'assertive');
+  }));
+
   it('should be able to get dismissed through the service', fakeAsync(() => {
     snackBar.open(simpleMessage);
     viewContainerFixture.detectChanges();

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

@@ -112,7 +112,10 @@ export class MatSnackBar {
     // Since the user doesn't have access to the component, we can
     // override the data to pass in our own message and action.
     _config.data = {message, action};
-    _config.announcementMessage = message;
+
+    if (!_config.announcementMessage) {
+      _config.announcementMessage = message;
+    }
 
     return this.openFromComponent(SimpleSnackBar, _config);
   }