feat(MenuSurface): Add opening event for menus.

BREAKING CHANGE: Adds #notifyOpening method to menu surface adapter.

PiperOrigin-RevId: 444830518

Author: yavanosta

packages/mdc-menu-surface/adapter.ts

@@ -64,4 +64,7 @@ export interface MDCMenuSurfaceAdapter {
 
   /** Emits an event when the menu surface is opened. */
   notifyOpen(): void;
+
+  /** Emits an event when the menu surface is opening. */
+  notifyOpening(): void;
 }

packages/mdc-menu-surface/component.ts

@@ -160,6 +160,8 @@ export class MDCMenuSurface extends MDCComponent<MDCMenuSurfaceFoundation> {
       },
       notifyOpen: () =>
           this.emit(MDCMenuSurfaceFoundation.strings.OPENED_EVENT, {}),
+      notifyOpening: () =>
+          this.emit(MDCMenuSurfaceFoundation.strings.OPENING_EVENT, {}),
       isElementInContainer: (el) => this.root.contains(el),
       isRtl: () =>
           getComputedStyle(this.root).getPropertyValue('direction') === 'rtl',

packages/mdc-menu-surface/constants.ts

@@ -36,6 +36,7 @@ const strings = {
   CLOSED_EVENT: 'MDCMenuSurface:closed',
   CLOSING_EVENT: 'MDCMenuSurface:closing',
   OPENED_EVENT: 'MDCMenuSurface:opened',
+  OPENING_EVENT: 'MDCMenuSurface:opening',
   FOCUSABLE_ELEMENTS: [
     'button:not(:disabled)',
     '[href]:not([aria-disabled="true"])',

packages/mdc-menu-surface/foundation.ts

@@ -82,8 +82,9 @@ export class MDCMenuSurfaceFoundation extends
       restoreFocus: () => undefined,
 
       notifyClose: () => undefined,
-      notifyOpen: () => undefined,
       notifyClosing: () => undefined,
+      notifyOpen: () => undefined,
+      notifyOpening: () => undefined,
     };
     // tslint:enable:object-literal-sort-keys
   }
@@ -238,6 +239,7 @@ export class MDCMenuSurfaceFoundation extends
       return;
     }
 
+    this.adapter.notifyOpening();
     this.adapter.saveFocus();
 
     if (this.isQuickOpen) {

packages/mdc-menu-surface/test/component.test.ts

@@ -316,6 +316,15 @@ describe('MDCMenuSurface', () => {
     expect(handler).toHaveBeenCalled();
   });
 
+  it(`adapter#notifyOpening fires an ${strings.OPENING_EVENT} custom event`,
+     () => {
+       const {root, component} = setupTest();
+       const handler = jasmine.createSpy('notifyOpening handler');
+       root.addEventListener(strings.OPENING_EVENT, handler);
+       (component.getDefaultFoundation() as any).adapter.notifyOpening();
+       expect(handler).toHaveBeenCalled();
+     });
+
   it('adapter#restoreFocus restores focus saved by adapter#saveFocus', () => {
     const {root, component} = setupTest({open: true});
     const button = document.createElement('button');

packages/mdc-menu-surface/test/foundation.test.ts

@@ -196,6 +196,7 @@ describe('MDCMenuSurfaceFoundation', () => {
       'notifyClose',
       'notifyClosing',
       'notifyOpen',
+      'notifyOpening',
       'isElementInContainer',
       'isRtl',
       'setTransformOrigin',
@@ -264,6 +265,13 @@ describe('MDCMenuSurfaceFoundation', () => {
             .toHaveBeenCalledWith(cssClasses.ANIMATING_OPEN);
       });
 
+  testFoundation(
+      '#open emits the opening event at the beginning of the animation',
+      ({foundation, mockAdapter}) => {
+        foundation.open();
+        expect(mockAdapter.notifyOpening).toHaveBeenCalled();
+      });
+
   testFoundation(
       '#open emits the open event at the end of the animation',
       ({foundation, mockAdapter}) => {