fix(select): support ctrl+a shortcut for multi-select

[Based on the accessibility guidelines](https://www.w3.org/TR/wai-aria-practices-1.1/#Listbox), multi-selection lists should support selecting/deselecting all using ctrl+a.

Author: crisbeto

src/lib/select/select.spec.ts

@@ -9,6 +9,7 @@ import {
   SPACE,
   TAB,
   UP_ARROW,
+  A,
 } from '@angular/cdk/keycodes';
 import {OverlayContainer} from '@angular/cdk/overlay';
 import {Platform} from '@angular/cdk/platform';
@@ -3823,6 +3824,96 @@ describe('MatSelect', () => {
       expect(fixture.componentInstance.select._keyManager.activeItemIndex).toBe(2);
     }));
 
+    it('should select all options when pressing ctrl + a', () => {
+      const selectElement = fixture.nativeElement.querySelector('mat-select');
+      const options = fixture.componentInstance.options.toArray();
+
+      expect(testInstance.control.value).toBeFalsy();
+      expect(options.every(option => option.selected)).toBe(false);
+
+      fixture.componentInstance.select.open();
+      fixture.detectChanges();
+
+      const event = createKeyboardEvent('keydown', A, selectElement);
+      Object.defineProperty(event, 'ctrlKey', {get: () => true});
+      dispatchEvent(selectElement, event);
+      fixture.detectChanges();
+
+      expect(options.every(option => option.selected)).toBe(true);
+      expect(testInstance.control.value).toEqual([
+        'steak-0',
+        'pizza-1',
+        'tacos-2',
+        'sandwich-3',
+        'chips-4',
+        'eggs-5',
+        'pasta-6',
+        'sushi-7'
+      ]);
+    });
+
+    it('should select all options when pressing ctrl + a when some options are selected', () => {
+      const selectElement = fixture.nativeElement.querySelector('mat-select');
+      const options = fixture.componentInstance.options.toArray();
+
+      options[0].select();
+      fixture.detectChanges();
+
+      expect(testInstance.control.value).toEqual(['steak-0']);
+      expect(options.some(option => option.selected)).toBe(true);
+
+      fixture.componentInstance.select.open();
+      fixture.detectChanges();
+
+      const event = createKeyboardEvent('keydown', A, selectElement);
+      Object.defineProperty(event, 'ctrlKey', {get: () => true});
+      dispatchEvent(selectElement, event);
+      fixture.detectChanges();
+
+      expect(options.every(option => option.selected)).toBe(true);
+      expect(testInstance.control.value).toEqual([
+        'steak-0',
+        'pizza-1',
+        'tacos-2',
+        'sandwich-3',
+        'chips-4',
+        'eggs-5',
+        'pasta-6',
+        'sushi-7'
+      ]);
+    });
+
+    it('should deselect all options with ctrl + a if all options are selected', () => {
+      const selectElement = fixture.nativeElement.querySelector('mat-select');
+      const options = fixture.componentInstance.options.toArray();
+
+      options.forEach(option => option.select());
+      fixture.detectChanges();
+
+      expect(testInstance.control.value).toEqual([
+        'steak-0',
+        'pizza-1',
+        'tacos-2',
+        'sandwich-3',
+        'chips-4',
+        'eggs-5',
+        'pasta-6',
+        'sushi-7'
+      ]);
+      expect(options.every(option => option.selected)).toBe(true);
+
+      fixture.componentInstance.select.open();
+      fixture.detectChanges();
+
+      const event = createKeyboardEvent('keydown', A, selectElement);
+      Object.defineProperty(event, 'ctrlKey', {get: () => true});
+      dispatchEvent(selectElement, event);
+      fixture.detectChanges();
+
+      expect(options.some(option => option.selected)).toBe(false);
+      expect(testInstance.control.value).toEqual([]);
+    });
+
   });
 });
 

src/lib/select/select.ts

@@ -19,6 +19,7 @@ import {
   RIGHT_ARROW,
   SPACE,
   UP_ARROW,
+  A,
 } from '@angular/cdk/keycodes';
 import {
   CdkConnectedOverlay,
@@ -695,6 +696,10 @@ export class MatSelect extends _MatSelectMixinBase implements AfterContentInit,
     } else if ((keyCode === ENTER || keyCode === SPACE) && manager.activeItem) {
       event.preventDefault();
       manager.activeItem._selectViaInteraction();
+    } else if (this._multiple && keyCode === A && event.ctrlKey) {
+      event.preventDefault();
+      const hasDeselectedOptions = this.options.some(option => !option.selected);
+      this.options.forEach(option => hasDeselectedOptions ? option.select() : option.deselect());
     } else {
       const previouslyFocusedIndex = manager.activeItemIndex;