feat(select): add ability to cycle through options with arrow keys when closed

* Adds the ability for users to select options by focusing on a closed `md-select` and pressing the up/down arrow keys.
* Fixes a bug that prevents the selection from going to the first item in a `ListKeyManager`, if there were no previously-selected items.
* Adds an extra null check to the `FocusKeyManager` to avoid issues where the focused item is cleared.

Fixes #2990.

Author: crisbeto

src/lib/core/a11y/focus-key-manager.ts

@@ -23,7 +23,10 @@ export class FocusKeyManager extends ListKeyManager<Focusable> {
    */
   setActiveItem(index: number): void {
     super.setActiveItem(index);
-    this.activeItem.focus();
+
+    if (this.activeItem) {
+      this.activeItem.focus();
+    }
   }
 
 }

src/lib/core/a11y/list-key-manager.spec.ts

@@ -240,6 +240,16 @@ describe('Key managers', () => {
         expect(TAB_EVENT.defaultPrevented).toBe(false);
       });
 
+      it('it should activate the first item when pressing down on a clean key manager', () => {
+        keyManager = new ListKeyManager<FakeFocusable>(itemList);
+
+        expect(keyManager.activeItemIndex).toBeNull('Expected active index to default to null.');
+
+        keyManager.onKeydown(DOWN_ARROW_EVENT);
+
+        expect(keyManager.activeItemIndex).toBe(0, 'Expected first item to become active.');
+      });
+
     });
 
     describe('programmatic focus', () => {

src/lib/core/a11y/list-key-manager.ts

@@ -16,7 +16,7 @@ export interface CanDisable {
  * of items, it will set the active item correctly when arrow events occur.
  */
 export class ListKeyManager<T extends CanDisable> {
-  private _activeItemIndex: number;
+  private _activeItemIndex: number = null;
   private _activeItem: T;
   private _tabOut: Subject<any> = new Subject();
   private _wrap: boolean = false;

src/lib/select/select.spec.ts

@@ -14,11 +14,12 @@ import {OverlayContainer} from '../core/overlay/overlay-container';
 import {MdSelect, MdSelectFloatPlaceholderType} from './select';
 import {MdOption} from '../core/option/option';
 import {Dir} from '../core/rtl/dir';
+import {DOWN_ARROW, UP_ARROW} from '../core/keyboard/keycodes';
 import {
   ControlValueAccessor, FormControl, FormsModule, NG_VALUE_ACCESSOR, ReactiveFormsModule
 } from '@angular/forms';
 import {ViewportRuler} from '../core/overlay/position/viewport-ruler';
-import {dispatchFakeEvent} from '../core/testing/dispatch-events';
+import {dispatchFakeEvent, dispatchKeyboardEvent} from '../core/testing/dispatch-events';
 
 describe('MdSelect', () => {
   let overlayContainerElement: HTMLElement;
@@ -1079,6 +1080,81 @@ describe('MdSelect', () => {
         expect(select.getAttribute('tabindex')).toEqual('0');
       });
 
+      it('should be able to select options via the arrow keys on a closed select', () => {
+        const formControl = fixture.componentInstance.control;
+        const options = fixture.componentInstance.options.toArray();
+
+        expect(formControl.value).toBeFalsy('Expected no initial value.');
+
+        dispatchKeyboardEvent(select, 'keydown', DOWN_ARROW);
+
+        expect(options[0].selected).toBe(true, 'Expected first option to be selected.');
+        expect(formControl.value).toBe(options[0].value,
+          'Expected value from first option to have been set on the model.');
+
+        dispatchKeyboardEvent(select, 'keydown', DOWN_ARROW);
+        dispatchKeyboardEvent(select, 'keydown', DOWN_ARROW);
+
+        // Note that the third option is skipped, because it is disabled.
+        expect(options[3].selected).toBe(true, 'Expected fourth option to be selected.');
+        expect(formControl.value).toBe(options[3].value,
+          'Expected value from fourth option to have been set on the model.');
+
+        dispatchKeyboardEvent(select, 'keydown', UP_ARROW);
+
+        expect(options[1].selected).toBe(true, 'Expected second option to be selected.');
+        expect(formControl.value).toBe(options[1].value,
+          'Expected value from second option to have been set on the model.');
+      });
+
+      it('should do nothing if the key manager did not change the active item', () => {
+        const formControl = fixture.componentInstance.control;
+
+        expect(formControl.value).toBeNull('Expected form control value to be empty.');
+        expect(formControl.pristine).toBe(true, 'Expected form control to be clean.');
+
+        dispatchKeyboardEvent(select, 'keydown', 16); // Press a random key.
+
+        expect(formControl.value).toBeNull('Expected form control value to stay empty.');
+        expect(formControl.pristine).toBe(true, 'Expected form control to stay clean.');
+      });
+
+      it('should continue from the selected option when the value is set programmatically', () => {
+        const formControl = fixture.componentInstance.control;
+
+        formControl.setValue('eggs-5');
+        fixture.detectChanges();
+
+        dispatchKeyboardEvent(select, 'keydown', DOWN_ARROW);
+
+        expect(formControl.value).toBe('pasta-6');
+        expect(fixture.componentInstance.options.toArray()[6].selected).toBe(true);
+      });
+
+      it('should not cycle through the options if the control is disabled', () => {
+        const formControl = fixture.componentInstance.control;
+
+        formControl.setValue('eggs-5');
+        formControl.disable();
+        dispatchKeyboardEvent(select, 'keydown', DOWN_ARROW);
+
+        expect(formControl.value).toBe('eggs-5', 'Expected value to remain unchaged.');
+      });
+
+      it('should not wrap selection around after reaching the end of the options', () => {
+        const lastOption = fixture.componentInstance.options.last;
+
+        fixture.componentInstance.options.forEach(() => {
+          dispatchKeyboardEvent(select, 'keydown', DOWN_ARROW);
+        });
+
+        expect(lastOption.selected).toBe(true, 'Expected last option to be selected.');
+
+        dispatchKeyboardEvent(select, 'keydown', DOWN_ARROW);
+
+        expect(lastOption.selected).toBe(true, 'Expected last option to stay selected.');
+      });
+
     });
 
     describe('for options', () => {

src/lib/select/select.ts

@@ -363,7 +363,8 @@ export class MdSelect implements AfterContentInit, ControlValueAccessor, OnDestr
     return this._dir ? this._dir.value === 'rtl' : false;
   }
 
-  /** The width of the trigger element. This is necessary to match
+  /**
+   * The width of the trigger element. This is necessary to match
    * the overlay width to the trigger width.
    */
   _getWidth(): number {
@@ -374,6 +375,21 @@ export class MdSelect implements AfterContentInit, ControlValueAccessor, OnDestr
   _handleKeydown(event: KeyboardEvent): void {
     if (event.keyCode === ENTER || event.keyCode === SPACE) {
       this.open();
+    } else if (!this.disabled) {
+      let prevActiveItem = this._keyManager.activeItem;
+
+      // TODO(crisbeto): native selects also cycle through the options with left/right arrows,
+      // however the key manager only supports up/down at the moment.
+      this._keyManager.onKeydown(event);
+
+      let currentActiveItem = this._keyManager.activeItem as MdOption;
+
+      // TODO(crisbeto): once #2722 gets in, this should open
+      // the panel instead of selecting in `multiple` mode.
+      if (currentActiveItem !== prevActiveItem) {
+        this._emitChangeEvent(currentActiveItem);
+        currentActiveItem.select();
+      }
     }
   }
 
@@ -435,6 +451,7 @@ export class MdSelect implements AfterContentInit, ControlValueAccessor, OnDestr
     for (let i = 0; i < this.options.length; i++) {
       if (options[i].value === value) {
         options[i].select();
+        this._keyManager.setActiveItem(i);
         return;
       }
     }
@@ -447,6 +464,7 @@ export class MdSelect implements AfterContentInit, ControlValueAccessor, OnDestr
   private _clearSelection(): void {
     this._selected = null;
     this._updateOptions();
+    this._keyManager.setActiveItem(null);
   }
 
   private _getTriggerRect(): ClientRect {
@@ -472,7 +490,7 @@ export class MdSelect implements AfterContentInit, ControlValueAccessor, OnDestr
   private _listenToOptions(): void {
     this.options.forEach((option: MdOption) => {
       const sub = option.onSelect.subscribe((event: MdOptionSelectEvent) => {
-        if (event.isUserInput && this._selected !== option) {
+        if (event.isUserInput) {
           this._emitChangeEvent(option);
         }
         this._onSelect(option);
@@ -489,8 +507,10 @@ export class MdSelect implements AfterContentInit, ControlValueAccessor, OnDestr
 
   /** Emits an event when the user selects an option. */
   private _emitChangeEvent(option: MdOption): void {
-    this._onChange(option.value);
-    this.change.emit(new MdSelectChange(this, option.value));
+    if (this._selected !== option) {
+      this._onChange(option.value);
+      this.change.emit(new MdSelectChange(this, option.value));
+    }
   }
 
   /** Records option IDs to pass to the aria-owns property. */