feat(autocomplete): emit event when an option is selected

Emits the `select` event when an option in the autocomplete is selected.

**Note:** I went with passing the selected option from the trigger to the panel, instead of listening to the `onSelectionChange` inside the panel, because it involves keeping track of less subscriptions and not having to re-construct them when the list of options changes.

Fixes angular#4094.
Fixes angular#3645.

src/lib/autocomplete/autocomplete-trigger.ts

@@ -432,6 +432,7 @@ export class MdAutocompleteTrigger implements ControlValueAccessor, OnDestroy {
       this._setTriggerValue(event.source.value);
       this._onChange(event.source.value);
       this._element.nativeElement.focus();
+      this.autocomplete._emitSelectEvent(event.source);
     }
 
     this.closePanel();

src/lib/autocomplete/autocomplete.spec.ts

@@ -20,6 +20,7 @@ import {
   MdAutocomplete,
   MdAutocompleteModule,
   MdAutocompleteTrigger,
+  MdAutocompleteSelect,
 } from './index';
 import {MdInputModule} from '../input/index';
 import {Subscription} from 'rxjs/Subscription';
@@ -55,7 +56,8 @@ describe('MdAutocomplete', () => {
         AutocompleteWithOnPushDelay,
         AutocompleteWithNativeInput,
         AutocompleteWithoutPanel,
-        AutocompleteWithFormsAndNonfloatingPlaceholder
+        AutocompleteWithFormsAndNonfloatingPlaceholder,
+        AutocompleteWithSelectEvent,
       ],
       providers: [
         {provide: OverlayContainer, useFactory: () => {
@@ -1463,6 +1465,29 @@ describe('MdAutocomplete', () => {
         expect(panel.classList).toContain(visibleClass, `Expected panel to be visible.`);
       });
     }));
+
+  it('should call emit an event when an option is selected', fakeAsync(() => {
+    let fixture = TestBed.createComponent(AutocompleteWithSelectEvent);
+
+    fixture.detectChanges();
+    fixture.componentInstance.trigger.openPanel();
+    tick();
+    fixture.detectChanges();
+
+    let options = overlayContainerElement.querySelectorAll('md-option') as NodeListOf<HTMLElement>;
+    let spy = fixture.componentInstance.select;
+
+    options[1].click();
+    tick();
+    fixture.detectChanges();
+
+    expect(spy).toHaveBeenCalledTimes(1);
+
+    let event = spy.calls.mostRecent().args[0] as MdAutocompleteSelect;
+
+    expect(event.source).toBe(fixture.componentInstance.autocomplete);
+    expect(event.option.value).toBe('Washington');
+  }));
 });
 
 @Component({
@@ -1706,3 +1731,25 @@ class AutocompleteWithoutPanel {
 class AutocompleteWithFormsAndNonfloatingPlaceholder {
   formControl = new FormControl('California');
 }
+
+@Component({
+  template: `
+    <md-input-container>
+      <input mdInput placeholder="State" [mdAutocomplete]="auto" [(ngModel)]="selectedState">
+    </md-input-container>
+
+    <md-autocomplete #auto="mdAutocomplete" (select)="select($event)">
+      <md-option *ngFor="let state of states" [value]="state">
+        <span>{{ state }}</span>
+      </md-option>
+    </md-autocomplete>
+  `
+})
+class AutocompleteWithSelectEvent {
+  selectedState: string;
+  states = ['New York', 'Washington', 'Oregon'];
+  select = jasmine.createSpy('select callback');
+
+  @ViewChild(MdAutocompleteTrigger) trigger: MdAutocompleteTrigger;
+  @ViewChild(MdAutocomplete) autocomplete: MdAutocomplete;
+}

src/lib/autocomplete/autocomplete.ts

@@ -18,16 +18,26 @@ import {
   ViewEncapsulation,
   ChangeDetectorRef,
   ChangeDetectionStrategy,
+  EventEmitter,
+  Output,
 } from '@angular/core';
 import {MdOption} from '../core';
 import {ActiveDescendantKeyManager} from '@angular/cdk/a11y';
 
+
 /**
  * Autocomplete IDs need to be unique across components, so this counter exists outside of
  * the component definition.
  */
 let _uniqueAutocompleteIdCounter = 0;
 
+/** Event object that is emitted when an autocomplete option is selected */
+export class MdAutocompleteSelect {
+  constructor(public source: MdAutocomplete, public option: MdOption) { }
+}
+
+export type AutocompletePositionY = 'above' | 'below';
+
 @Component({
   moduleId: module.id,
   selector: 'md-autocomplete, mat-autocomplete',
@@ -60,6 +70,9 @@ export class MdAutocomplete implements AfterContentInit {
   /** Function that maps an option's control value to its display value in the trigger. */
   @Input() displayWith: ((value: any) => string) | null = null;
 
+  /** Event that is emitted whenever an option from the list is selected. */
+  @Output() select: EventEmitter<MdAutocompleteSelect> = new EventEmitter<MdAutocompleteSelect>();
+
   /** Unique ID to be used by autocomplete trigger's "aria-owns" property. */
   id: string = `md-autocomplete-${_uniqueAutocompleteIdCounter++}`;
 
@@ -85,13 +98,19 @@ export class MdAutocomplete implements AfterContentInit {
   }
 
   /** Panel should hide itself when the option list is empty. */
-  _setVisibility() {
+  _setVisibility(): void {
     Promise.resolve().then(() => {
       this.showPanel = !!this.options.length;
       this._changeDetectorRef.markForCheck();
     });
   }
 
+  /** Emits the `select` event. */
+  _emitSelectEvent(option: MdOption): void {
+    const selectEvent = new MdAutocompleteSelect(this, option);
+    this.select.emit(selectEvent);
+  }
+
   /** Sets a class on the panel based on whether it is visible. */
   _getClassList() {
     return {