feat(autocomplete): add event when option is activated

src/material/autocomplete/autocomplete.spec.ts

@@ -2496,6 +2496,34 @@ describe('MatAutocomplete', () => {
     expect(event.option.value).toBe('Puerto Rico');
   }));
 
+  it('should emit an event when an option is activated', fakeAsync(() => {
+    const fixture = createComponent(AutocompleteWithActivatedEvent);
+
+    fixture.detectChanges();
+    fixture.componentInstance.trigger.openPanel();
+    zone.simulateZoneExit();
+    fixture.detectChanges();
+
+    const input = fixture.nativeElement.querySelector('input');
+    const spy = fixture.componentInstance.optionActivated;
+    const autocomplete = fixture.componentInstance.autocomplete;
+    const options = fixture.componentInstance.options.toArray();
+
+    expect(spy).not.toHaveBeenCalled();
+
+    dispatchKeyboardEvent(input, 'keydown', DOWN_ARROW);
+    fixture.detectChanges();
+    expect(spy.calls.mostRecent().args[0]).toEqual({source: autocomplete, option: options[0]});
+
+    dispatchKeyboardEvent(input, 'keydown', DOWN_ARROW);
+    fixture.detectChanges();
+    expect(spy.calls.mostRecent().args[0]).toEqual({source: autocomplete, option: options[1]});
+
+    dispatchKeyboardEvent(input, 'keydown', DOWN_ARROW);
+    fixture.detectChanges();
+    expect(spy.calls.mostRecent().args[0]).toEqual({source: autocomplete, option: options[2]});
+  }));
+
   it('should be able to set a custom panel connection element', () => {
     const fixture = createComponent(AutocompleteWithDifferentOrigin);
 
@@ -2978,3 +3006,24 @@ class AutocompleteWithNativeAutocompleteAttribute {
 })
 class InputWithoutAutocompleteAndDisabled {
 }
+
+
+@Component({
+  template: `
+    <mat-form-field>
+      <input matInput [matAutocomplete]="auto">
+    </mat-form-field>
+
+    <mat-autocomplete #auto="matAutocomplete" (optionActivated)="optionActivated($event)">
+      <mat-option *ngFor="let state of states" [value]="state">{{ state }}</mat-option>
+    </mat-autocomplete>
+  `
+})
+class AutocompleteWithActivatedEvent {
+  states = ['California', 'West Virginia', 'Florida'];
+  optionActivated = jasmine.createSpy('optionActivated callback');
+
+  @ViewChild(MatAutocompleteTrigger) trigger: MatAutocompleteTrigger;
+  @ViewChild(MatAutocomplete) autocomplete: MatAutocomplete;
+  @ViewChildren(MatOption) options: QueryList<MatOption>;
+}

src/material/autocomplete/autocomplete.ts

@@ -24,6 +24,7 @@ import {
   TemplateRef,
   ViewChild,
   ViewEncapsulation,
+  OnDestroy,
 } from '@angular/core';
 import {
   CanDisableRipple,
@@ -33,6 +34,7 @@ import {
   MatOption,
   mixinDisableRipple,
 } from '@angular/material/core';
+import {Subscription} from 'rxjs';
 
 
 /**
@@ -50,6 +52,14 @@ export class MatAutocompleteSelectedEvent {
     public option: MatOption) { }
 }
 
+/** Event object that is emitted when an autocomplete option is activated. */
+export interface MatAutocompleteActivatedEvent {
+  /** Reference to the autocomplete panel that emitted the event. */
+  source: MatAutocomplete;
+
+  /** Option that was selected. */
+  option: MatOption|null;
+}
 
 // Boilerplate for applying mixins to MatAutocomplete.
 /** @docs-private */
@@ -91,7 +101,8 @@ export function MAT_AUTOCOMPLETE_DEFAULT_OPTIONS_FACTORY(): MatAutocompleteDefau
   ]
 })
 export class MatAutocomplete extends _MatAutocompleteMixinBase implements AfterContentInit,
-  CanDisableRipple {
+  CanDisableRipple, OnDestroy {
+    private _activeOptionChanges = Subscription.EMPTY;
 
   /** Manages active item in option list based on key events. */
   _keyManager: ActiveDescendantKeyManager<MatOption>;
@@ -149,6 +160,10 @@ export class MatAutocomplete extends _MatAutocompleteMixinBase implements AfterC
   /** Event that is emitted when the autocomplete panel is closed. */
   @Output() readonly closed: EventEmitter<void> = new EventEmitter<void>();
 
+  /** Emits whenever an option is activated using the keyboard. */
+  @Output() readonly optionActivated: EventEmitter<MatAutocompleteActivatedEvent> =
+      new EventEmitter<MatAutocompleteActivatedEvent>();
+
   /**
    * Takes classes set on the host mat-autocomplete element and applies them to the panel
    * inside the overlay container to allow for easy styling.
@@ -183,10 +198,18 @@ export class MatAutocomplete extends _MatAutocompleteMixinBase implements AfterC
 
   ngAfterContentInit() {
     this._keyManager = new ActiveDescendantKeyManager<MatOption>(this.options).withWrap();
+    this._activeOptionChanges = this._keyManager.change.subscribe(index => {
+      this.optionActivated.emit({source: this, option: this.options.toArray()[index] || null});
+    });
+
     // Set the initial visibility state.
     this._setVisibility();
   }
 
+  ngOnDestroy() {
+    this._activeOptionChanges.unsubscribe();
+  }
+
   /**
    * Sets the panel scrollTop. This allows us to manually scroll to display options
    * above or below the fold, as they are not actually being focused when active.

tools/public_api_guard/material/autocomplete.d.ts

@@ -20,7 +20,7 @@ export declare const MAT_AUTOCOMPLETE_SCROLL_STRATEGY_FACTORY_PROVIDER: {
 
 export declare const MAT_AUTOCOMPLETE_VALUE_ACCESSOR: any;
 
-export declare class MatAutocomplete extends _MatAutocompleteMixinBase implements AfterContentInit, CanDisableRipple {
+export declare class MatAutocomplete extends _MatAutocompleteMixinBase implements AfterContentInit, CanDisableRipple, OnDestroy {
     _classList: {
         [key: string]: boolean;
     };
@@ -34,6 +34,7 @@ export declare class MatAutocomplete extends _MatAutocompleteMixinBase implement
     id: string;
     get isOpen(): boolean;
     readonly opened: EventEmitter<void>;
+    readonly optionActivated: EventEmitter<MatAutocompleteActivatedEvent>;
     optionGroups: QueryList<MatOptgroup>;
     readonly optionSelected: EventEmitter<MatAutocompleteSelectedEvent>;
     options: QueryList<MatOption>;
@@ -47,12 +48,18 @@ export declare class MatAutocomplete extends _MatAutocompleteMixinBase implement
     _setScrollTop(scrollTop: number): void;
     _setVisibility(): void;
     ngAfterContentInit(): void;
+    ngOnDestroy(): void;
     static ngAcceptInputType_autoActiveFirstOption: BooleanInput;
     static ngAcceptInputType_disableRipple: BooleanInput;
-    static ɵcmp: i0.ɵɵComponentDefWithMeta<MatAutocomplete, "mat-autocomplete", ["matAutocomplete"], { "disableRipple": "disableRipple"; "displayWith": "displayWith"; "autoActiveFirstOption": "autoActiveFirstOption"; "panelWidth": "panelWidth"; "classList": "class"; }, { "optionSelected": "optionSelected"; "opened": "opened"; "closed": "closed"; }, ["options", "optionGroups"]>;
+    static ɵcmp: i0.ɵɵComponentDefWithMeta<MatAutocomplete, "mat-autocomplete", ["matAutocomplete"], { "disableRipple": "disableRipple"; "displayWith": "displayWith"; "autoActiveFirstOption": "autoActiveFirstOption"; "panelWidth": "panelWidth"; "classList": "class"; }, { "optionSelected": "optionSelected"; "opened": "opened"; "closed": "closed"; "optionActivated": "optionActivated"; }, ["options", "optionGroups"]>;
     static ɵfac: i0.ɵɵFactoryDef<MatAutocomplete>;
 }
 
+export interface MatAutocompleteActivatedEvent {
+    option: MatOption | null;
+    source: MatAutocomplete;
+}
+
 export interface MatAutocompleteDefaultOptions {
     autoActiveFirstOption?: boolean;
 }