fix(material/autocomplete): don't assign to model value while typing when requireSelection is enabled

Follow-up to #27423 based on the feedback. Usually `mat-autocomplete` assigns to the model as the user is typing which may not be desired when `requireSelection` is enabled, because at the end of the selection either an option value will set or it'll be reset.

These changes add a condition so that the value isn't assigned while typing and `requireSelection` is enabled.

Author: crisbeto

src/components-examples/material/autocomplete/autocomplete-require-selection/autocomplete-require-selection-example.css

@@ -2,6 +2,7 @@
   min-width: 150px;
   max-width: 500px;
   width: 100%;
+  margin-top: 16px;
 }
 
 .example-full-width {

src/components-examples/material/autocomplete/autocomplete-require-selection/autocomplete-require-selection-example.html

@@ -1,18 +1,20 @@
+Control value: {{myControl.value || 'empty'}}
+
 <form class="example-form">
   <mat-form-field class="example-full-width">
     <mat-label>Number</mat-label>
-    <input type="text"
+    <input #input
+           type="text"
            placeholder="Pick one"
-           aria-label="Number"
            matInput
            [formControl]="myControl"
-           [matAutocomplete]="auto">
+           [matAutocomplete]="auto"
+           (input)="filter()"
+           (focus)="filter()">
     <mat-autocomplete requireSelection #auto="matAutocomplete">
-      <mat-option *ngFor="let option of filteredOptions | async" [value]="option">
+      <mat-option *ngFor="let option of filteredOptions" [value]="option">
         {{option}}
       </mat-option>
     </mat-autocomplete>
   </mat-form-field>
 </form>
-
-Control value: {{myControl.value}}

src/components-examples/material/autocomplete/autocomplete-require-selection/autocomplete-require-selection-example.ts

@@ -1,14 +1,12 @@
-import {Component, OnInit} from '@angular/core';
+import {Component, ElementRef, ViewChild} from '@angular/core';
 import {FormControl, FormsModule, ReactiveFormsModule} from '@angular/forms';
-import {Observable} from 'rxjs';
-import {map, startWith} from 'rxjs/operators';
 import {NgFor, AsyncPipe} from '@angular/common';
 import {MatAutocompleteModule} from '@angular/material/autocomplete';
 import {MatInputModule} from '@angular/material/input';
 import {MatFormFieldModule} from '@angular/material/form-field';
 
 /**
- * @title Require an autocomplete option to be selected.
+ * @title Require an autocomplete option to be selected
  */
 @Component({
   selector: 'autocomplete-require-selection-example',
@@ -25,21 +23,18 @@ import {MatFormFieldModule} from '@angular/material/form-field';
     AsyncPipe,
   ],
 })
-export class AutocompleteRequireSelectionExample implements OnInit {
+export class AutocompleteRequireSelectionExample {
+  @ViewChild('input') input: ElementRef<HTMLInputElement>;
   myControl = new FormControl('');
-  options: string[] = ['One', 'Two', 'Three', 'Three', 'Four'];
-  filteredOptions: Observable<string[]>;
+  options: string[] = ['One', 'Two', 'Three', 'Four', 'Five'];
+  filteredOptions: string[];
 
-  ngOnInit() {
-    this.filteredOptions = this.myControl.valueChanges.pipe(
-      startWith(''),
-      map(value => this._filter(value || '')),
-    );
+  constructor() {
+    this.filteredOptions = this.options.slice();
   }
 
-  private _filter(value: string): string[] {
-    const filterValue = value.toLowerCase();
-
-    return this.options.filter(option => option.toLowerCase().includes(filterValue));
+  filter(): void {
+    const filterValue = this.input.nativeElement.value.toLowerCase();
+    this.filteredOptions = this.options.filter(o => o.toLowerCase().includes(filterValue));
   }
 }

src/dev-app/autocomplete/autocomplete-demo.html

@@ -1,21 +1,27 @@
 Space above cards: <input type="number" [formControl]="topHeightCtrl">
 <div [style.height.px]="topHeightCtrl.value"></div>
 <div class="demo-autocomplete">
-  <mat-card *ngIf="(reactiveStates | async) as tempStates">
-    Reactive length: {{ tempStates?.length }}
+  <mat-card>
+    Reactive length: {{ reactiveStates.length }}
     <div>Reactive value: {{ stateCtrl.value | json }}</div>
     <div>Reactive dirty: {{ stateCtrl.dirty }}</div>
 
     <mat-form-field [color]="reactiveStatesTheme">
       <mat-label>State</mat-label>
-      <input matInput [matAutocomplete]="reactiveAuto" [formControl]="stateCtrl">
+      <input
+        #reactiveInput
+        matInput
+        [matAutocomplete]="reactiveAuto"
+        [formControl]="stateCtrl"
+        (input)="reactiveStates = filterStates(reactiveInput.value)"
+        (focus)="reactiveStates = filterStates(reactiveInput.value)">
     </mat-form-field>
     <mat-autocomplete #reactiveAuto="matAutocomplete"
       [displayWith]="displayFn"
       [hideSingleSelectionIndicator]="reactiveHideSingleSelectionIndicator"
       [autoActiveFirstOption]="reactiveAutoActiveFirstOption"
       [requireSelection]="reactiveRequireSelection">
-      <mat-option *ngFor="let state of tempStates; let index = index" [value]="state"
+      <mat-option *ngFor="let state of reactiveStates; let index = index" [value]="state"
         [disabled]="reactiveIsStateDisabled(state.index)">
         <span>{{ state.name }}</span>
         <span class="demo-secondary-text"> ({{ state.code }}) </span>

src/dev-app/autocomplete/autocomplete-demo.ts

@@ -14,8 +14,6 @@ import {MatButtonModule} from '@angular/material/button';
 import {MatCardModule} from '@angular/material/card';
 import {MatCheckboxModule} from '@angular/material/checkbox';
 import {MatInputModule} from '@angular/material/input';
-import {Observable} from 'rxjs';
-import {map, startWith} from 'rxjs/operators';
 import {ThemePalette} from '@angular/material/core';
 import {MatDialog, MatDialogModule, MatDialogRef} from '@angular/material/dialog';
 
@@ -50,12 +48,12 @@ type DisableStateOption = 'none' | 'first-middle-last' | 'all';
   ],
 })
 export class AutocompleteDemo {
-  stateCtrl = new FormControl({code: 'CA', name: 'California'});
+  stateCtrl = new FormControl();
   currentState = '';
   currentGroupedState = '';
   topHeightCtrl = new FormControl(0);
 
-  reactiveStates: Observable<State[]>;
+  reactiveStates: State[];
   tdStates: State[];
 
   tdDisabled = false;
@@ -138,12 +136,8 @@ export class AutocompleteDemo {
   ].map((state, index) => ({...state, index}));
 
   constructor() {
-    this.tdStates = this.states;
-    this.reactiveStates = this.stateCtrl.valueChanges.pipe(
-      startWith(this.stateCtrl.value),
-      map(val => this.displayFn(val)),
-      map(name => this.filterStates(name)),
-    );
+    this.tdStates = this.states.slice();
+    this.reactiveStates = this.states.slice();
 
     this.filteredGroupedStates = this.groupedStates = this.states.reduce<StateGroup[]>(
       (groups, state) => {

src/material/autocomplete/autocomplete-trigger.ts

@@ -470,7 +470,13 @@ export abstract class _MatAutocompleteTriggerBase
     if (this._previousValue !== value) {
       this._previousValue = value;
       this._pendingAutoselectedOption = null;
-      this._onChange(value);
+
+      // If selection is required we don't write to the CVA while the user is typing.
+      // At the end of the selection either the user will have picked something
+      // or we'll reset the value back to null.
+      if (!this.autocomplete || !this.autocomplete.requireSelection) {
+        this._onChange(value);
+      }
 
       if (!value) {
         this._clearPreviousSelectedOption(null, false);

src/material/autocomplete/autocomplete.spec.ts

@@ -2622,7 +2622,7 @@ describe('MDC-based MatAutocomplete', () => {
       tick();
 
       expect(input.value).toBe('Cali');
-      expect(stateCtrl.value).toBe('Cali');
+      expect(stateCtrl.value).toEqual({code: 'CA', name: 'California'});
       expect(spy).not.toHaveBeenCalled();
 
       dispatchFakeEvent(document, 'click');