fix(autocomplete): make writeValue method synchronous

Refactors the `MatAutocompleteTrigger.writeValue` method to avoid having to use a `Promise.resolve` to defer rendering. This makes it easier to test and avoids potential race conditions. It seems like the reason it was added in the first place was to be able to handle components that have a preselected value through a `FormControl` as well as a custom `displayWith` function.

Fixes angular#3250.

src/lib/autocomplete/autocomplete-trigger.ts

@@ -19,6 +19,7 @@ import {TemplatePortal} from '@angular/cdk/portal';
 import {DOCUMENT} from '@angular/common';
 import {filter, take, switchMap, delay, tap, map} from 'rxjs/operators';
 import {
+  AfterContentChecked,
   ChangeDetectorRef,
   Directive,
   ElementRef,
@@ -116,7 +117,8 @@ export function getMatAutocompleteMissingPanelError(): Error {
   exportAs: 'matAutocompleteTrigger',
   providers: [MAT_AUTOCOMPLETE_VALUE_ACCESSOR]
 })
-export class MatAutocompleteTrigger implements ControlValueAccessor, OnDestroy {
+export class MatAutocompleteTrigger implements ControlValueAccessor, AfterContentChecked,
+  OnDestroy {
   private _overlayRef: OverlayRef | null;
   private _portal: TemplatePortal;
   private _componentDestroyed = false;
@@ -145,6 +147,12 @@ export class MatAutocompleteTrigger implements ControlValueAccessor, OnDestroy {
    */
   private _canOpenOnNextFocus = true;
 
+  /** Whether the component has been initializied. */
+  private _isInitialized: boolean;
+
+  /** Initial value that should be shown after the component is initialized. */
+  private _initialValueToSelect: any;
+
   /** Stream of keyboard events that can close the panel. */
   private readonly _closeKeyEventStream = new Subject<void>();
 
@@ -211,6 +219,15 @@ export class MatAutocompleteTrigger implements ControlValueAccessor, OnDestroy {
     this._scrollStrategy = scrollStrategy;
   }
 
+  ngAfterContentChecked() {
+    if (!this._isInitialized && typeof this._initialValueToSelect !== 'undefined') {
+      this._setTriggerValue(this._initialValueToSelect);
+      this._initialValueToSelect = undefined;
+    }
+
+    this._isInitialized = true;
+  }
+
   ngOnDestroy() {
     if (typeof window !== 'undefined') {
       window.removeEventListener('blur', this._windowBlurHandler);
@@ -340,7 +357,14 @@ export class MatAutocompleteTrigger implements ControlValueAccessor, OnDestroy {
 
   // Implemented as part of ControlValueAccessor.
   writeValue(value: any): void {
-    Promise.resolve(null).then(() => this._setTriggerValue(value));
+    if (this._isInitialized) {
+      this._setTriggerValue(value);
+    } else {
+      // If the component isn't initialized yet, defer until the first CD pass, otherwise we'll
+      // miss the initial `displayWith` value. By deferring until the first `AfterContentChecked`
+      // we avoid making the method async while preventing "changed after checked" errors.
+      this._initialValueToSelect = value;
+    }
   }
 
   // Implemented as part of ControlValueAccessor.

src/lib/autocomplete/autocomplete.spec.ts

@@ -2319,6 +2319,16 @@ describe('MatAutocomplete', () => {
     expect(formControl.value).toBe('Cal', 'Expected new value to be propagated to model');
   }));
 
+  it('should evaluate `displayWith` before assigning the initial value', fakeAsync(() => {
+    const fixture = createComponent(PreselectedAutocompleteDisplayWith);
+    const input = fixture.nativeElement.querySelector('input');
+
+    fixture.detectChanges();
+    flush();
+
+    expect(input.value).toBe('Alaska');
+  }));
+
 });
 
 @Component({
@@ -2707,3 +2717,29 @@ class AutocompleteWithNativeAutocompleteAttribute {
 })
 class InputWithoutAutocompleteAndDisabled {
 }
+
+
+@Component({
+  template: `
+    <mat-form-field>
+      <input matInput [matAutocomplete]="auto" [formControl]="stateCtrl">
+    </mat-form-field>
+
+    <mat-autocomplete #auto="matAutocomplete" [displayWith]="displayFn">
+      <mat-option *ngFor="let state of states" [value]="state">
+        <span>{{ state.name }}</span>
+      </mat-option>
+    </mat-autocomplete>
+  `
+})
+class PreselectedAutocompleteDisplayWith {
+  stateCtrl = new FormControl({code: 'AK', name: 'Alaska'});
+  states = [
+    {code: 'AL', name: 'Alabama'},
+    {code: 'AK', name: 'Alaska'}
+  ];
+
+  displayFn(value: any): string {
+    return value && typeof value === 'object' ? value.name : value;
+  }
+}