fix (forms): clear selected options when model is not an array (#12519)

When an invalid model value (eg empty string) was preset ngModel on
select[multiple] would throw an error, which is inconsistent with how it
works on other user input elements. Setting the model value to null or
undefined would also have no effect on what was already selected in the
UI. Fix this by clearing selected options when model set to null,
undefined or a type other than Array.

Closes #11926

modules/@angular/forms/src/directives/select_multiple_control_value_accessor.ts

@@ -66,11 +66,15 @@ export class SelectMultipleControlValueAccessor implements ControlValueAccessor
 
   writeValue(value: any): void {
     this.value = value;
-    if (value == null) return;
-    const values: Array<any> = <Array<any>>value;
+    let optionSelectedStateSetter: (opt: NgSelectMultipleOption, o: any) => void;
+    if (Array.isArray(value)) {
       // convert values to ids
-    const ids = values.map((v) => this._getOptionId(v));
-    this._optionMap.forEach((opt, o) => { opt._setSelected(ids.indexOf(o.toString()) > -1); });
+      const ids = value.map((v) => this._getOptionId(v));
+      optionSelectedStateSetter = (opt, o) => { opt._setSelected(ids.indexOf(o.toString()) > -1); };
+    } else {
+      optionSelectedStateSetter = (opt, o) => { opt._setSelected(false); };
+    }
+    this._optionMap.forEach(optionSelectedStateSetter);
   }
 
   registerOnChange(fn: (value: any) => any): void {

modules/@angular/forms/test/template_integration_spec.ts

@@ -7,7 +7,7 @@
  */
 
 import {Component, Directive, Input, forwardRef} from '@angular/core';
-import {TestBed, async, fakeAsync, tick} from '@angular/core/testing';
+import {ComponentFixture, TestBed, async, fakeAsync, tick} from '@angular/core/testing';
 import {AbstractControl, ControlValueAccessor, FormsModule, NG_ASYNC_VALIDATORS, NG_VALUE_ACCESSOR, NgForm, Validator} from '@angular/forms';
 import {By} from '@angular/platform-browser/src/dom/debug/by';
 import {getDOM} from '@angular/platform-browser/src/dom/dom_adapter';
@@ -23,7 +23,7 @@ export function main() {
           NgModelRadioForm, NgModelRangeForm, NgModelSelectForm, NgNoFormComp, InvalidNgModelNoName,
           NgModelOptionsStandalone, NgModelCustomComp, NgModelCustomWrapper,
           NgModelValidationBindings, NgModelMultipleValidators, NgAsyncValidator,
-          NgModelAsyncValidation, NgModelSelectWithNullForm
+          NgModelAsyncValidation, NgModelSelectMultipleForm, NgModelSelectWithNullForm
         ],
         imports: [FormsModule]
       });
@@ -740,6 +740,65 @@ export function main() {
          }));
     });
 
+    describe('select multiple controls', () => {
+      let fixture: ComponentFixture<NgModelSelectMultipleForm>;
+      let comp: NgModelSelectMultipleForm;
+
+      beforeEach(() => {
+        fixture = TestBed.createComponent(NgModelSelectMultipleForm);
+        comp = fixture.componentInstance;
+        comp.cities = [{'name': 'SF'}, {'name': 'NYC'}, {'name': 'Buffalo'}];
+      });
+
+      const setSelectedCities = (selectedCities: any): void => {
+        comp.selectedCities = selectedCities;
+        fixture.detectChanges();
+        tick();
+      };
+
+      const assertOptionElementSelectedState = (selectedStates: boolean[]): void => {
+        const options = fixture.debugElement.queryAll(By.css('option'));
+        if (options.length !== selectedStates.length) {
+          throw 'the selected state values to assert does not match the number of options';
+        }
+        for (let i = 0; i < selectedStates.length; i++) {
+          expect(options[i].nativeElement.selected).toBe(selectedStates[i]);
+        }
+      };
+
+      const testNewModelValueUnselectsAllOptions = (modelValue: any): void => {
+        setSelectedCities([comp.cities[1]]);
+        assertOptionElementSelectedState([false, true, false]);
+
+        setSelectedCities(modelValue);
+
+        const select = fixture.debugElement.query(By.css('select'));
+        expect(select.nativeElement.value).toEqual('');
+        assertOptionElementSelectedState([false, false, false]);
+      };
+
+      it('should support setting value to null and undefined', fakeAsync(() => {
+           testNewModelValueUnselectsAllOptions(null);
+           testNewModelValueUnselectsAllOptions(undefined);
+         }));
+
+      it('should clear all selected option elements when value of wrong type supplied',
+         fakeAsync(() => { testNewModelValueUnselectsAllOptions(''); }));
+
+      it('should set option elements to selected that are present in model', fakeAsync(() => {
+           setSelectedCities([comp.cities[0], comp.cities[2]]);
+           assertOptionElementSelectedState([true, false, true]);
+         }));
+
+      it('should clear selected option elements since removed from model', fakeAsync(() => {
+           const selectedCities: [{'name:string': string}] = <[{'name:string': string}]>[];
+           selectedCities.push.apply(selectedCities, comp.cities);
+           setSelectedCities(selectedCities);
+           setSelectedCities([comp.cities[1]]);
+           assertOptionElementSelectedState([false, true, false]);
+         }));
+    });
+
     describe('custom value accessors', () => {
       it('should support standard writing to view and model', async(() => {
            const fixture = TestBed.createComponent(NgModelCustomWrapper);
@@ -1153,6 +1212,19 @@ class NgModelSelectWithNullForm {
   cities: any[] = [];
 }
 
+@Component({
+  selector: 'ng-model-select-multiple-form',
+  template: `
+    <select multiple [(ngModel)]="selectedCities">
+      <option *ngFor="let c of cities" [ngValue]="c"> {{c.name}} </option>
+    </select>
+  `
+})
+class NgModelSelectMultipleForm {
+  selectedCities: any[];
+  cities: any[] = [];
+}
+
 @Component({
   selector: 'ng-model-custom-comp',
   template: `