fix(slide-toggle): invalid required validator in template-driven forms

Currently using the `slide-toggle` in a form using template-driven forms causes
the slide-toggle to retrieve a wrong validator if the `required` attribute is set.

This is because by default `@angular/forms` uses an input validator
that ensures that the value is just defined. This is always the case for
a slide-toggle since the value is always `true` or `false`.

The solution to this problem is that we need to provide the checkbox validator
for required slide-toggle components. The checkbox validator from the forms
package ensures that the control is only valid if the slide-toggle is checked.

Author: devversion

src/material-experimental/mdc-slide-toggle/BUILD.bazel

@@ -15,6 +15,7 @@ ng_module(
     deps = [
         "//src/cdk/coercion",
         "//src/material/core",
+        "//src/material/slide-toggle",
         "@npm//@angular/animations",
         "@npm//@angular/common",
         "@npm//@angular/core",

src/material-experimental/mdc-slide-toggle/module.ts

@@ -9,11 +9,21 @@
 import {CommonModule} from '@angular/common';
 import {NgModule} from '@angular/core';
 import {MatCommonModule, MatRippleModule} from '@angular/material/core';
+import {_MatSlideToggleRequiredValidatorModule} from '@angular/material/slide-toggle';
 import {MatSlideToggle} from './slide-toggle';
 
 @NgModule({
-  imports: [MatCommonModule, MatRippleModule, CommonModule],
-  exports: [MatSlideToggle, MatCommonModule],
+  imports: [
+    _MatSlideToggleRequiredValidatorModule,
+    MatCommonModule,
+    MatRippleModule,
+    CommonModule
+  ],
+  exports: [
+    _MatSlideToggleRequiredValidatorModule,
+    MatSlideToggle,
+    MatCommonModule
+  ],
   declarations: [MatSlideToggle],
 })
 export class MatSlideToggleModule {

src/material-experimental/mdc-slide-toggle/slide-toggle.spec.ts

@@ -650,6 +650,32 @@ describe('MatSlideToggle with forms', () => {
 
       expect(testComponent.isSubmitted).toBe(true);
     });
+
+    it('should have proper invalid state if unchecked', () => {
+      testComponent.isRequired = true;
+      fixture.detectChanges();
+
+      const slideToggleEl = fixture.nativeElement.querySelector('.mat-mdc-slide-toggle');
+
+      expect(slideToggleEl.classList).toContain('ng-invalid');
+      expect(slideToggleEl.classList).not.toContain('ng-valid');
+
+      // The required slide-toggle will be checked and the form control
+      // should become valid.
+      inputElement.click();
+      fixture.detectChanges();
+
+      expect(slideToggleEl.classList).not.toContain('ng-invalid');
+      expect(slideToggleEl.classList).toContain('ng-valid');
+
+      // The required slide-toggle will be unchecked and the form control
+      // should become invalid.
+      inputElement.click();
+      fixture.detectChanges();
+
+      expect(slideToggleEl.classList).toContain('ng-invalid');
+      expect(slideToggleEl.classList).not.toContain('ng-valid');
+    });
   });
 
   describe('with model and change event', () => {

src/material/slide-toggle/slide-toggle-module.ts

@@ -11,11 +11,29 @@ import {NgModule} from '@angular/core';
 import {GestureConfig, MatCommonModule, MatRippleModule} from '@angular/material/core';
 import {HAMMER_GESTURE_CONFIG} from '@angular/platform-browser';
 import {MatSlideToggle} from './slide-toggle';
+import {MatSlideToggleRequiredValidator} from './slide-toggle-required-validator';
 
+/** This module is used by both original and MDC-based slide-toggle implementations. */
+@NgModule({
+  exports: [MatSlideToggleRequiredValidator],
+  declarations: [MatSlideToggleRequiredValidator],
+})
+// tslint:disable-next-line:class-name
+export class _MatSlideToggleRequiredValidatorModule {
+}
 
 @NgModule({
-  imports: [MatRippleModule, MatCommonModule, ObserversModule],
-  exports: [MatSlideToggle, MatCommonModule],
+  imports: [
+    _MatSlideToggleRequiredValidatorModule,
+    MatRippleModule,
+    MatCommonModule,
+    ObserversModule,
+  ],
+  exports: [
+    _MatSlideToggleRequiredValidatorModule,
+    MatSlideToggle,
+    MatCommonModule
+  ],
   declarations: [MatSlideToggle],
   providers: [
     {provide: HAMMER_GESTURE_CONFIG, useClass: GestureConfig}

src/material/slide-toggle/slide-toggle-required-validator.ts

@@ -0,0 +1,39 @@
+/**
+ * @license
+ * Copyright Google LLC All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+import {
+  Directive,
+  forwardRef,
+  Provider,
+} from '@angular/core';
+import {
+  CheckboxRequiredValidator,
+  NG_VALIDATORS,
+} from '@angular/forms';
+
+export const MAT_SLIDE_TOGGLE_REQUIRED_VALIDATOR: Provider = {
+  provide: NG_VALIDATORS,
+  useExisting: forwardRef(() => MatSlideToggleRequiredValidator),
+  multi: true
+};
+
+/**
+ * Validator for Material slide-toggle components with the required attribute in a
+ * template-driven form. The default validator for required form controls asserts
+ * that the control value is not undefined but that is not appropriate for a slide-toggle
+ * where the value is always defined.
+ *
+ * Required slide-toggle form controls are valid when checked.
+ */
+@Directive({
+  selector: `mat-slide-toggle[required][formControlName],
+             mat-slide-toggle[required][formControl], mat-slide-toggle[required][ngModel]`,
+  providers: [MAT_SLIDE_TOGGLE_REQUIRED_VALIDATOR],
+  host: {'[attr.required]': 'required ? "" : null'}
+})
+export class MatSlideToggleRequiredValidator extends CheckboxRequiredValidator {}

src/material/slide-toggle/slide-toggle.spec.ts

@@ -1041,6 +1041,32 @@ describe('MatSlideToggle with forms', () => {
 
       expect(testComponent.isSubmitted).toBe(true);
     });
+
+    it('should have proper invalid state if unchecked', () => {
+      testComponent.isRequired = true;
+      fixture.detectChanges();
+
+      const slideToggleEl = fixture.nativeElement.querySelector('.mat-slide-toggle');
+
+      expect(slideToggleEl.classList).toContain('ng-invalid');
+      expect(slideToggleEl.classList).not.toContain('ng-valid');
+
+      // The required slide-toggle will be checked and the form control
+      // should become valid.
+      inputElement.click();
+      fixture.detectChanges();
+
+      expect(slideToggleEl.classList).not.toContain('ng-invalid');
+      expect(slideToggleEl.classList).toContain('ng-valid');
+
+      // The required slide-toggle will be unchecked and the form control
+      // should become invalid.
+      inputElement.click();
+      fixture.detectChanges();
+
+      expect(slideToggleEl.classList).toContain('ng-invalid');
+      expect(slideToggleEl.classList).not.toContain('ng-valid');
+    });
   });
 
   describe('with model and change event', () => {