fix(material-experimental/mdc-form-field): ensure validity styling is not reset by foundation (#18266)

Currently, whenever the abstract form-field control turns invalid, the
text-field foundation does not know about the validity change. Since it
refreshes the validity upon focus/blur, it could end up accidentally
removing the invalid class. We fix this, by ensuring that the foundation
shares the validity state with the one from the form-field control.

Author: devversion

src/material-experimental/mdc-form-field/form-field.ts

@@ -283,6 +283,12 @@ export class MatFormField implements AfterViewInit, OnDestroy, AfterContentCheck
       get: () => this._shouldLabelFloat(),
     });
 
+    // By default, the foundation determines the validity of the text-field from the
+    // specified native input. Since we don't pass a native input to the foundation because
+    // abstract form controls are not necessarily consisting of an input, we handle the
+    // text-field validity through the abstract form-field control state.
+    this._foundation.isValid = () => !this._control.errorState;
+
     // Initial focus state sync. This happens rarely, but we want to account for
     // it in case the form-field control has "focused" set to true on init.
     this._updateFocusState();

src/material-experimental/mdc-input/input.spec.ts

@@ -830,6 +830,21 @@ describe('MatMdcInput with forms', () => {
         .toBe('true', 'Expected aria-invalid to be set to "true".');
     }));
 
+    it('should not reset text-field validity if focus changes for an invalid input',
+        fakeAsync(() => {
+          // Mark the control as touched, so that the form-field displays as invalid.
+          testComponent.formControl.markAsTouched();
+          fixture.detectChanges();
+          flush();
+
+          const wrapperEl = containerEl.querySelector('.mdc-text-field')!;
+          expect(wrapperEl.classList).toContain('mdc-text-field--invalid');
+
+          dispatchFakeEvent(inputEl, 'focus');
+          dispatchFakeEvent(inputEl, 'blur');
+          expect(wrapperEl.classList).toContain('mdc-text-field--invalid');
+    }));
+
     it('should display an error message when the parent form is submitted', fakeAsync(() => {
       expect(testComponent.form.submitted).toBe(false, 'Expected form not to have been submitted');
       expect(testComponent.formControl.invalid).toBe(true, 'Expected form control to be invalid');