fix(material-experimental/mdc-chips): Mirror aria-describedby to matChipInput

Updates mat-chip-grid to associate any ids set for aria-describedby to
the matChipInput instance within the grid, if one exists.

Removes the aria-describedby attribute on the grid itself since it never
receives focus.

Fixes #24542

Author: ByzantineFailure

src/material-experimental/mdc-chips/chip-grid.spec.ts

@@ -853,12 +853,14 @@ describe('MDC-based MatChipGrid', () => {
     let errorTestComponent: ChipGridWithFormErrorMessages;
     let containerEl: HTMLElement;
     let chipGridEl: HTMLElement;
+    let inputEl: HTMLElement;
 
     beforeEach(() => {
       fixture = createComponent(ChipGridWithFormErrorMessages);
       errorTestComponent = fixture.componentInstance;
       containerEl = fixture.debugElement.query(By.css('mat-form-field'))!.nativeElement;
       chipGridEl = fixture.debugElement.query(By.css('mat-chip-grid'))!.nativeElement;
+      inputEl = fixture.debugElement.query(By.css('input'))!.nativeElement;
     });
 
     it('should not show any errors if the user has not interacted', () => {
@@ -966,11 +968,11 @@ describe('MDC-based MatChipGrid', () => {
       expect(containerEl.querySelector('mat-error')!.getAttribute('aria-live')).toBe('polite');
     });
 
-    it('sets the aria-describedby to reference errors when in error state', () => {
+    it('sets the aria-describedby on the input to reference errors when in error state', () => {
       let hintId = fixture.debugElement
         .query(By.css('.mat-mdc-form-field-hint'))!
         .nativeElement.getAttribute('id');
-      let describedBy = chipGridEl.getAttribute('aria-describedby');
+      let describedBy = inputEl.getAttribute('aria-describedby');
 
       expect(hintId).withContext('hint should be shown').toBeTruthy();
       expect(describedBy).toBe(hintId);
@@ -982,7 +984,7 @@ describe('MDC-based MatChipGrid', () => {
         .queryAll(By.css('.mat-mdc-form-field-error'))
         .map(el => el.nativeElement.getAttribute('id'))
         .join(' ');
-      describedBy = chipGridEl.getAttribute('aria-describedby');
+      describedBy = inputEl.getAttribute('aria-describedby');
 
       expect(errorIds).withContext('errors should be shown').toBeTruthy();
       expect(describedBy).toBe(errorIds);

src/material-experimental/mdc-chips/chip-grid.ts

@@ -108,8 +108,6 @@ const _MatChipGridMixinBase = mixinErrorState(MatChipGridBase);
     'class': 'mat-mdc-chip-set mat-mdc-chip-grid mdc-evolution-chip-set',
     '[attr.role]': 'role',
     '[tabIndex]': '_chips && _chips.length === 0 ? -1 : tabIndex',
-    // TODO: replace this binding with use of AriaDescriber
-    '[attr.aria-describedby]': '_ariaDescribedby || null',
     '[attr.aria-disabled]': 'disabled.toString()',
     '[attr.aria-invalid]': 'errorState',
     '[class.mat-mdc-chip-list-disabled]': 'disabled',
@@ -145,6 +143,11 @@ export class MatChipGrid
 
   protected override _defaultRole = 'grid';
 
+  /**
+   * List of element ids to propagate to the chipInput's aria-describedby attribute.
+   */
+  private _ariaDescribedbyIds: string[] = [];
+
   /**
    * Function when touched. Set as part of ControlValueAccessor implementation.
    * @docs-private
@@ -337,6 +340,7 @@ export class MatChipGrid
   /** Associates an HTML input element with this chip grid. */
   registerInput(inputElement: MatChipTextControl): void {
     this._chipInput = inputElement;
+    this._chipInput.setDescribedByIds(this._ariaDescribedbyIds);
   }
 
   /**
@@ -378,7 +382,13 @@ export class MatChipGrid
    * @docs-private
    */
   setDescribedByIds(ids: string[]) {
-    this._ariaDescribedby = ids.join(' ');
+    // We must keep this up to date to handle the case where ids are set
+    // before the chip input is registered.
+    this._ariaDescribedbyIds = ids;
+
+    if (this._chipInput) {
+      this._chipInput.setDescribedByIds(ids);
+    }
   }
 
   /**

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

@@ -230,6 +230,24 @@ describe('MDC-based MatChipInput', () => {
       dispatchKeyboardEvent(inputNativeElement, 'keydown', ENTER, undefined, {shift: true});
       expect(testChipInput.add).not.toHaveBeenCalled();
     });
+
+    it('should set aria-describedby correctly when a non-empty list of ids is passed to setDescribedByIds', () => {
+      const ids = ['a', 'b', 'c'];
+
+      testChipInput.chipGridInstance.setDescribedByIds(ids);
+      fixture.detectChanges();
+
+      expect(inputNativeElement.getAttribute('aria-describedby')).toEqual('a b c');
+    });
+
+    it('should set aria-describedby correctly when an empty list of ids is passed to setDescribedByIds', () => {
+      const ids: string[] = [];
+
+      testChipInput.chipGridInstance.setDescribedByIds(ids);
+      fixture.detectChanges();
+
+      expect(inputNativeElement.getAttribute('aria-describedby')).toBeNull();
+    });
   });
 });
 

src/material-experimental/mdc-chips/chip-input.ts

@@ -65,6 +65,7 @@ let nextUniqueId = 0;
     '[attr.disabled]': 'disabled || null',
     '[attr.placeholder]': 'placeholder || null',
     '[attr.aria-invalid]': '_chipGrid && _chipGrid.ngControl ? _chipGrid.ngControl.invalid : null',
+    '[attr.aria-describedby]': '_ariaDescribedby || null',
     '[attr.aria-required]': '_chipGrid && _chipGrid.required || null',
     '[attr.required]': '_chipGrid && _chipGrid.required || null',
   },
@@ -73,6 +74,9 @@ export class MatChipInput implements MatChipTextControl, AfterContentInit, OnCha
   /** Used to prevent focus moving to chips while user is holding backspace */
   private _focusLastChipOnBackspace: boolean;
 
+  /** Value for ariaDescribedby property */
+  _ariaDescribedby?: string;
+
   /** Whether the control is focused. */
   focused: boolean = false;
   _chipGrid: MatChipGrid;
@@ -240,6 +244,10 @@ export class MatChipInput implements MatChipTextControl, AfterContentInit, OnCha
     this._focusLastChipOnBackspace = true;
   }
 
+  setDescribedByIds(ids: string[]): void {
+    this._ariaDescribedby = ids.join(' ');
+  }
+
   /** Checks whether a keycode is one of the configured separators. */
   private _isSeparatorKey(event: KeyboardEvent) {
     return !hasModifierKey(event) && new Set(this.separatorKeyCodes).has(event.keyCode);

src/material-experimental/mdc-chips/chip-set.ts

@@ -65,8 +65,6 @@ const _MatChipSetMixinBase = mixinTabIndex(MatChipSetBase);
   host: {
     'class': 'mat-mdc-chip-set mdc-evolution-chip-set',
     '[attr.role]': 'role',
-    // TODO: replace this binding with use of AriaDescriber
-    '[attr.aria-describedby]': '_ariaDescribedby || null',
   },
   encapsulation: ViewEncapsulation.None,
   changeDetection: ChangeDetectionStrategy.OnPush,
@@ -138,9 +136,6 @@ export class MatChipSet
     },
   };
 
-  /** The aria-describedby attribute on the chip list for improved a11y. */
-  _ariaDescribedby: string;
-
   /**
    * Map from class to whether the class is enabled.
    * Enabled classes are set on the MDC chip-set div.

src/material-experimental/mdc-chips/chip-text-control.ts

@@ -22,4 +22,7 @@ export interface MatChipTextControl {
 
   /** Focuses the text control. */
   focus(): void;
+
+  /** Sets the list of ids the input is described by. */
+  setDescribedByIds(ids: string[]): void;
 }