fix(material/button-toggle): use radio pattern for single select Mat toggle button group

Author: clamli

src/material/button-toggle/button-toggle.html

@@ -1,8 +1,10 @@
 <button #button class="mat-button-toggle-button mat-focus-indicator"
         type="button"
         [id]="buttonId"
+        [attr.role]="_isSingleSelector() ? 'radio' : 'button'"
         [attr.tabindex]="disabled ? -1 : tabIndex"
-        [attr.aria-pressed]="checked"
+        [attr.aria-pressed]="!_isSingleSelector() ? checked : null"
+        [attr.aria-checked]="_isSingleSelector() ? checked : null"
         [disabled]="disabled || null"
         [attr.name]="_getButtonName()"
         [attr.aria-label]="ariaLabel"

src/material/button-toggle/button-toggle.ts

@@ -8,6 +8,7 @@
 
 import {FocusMonitor} from '@angular/cdk/a11y';
 import {SelectionModel} from '@angular/cdk/collections';
+import {DOWN_ARROW, LEFT_ARROW, RIGHT_ARROW, UP_ARROW, SPACE, ENTER} from '@angular/cdk/keycodes';
 import {
   AfterContentInit,
   Attribute,
@@ -106,8 +107,9 @@ export class MatButtonToggleChange {
     {provide: MAT_BUTTON_TOGGLE_GROUP, useExisting: MatButtonToggleGroup},
   ],
   host: {
-    'role': 'group',
     'class': 'mat-button-toggle-group',
+    '(keydown)': '_keydown($event)',
+    '[role]': "multiple ? 'group' : 'radiogroup'",
     '[attr.aria-disabled]': 'disabled',
     '[class.mat-button-toggle-vertical]': 'vertical',
     '[class.mat-button-toggle-group-appearance-standard]': 'appearance === "standard"',
@@ -231,6 +233,9 @@ export class MatButtonToggleGroup implements ControlValueAccessor, OnInit, After
 
   ngAfterContentInit() {
     this._selectionModel.select(...this._buttonToggles.filter(toggle => toggle.checked));
+    if (!this.multiple) {
+      this._initializeTabIndex();
+    }
   }
 
   /**
@@ -257,6 +262,53 @@ export class MatButtonToggleGroup implements ControlValueAccessor, OnInit, After
     this.disabled = isDisabled;
   }
 
+  /** Handle keydown event calling to single-select button toggle. */
+  _keydown(event: KeyboardEvent) {
+    if (this.multiple || this.disabled) {
+      return;
+    }
+
+    const target = event.target as HTMLButtonElement;
+    const buttonId = target.id;
+    const index = this._buttonToggles.toArray().findIndex(toggle => {
+      return toggle.buttonId === buttonId;
+    });
+
+    let nextButton;
+    switch (event.keyCode) {
+      case SPACE:
+      case ENTER:
+        nextButton = this._buttonToggles.get(index);
+        break;
+      case UP_ARROW:
+      case LEFT_ARROW:
+        nextButton = this._buttonToggles.get(this._getNextIndex(index, -1));
+        break;
+      case DOWN_ARROW:
+      case RIGHT_ARROW:
+        nextButton = this._buttonToggles.get(this._getNextIndex(index, 1));
+        break;
+      default:
+        return;
+    }
+
+    event.preventDefault();
+    nextButton?._onButtonClick();
+    nextButton?.focus();
+  }
+
+  /** Obtain the subsequent index to which the focus shifts. */
+  _getNextIndex(index: number, offset: number): number {
+    let nextIndex = index + offset;
+    if (nextIndex === this._buttonToggles.length) {
+      nextIndex = 0;
+    }
+    if (nextIndex === -1) {
+      nextIndex = this._buttonToggles.length - 1;
+    }
+    return nextIndex;
+  }
+
   /** Dispatch change event with current selection and group value. */
   _emitChangeEvent(toggle: MatButtonToggle): void {
     const event = new MatButtonToggleChange(toggle, this.value);
@@ -322,6 +374,18 @@ export class MatButtonToggleGroup implements ControlValueAccessor, OnInit, After
     return toggle.value === this._rawValue;
   }
 
+  /** Initializes the tabindex attribute using the radio pattern. */
+  private _initializeTabIndex() {
+    this._buttonToggles.forEach(toggle => {
+      toggle.tabIndex = -1;
+    });
+    if (this.selected) {
+      (this.selected as MatButtonToggle).tabIndex = 0;
+    } else if (this._buttonToggles.length > 0) {
+      this._buttonToggles.get(0)!.tabIndex = 0;
+    }
+  }
+
   /** Updates the selection state of the toggles in the group based on a value. */
   private _setSelectionByValue(value: any | any[]) {
     this._rawValue = value;
@@ -346,7 +410,13 @@ export class MatButtonToggleGroup implements ControlValueAccessor, OnInit, After
   /** Clears the selected toggles. */
   private _clearSelection() {
     this._selectionModel.clear();
-    this._buttonToggles.forEach(toggle => (toggle.checked = false));
+    this._buttonToggles.forEach(toggle => {
+      toggle.checked = false;
+      // If the button toggle is in single select mode, initialize the tabIndex.
+      if (!this.multiple) {
+        toggle.tabIndex = -1;
+      }
+    });
   }
 
   /** Selects a value if there's a toggle that corresponds to it. */
@@ -358,6 +428,10 @@ export class MatButtonToggleGroup implements ControlValueAccessor, OnInit, After
     if (correspondingOption) {
       correspondingOption.checked = true;
       this._selectionModel.select(correspondingOption);
+      if (!this.multiple) {
+        // If the button toggle is in single select mode, reset the tabIndex.
+        correspondingOption.tabIndex = 0;
+      }
     }
   }
 
@@ -437,8 +511,16 @@ export class MatButtonToggle implements OnInit, AfterViewInit, OnDestroy {
   /** MatButtonToggleGroup reads this to assign its own value. */
   @Input() value: any;
 
-  /** Tabindex for the toggle. */
-  @Input() tabIndex: number | null;
+  /** The tabindex of the button. */
+  @Input()
+  get tabIndex(): number | null {
+    return this._tabIndex;
+  }
+  set tabIndex(value: number | null) {
+    this._tabIndex = value;
+    this._changeDetectorRef.markForCheck();
+  }
+  private _tabIndex: number | null;
 
   /** Whether ripples are disabled on the button toggle. */
   @Input({transform: booleanAttribute}) disableRipple: boolean;
@@ -550,6 +632,19 @@ export class MatButtonToggle implements OnInit, AfterViewInit, OnDestroy {
         this.buttonToggleGroup._onTouched();
       }
     }
+
+    if (this._isSingleSelector()) {
+      const focusable = this.buttonToggleGroup._buttonToggles.find(toggle => {
+        return toggle.tabIndex === 0;
+      });
+      // Modify the tabindex attribute of the last focusable button toggle to -1.
+      if (focusable) {
+        focusable.tabIndex = -1;
+      }
+      // Modify the tabindex attribute of the presently selected button toggle to 0.
+      this.tabIndex = 0;
+    }
+
     // Emit a change event when it's the single selector
     this.change.emit(new MatButtonToggleChange(this, this.value));
   }
@@ -574,7 +669,7 @@ export class MatButtonToggle implements OnInit, AfterViewInit, OnDestroy {
   }
 
   /** Whether the toggle is in single selection mode. */
-  private _isSingleSelector(): boolean {
+  _isSingleSelector(): boolean {
     return this.buttonToggleGroup && !this.buttonToggleGroup.multiple;
   }
 }

src/material/button-toggle/testing/button-toggle-harness.ts

@@ -6,7 +6,7 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
-import {ComponentHarness, HarnessPredicate} from '@angular/cdk/testing';
+import {ComponentHarness, HarnessPredicate, parallel} from '@angular/cdk/testing';
 import {coerceBooleanProperty} from '@angular/cdk/coercion';
 import {MatButtonToggleAppearance} from '@angular/material/button-toggle';
 import {ButtonToggleHarnessFilters} from './button-toggle-harness-filters';
@@ -45,8 +45,12 @@ export class MatButtonToggleHarness extends ComponentHarness {
 
   /** Gets a boolean promise indicating if the button toggle is checked. */
   async isChecked(): Promise<boolean> {
-    const checked = (await this._button()).getAttribute('aria-pressed');
-    return coerceBooleanProperty(await checked);
+    const button = await this._button();
+    const [checked, pressed] = await parallel(() => [
+      button.getAttribute('aria-checked'),
+      button.getAttribute('aria-pressed'),
+    ]);
+    return coerceBooleanProperty(checked) || coerceBooleanProperty(pressed);
   }
 
   /** Gets a boolean promise indicating if the button toggle is disabled. */

tools/public_api_guard/material/button-toggle.md

@@ -46,6 +46,7 @@ export class MatButtonToggle implements OnInit, AfterViewInit, OnDestroy {
     focus(options?: FocusOptions): void;
     _getButtonName(): string | null;
     id: string;
+    _isSingleSelector(): boolean;
     _markForCheck(): void;
     name: string;
     // (undocumented)
@@ -61,7 +62,8 @@ export class MatButtonToggle implements OnInit, AfterViewInit, OnDestroy {
     // (undocumented)
     ngOnInit(): void;
     _onButtonClick(): void;
-    tabIndex: number | null;
+    get tabIndex(): number | null;
+    set tabIndex(value: number | null);
     value: any;
     // (undocumented)
     static ɵcmp: i0.ɵɵComponentDeclaration<MatButtonToggle, "mat-button-toggle", ["matButtonToggle"], { "ariaLabel": { "alias": "aria-label"; "required": false; }; "ariaLabelledby": { "alias": "aria-labelledby"; "required": false; }; "id": { "alias": "id"; "required": false; }; "name": { "alias": "name"; "required": false; }; "value": { "alias": "value"; "required": false; }; "tabIndex": { "alias": "tabIndex"; "required": false; }; "disableRipple": { "alias": "disableRipple"; "required": false; }; "appearance": { "alias": "appearance"; "required": false; }; "checked": { "alias": "checked"; "required": false; }; "disabled": { "alias": "disabled"; "required": false; }; }, { "change": "change"; }, never, ["*"], true, never>;
@@ -96,8 +98,10 @@ export class MatButtonToggleGroup implements ControlValueAccessor, OnInit, After
     get disabled(): boolean;
     set disabled(value: boolean);
     _emitChangeEvent(toggle: MatButtonToggle): void;
+    _getNextIndex(index: number, offset: number): number;
     _isPrechecked(toggle: MatButtonToggle): boolean;
     _isSelected(toggle: MatButtonToggle): boolean;
+    _keydown(event: KeyboardEvent): void;
     get multiple(): boolean;
     set multiple(value: boolean);
     get name(): string;