fix(checkbox): added aria-checked=mixed to indeterminate state

packages/mdc-checkbox/adapter.js

@@ -43,6 +43,19 @@ class MDCCheckboxAdapter {
   /** @param {string} className */
   removeClass(className) {}
 
+  /**
+   * Sets an attribute with a given value on the input element.
+   * @param {string} attr
+   * @param {string} value
+   */
+  setNativeControlAttr(attr, value) {}
+
+  /**
+   * Removes an attribute from the input element.
+   * @param {string} attr
+   */
+  removeNativeControlAttr(attr) {}
+
   /** @param {!EventListener} handler */
   registerAnimationEndHandler(handler) {}
 

packages/mdc-checkbox/constants.js

@@ -39,6 +39,8 @@ const strings = {
   TRANSITION_STATE_CHECKED: 'checked',
   TRANSITION_STATE_UNCHECKED: 'unchecked',
   TRANSITION_STATE_INDETERMINATE: 'indeterminate',
+  ARIA_CHECKED_ATTR: 'aria-checked',
+  ARIA_CHECKED_INDETERMINATE_VALUE: 'mixed',
 };
 
 /** @enum {number} */

packages/mdc-checkbox/foundation.js

@@ -49,6 +49,8 @@ class MDCCheckboxFoundation extends MDCFoundation {
     return /** @type {!MDCCheckboxAdapter} */ ({
       addClass: (/* className: string */) => {},
       removeClass: (/* className: string */) => {},
+      setNativeControlAttr: () => {},
+      removeNativeControlAttr: () => {},
       registerAnimationEndHandler: (/* handler: EventListener */) => {},
       deregisterAnimationEndHandler: (/* handler: EventListener */) => {},
       registerChangeHandler: (/* handler: EventListener */) => {},
@@ -203,6 +205,14 @@ class MDCCheckboxFoundation extends MDCFoundation {
       return;
     }
 
+    // Ensure aria-checked is set to mixed if checkbox is in indeterminate state.
+    if (this.isIndeterminate()) {
+      this.adapter_.setNativeControlAttr(
+        strings.ARIA_CHECKED_ATTR, strings.ARIA_CHECKED_INDETERMINATE_VALUE);
+    } else {
+      this.adapter_.removeNativeControlAttr(strings.ARIA_CHECKED_ATTR);
+    }
+
     // Check to ensure that there isn't a previously existing animation class, in case for example
     // the user interacted with the checkbox before the animation was finished.
     if (this.currentAnimationClass_.length > 0) {

test/unit/mdc-checkbox/foundation.test.js

@@ -110,9 +110,9 @@ test('exports numbers', () => {
 
 test('defaultAdapter returns a complete adapter implementation', () => {
   verifyDefaultAdapter(MDCCheckboxFoundation, [
-    'addClass', 'removeClass', 'registerAnimationEndHandler', 'deregisterAnimationEndHandler',
-    'registerChangeHandler', 'deregisterChangeHandler', 'getNativeControl', 'forceLayout',
-    'isAttachedToDOM',
+    'addClass', 'removeClass', 'setNativeControlAttr', 'removeNativeControlAttr', 'registerAnimationEndHandler',
+    'deregisterAnimationEndHandler', 'registerChangeHandler', 'deregisterChangeHandler', 'getNativeControl',
+    'forceLayout', 'isAttachedToDOM',
   ]);
 });
 
@@ -424,6 +424,16 @@ test('change handler triggers layout for changes within the same frame to correc
   td.verify(mockAdapter.forceLayout());
 });
 
+test('change handler updates aria-checked attribute correctly.', () => {
+  const {mockAdapter, change} = setupChangeHandlerTest();
+
+  change({checked: true, indeterminate: true});
+  td.verify(mockAdapter.setNativeControlAttr('aria-checked', 'mixed'));
+
+  change({checked: true, indeterminate: false});
+  td.verify(mockAdapter.removeNativeControlAttr('aria-checked'));
+});
+
 test('change handler does not add animation classes when isAttachedToDOM() is falsy', () => {
   const {mockAdapter, change} = setupChangeHandlerTest();
   const animClassArg = td.matchers.argThat((cls) => cls.indexOf('mdc-checkbox--anim') >= 0);