feat(list): Implement getAttributeForElementIndex (#948)

Author: 태재영

packages/list/index.tsx

@@ -217,15 +217,16 @@ export default class List extends React.Component<ListProps, ListState> {
       getListItemCount: () => this.listElements.length,
       getFocusedElementIndex: () =>
         this.listElements.indexOf(document.activeElement as HTMLLIElement),
+      getAttributeForElementIndex: (index, attr) => {
+        const listItem = this.listElements[index];
+        return listItem.getAttribute(attr);
+      },
       setAttributeForElementIndex: (index, attr, value) => {
         const listItem = this.listElements[index];
         if (listItem) {
           listItem.setAttribute(attr, value);
         }
       },
-      // TODO: implement
-      // https://github.com/material-components/material-components-web-react/issues/822
-      getAttributeForElementIndex: () => '',
       /**
        * Pushes class name to state.listItemClassNames[listItemIndex] if it doesn't yet exist.
        */

test/unit/list/index.test.tsx

@@ -161,6 +161,14 @@ test('#adapter.getListItemCount returns correct number of list items if List has
   assert.equal(wrapper.instance().adapter.getListItemCount(), 3);
 });
 
+test('#adapter.getAttributeForElementIndex can get value of attribute', () => {
+  const wrapper = mount<List>(<List>{children()}</List>);
+  const adapter = wrapper.instance().adapter;
+  assert.equal(null, adapter.getAttributeForElementIndex(0, 'role'));
+  adapter.setAttributeForElementIndex(0, 'role', 'menu');
+  assert.equal('menu', adapter.getAttributeForElementIndex(0, 'role'));
+});
+
 test('#adapter.setAttributeForElementIndex calls setAttribute on listItem', () => {
   const wrapper = mount<List>(<List>{children()}</List>);
   wrapper.instance().listElements[0].setAttribute = coerceForTesting<