change role input to always be treeitem

Author: annieyw

src/cdk/tree/tree.ts

@@ -314,7 +314,7 @@ export class CdkTreeNode<T> implements FocusableOption, OnDestroy {
   set data(value: T) {
     if (value !== this._data) {
       this._data = value;
-      this._setRoleFromData();
+      this._setRole();
       this._dataChanges.next();
     }
   }
@@ -329,8 +329,7 @@ export class CdkTreeNode<T> implements FocusableOption, OnDestroy {
   }
 
   /**
-   * The role of the node should be 'group' if it's an internal node,
-   * and 'treeitem' if it's a leaf node.
+   * The role of the node should always be 'treeitem'.
    */
   @Input() role: 'treeitem' | 'group' = 'treeitem';
 
@@ -356,24 +355,10 @@ export class CdkTreeNode<T> implements FocusableOption, OnDestroy {
     this._elementRef.nativeElement.focus();
   }
 
-  protected _setRoleFromData(): void {
-    if (this._tree.treeControl.isExpandable) {
-      this.role = this._tree.treeControl.isExpandable(this._data) ? 'group' : 'treeitem';
-    } else {
-      if (!this._tree.treeControl.getChildren) {
-        throw getTreeControlFunctionsMissingError();
-      }
-      const childrenNodes = this._tree.treeControl.getChildren(this._data);
-      if (Array.isArray(childrenNodes)) {
-        this._setRoleFromChildren(childrenNodes as T[]);
-      } else if (childrenNodes instanceof Observable) {
-        childrenNodes.pipe(takeUntil(this._destroyed))
-            .subscribe(children => this._setRoleFromChildren(children));
-      }
+  protected _setRole(): void {
+    if (this._tree.treeControl.isExpandable && !this._tree.treeControl.getChildren) {
+      throw getTreeControlFunctionsMissingError();
     }
-  }
-
-  protected _setRoleFromChildren(children: T[]) {
-    this.role = children && children.length ? 'group' : 'treeitem';
+    this.role = 'treeitem';
   }
 }

src/material/tree/node.ts

@@ -45,7 +45,7 @@ const _MatTreeNodeMixinBase: HasTabIndexCtor & CanDisableCtor & typeof CdkTreeNo
   host: {
     '[attr.aria-expanded]': 'isExpanded',
     '[attr.aria-level]': 'level + 1',
-    '[attr.role]': `'treeitem'`,
+    '[attr.role]': 'role',
     'class': 'mat-tree-node'
   },
   providers: [{provide: CdkTreeNode, useExisting: MatTreeNode}]
@@ -87,7 +87,7 @@ export class MatTreeNodeDef<T> extends CdkTreeNodeDef<T> {
   exportAs: 'matNestedTreeNode',
   host: {
     '[attr.aria-expanded]': 'isExpanded',
-    '[attr.role]': `'treeitem'`,
+    '[attr.role]': 'role',
     'class': 'mat-nested-tree-node',
   },
   providers: [