Method _getAriaExpanded

src/cdk/tree/tree.ts:1361–1366

* Determines the value for `aria-expanded`. * * For non-expandable nodes, this is `null`.

()

   * For non-expandable nodes, this is `null`.
   */
  _getAriaExpanded(): string | null {
    if (!this._isExpandable()) {
      return null;
    }
    return String(this.isExpanded);
  }

  /**
   * Determines the size of this node's parent's child set.