walkTree = (el, level, callback) => {
    el.items.forEach((item, index) => {
      callback(item, level, index);
      if (item.items.length > 0) {
        walkTree(item, level + 1, callback);
      }
    });
  }
Fired when selection is changed by user interaction
in <code>SingleSelect</code>, <code>SingleSelectBegin</code>, <code>SingleSelectEnd</code> and <code>MultiSelect</code> modes.

@event sap.ui.webc.main.Tree#selection-change
@param {Array} selectedItems An array of the selected items.
@param {Array} previouslySelectedItems An array of the previously selected items.
@param {HTMLElement} targetItem The item triggering the event.
@public