title: How to add children to existing Angular Mat-Tree nodes
body: <p>I am creating an Angular Mat-tree to show the directory structure of my file system.
  I can get a list of all root folders and show them in Mat-tree using the official example from the following link <a href="https://material.angular.io/components/tree/examples" rel="nofollow noreferrer">example</a></p>
  <p>But how can I add child items to the existing parent nodes and then add another level by adding new data as the user expands the items?</p>
  <p>I Currently have this</p>
  <pre><code>  treeControl = new FlatTreeControl&lt;DirectoryItemFlatNode&gt;(
      node =&gt; node.level,
      node =&gt; node.expandable,
    );
  
    treeFlattener = new MatTreeFlattener(
      this._transformer,
      node =&gt; node.level,
      node =&gt; node.expandable,
      node =&gt; node.children,
    );
  
    dataSource = new MatTreeFlatDataSource(this.treeControl, this.treeFlattener);
  
  this.dataSource.data = TREE_DATA;
  </code></pre>