flutter ExpansionTile 层级菜单的实现

 @override
 Widget build(BuildContext context) {
  return ListTile(

   title: _buildItem(widget.bean),
  );
 }

 Widget _buildItem(NameBean bean){
  if(bean.children.isEmpty){
   return ListTile(
    title: Text(bean.name),
    onTap: (){
     _showSeletedName(bean.name);
    },
   );
  }
  return ExpansionTile(
   key: PageStorageKey<NameBean>(bean),
   title: Text(bean.name),
   children: bean.children.map<Widget>(_buildItem).toList(),
   leading: CircleAvatar(
    backgroundColor: Colors.green,
    child: Text(bean.name.substring(0,1),style: TextStyle(color: Colors.white),),
   ),
  );
 }

/// A single-line [ListTile] with a trailing button that expands or collapses
/// the tile to reveal or hide the [children].
///
/// This widget is typically used with [ListView] to create an
/// "expand / collapse" list entry. When used with scrolling widgets like
/// [ListView], a unique [PageStorageKey] must be specified to enable the
/// [ExpansionTile] to save and restore its expanded state when it is scrolled
/// in and out of view.
///
/// See also:
///
/// * [ListTile], useful for creating expansion tile [children] when the
///  expansion tile represents a sublist.
/// * The "Expand/collapse" section of
///  <https://material.io/guidelines/components/lists-controls.html>.
class ExpansionTile extends StatefulWidget {