Flutter 系统是如何实现ExpansionPanelList的示例代码

SingleChildScrollView(
 child: MergeableMaterial(
 children: [
  MaterialSlice(
   key: ValueKey(1),
   child: Container(
   height: 45,
   color: Colors.primaries[1 % Colors.primaries.length],
   )),
  MaterialGap(key: ValueKey(2)),
  MaterialSlice(
   key: ValueKey(3),
   child: Container(
   height: 45,
   color: Colors.primaries[1 % Colors.primaries.length],
   )),
  MaterialGap(key: ValueKey(4)),
  MaterialSlice(
   key: ValueKey(5),
   child: Container(
   height: 45,
   color: Colors.primaries[1 % Colors.primaries.length],
   )),
 ],
 ),
)

List<MergeableMaterialItem> items = [];
List.generate(_count, (index) {
 items.add(MaterialSlice(
  key: ValueKey(index * 2),
  child: Container(
  height: 45,
  color: Colors.primaries[index % Colors.primaries.length],
  )));
});

return SingleChildScrollView(
 child: MergeableMaterial(
 children: items,
 ),
)

MergeableMaterial(
 hasDividers: true,
 elevation: 24,
 children: items,
)

bool _expand = false;

@override
Widget build(BuildContext context) {
 return Column(
 children: <Widget>[
  Container(
  height: 45,
  color: Colors.green.withOpacity(.3),
  alignment: Alignment.centerRight,
  child: IconButton(
   icon: Icon(Icons.arrow_drop_down),
   onPressed: () {
   setState(() {
    _expand = !_expand;
   });
   },
  ),
  ),
  _expand
   ? MergeableMaterial(
    hasDividers: true,
    elevation: 24,
    children: [
    MaterialSlice(
     key: ValueKey(1),
     child: Container(
      height: 200,
      color: Colors.green.withOpacity(.3),
     ))
    ],
   )
   : Container(),
  Container(
  height: 45,
  color: Colors.red.withOpacity(.3),
  ),
 ],
 );
}