/**
* 集成自Stack，用来显示第index个child，隐藏其他的child
* IndexedStack({
Key key,
AlignmentGeometry alignment = AlignmentDirectional.topStart,
TextDirection textDirection,
StackFit sizing = StackFit.loose,
this.index = 0,
List<Widget> children = const <Widget>[],
})
*/
body: IndexedStack(
index: 2,
alignment: Alignment.center,
children: <Widget>[
Text("第一层"),
Text("第二层"),
Text("第三层"),
],
)
