Flutter-如何在列表视图内将小部件居中

垂直居中和水平居中：

Scaffold(  appBar: new AppBar(),  body: Center(    child: new ListView(      shrinkWrap: true,        padding: const EdgeInsets.all(20.0),        children: [          Center(child: new Text('ABC'))        ]    ),  ),);

仅垂直中心

Scaffold(  appBar: new AppBar(),  body: Center(    child: new ListView(      shrinkWrap: true,        padding: const EdgeInsets.all(20.0),        children: [          new Text('ABC')        ]    ),  ),);