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')
]
),
),
);
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