Text ------widget树由两个widget:Center(及其子widget)和Text组成
import 'package:flutter/material.dart'; void main() => runApp(new Center( child: new Text( 'Hello, world!', textDirection: TextDirection.ltr, textAlign: TextAlign.center, maxLines: 1, overflow: TextOverflow.ellipsis, style: TextStyle(fontSize: 80.0, color: Colors.yellow), )));