Stack控件的每一个子控件都是定位或不定位，定位的子控件是被Positioned控件包裹的。Stack控件本身包含所有不定位的子控件，其根据alignment定位（默认为左上角）。然后根据定位的子控件的top、right、bottom和left属性将它们放置在Stack控件上。
import 'package:flutter/material.dart';class LayoutDemo extends StatelessWidget { @override Widget build(BuildContext context) { return new Scaffold( appBar: new AppBar( title: new Text('层叠定位布局'), ), body:new Center( child: new Stack( children: <Widget>[ new Image.network('http://img2.cxtuku.com/00/13/12/s97783873391.jpg'), new Positioned( left: 35.0, right: 35.0, top: 45.0, child: new Text( 'Whatever is worth doing is worth doing well. ๑•ิ.•ั๑', style: new TextStyle( fontSize: 20.0, fontFamily: 'serif', ), ), ), ] ), ), ); }}void main() { runApp( new MaterialApp( title: 'Flutter教程', home: new LayoutDemo(), ), );} 
