先上代码

import 'package:flutter/material.dart';

void main()=> runApp(new MyApp());

class MyApp extends StatelessWidget{
  @override
  Widget build(BuildContext context){
    return MaterialApp(
      title: "Align组件",
      home:Scaffold(//Scaffold定义页面的结构
          appBar:AppBar(
            title: Text("Align组件"),
          ),
        body:Container(
          // width: 200.0,
          // height: 200.0,
          color: Colors.blue,
          child: Align(
            widthFactor: 2.0,
            heightFactor: 3.0,
            alignment: Alignment.center,
            child: Container(
              width: 50.0,
              height: 50.0,
              color: Colors.red,
              child: Text("居中",style: TextStyle(color: Colors.white),),
            ),
          ),
        )
      )

    );
  }
}

运行效果图

Align组件

属性主要有两类：

1. alignment:设置子组件在父组件里的对齐方式属性值有:
   center：上下左右居中，
   topRight:父组件的右上角
   topCenter:顶部中间
   topLeft：左上
   。。。。以此类推centerLeft，centerRight等等
   还有一种用法是alignment: Alignment(x，y)，x是横向，y是纵向，值的范围是0-1，
   alignment: Alignment(0.0，0.0)等同于alignment: Alignment.center
   alignment: Alignment(1.0，1.0)等同于alignment: Alignment.topRight
   alignment: Alignment(-1.0-1.0)等同于alignment: Alignment.bottomLeft
2. widthFactor和heightFactor
   是设置父容器的宽高是子组件宽高的几倍（前提是父组件没有设置宽高，否则设置无效）
   示例代码：

import 'package:flutter/material.dart';

void main()=> runApp(new MyApp());

class MyApp extends StatelessWidget{
  @override
  Widget build(BuildContext context){
    return MaterialApp(
      title: "Align组件",
      home:Scaffold(
          appBar:AppBar(
            title: Text("Align组件"),
          ),
        body:Container(
          // width: 200.0,
          // height: 200.0,
          color: Colors.blue,
          child: Align(
            widthFactor: 2.0,
            heightFactor: 3.0,
            alignment: Alignment(0.0,0.0),
            child: Container(
              width: 100.0,
              height: 100.0,
              color: Colors.red,
              child: Text("居中",style: TextStyle(color: Colors.white),),
            ),
          ),
        )
      )

    );
  }
}

效果图