stack绝对定位

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';

main(){
  runApp(MyApp());
}
class MyApp2 extends StatelessWidget{
  @override
  Widget build(BuildContext context) {
  return Scaffold(
      body: Container(
        width: MediaQuery.of(context).size.width,
        height: MediaQuery.of(context).size.height,
        margin: EdgeInsets.all(15),
        child: Stack(
          children: [
            MaterialButton(onPressed: (){

            },
              color: Colors.grey,
            ),
            Positioned(child: MaterialButton(onPressed: (){},color: Colors.blue,),
            left: MediaQuery.of(context).size.width/2,
            top: MediaQuery.of(context).size.height/2,
            ),
            Positioned(child: MaterialButton(onPressed: (){},color: Colors.black12,),
              left: MediaQuery.of(context).size.width/2,
              top: MediaQuery.of(context).size.height/3,
            ),
            Positioned(child: MaterialButton(onPressed: (){},color: Colors.black12,),
              left: 50,
              right: 50,
            )
          ],
        ),
      ),
    );
  }
}
class MyApp extends StatelessWidget{
  @override
  Widget build(BuildContext context) {
   return MaterialApp(
     home: MyApp2(),
   );
  }

}