flutter SnackBar 底部消息提示

import 'package:flutter/material.dart';
class SnackBarDemo extendsStatefulWidget {
  @override
  _SnackBarDemoState createState() =>_SnackBarDemoState();
}
class _SnackBarDemoState extends State<SnackBarDemo>{
  @override
  Widget build(BuildContext context) {
    returnScaffold(
      appBar: AppBar(
        title: Text('SnackBarDemo'),
        elevation: 0.0,
      ),
      body: Container(
        padding: EdgeInsets.all(16.0),
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Row(
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
                SnackBarButton(),
              ]
            ),
          ],
        ),
      ),
    );
  }
}
class SnackBarButton extendsStatelessWidget {
  @override
  Widget build(BuildContext context) {
    returnFlatButton(
      child: Text('Open SnackBar'),
      onPressed: () {
        Scaffold.of(context).showSnackBar(
          SnackBar(
            content: Text('Processing...'),
            action: SnackBarAction(
              label: 'OK',
              onPressed: () {},
            ),
          )
        );
      },
    );
  }
}