具有可选操作的轻量级消息提示，在屏幕的底部显示

文档：https://api.flutter.dev/flutter/material/SnackBar-class.html

demo:

import 'package:flutter/material.dart';

class SnackBarDemo extends StatefulWidget {
  @override
  _SnackBarDemoState createState() => _SnackBarDemoState();
}

class _SnackBarDemoState extends State<SnackBarDemo> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      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 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return FlatButton(
      
      child: Text('Open SnackBar'),
      onPressed: () {
        Scaffold.of(context).showSnackBar(
          SnackBar(
            content: Text('Processing...'),
            action: SnackBarAction(
              label: 'OK',
              onPressed: () {},
            ),
           
          )
        );
      },
    );
  }
}

 效果：