SnackBar使用

注意：
build(BuildContext context)在 Scaffold.of(context)之前时，会报错，解决办法：
通过build widget来解决，如下代码。

import 'package:flutter/material.dart';

main() => runApp(MaterialApp(
      home: MyHome(),
    ));

class MyHome extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    // TODO: implement createState
    return MyHomePageState();
  }
}

class MyHomePageState extends State<MyHome> {
  Widget sdtSnack = SnackBar(content: Text('sdt'));

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('BottomNavigationBar Sample'),
      ),
      body: Center(
        child: new ListView(
          children: <Widget>[

            new FlatButton(
              onPressed: (){
                Scaffold.of(context).showSnackBar(sdtSnack);
              },
              child: new Text('我是按钮'),
            ),


            Builder(builder: (BuildContext context) {
              return new Center(
                child: new GestureDetector(
                  onTap: () {

                    final mySnackBar = SnackBar(
                      content: new Text('我是SnackBar'),
                      duration: Duration(seconds: 4),
                      action: new SnackBarAction(
                          label: '我是scackbar按钮',
                          onPressed: () {
                            print('点击了snackbar按钮');
                          }),
                    );

                    Scaffold.of(context).showSnackBar(mySnackBar);
                  },
                  child: new Text('点我显示SnackBar'),
                ),
              );
            }),



          ],
        ),
      ),
    );
  }
}