如何在类中显示SnackBar

import 'package:flutter/material.dart';

class MyWidget extends StatefulWidget {
  @override
  _MyWidgetState createState() => _MyWidgetState();
}

class _MyWidgetState extends State<MyWidget> {
  final GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();

  void _showSnackBar() {
    _scaffoldKey.currentState.showSnackBar(
      SnackBar(
        content: Text('这是一个SnackBar'),
      ),
    );
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      key: _scaffoldKey,
      appBar: AppBar(
        title: Text('显示SnackBar示例'),
      ),
      body: Center(
        child: RaisedButton(
          onPressed: _showSnackBar,
          child: Text('显示SnackBar'),
        ),
      ),
    );
  }
}