互联网连接检查不起作用，连接插件

当您启动应用程序时，中心的铭文 Hello World!。单击“检查”按钮后，理论上应该会出现一条关于 Internet 连接或不存在的消息。什么都没发生。为什么？下面是完整的代码和截图。

主要.dart

import 'package:flutter/material.dart';

import 'package:connectivity/connectivity.dart';

import 'dart:async';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
 @override
 Widget build(BuildContext context) {
  return MaterialApp(
   debugShowCheckedModeBanner: false,
   title: 'Name App',
   theme: ThemeData(
    primarySwatch: Colors.blue,
   ),
   home: Scaffold(
    appBar: AppBar(
     title: Text('Name Page'),
    ),
    body: MyHomePage(),
   ),
  );
 }
}

class MyHomePage extends StatefulWidget {
 @override
 _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {

 String _str = 'Hello World!';

 @override
 void initState() {
  super.initState();

 }

 @override
 void dispose() {
  super.dispose();

 }

 void _f1() {
  setState(() {

  });
 }

 Future<void> _funCheckInternetConnection() async {

  ConnectivityResult result = await Connectivity().checkConnectivity();

  if(result == ConnectivityResult.mobile) {
   setState(() {
    _str = 'connected to a mobile network';
   });
  } else if(result == ConnectivityResult.wifi) {
   setState(() {
    _str = 'connected to a wifi network';
   });
  } else if(result == ConnectivityResult.none) {
   setState(() {
    _str = 'disconnected to a network';
   });
  }

 }

 @override
 Widget build(BuildContext context) {
  return Stack(
   children: [
    Align(
     alignment: Alignment.topCenter,
     child: TextButton(
      onPressed: () { _funCheckInternetConnection(); },
      child: Container(
       width: 100.0,
       height: 50.0,
       decoration: BoxDecoration(
        color: Colors.black12,
        borderRadius: BorderRadius.all(
         Radius.circular(10.0)
        )
       ),
       alignment: Alignment.center,
       child: Text(
        'Check',
        style: TextStyle(
         fontSize: 20,
         color: Colors.deepPurpleAccent
        ),
       ),
      ),
     ),
    ),
    Align(
     alignment: Alignment.center,
     child: Text(
      _str,
      style: TextStyle(
       fontSize: 26.0,
       color: Colors.pink
      ),
     ),
    ),
   ],
  );
 }

}