Flutter Flatter SharedReferences在应用程序重启后获取数据

我已经实现了用于存储当前登录用户的共享首选项，在用户每次登录后，我都可以获取用户名，但是在重新启动应用程序后，用户名消失了，但当前用户仍然登录。我附上了gif以便更好地解释。下面是我的代码

main.dart

Future<void> main() async {
SharedPreferences prefs = await SharedPreferences.getInstance();
var usernamee = prefs.getString('yourusername');
print(usernamee);
runApp(MaterialApp(
  home: usernamee == null
      ? LoginPage()
      : MainPage(
          username: username,
        )));
}

String username = '';

class LoginPage extends StatefulWidget {
@override
LoginPageState createState() => LoginPageState();
}

class LoginPageState extends State<LoginPage> {

TextEditingController user = new TextEditingController();
TextEditingController pass = new TextEditingController(); 

String txtmsg = '';

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

Future<bool> _onWillPop() {
return showDialog(
      context: context,
      builder: (context) => new AlertDialog(
            title: new Text('Are you sure?'),
            content: new Text('Do you want to exit an App'),
            actions: <Widget>[
              new FlatButton(
                onPressed: () => Navigator.of(context).pop(false),
                child: new Text('No'),
              ),
              new FlatButton(
                onPressed: () => Navigator.of(context).pop(true),
                child: new Text('Yes'),
              ),
            ],
          ),
    ) ??
    false;
}

Future<List> _login() async {
SharedPreferences prefs = await SharedPreferences.getInstance();

final response =
    await http.post("http://192.168.1.2/login/login.php", body: {
  "username": user.text,
  "password": pass.text,
});

var datauser = json.decode(response.body);

if (datauser.length == 0) {
  setState(() {
    txtmsg = "Username or password is wrong, please try again.";
    Fluttertoast.showToast(msg: txtmsg);
  });
} else {
  if (datauser[0]['level'] == 'admin') {
    prefs.setString('yourusername', '$username');
    Navigator.pushReplacement(
        context,
        MaterialPageRoute(
            builder: (BuildContext ctx) => MainPage(
                  username: username,
                )));
  }

  setState(() {
    username = datauser[0]['username'];
  });
}
print(username);
return datauser;
}

final logo = Hero(
tag: 'hero',
child: CircleAvatar(
  backgroundColor: Colors.transparent,
  radius: 48.0,
  child: Image.asset('assets/images/ic_launcher.png'),
),
);

final forgotLabel = FlatButton(
child: Text(
  'Forgot password?',
  style: TextStyle(color: Colors.black54),
),
onPressed: () {},
);

@override
Widget build(BuildContext context) {
ScreenUtil.instance = ScreenUtil.getInstance()..init(context);
ScreenUtil.instance =
    ScreenUtil(width: 750, height: 1334, allowFontScaling: true);

return WillPopScope(
    onWillPop: _onWillPop,
    child: Scaffold(
        body: Center(
      child: ListView(
        shrinkWrap: true,
        padding: EdgeInsets.only(left: 24.0, right: 24.0),
        children: <Widget>[
          logo,
          SizedBox(height: 48.0),
          Center(
            child: FormUI(),
          )
        ],
      ),
    )));
}

Widget FormUI() {
return new Column(
  children: <Widget>[
    TextFormField(
      controller: user,
      decoration: InputDecoration(
        hintText: 'Username',
        contentPadding: EdgeInsets.fromLTRB(20.0, 10.0, 20.0, 20.0),
        border:
            OutlineInputBorder(borderRadius: BorderRadius.circular(20.0)),
      ),
      validator: validateusername,
    ),
    SizedBox(height: 18.0),
    TextFormField(
      controller: pass,
      obscureText: true,
      decoration: InputDecoration(
        hintText: 'Password',
        contentPadding: EdgeInsets.fromLTRB(20.0, 10.0, 20.0, 20.0),
        border:
            OutlineInputBorder(borderRadius: BorderRadius.circular(20.0)),
      ),
      validator: validatepassword,
    ),
    SizedBox(
      height: ScreenUtil.getInstance().setHeight(40),
    ),
    RaisedButton(
      shape: RoundedRectangleBorder(
        borderRadius: BorderRadius.circular(24),
      ),
      onPressed: () => _login(),
      padding: EdgeInsets.all(12),
      color: Colors.lightBlueAccent,
      child: Text('Log In', style: TextStyle(color: Colors.white)),
    ),
    forgotLabel,
  ],
);
}

String validateusername(String value) {
if (value.length == 0) {
  return "Name is Required";
}

return null;
}

String validatepassword(String value) {
if (value.length == 0) {
  return "Password is Required";
}

return null;
}
}

class MainPage extends StatefulWidget {
MainPage({this.username});
final String username;

@override
MainPageState createState() => MainPageState();
}

class MainPageState extends State<MainPage> {
Future<bool> _onWillPop() {
return showDialog(
      context: context,
      builder: (context) => new AlertDialog(
            title: new Text('Are you sure?'),
            content: new Text('Do you want to exit an App'),
            actions: <Widget>[
              new FlatButton(
                onPressed: () => Navigator.of(context).pop(false),
                child: new Text('No'),
              ),
              new FlatButton(
                onPressed: () => Navigator.of(context).pop(true),
                child: new Text('Yes'),
              ),
            ],
          ),
    ) ??
    false;
}

void showWifiAlert() async {
var wifiEnabled = await getWifiStatus();
if (wifiEnabled) {
  //Do stuff
} else {
  AlertDialog(
    title: Center(
      child: Text('Alert'),
    ),
    content: Row(
      mainAxisAlignment: MainAxisAlignment.center,
      crossAxisAlignment: CrossAxisAlignment.center,
      children: <Widget>[
        Expanded(
          child: Text(
            'Please connect to the internet',
            textAlign: TextAlign.center,
            style: TextStyle(
              color: Colors.red,
            ),
          ),
        )
      ],
    ),
    actions: <Widget>[
      FlatButton(
        child: Text(
          'Ok',
        ),
        onPressed: () {
          Navigator.of(context).pop();
        },
      ),
    ],
  );
}
}

Future<bool> getWifiStatus() async {
try {
  final result = await InternetAddress.lookup('google.com');
  if (result.isNotEmpty && result[0].rawAddress.isNotEmpty) {
    print('connected');
    return true;
  }
} on SocketException catch (_) {
  print('not connected');
  return false;
}
}

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

}

@override
Widget build(BuildContext context) {
String username;
print(username);
showWifiAlert();

return WillPopScope(
  onWillPop: _onWillPop,
  child: Scaffold(
      appBar: AppBar(
        title: Text('My App'),
      ),
      body: Center(
        child: Text(
          widget.username,
        ),
      ),
      drawer: new DrawerOnly()),
);
}
}

class DrawerOnly extends StatefulWidget {
@override
DrawerOnlyState createState() => DrawerOnlyState();
}

class DrawerOnlyState extends State<DrawerOnly> {
@override
Widget build(BuildContext ctxt) {
return new Drawer(
    child: new ListView(
  children: <Widget>[
    new UserAccountsDrawerHeader(
      accountName: Text('$username'),
      accountEmail: null,
      currentAccountPicture: CircleAvatar(
        child: FlutterLogo(size: 42.0),
        backgroundColor: Colors.white,
      ),
    ),
    new ListTile(
      leading: Icon(Icons.library_music),
      title: Text('Fragment 1'),
      onTap: () {
        Navigator.pop(ctxt);
        Navigator.push(ctxt,
            new MaterialPageRoute(builder: (ctxt) => new FirstFragment()));
      },
    ),
    new ListTile(
      leading: Icon(Icons.movie),
      title: Text('Fragment 2'),
      onTap: () {
        Navigator.pop(ctxt);
        Navigator.push(ctxt,
            new MaterialPageRoute(builder: (ctxt) => new SecondFragment()));
      },
    ),
    new ListTile(
      leading: Icon(Icons.exit_to_app),
      title: Text('Logout'),
      onTap: () async {
        SharedPreferences prefs = await SharedPreferences.getInstance();
        prefs.remove('yourusername');
        Navigator.pushReplacement(ctxt,
            MaterialPageRoute(builder: (BuildContext ctx) => LoginPage()));
        /* Navigator.pop(ctxt);
        Navigator.pushReplacementNamed(ctxt, '/landingpage'); */
      },
    ),
  ],
));
}
}

Future main（）异步{
SharedReferences prefs=等待SharedReferences.getInstance（）；
var usernamee=prefs.getString（'yourusername'）；
打印（用户名）；
runApp（材料应用程序）(
主页：usernamee==null
？登录页面（）
：主页(
用户名：用户名，
)));
}
字符串用户名=“”；

loginpage.dart

Future<void> main() async {
SharedPreferences prefs = await SharedPreferences.getInstance();
var usernamee = prefs.getString('yourusername');
print(usernamee);
runApp(MaterialApp(
  home: usernamee == null
      ? LoginPage()
      : MainPage(
          username: username,
        )));
}

String username = '';

class LoginPage extends StatefulWidget {
@override
LoginPageState createState() => LoginPageState();
}

class LoginPageState extends State<LoginPage> {

TextEditingController user = new TextEditingController();
TextEditingController pass = new TextEditingController(); 

String txtmsg = '';

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

Future<bool> _onWillPop() {
return showDialog(
      context: context,
      builder: (context) => new AlertDialog(
            title: new Text('Are you sure?'),
            content: new Text('Do you want to exit an App'),
            actions: <Widget>[
              new FlatButton(
                onPressed: () => Navigator.of(context).pop(false),
                child: new Text('No'),
              ),
              new FlatButton(
                onPressed: () => Navigator.of(context).pop(true),
                child: new Text('Yes'),
              ),
            ],
          ),
    ) ??
    false;
}

Future<List> _login() async {
SharedPreferences prefs = await SharedPreferences.getInstance();

final response =
    await http.post("http://192.168.1.2/login/login.php", body: {
  "username": user.text,
  "password": pass.text,
});

var datauser = json.decode(response.body);

if (datauser.length == 0) {
  setState(() {
    txtmsg = "Username or password is wrong, please try again.";
    Fluttertoast.showToast(msg: txtmsg);
  });
} else {
  if (datauser[0]['level'] == 'admin') {
    prefs.setString('yourusername', '$username');
    Navigator.pushReplacement(
        context,
        MaterialPageRoute(
            builder: (BuildContext ctx) => MainPage(
                  username: username,
                )));
  }

  setState(() {
    username = datauser[0]['username'];
  });
}
print(username);
return datauser;
}

final logo = Hero(
tag: 'hero',
child: CircleAvatar(
  backgroundColor: Colors.transparent,
  radius: 48.0,
  child: Image.asset('assets/images/ic_launcher.png'),
),
);

final forgotLabel = FlatButton(
child: Text(
  'Forgot password?',
  style: TextStyle(color: Colors.black54),
),
onPressed: () {},
);

@override
Widget build(BuildContext context) {
ScreenUtil.instance = ScreenUtil.getInstance()..init(context);
ScreenUtil.instance =
    ScreenUtil(width: 750, height: 1334, allowFontScaling: true);

return WillPopScope(
    onWillPop: _onWillPop,
    child: Scaffold(
        body: Center(
      child: ListView(
        shrinkWrap: true,
        padding: EdgeInsets.only(left: 24.0, right: 24.0),
        children: <Widget>[
          logo,
          SizedBox(height: 48.0),
          Center(
            child: FormUI(),
          )
        ],
      ),
    )));
}

Widget FormUI() {
return new Column(
  children: <Widget>[
    TextFormField(
      controller: user,
      decoration: InputDecoration(
        hintText: 'Username',
        contentPadding: EdgeInsets.fromLTRB(20.0, 10.0, 20.0, 20.0),
        border:
            OutlineInputBorder(borderRadius: BorderRadius.circular(20.0)),
      ),
      validator: validateusername,
    ),
    SizedBox(height: 18.0),
    TextFormField(
      controller: pass,
      obscureText: true,
      decoration: InputDecoration(
        hintText: 'Password',
        contentPadding: EdgeInsets.fromLTRB(20.0, 10.0, 20.0, 20.0),
        border:
            OutlineInputBorder(borderRadius: BorderRadius.circular(20.0)),
      ),
      validator: validatepassword,
    ),
    SizedBox(
      height: ScreenUtil.getInstance().setHeight(40),
    ),
    RaisedButton(
      shape: RoundedRectangleBorder(
        borderRadius: BorderRadius.circular(24),
      ),
      onPressed: () => _login(),
      padding: EdgeInsets.all(12),
      color: Colors.lightBlueAccent,
      child: Text('Log In', style: TextStyle(color: Colors.white)),
    ),
    forgotLabel,
  ],
);
}

String validateusername(String value) {
if (value.length == 0) {
  return "Name is Required";
}

return null;
}

String validatepassword(String value) {
if (value.length == 0) {
  return "Password is Required";
}

return null;
}
}

class MainPage extends StatefulWidget {
MainPage({this.username});
final String username;

@override
MainPageState createState() => MainPageState();
}

class MainPageState extends State<MainPage> {
Future<bool> _onWillPop() {
return showDialog(
      context: context,
      builder: (context) => new AlertDialog(
            title: new Text('Are you sure?'),
            content: new Text('Do you want to exit an App'),
            actions: <Widget>[
              new FlatButton(
                onPressed: () => Navigator.of(context).pop(false),
                child: new Text('No'),
              ),
              new FlatButton(
                onPressed: () => Navigator.of(context).pop(true),
                child: new Text('Yes'),
              ),
            ],
          ),
    ) ??
    false;
}

void showWifiAlert() async {
var wifiEnabled = await getWifiStatus();
if (wifiEnabled) {
  //Do stuff
} else {
  AlertDialog(
    title: Center(
      child: Text('Alert'),
    ),
    content: Row(
      mainAxisAlignment: MainAxisAlignment.center,
      crossAxisAlignment: CrossAxisAlignment.center,
      children: <Widget>[
        Expanded(
          child: Text(
            'Please connect to the internet',
            textAlign: TextAlign.center,
            style: TextStyle(
              color: Colors.red,
            ),
          ),
        )
      ],
    ),
    actions: <Widget>[
      FlatButton(
        child: Text(
          'Ok',
        ),
        onPressed: () {
          Navigator.of(context).pop();
        },
      ),
    ],
  );
}
}

Future<bool> getWifiStatus() async {
try {
  final result = await InternetAddress.lookup('google.com');
  if (result.isNotEmpty && result[0].rawAddress.isNotEmpty) {
    print('connected');
    return true;
  }
} on SocketException catch (_) {
  print('not connected');
  return false;
}
}

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

}

@override
Widget build(BuildContext context) {
String username;
print(username);
showWifiAlert();

return WillPopScope(
  onWillPop: _onWillPop,
  child: Scaffold(
      appBar: AppBar(
        title: Text('My App'),
      ),
      body: Center(
        child: Text(
          widget.username,
        ),
      ),
      drawer: new DrawerOnly()),
);
}
}

class DrawerOnly extends StatefulWidget {
@override
DrawerOnlyState createState() => DrawerOnlyState();
}

class DrawerOnlyState extends State<DrawerOnly> {
@override
Widget build(BuildContext ctxt) {
return new Drawer(
    child: new ListView(
  children: <Widget>[
    new UserAccountsDrawerHeader(
      accountName: Text('$username'),
      accountEmail: null,
      currentAccountPicture: CircleAvatar(
        child: FlutterLogo(size: 42.0),
        backgroundColor: Colors.white,
      ),
    ),
    new ListTile(
      leading: Icon(Icons.library_music),
      title: Text('Fragment 1'),
      onTap: () {
        Navigator.pop(ctxt);
        Navigator.push(ctxt,
            new MaterialPageRoute(builder: (ctxt) => new FirstFragment()));
      },
    ),
    new ListTile(
      leading: Icon(Icons.movie),
      title: Text('Fragment 2'),
      onTap: () {
        Navigator.pop(ctxt);
        Navigator.push(ctxt,
            new MaterialPageRoute(builder: (ctxt) => new SecondFragment()));
      },
    ),
    new ListTile(
      leading: Icon(Icons.exit_to_app),
      title: Text('Logout'),
      onTap: () async {
        SharedPreferences prefs = await SharedPreferences.getInstance();
        prefs.remove('yourusername');
        Navigator.pushReplacement(ctxt,
            MaterialPageRoute(builder: (BuildContext ctx) => LoginPage()));
        /* Navigator.pop(ctxt);
        Navigator.pushReplacementNamed(ctxt, '/landingpage'); */
      },
    ),
  ],
));
}
}

class LoginPage扩展StatefulWidget{
@凌驾
LoginPagentate createState（）=>LoginPagentate（）；
}
类LoginPagentate扩展了状态{
TextEditingController用户=新建TextEditingController（）；
TextEditingController pass=新的TextEditingController（）；
字符串txtmsg=''；
@凌驾
void initState（）{
super.initState（）；
}
未来(willpop){
返回显示对话框(
上下文：上下文，
生成器：（上下文）=>新建警报对话框(
标题：新文本（“你确定吗？”），
内容：新文本（“是否要退出应用程序”），
行动：[
新扁平按钮(
onPressed:（）=>Navigator.of（context.pop）（false），
子项：新文本（“否”），
),
新扁平按钮(
onPressed:（）=>Navigator.of（context.pop）（true），
子项：新文本（“是”），
),
],
),
) ??
虚假的；
}
Future\u login（）异步{
SharedReferences prefs=等待SharedReferences.getInstance（）；
最后答复=
等待http.post（“http://192.168.1.2/login/login.php“，主体：{
“用户名”：user.text，
“密码”：pass.text，
});
var datauser=json.decode（response.body）；
如果（datauser.length==0）{
设置状态（）{
txtmsg=“用户名或密码错误，请重试。”；
Toast.showtoos（msg:txtmsg）；
});
}否则{
如果（数据用户[0]['level']=='admin'）{
prefs.setString（'yourusername'，'$username'）；
导航器。更换(
上下文
材料路线(
生成器：（BuildContext ctx）=>MainPage(
用户名：用户名，
)));
}
设置状态（）{
用户名=数据用户[0]['username']；
});
}
打印（用户名）；
返回数据用户；
}
最终标志=英雄(
标签：“英雄”，
孩子：圆环星(
背景颜色：颜色。透明，
半径：48.0，
子项：Image.asset（'assets/images/ic_launcher.png'），
),
);
最终锻件标签=扁平按钮(
子：文本(
“忘记密码了吗？”，
样式：TextStyle（颜色：Colors.black54），
),
按下：（）{}，
);
@凌驾
小部件构建（构建上下文）{
ScreenUtil.instance=ScreenUtil.getInstance（）…初始化（上下文）；
ScreenUtil.instance=
ScreenUtil（宽度：750，高度：1334，allowFontScaling:true）；
返回式示波器(
onWillPop:_onWillPop，
孩子：脚手架(
正文：中(
子：ListView(
收缩膜：对，
填充：仅限边设置（左：24.0，右：24.0），
儿童：[
标志
尺寸箱（高度：48.0），
居中(
子项：FormUI（），
)
],
),
)));
}
控件FormUI（）{
返回新列(
儿童：[
TextFormField(
控制器：用户，
装饰：输入装饰(
hintText:'用户名'，
内容填充：来自LTRB（20.0、10.0、20.0、20.0）的EdgeInsets，
边界：
OutlineInputBorder（borderRadius:borderRadius.circular（20.0）），
),
验证程序：validateusername，
),
尺寸箱（高度：18.0），
TextFormField(
控制员：通过，
蒙昧文字：对，
装饰：输入装饰(
hintText:'密码'，
内容填充：来自LTRB（20.0、10.0、20.0、20.0）的EdgeInsets，
边界：
OutlineInputBorder（borderRadius:borderRadius.circular（20.0）），
),
validator:validatepassword，
),
大小盒子(
高度：ScreenUtil.getInstance（）.setHeight（40），
),
升起的按钮(
形状：圆形矩形边框(
边界半径：边界半径。圆形（24），
),
onPressed:（）=>\u login（），
填充：边缘设置。全部（12），
颜色：Colors.lightBlueAccent，
子项：文本（'登录'，样式：TextStyle（颜色：Colors.white）），
),
放弃标签，
],
);
}
字符串validateusername（字符串值）{
如果（value.length==0）{
返回“名称为必填项”；
}
返回null；
}
字符串验证密码（字符串值）{
如果（value.length==0）{
返回“需要密码”；
}
返回null；
}
}

mainpage.dart

Future<void> main() async {
SharedPreferences prefs = await SharedPreferences.getInstance();
var usernamee = prefs.getString('yourusername');
print(usernamee);
runApp(MaterialApp(
  home: usernamee == null
      ? LoginPage()
      : MainPage(
          username: username,
        )));
}

String username = '';

class LoginPage extends StatefulWidget {
@override
LoginPageState createState() => LoginPageState();
}

class LoginPageState extends State<LoginPage> {

TextEditingController user = new TextEditingController();
TextEditingController pass = new TextEditingController(); 

String txtmsg = '';

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

Future<bool> _onWillPop() {
return showDialog(
      context: context,
      builder: (context) => new AlertDialog(
            title: new Text('Are you sure?'),
            content: new Text('Do you want to exit an App'),
            actions: <Widget>[
              new FlatButton(
                onPressed: () => Navigator.of(context).pop(false),
                child: new Text('No'),
              ),
              new FlatButton(
                onPressed: () => Navigator.of(context).pop(true),
                child: new Text('Yes'),
              ),
            ],
          ),
    ) ??
    false;
}

Future<List> _login() async {
SharedPreferences prefs = await SharedPreferences.getInstance();

final response =
    await http.post("http://192.168.1.2/login/login.php", body: {
  "username": user.text,
  "password": pass.text,
});

var datauser = json.decode(response.body);

if (datauser.length == 0) {
  setState(() {
    txtmsg = "Username or password is wrong, please try again.";
    Fluttertoast.showToast(msg: txtmsg);
  });
} else {
  if (datauser[0]['level'] == 'admin') {
    prefs.setString('yourusername', '$username');
    Navigator.pushReplacement(
        context,
        MaterialPageRoute(
            builder: (BuildContext ctx) => MainPage(
                  username: username,
                )));
  }

  setState(() {
    username = datauser[0]['username'];
  });
}
print(username);
return datauser;
}

final logo = Hero(
tag: 'hero',
child: CircleAvatar(
  backgroundColor: Colors.transparent,
  radius: 48.0,
  child: Image.asset('assets/images/ic_launcher.png'),
),
);

final forgotLabel = FlatButton(
child: Text(
  'Forgot password?',
  style: TextStyle(color: Colors.black54),
),
onPressed: () {},
);

@override
Widget build(BuildContext context) {
ScreenUtil.instance = ScreenUtil.getInstance()..init(context);
ScreenUtil.instance =
    ScreenUtil(width: 750, height: 1334, allowFontScaling: true);

return WillPopScope(
    onWillPop: _onWillPop,
    child: Scaffold(
        body: Center(
      child: ListView(
        shrinkWrap: true,
        padding: EdgeInsets.only(left: 24.0, right: 24.0),
        children: <Widget>[
          logo,
          SizedBox(height: 48.0),
          Center(
            child: FormUI(),
          )
        ],
      ),
    )));
}

Widget FormUI() {
return new Column(
  children: <Widget>[
    TextFormField(
      controller: user,
      decoration: InputDecoration(
        hintText: 'Username',
        contentPadding: EdgeInsets.fromLTRB(20.0, 10.0, 20.0, 20.0),
        border:
            OutlineInputBorder(borderRadius: BorderRadius.circular(20.0)),
      ),
      validator: validateusername,
    ),
    SizedBox(height: 18.0),
    TextFormField(
      controller: pass,
      obscureText: true,
      decoration: InputDecoration(
        hintText: 'Password',
        contentPadding: EdgeInsets.fromLTRB(20.0, 10.0, 20.0, 20.0),
        border:
            OutlineInputBorder(borderRadius: BorderRadius.circular(20.0)),
      ),
      validator: validatepassword,
    ),
    SizedBox(
      height: ScreenUtil.getInstance().setHeight(40),
    ),
    RaisedButton(
      shape: RoundedRectangleBorder(
        borderRadius: BorderRadius.circular(24),
      ),
      onPressed: () => _login(),
      padding: EdgeInsets.all(12),
      color: Colors.lightBlueAccent,
      child: Text('Log In', style: TextStyle(color: Colors.white)),
    ),
    forgotLabel,
  ],
);
}

String validateusername(String value) {
if (value.length == 0) {
  return "Name is Required";
}

return null;
}

String validatepassword(String value) {
if (value.length == 0) {
  return "Password is Required";
}

return null;
}
}

class MainPage extends StatefulWidget {
MainPage({this.username});
final String username;

@override
MainPageState createState() => MainPageState();
}

class MainPageState extends State<MainPage> {
Future<bool> _onWillPop() {
return showDialog(
      context: context,
      builder: (context) => new AlertDialog(
            title: new Text('Are you sure?'),
            content: new Text('Do you want to exit an App'),
            actions: <Widget>[
              new FlatButton(
                onPressed: () => Navigator.of(context).pop(false),
                child: new Text('No'),
              ),
              new FlatButton(
                onPressed: () => Navigator.of(context).pop(true),
                child: new Text('Yes'),
              ),
            ],
          ),
    ) ??
    false;
}

void showWifiAlert() async {
var wifiEnabled = await getWifiStatus();
if (wifiEnabled) {
  //Do stuff
} else {
  AlertDialog(
    title: Center(
      child: Text('Alert'),
    ),
    content: Row(
      mainAxisAlignment: MainAxisAlignment.center,
      crossAxisAlignment: CrossAxisAlignment.center,
      children: <Widget>[
        Expanded(
          child: Text(
            'Please connect to the internet',
            textAlign: TextAlign.center,
            style: TextStyle(
              color: Colors.red,
            ),
          ),
        )
      ],
    ),
    actions: <Widget>[
      FlatButton(
        child: Text(
          'Ok',
        ),
        onPressed: () {
          Navigator.of(context).pop();
        },
      ),
    ],
  );
}
}

Future<bool> getWifiStatus() async {
try {
  final result = await InternetAddress.lookup('google.com');
  if (result.isNotEmpty && result[0].rawAddress.isNotEmpty) {
    print('connected');
    return true;
  }
} on SocketException catch (_) {
  print('not connected');
  return false;
}
}

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

}

@override
Widget build(BuildContext context) {
String username;
print(username);
showWifiAlert();

return WillPopScope(
  onWillPop: _onWillPop,
  child: Scaffold(
      appBar: AppBar(
        title: Text('My App'),
      ),
      body: Center(
        child: Text(
          widget.username,
        ),
      ),
      drawer: new DrawerOnly()),
);
}
}

class DrawerOnly extends StatefulWidget {
@override
DrawerOnlyState createState() => DrawerOnlyState();
}

class DrawerOnlyState extends State<DrawerOnly> {
@override
Widget build(BuildContext ctxt) {
return new Drawer(
    child: new ListView(
  children: <Widget>[
    new UserAccountsDrawerHeader(
      accountName: Text('$username'),
      accountEmail: null,
      currentAccountPicture: CircleAvatar(
        child: FlutterLogo(size: 42.0),
        backgroundColor: Colors.white,
      ),
    ),
    new ListTile(
      leading: Icon(Icons.library_music),
      title: Text('Fragment 1'),
      onTap: () {
        Navigator.pop(ctxt);
        Navigator.push(ctxt,
            new MaterialPageRoute(builder: (ctxt) => new FirstFragment()));
      },
    ),
    new ListTile(
      leading: Icon(Icons.movie),
      title: Text('Fragment 2'),
      onTap: () {
        Navigator.pop(ctxt);
        Navigator.push(ctxt,
            new MaterialPageRoute(builder: (ctxt) => new SecondFragment()));
      },
    ),
    new ListTile(
      leading: Icon(Icons.exit_to_app),
      title: Text('Logout'),
      onTap: () async {
        SharedPreferences prefs = await SharedPreferences.getInstance();
        prefs.remove('yourusername');
        Navigator.pushReplacement(ctxt,
            MaterialPageRoute(builder: (BuildContext ctx) => LoginPage()));
        /* Navigator.pop(ctxt);
        Navigator.pushReplacementNamed(ctxt, '/landingpage'); */
      },
    ),
  ],
));
}
}

class主页扩展StatefulWidget{
主页（{this.username}）；
最终字符串用户名；
@凌驾
MainPageState createState（）=>MainPageState（）；
}
类MainPageState扩展状态{
未来(willpop){
返回显示对话框(
上下文：上下文，
生成器：（上下文）=>新建警报对话框(
标题：新文本（“你确定吗？”），
内容：新文本（“是否要退出应用程序”），
行动：[
新扁平按钮(
onPressed:（）=>Navigator.of（context.pop）（false），
子项：新文本（“否”），
),
新扁平按钮(
onPressed:（）=>Navigator.of（context.pop）（true），
子项：新文本（“是”），
),
],
),
) ??
虚假的；
}
void showWifiAlert（）异步{
var wifiEnabled=wait getWifiStatus（）；
如果（可连接）{
//做事
}否则{
警报对话框(
标题：中心(
子项：文本（“警报”），
),
内容：行(
mainAxisAlignment:mainAxisAlignment.center，
crossAxisAlignment:crossAxisAlignment.center，
儿童：[
扩大(
子：文本(
“请连接到internet”，