Flutter pressing back button pops up previous snackBar from Login page again - flutter

Question

I have a LoginPage in Flutter. After login, it shows a small snackbar with "success" or "failure.." if password is wrong, then it navigates to the todo list.
When I now press the "back" button on an Android device, it navigates back to the login screen. However, there is still the snackbar popping up and saying "Login successful, redirecting..", and also, my textfields are not emptied and still have the values from the first login, why? That should not happen, but I cannot figure out why that is... here is my code:
import 'package:flutter/material.dart';
import 'package:todoey_flutter/components/rounded_button.dart';
import 'package:shared_preferences/shared_preferences.dart';
import 'package:todoey_flutter/util/file_handler.dart';
import 'package:provider/provider.dart';
class LoginScreen extends StatefulWidget {
#override
_LoginScreenState createState() => _LoginScreenState();
}
class _LoginScreenState extends State<LoginScreen> {
String username;
String password;
String hashedPW;
// Future<SharedPreferences> _prefs = SharedPreferences.getInstance();
var _nameController = TextEditingController();
var _pwController = TextEditingController();
#override
Widget build(BuildContext context) {
CryptOid cy = Provider.of<CryptOid>(context, listen: true);
FileHandler fh = Provider.of<FileHandler>(context, listen: true);
return Scaffold(
backgroundColor: Colors.white,
body: Builder(
builder: (BuildContext scaffoldBuildContext) {
return Container(
//inAsyncCall: isSpinning,
child: Padding(
padding: EdgeInsets.symmetric(horizontal: 34.0),
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
crossAxisAlignment: CrossAxisAlignment.stretch,
children: <Widget>[
/*
Flexible(
child: Hero(
tag: 'logo',
child: Container(
height: 200.0,
child: Image.asset('images/logo.png'),
),
),
),*/
SizedBox(
height: 48.0,
),
TextField(
controller: _nameController,
style: TextStyle(color: Colors.black54),
onChanged: (value) {
//Do something with the user input.
username = value.toLowerCase();
},
decoration: InputDecoration(
hintText: 'Enter your username',
),
),
SizedBox(
height: 8.0,
),
TextField(
controller: _pwController,
obscureText: true,
style: TextStyle(color: Colors.black54),
onChanged: (value) {
//Do something with the user input.
password = value;
},
decoration: InputDecoration(
hintText: 'Enter your password',
),
),
SizedBox(
height: 24.0,
),
RoundedButton(
title: 'Login',
colour: Colors.lightBlueAccent,
onPressed: () async {
Scaffold.of(scaffoldBuildContext).removeCurrentSnackBar();
print("user: $username, pw: $password");
if ((username != '' && username != null) && (password != '' && password != null)) {
SharedPreferences prefs = await SharedPreferences.getInstance();
// cy.test();
if ((username != '' && username != null) && prefs.containsKey(username)) {
hashedPW = prefs.getString(username);
bool decryptPW = await cy.deHash(hashedPW, password);
if (decryptPW) {
cy.setUsername(username);
fh.setUser(username);
prefs.setString('activeUser', username);
Scaffold.of(scaffoldBuildContext).showSnackBar(
SnackBar(
content: Text("Login successful! redirecting.."),
),
);
Navigator.pushNamed(context, 'taskScreen');
} else {
Scaffold.of(scaffoldBuildContext).showSnackBar(
SnackBar(
content: Text("Wrong password for user $username!"),
),
);
}
} else {
String hashedPW = await cy.hashPW(password);
prefs.setString('activeUser', username);
prefs.setString(username, hashedPW);
cy.setUsername(username);
fh.setUser(username);
Scaffold.of(scaffoldBuildContext).showSnackBar(
SnackBar(
content: Text("User created successful! redirecting.."),
),
);
Navigator.pushNamed(context, 'taskScreen');
//prefs.setString(username, hashedPW);
}
_nameController.clear();
_pwController.clear();
} else {
Scaffold.of(scaffoldBuildContext).showSnackBar(
SnackBar(
content: Text("User and password may not be empty.."),
),
);
_nameController.clear();
_pwController.clear();
return;
}
},
),
],
),
),
);
},
),
);
}
}

Answer 1

You should create a ScaffoldState GlobalKey then assign the to the scaffold.
final GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();
#override
Widget build(BuildContext context) {
return Scaffold(
key: _scaffoldKey,
body: Container());
}
The use the key to showSnackBar
void _showInSnackBar(String value) {
_scaffoldKey.currentState
.showSnackBar(new SnackBar(content: new Text(value)));
}
So your full code would look like this:
import 'package:flutter/material.dart';
import 'package:todoey_flutter/components/rounded_button.dart';
import 'package:shared_preferences/shared_preferences.dart';
import 'package:todoey_flutter/util/file_handler.dart';
import 'package:provider/provider.dart';
class LoginScreen extends StatefulWidget {
#override
_LoginScreenState createState() => _LoginScreenState();
}
class _LoginScreenState extends State<LoginScreen> {
final GlobalKey<ScaffoldState> _scaffoldKey = GlobalKey<ScaffoldState>();
String username;
String password;
String hashedPW;
// Future<SharedPreferences> _prefs = SharedPreferences.getInstance();
var _nameController = TextEditingController();
var _pwController = TextEditingController();
#override
Widget build(BuildContext context) {
CryptOid cy = Provider.of<CryptOid>(context, listen: true);
FileHandler fh = Provider.of<FileHandler>(context, listen: true);
return Scaffold(
key: _scaffoldKey,
backgroundColor: Colors.white,
body: Builder(
builder: (BuildContext scaffoldBuildContext) {
return Container(
//inAsyncCall: isSpinning,
child: Padding(
padding: EdgeInsets.symmetric(horizontal: 34.0),
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
crossAxisAlignment: CrossAxisAlignment.stretch,
children: <Widget>[
/*
Flexible(
child: Hero(
tag: 'logo',
child: Container(
height: 200.0,
child: Image.asset('images/logo.png'),
),
),
),*/
SizedBox(
height: 48.0,
),
TextField(
controller: _nameController,
style: TextStyle(color: Colors.black54),
onChanged: (value) {
//Do something with the user input.
username = value.toLowerCase();
},
decoration: InputDecoration(
hintText: 'Enter your username',
),
),
SizedBox(
height: 8.0,
),
TextField(
controller: _pwController,
obscureText: true,
style: TextStyle(color: Colors.black54),
onChanged: (value) {
//Do something with the user input.
password = value;
},
decoration: InputDecoration(
hintText: 'Enter your password',
),
),
SizedBox(
height: 24.0,
),
RoundedButton(
title: 'Login',
colour: Colors.lightBlueAccent,
onPressed: () async {
_scaffoldKey.currentState.removeCurrentSnackBar();
print("user: $username, pw: $password");
if ((username != '' && username != null) &&
(password != '' && password != null)) {
SharedPreferences prefs =
await SharedPreferences.getInstance();
// cy.test();
if ((username != '' && username != null) &&
prefs.containsKey(username)) {
hashedPW = prefs.getString(username);
bool decryptPW = await cy.deHash(hashedPW, password);
if (decryptPW) {
cy.setUsername(username);
fh.setUser(username);
prefs.setString('activeUser', username);
_showInSnackBar("Login successful! redirecting..");
Navigator.pushNamed(context, 'taskScreen');
} else {
_showInSnackBar(
"Wrong password for user $username!");
}
} else {
String hashedPW = await cy.hashPW(password);
prefs.setString('activeUser', username);
prefs.setString(username, hashedPW);
cy.setUsername(username);
fh.setUser(username);
_showInSnackBar(
"User created successful! redirecting..");
Navigator.pushNamed(context, 'taskScreen');
//prefs.setString(username, hashedPW);
}
_nameController.clear();
_pwController.clear();
} else {
_showInSnackBar("User and password may not be empty..");
_nameController.clear();
_pwController.clear();
return;
}
},
),
],
),
),
);
},
),
);
}
void _showInSnackBar(String value) {
_scaffoldKey.currentState
.showSnackBar(new SnackBar(content: new Text(value)));
}
}