NoSuchMethodError: The method 'add' was called on null - list

Question

I'm a newbie of Flutter. I've written some codes which enable users to login or register with their own account and password. But when I want to add data to lists such as Username, Account and ID, I always get this error : "NoSuchMethodError: The method 'add' was called on null.". I've read some similar questions people asked. Most of them got the error just because they didn't initialize the List. Unlike them, I've initialized every List, but I still got this error continually. Please help me with the problem, tks.
The function which caused the error:
Future Register() async {
SharedPreferences prefs = await SharedPreferences.getInstance();
Username.add(_RtextFieldUsername.value.text.toString());
Account.add(_RtextFieldAccount.value.text.toString());
ID.add(_RtextFieldID.value.text.toString());
await prefs.setStringList(_StorageAccount, Account);
await prefs.setStringList(_StorageID, ID);
await prefs.setStringList(_StorageUsername, Username);
}
Entire codes:
import 'package:flutter/material.dart';
import 'package:shared_preferences/shared_preferences.dart';
import 'Uhome.dart';
import 'Thome.dart';
class ProfilePage extends StatefulWidget{
List<String> Username = List();
List<String> Account = List();
List<String> ID = List();
ProfilePage({this.Username, this.Account, this.ID});
#override
_ProfilePageState createState() => _ProfilePageState(Username, Account, ID);
}
class _ProfilePageState extends State<ProfilePage>{
List<String> Username = List();
List<String> Account = List();
List<String> ID = List();
_ProfilePageState(this.Username, this.Account, this.ID);
int _currentIndex = 2;
final _Pages = <Widget>[
UniversityPage(),
TransportationPage(),
ProfilePage(),
];
var _LtextFieldAccount = new TextEditingController();
var _LtextFieldID = new TextEditingController();
final _StorageUsername = 'Username';
final _StorageAccount = 'Account';
final _StorageID = 'ID';
String LoginedUsername = '';
Future Login() async {
SharedPreferences prefs = await SharedPreferences.getInstance();
bool checked = false;
String InsertAccount = _LtextFieldAccount.value.text.toString();
String InsertID = _LtextFieldID.value.text.toString();
List<String> GetAccountList = (prefs.getStringList(_StorageAccount) ?? List<String>());
List<String> GetIDList = (prefs.getStringList(_StorageID) ?? List<String>());
for(int i = 0 ; i < GetAccountList.length ; i++){
if(GetAccountList[i] == InsertAccount && GetIDList[i] == InsertID){
List<String> GetUsernameList = (prefs.getStringList(_StorageUsername) ?? List<String>());
checked = true;
LoginedUsername = GetUsernameList[i];
break;
}
}
if(checked == true){
print('Succeed');// Navigate to profile page
}
else{
Alert(context);// stay at the same page
}
}
void Alert(BuildContext context) {
AlertDialog dialog = AlertDialog(
backgroundColor: Colors.white,
shape: RoundedRectangleBorder(
borderRadius: BorderRadius.all(Radius.circular(15)),
),
content: Container(
padding: EdgeInsets.only(left: 5, top: 5, bottom: 5),
child: Row(
children: <Widget>[
Icon(
Icons.warning,
color: Colors.black54,
size: 30,
),
Padding(padding: EdgeInsets.only(right: 10)),
Text("帳號或密碼輸入錯誤", style: TextStyle(color: Colors.black, fontSize: 20,),
),
],
),
),
actions: <Widget>[
FlatButton(
onPressed: () {
Navigator.pop(context, true);
},
child: Text("確定", style: TextStyle(color: Colors.blueAccent),),
),
],
);
showDialog(
barrierDismissible: false,
context: context,
builder: (BuildContext context) => dialog,
);
}
#override
Widget build(BuildContext context) {
return Scaffold(
body: Stack(
children: <Widget>[
Container(
height: double.infinity,
width: double.infinity,
decoration: BoxDecoration(
gradient: LinearGradient(
begin: Alignment.topCenter,
end: Alignment.bottomCenter,
colors: [
Color(0xFF73AEF5),
Color(0xFF61A4F1),
Color(0xFF478DE0),
Color(0xFF398AE5),
],
stops: [0.1, 0.4, 0.7, 0.9],
)
),
child: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
SizedBox(height: 10,),
Icon(Icons.account_circle, size: 150.0,),
SizedBox(height: 30),
Container(
width: 300,
child: Theme(
data: ThemeData(
primaryColor: Colors.white,
primaryColorDark: Colors.yellowAccent,
),
child: TextField(
controller: _LtextFieldAccount,
decoration: InputDecoration(
border: OutlineInputBorder(),
hintText: '請輸入帳號',
prefixIcon: const Icon(Icons.person)
),
)
),
),
SizedBox(height: 20),
Container(
width: 300,
child: Theme(
data: ThemeData(
primaryColor: Colors.white,
primaryColorDark: Colors.yellowAccent,
),
child: TextField(
controller: _LtextFieldID,
decoration: InputDecoration(
border: OutlineInputBorder(),
hintText: '請輸入密碼',
prefixIcon: const Icon(Icons.lock)
),
)
),
),
SizedBox(height: 30),
RaisedButton(
shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(25.0)),
elevation: 4.0,
color: Colors.white,
child: Container(
padding: EdgeInsets.only(left: 10, right: 10, top: 10, bottom: 10),
child: Text('登入', style: TextStyle(fontSize: 25, color: Colors.blueAccent),),
),
onPressed: Login,
),
SizedBox(height: 20),
RaisedButton(
shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(25.0)),
elevation: 4.0,
color: Colors.white,
child: Container(
padding: EdgeInsets.only(left: 10, right: 10, top: 10, bottom: 10),
child: Text('新用戶註冊', style: TextStyle(fontSize: 25, color: Colors.blueAccent),),
),
onPressed: (){
Navigator.of(context).push(MaterialPageRoute(
builder: (context) => RegisterPage(Username: Username, Account: Account, ID: ID,)
));
}
)
],
),
),
),
],
),
bottomNavigationBar: BottomNavigationBar(
onTap: onTabTapped,
currentIndex: _currentIndex,
// this will be set when a new tab is tapped
items: [
new BottomNavigationBarItem(
icon: new Icon(Icons.school),
title: new Text('大學'),
),
new BottomNavigationBarItem(
icon: new Icon(Icons.directions_subway),
title: new Text('交通'),
),
new BottomNavigationBarItem(
icon: new Icon(Icons.person),
title: new Text('個人'),
)
],
),
);
}
void onTabTapped(int index){
setState(() {
_currentIndex = index;
Navigator.push(
context,
PageRouteBuilder(
pageBuilder: (context, animation1, animation2) => _Pages[index],
transitionDuration: Duration(seconds: 0),
),
);
});
}
}
// LoginPage-----------------------------------------------------------------------------------------------------------------------------------------------------------------
class RegisterPage extends StatefulWidget{
List<String> Username = List();
List<String> Account = List();
List<String> ID = List();
RegisterPage({this.Username, this.Account, this.ID});
RegisterPageState createState() => RegisterPageState(Username, Account, ID);
}
class RegisterPageState extends State<RegisterPage>{
List<String> Username = List();
List<String> Account = List();
List<String> ID = List();
RegisterPageState(this.Username, this.Account, this.ID);
int _currentIndex = 2;
final _Pages = <Widget>[
UniversityPage(),
TransportationPage(),
ProfilePage(),
];
var _RtextFieldUsername = new TextEditingController();
var _RtextFieldAccount = new TextEditingController();
var _RtextFieldID = new TextEditingController();
final _StorageUsername = 'Username';
final _StorageAccount = 'Account';
final _StorageID = 'ID';
// register & store data
Future Register() async {
SharedPreferences prefs = await SharedPreferences.getInstance();
Username.add(_RtextFieldUsername.value.text.toString());
Account.add(_RtextFieldAccount.value.text.toString());
ID.add(_RtextFieldID.value.text.toString());
await prefs.setStringList(_StorageAccount, Account);
await prefs.setStringList(_StorageID, ID);
await prefs.setStringList(_StorageUsername, Username);
}
void Alert(BuildContext context) {
AlertDialog dialog = AlertDialog(
backgroundColor: Colors.white,
shape: RoundedRectangleBorder(
borderRadius: BorderRadius.all(Radius.circular(15)),
),
content: Container(
padding: EdgeInsets.only(left: 5, top: 5, bottom: 5),
child: Row(
children: <Widget>[
Icon(
Icons.check,
color: Colors.black54,
size: 30,
),
Padding(padding: EdgeInsets.only(right: 10)),
Text("註冊成功! =D\n請重新登入", style: TextStyle(color: Colors.black, fontSize: 20,),
),
],
),
),
actions: <Widget>[
FlatButton(
onPressed: () {
Navigator.pop(context, true);
Navigator.of(context).push(MaterialPageRoute(
builder: (context) => ProfilePage(Username: Username, Account: Account, ID: ID,)
));
},
child: Text("確定", style: TextStyle(color: Colors.blueAccent),),
),
],
);
showDialog(
barrierDismissible: false,
context: context,
builder: (BuildContext context) => dialog,
);
}
void RegisterAlert(BuildContext context, int num) {
List<String> AlertText = ['欄位不可為空', '帳號需至少8個字元', '密碼需至少8個字元'];
AlertDialog dialog = AlertDialog(
backgroundColor: Colors.white,
shape: RoundedRectangleBorder(
borderRadius: BorderRadius.all(Radius.circular(15)),
),
content: Container(
padding: EdgeInsets.only(left: 5, top: 5, bottom: 5),
child: Row(
children: <Widget>[
Icon(
Icons.warning,
color: Colors.black54,
size: 30,
),
Padding(padding: EdgeInsets.only(right: 10)),
Text(AlertText[num], style: TextStyle(color: Colors.black, fontSize: 20,),
),
],
),
),
actions: <Widget>[
FlatButton(
onPressed: () {
Navigator.pop(context, true);
},
child: Text("確定", style: TextStyle(color: Colors.blueAccent),),
),
],
);
showDialog(
barrierDismissible: false,
context: context,
builder: (BuildContext context) => dialog,
);
}
#override
Widget build(BuildContext context) {
return Scaffold(
body: Stack(
children: <Widget>[
Container(
height: double.infinity,
width: double.infinity,
decoration: BoxDecoration(
gradient: LinearGradient(
begin: Alignment.topCenter,
end: Alignment.bottomCenter,
colors: [
Color(0xFF73AEF5),
Color(0xFF61A4F1),
Color(0xFF478DE0),
Color(0xFF398AE5),
],
stops: [0.1, 0.4, 0.7, 0.9],
)
),
child: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
SizedBox(height: 10,),
Icon(Icons.person_add, size: 150.0,),
SizedBox(height: 30),
Container(
width: 300,
child: Theme(
data: ThemeData(
primaryColor: Colors.white,
primaryColorDark: Colors.yellowAccent,
),
child: TextField(
controller: _RtextFieldUsername,
decoration: InputDecoration(
border: OutlineInputBorder(),
hintText: '請輸入用戶名',
prefixIcon: const Icon(Icons.face)
),
)
),
),
SizedBox(height: 20),
Container(
width: 300,
child: Theme(
data: ThemeData(
primaryColor: Colors.white,
primaryColorDark: Colors.yellowAccent,
),
child: TextField(
controller: _RtextFieldAccount,
decoration: InputDecoration(
border: OutlineInputBorder(),
hintText: '請輸入註冊帳號',
prefixIcon: const Icon(Icons.person)
),
)
),
),
SizedBox(height: 20),
Container(
width: 300,
child: Theme(
data: ThemeData(
primaryColor: Colors.white,
primaryColorDark: Colors.yellowAccent,
),
child: TextField(
controller: _RtextFieldID,
decoration: InputDecoration(
border: OutlineInputBorder(),
hintText: '請輸入註冊密碼',
prefixIcon: const Icon(Icons.lock)
),
)
),
),
SizedBox(height: 30),
RaisedButton(
shape: RoundedRectangleBorder(borderRadius: BorderRadius.circular(25.0)),
elevation: 4.0,
color: Colors.white,
child: Container(
padding: EdgeInsets.only(left: 10, right: 10, top: 10, bottom: 10),
child: Text('註冊', style: TextStyle(fontSize: 25, color: Colors.blueAccent),),
),
onPressed: (){
if(_RtextFieldUsername.value.text.toString() == '' || _RtextFieldAccount.value.text.toString() == '' || _RtextFieldID.value.text.toString() == '' ){
RegisterAlert(context, 0);
}
else if(_RtextFieldAccount.value.text.toString().length < 8){
RegisterAlert(context, 1);
}
else if(_RtextFieldID.value.text.toString().length < 8){
RegisterAlert(context, 2);
}
else{
Register();
Alert(context);
}
},
),
],
),
),
),
],
),
bottomNavigationBar: BottomNavigationBar(
onTap: onTabTapped,
currentIndex: _currentIndex,
// this will be set when a new tab is tapped
items: [
new BottomNavigationBarItem(
icon: new Icon(Icons.school),
title: new Text('大學'),
),
new BottomNavigationBarItem(
icon: new Icon(Icons.directions_subway),
title: new Text('交通'),
),
new BottomNavigationBarItem(
icon: new Icon(Icons.person),
title: new Text('個人'),
)
],
),
);
}
void onTabTapped(int index){
setState(() {
_currentIndex = index;
Navigator.push(
context,
PageRouteBuilder(
pageBuilder: (context, animation1, animation2) => _Pages[index],
transitionDuration: Duration(seconds: 0),
),
);
});
}
}
Error message:
E/flutter (20665): [ERROR:flutter/lib/ui/ui_dart_state.cc(157)] Unhandled Exception: NoSuchMethodError: The method 'add' was called on null.
E/flutter (20665): Receiver: null
E/flutter (20665): Tried calling: add("username")
E/flutter (20665): #0 Object.noSuchMethod (dart:core-patch/object_patch.dart:53:5)
E/flutter (20665): #1 RegisterPageState.Register (package:roadtouniversity/profile.dart:281:14)
E/flutter (20665): <asynchronous suspension>
E/flutter (20665): #2 RegisterPageState.build.<anonymous closure> (package:roadtouniversity/profile.dart:473:25)
E/flutter (20665): #3 _InkResponseState._handleTap (package:flutter/src/material/ink_well.dart:779:19)
E/flutter (20665): #4 _InkResponseState.build.<anonymous closure> (package:flutter/src/material/ink_well.dart:862:36)
E/flutter (20665): #5 GestureRecognizer.invokeCallback (package:flutter/src/gestures/recognizer.dart:182:24)
E/flutter (20665): #6 TapGestureRecognizer.handleTapUp (package:flutter/src/gestures/tap.dart:504:11)
E/flutter (20665): #7 BaseTapGestureRecognizer._checkUp (package:flutter/src/gestures/tap.dart:282:5)
E/flutter (20665): #8 BaseTapGestureRecognizer.handlePrimaryPointer (package:flutter/src/gestures/tap.dart:217:7)
E/flutter (20665): #9 PrimaryPointerGestureRecognizer.handleEvent (package:flutter/src/gestures/recognizer.dart:475:9)
E/flutter (20665): #10 PointerRouter._dispatch (package:flutter/src/gestures/pointer_router.dart:76:12)
E/flutter (20665): #11 PointerRouter._dispatchEventToRoutes.<anonymous closure> (package:flutter/src/gestures/pointer_router.dart:122:9)
E/flutter (20665): #12 _LinkedHashMapMixin.forEach (dart:collection-patch/compact_hash.dart:379:8)
E/flutter (20665): #13 PointerRouter._dispatchEventToRoutes (package:flutter/src/gestures/pointer_router.dart:120:18)
E/flutter (20665): #14 PointerRouter.route (package:flutter/src/gestures/pointer_router.dart:106:7)
E/flutter (20665): #15 GestureBinding.handleEvent (package:flutter/src/gestures/binding.dart:218:19)
E/flutter (20665): #16 GestureBinding.dispatchEvent (package:flutter/src/gestures/binding.dart:198:22)
E/flutter (20665): #17 GestureBinding._handlePointerEvent (package:flutter/src/gestures/binding.dart:156:7)
E/flutter (20665): #18 GestureBinding._flushPointerEventQueue (package:flutter/src/gestures/binding.dart:102:7)
E/flutter (20665): #19 GestureBinding._handlePointerDataPacket (package:flutter/src/gestures/binding.dart:86:7)
E/flutter (20665): #20 _rootRunUnary (dart:async/zone.dart:1196:13)
E/flutter (20665): #21 _CustomZone.runUnary (dart:async/zone.dart:1085:19)
E/flutter (20665): #22 _CustomZone.runUnaryGuarded (dart:async/zone.dart:987:7)
E/flutter (20665): #23 _invoke1 (dart:ui/hooks.dart:275:10)
E/flutter (20665): #24 _dispatchPointerDataPacket (dart:ui/hooks.dart:184:5)
E/flutter (20665):

Answer 1

You can copy paste run full code below
Step 1: Call ProfilePage with init value
home: ProfilePage(
Username: [],
Account: [],
ID: [],
),
Step 2: You do not need to init List<String> again
class ProfilePage extends StatefulWidget {
List<String> Username;
List<String> Account;
List<String> ID;
...
class _ProfilePageState extends State<ProfilePage> {
List<String> Username;
List<String> Account;
List<String> ID;
...
class RegisterPage extends StatefulWidget {
List<String> Username;
List<String> Account;
List<String> ID;
...
class RegisterPageState extends State<RegisterPage> {
List<String> Username;
List<String> Account;
List<String> ID;
...
Step 3: Use SingleChildScrollView to avoid overflow
child: SingleChildScrollView(
child: Center(
working demo
full code
import 'package:flutter/material.dart';
import 'package:shared_preferences/shared_preferences.dart';
class ProfilePage extends StatefulWidget {
List<String> Username;
List<String> Account;
List<String> ID;
ProfilePage({this.Username, this.Account, this.ID});
#override
_ProfilePageState createState() => _ProfilePageState(Username, Account, ID);
}
class _ProfilePageState extends State<ProfilePage> {
List<String> Username;
List<String> Account;
List<String> ID;
_ProfilePageState(this.Username, this.Account, this.ID);
int _currentIndex = 2;
final _Pages = <Widget>[
UniversityPage(),
TransportationPage(),
ProfilePage(),
];
var _LtextFieldAccount = new TextEditingController();
var _LtextFieldID = new TextEditingController();
final _StorageUsername = 'Username';
final _StorageAccount = 'Account';
final _StorageID = 'ID';
String LoginedUsername = '';
Future Login() async {
SharedPreferences prefs = await SharedPreferences.getInstance();
bool checked = false;
String InsertAccount = _LtextFieldAccount.value.text.toString();
String InsertID = _LtextFieldID.value.text.toString();
List<String> GetAccountList =
(prefs.getStringList(_StorageAccount) ?? List<String>());
List<String> GetIDList =
(prefs.getStringList(_StorageID) ?? List<String>());
for (int i = 0; i < GetAccountList.length; i++) {
if (GetAccountList[i] == InsertAccount && GetIDList[i] == InsertID) {
List<String> GetUsernameList =
(prefs.getStringList(_StorageUsername) ?? List<String>());
checked = true;
LoginedUsername = GetUsernameList[i];
break;
}
}
if (checked == true) {
print('Succeed'); // Navigate to profile page
} else {
Alert(context); // stay at the same page
}
}
void Alert(BuildContext context) {
AlertDialog dialog = AlertDialog(
backgroundColor: Colors.white,
shape: RoundedRectangleBorder(
borderRadius: BorderRadius.all(Radius.circular(15)),
),
content: Container(
padding: EdgeInsets.only(left: 5, top: 5, bottom: 5),
child: Row(
children: <Widget>[
Icon(
Icons.warning,
color: Colors.black54,
size: 30,
),
Padding(padding: EdgeInsets.only(right: 10)),
Text(
"帳號或密碼輸入錯誤",
style: TextStyle(
color: Colors.black,
fontSize: 20,
),
),
],
),
),
actions: <Widget>[
FlatButton(
onPressed: () {
Navigator.pop(context, true);
},
child: Text(
"確定",
style: TextStyle(color: Colors.blueAccent),
),
),
],
);
showDialog(
barrierDismissible: false,
context: context,
builder: (BuildContext context) => dialog,
);
}
#override
Widget build(BuildContext context) {
return Scaffold(
body: Stack(
children: <Widget>[
Container(
height: double.infinity,
width: double.infinity,
decoration: BoxDecoration(
gradient: LinearGradient(
begin: Alignment.topCenter,
end: Alignment.bottomCenter,
colors: [
Color(0xFF73AEF5),
Color(0xFF61A4F1),
Color(0xFF478DE0),
Color(0xFF398AE5),
],
stops: [0.1, 0.4, 0.7, 0.9],
)),
child: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
SizedBox(
height: 10,
),
Icon(
Icons.account_circle,
size: 150.0,
),
SizedBox(height: 30),
Container(
width: 300,
child: Theme(
data: ThemeData(
primaryColor: Colors.white,
primaryColorDark: Colors.yellowAccent,
),
child: TextField(
controller: _LtextFieldAccount,
decoration: InputDecoration(
border: OutlineInputBorder(),
hintText: '請輸入帳號',
prefixIcon: const Icon(Icons.person)),
)),
),
SizedBox(height: 20),
Container(
width: 300,
child: Theme(
data: ThemeData(
primaryColor: Colors.white,
primaryColorDark: Colors.yellowAccent,
),
child: TextField(
controller: _LtextFieldID,
decoration: InputDecoration(
border: OutlineInputBorder(),
hintText: '請輸入密碼',
prefixIcon: const Icon(Icons.lock)),
)),
),
SizedBox(height: 30),
RaisedButton(
shape: RoundedRectangleBorder(
borderRadius: BorderRadius.circular(25.0)),
elevation: 4.0,
color: Colors.white,
child: Container(
padding: EdgeInsets.only(
left: 10, right: 10, top: 10, bottom: 10),
child: Text(
'登入',
style:
TextStyle(fontSize: 25, color: Colors.blueAccent),
),
),
onPressed: Login,
),
SizedBox(height: 20),
RaisedButton(
shape: RoundedRectangleBorder(
borderRadius: BorderRadius.circular(25.0)),
elevation: 4.0,
color: Colors.white,
child: Container(
padding: EdgeInsets.only(
left: 10, right: 10, top: 10, bottom: 10),
child: Text(
'新用戶註冊',
style:
TextStyle(fontSize: 25, color: Colors.blueAccent),
),
),
onPressed: () {
Navigator.of(context).push(MaterialPageRoute(
builder: (context) => RegisterPage(
Username: Username,
Account: Account,
ID: ID,
)));
})
],
),
),
),
],
),
bottomNavigationBar: BottomNavigationBar(
onTap: onTabTapped,
currentIndex: _currentIndex,
// this will be set when a new tab is tapped
items: [
new BottomNavigationBarItem(
icon: new Icon(Icons.school),
title: new Text('大學'),
),
new BottomNavigationBarItem(
icon: new Icon(Icons.directions_subway),
title: new Text('交通'),
),
new BottomNavigationBarItem(
icon: new Icon(Icons.person),
title: new Text('個人'),
)
],
),
);
}
void onTabTapped(int index) {
setState(() {
_currentIndex = index;
Navigator.push(
context,
PageRouteBuilder(
pageBuilder: (context, animation1, animation2) => _Pages[index],
transitionDuration: Duration(seconds: 0),
),
);
});
}
}
// LoginPage-----------------------------------------------------------------------------------------------------------------------------------------------------------------
class RegisterPage extends StatefulWidget {
List<String> Username;
List<String> Account;
List<String> ID;
RegisterPage({this.Username, this.Account, this.ID});
RegisterPageState createState() => RegisterPageState(Username, Account, ID);
}
class RegisterPageState extends State<RegisterPage> {
List<String> Username;
List<String> Account;
List<String> ID;
RegisterPageState(this.Username, this.Account, this.ID);
int _currentIndex = 2;
final _Pages = <Widget>[
UniversityPage(),
TransportationPage(),
ProfilePage(),
];
var _RtextFieldUsername = new TextEditingController();
var _RtextFieldAccount = new TextEditingController();
var _RtextFieldID = new TextEditingController();
final _StorageUsername = 'Username';
final _StorageAccount = 'Account';
final _StorageID = 'ID';
// register & store data
Future Register() async {
SharedPreferences prefs = await SharedPreferences.getInstance();
Username.add(_RtextFieldUsername.value.text.toString());
Account.add(_RtextFieldAccount.value.text.toString());
ID.add(_RtextFieldID.value.text.toString());
await prefs.setStringList(_StorageAccount, Account);
await prefs.setStringList(_StorageID, ID);
await prefs.setStringList(_StorageUsername, Username);
}
void Alert(BuildContext context) {
AlertDialog dialog = AlertDialog(
backgroundColor: Colors.white,
shape: RoundedRectangleBorder(
borderRadius: BorderRadius.all(Radius.circular(15)),
),
content: Container(
padding: EdgeInsets.only(left: 5, top: 5, bottom: 5),
child: Row(
children: <Widget>[
Icon(
Icons.check,
color: Colors.black54,
size: 30,
),
Padding(padding: EdgeInsets.only(right: 10)),
Text(
"註冊成功! =D\n請重新登入",
style: TextStyle(
color: Colors.black,
fontSize: 20,
),
),
],
),
),
actions: <Widget>[
FlatButton(
onPressed: () {
Navigator.pop(context, true);
Navigator.of(context).push(MaterialPageRoute(
builder: (context) => ProfilePage(
Username: Username,
Account: Account,
ID: ID,
)));
},
child: Text(
"確定",
style: TextStyle(color: Colors.blueAccent),
),
),
],
);
showDialog(
barrierDismissible: false,
context: context,
builder: (BuildContext context) => dialog,
);
}
void RegisterAlert(BuildContext context, int num) {
List<String> AlertText = ['欄位不可為空', '帳號需至少8個字元', '密碼需至少8個字元'];
AlertDialog dialog = AlertDialog(
backgroundColor: Colors.white,
shape: RoundedRectangleBorder(
borderRadius: BorderRadius.all(Radius.circular(15)),
),
content: Container(
padding: EdgeInsets.only(left: 5, top: 5, bottom: 5),
child: Row(
children: <Widget>[
Icon(
Icons.warning,
color: Colors.black54,
size: 30,
),
Padding(padding: EdgeInsets.only(right: 10)),
Text(
AlertText[num],
style: TextStyle(
color: Colors.black,
fontSize: 20,
),
),
],
),
),
actions: <Widget>[
FlatButton(
onPressed: () {
Navigator.pop(context, true);
},
child: Text(
"確定",
style: TextStyle(color: Colors.blueAccent),
),
),
],
);
showDialog(
barrierDismissible: false,
context: context,
builder: (BuildContext context) => dialog,
);
}
#override
Widget build(BuildContext context) {
return Scaffold(
body: Stack(
children: <Widget>[
Container(
height: double.infinity,
width: double.infinity,
decoration: BoxDecoration(
gradient: LinearGradient(
begin: Alignment.topCenter,
end: Alignment.bottomCenter,
colors: [
Color(0xFF73AEF5),
Color(0xFF61A4F1),
Color(0xFF478DE0),
Color(0xFF398AE5),
],
stops: [0.1, 0.4, 0.7, 0.9],
)),
child: SingleChildScrollView(
child: Center(
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
SizedBox(
height: 10,
),
Icon(
Icons.person_add,
size: 150.0,
),
SizedBox(height: 30),
Container(
width: 300,
child: Theme(
data: ThemeData(
primaryColor: Colors.white,
primaryColorDark: Colors.yellowAccent,
),
child: TextField(
controller: _RtextFieldUsername,
decoration: InputDecoration(
border: OutlineInputBorder(),
hintText: '請輸入用戶名',
prefixIcon: const Icon(Icons.face)),
)),
),
SizedBox(height: 20),
Container(
width: 300,
child: Theme(
data: ThemeData(
primaryColor: Colors.white,
primaryColorDark: Colors.yellowAccent,
),
child: TextField(
controller: _RtextFieldAccount,
decoration: InputDecoration(
border: OutlineInputBorder(),
hintText: '請輸入註冊帳號',
prefixIcon: const Icon(Icons.person)),
)),
),
SizedBox(height: 20),
Container(
width: 300,
child: Theme(
data: ThemeData(
primaryColor: Colors.white,
primaryColorDark: Colors.yellowAccent,
),
child: TextField(
controller: _RtextFieldID,
decoration: InputDecoration(
border: OutlineInputBorder(),
hintText: '請輸入註冊密碼',
prefixIcon: const Icon(Icons.lock)),
)),
),
SizedBox(height: 30),
RaisedButton(
shape: RoundedRectangleBorder(
borderRadius: BorderRadius.circular(25.0)),
elevation: 4.0,
color: Colors.white,
child: Container(
padding: EdgeInsets.only(
left: 10, right: 10, top: 10, bottom: 10),
child: Text(
'註冊',
style:
TextStyle(fontSize: 25, color: Colors.blueAccent),
),
),
onPressed: () {
if (_RtextFieldUsername.value.text.toString() == '' ||
_RtextFieldAccount.value.text.toString() == '' ||
_RtextFieldID.value.text.toString() == '') {
RegisterAlert(context, 0);
} else if (_RtextFieldAccount.value.text
.toString()
.length <
8) {
RegisterAlert(context, 1);
} else if (_RtextFieldID.value.text.toString().length <
8) {
RegisterAlert(context, 2);
} else {
Register();
Alert(context);
}
},
),
],
),
),
),
),
],
),
bottomNavigationBar: BottomNavigationBar(
onTap: onTabTapped,
currentIndex: _currentIndex,
// this will be set when a new tab is tapped
items: [
new BottomNavigationBarItem(
icon: new Icon(Icons.school),
title: new Text('大學'),
),
new BottomNavigationBarItem(
icon: new Icon(Icons.directions_subway),
title: new Text('交通'),
),
new BottomNavigationBarItem(
icon: new Icon(Icons.person),
title: new Text('個人'),
)
],
),
);
}
void onTabTapped(int index) {
setState(() {
_currentIndex = index;
Navigator.push(
context,
PageRouteBuilder(
pageBuilder: (context, animation1, animation2) => _Pages[index],
transitionDuration: Duration(seconds: 0),
),
);
});
}
}
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
title: 'Flutter Demo',
theme: ThemeData(
primarySwatch: Colors.blue,
visualDensity: VisualDensity.adaptivePlatformDensity,
),
home: ProfilePage(
Username: [],
Account: [],
ID: [],
),
);
}
}
class UniversityPage extends StatelessWidget {
#override
Widget build(BuildContext context) {
return Text("UniversityPage");
}
}
class TransportationPage extends StatelessWidget {
#override
Widget build(BuildContext context) {
return Text("TransportationPage");
}
}

Answer 2

You're passing the lists from one class to another
ProfilePage > _ProfilePageState > RegisterPage > RegisterPageState
But that doesn't mean at the beggining the lists cannot be null.
class ProfilePage extends StatefulWidget{
List<String> Username = List();
List<String> Account = List();
List<String> ID = List();
ProfilePage({this.Username, this.Account, this.ID});
#override
_ProfilePageState createState() => _ProfilePageState(Username, Account, ID);
}
Initializing them as List but calling them in the constructor means that they will be override with the value of the constructor. If you start with ProfilePage(Username: null, Account: null, ID: null) or just call it ProfilePage() then those attributes are null, despite the value you gave them at the beggining. You have to be sure that when the class ProfilePage is created the attributes passed to aren't null, or else the whole logic will be null
Also just as others point out you don't need to init them in the StatefulWidget and State
class RegisterPage extends StatefulWidget{
List<String> Username = List();
List<String> Account = List();
List<String> ID = List();
RegisterPage({this.Username, this.Account, this.ID});
RegisterPageState createState() => RegisterPageState(Username, Account, ID);
}
class RegisterPageState extends State<RegisterPage>{
List<String> Username = List(); //This is pointless
List<String> Account = List(); //This is pointless
List<String> ID = List(); //This is pointless
...
}
If you want to use/modify the one you just pass from the previous page just use them like this
class RegisterPage extends StatefulWidget{
List<String> Username = List(); //you wanted to use this
List<String> Account = List(); //you wanted to use this
List<String> ID = List(); //you wanted to use this
RegisterPage({this.Username, this.Account, this.ID});
RegisterPageState createState() => RegisterPageState(); //no need to pass the parameters
}
class RegisterPageState extends State<RegisterPage>{
Future Register() async {
SharedPreferences prefs = await SharedPreferences.getInstance();
widget.Username.add(_RtextFieldUsername.value.text.toString());
widget.Account.add(_RtextFieldAccount.value.text.toString());
widget.ID.add(_RtextFieldID.value.text.toString());
/*
widget.value let you use the objects/variables defined in the StatefulWidget
so you don't need to create new ones in the State
*/
await prefs.setStringList(_StorageAccount, Account);
await prefs.setStringList(_StorageID, ID);
await prefs.setStringList(_StorageUsername, Username);
}
....
}