【发布时间】：2020-06-23 11:24:59
【问题描述】：
想要在接收来自服务器端的响应时登录应用程序，我创建了两个文本字段，我希望它在收到 0 状态后显示无效值并导航到状态 1 的下一页。我创建了一个用户类具有名称为 status 的对象，该对象从服务器端接收状态值。但事实是在实现并调用该对象进入登录页面后收到错误。
Unhandled Exception: NoSuchMethodError: The getter 'length' was called on null.
E/flutter (29946): Receiver: null
E/flutter (29946): Tried calling: length
如果有人理解，我不知道原因，我一定会很感激答案。谢谢！
登录页面代码：
import 'package:flutter/material.dart';
import 'package:http/http.dart';
import 'package:login_page_flutter/Models/pages/user_profile.dart';
import 'Models/myconstant.dart';
import 'Models/user.dart';
class LoginPage extends StatefulWidget {
@override
_LoginPageState createState() => _LoginPageState();
}
class _LoginPageState extends State<LoginPage> {
TextEditingController txtusername = TextEditingController();
TextEditingController txtpassword = TextEditingController();
bool isUploading = false;
bool validation = false;
@override
void dispose() {
super.dispose();
}
@override
Widget build(BuildContext context) {
isUploading = false;
return Scaffold(
appBar: AppBar(
elevation: 0,
brightness: Brightness.light,
backgroundColor: Colors.white,
leading: IconButton(
icon: Icon(Icons.arrow_back, size: 20, color: Colors.pink[80]),
onPressed: () {
Navigator.pop(context);
}),
),
body: SafeArea(
child: Padding(
padding: EdgeInsets.all(20.0),
child: ListView(
children: <Widget>[
SizedBox(height: 10),
SizedBox(height: 10),
Card(
child: Column(
children: <Widget>[
TextField(
decoration: InputDecoration(
errorText: validation ? 'Must Fill Input Field' : null,
border: InputBorder.none,
hintText: "Enter UserName",
hintStyle: TextStyle(
fontSize: 12,
color: Colors.pink[800].withOpacity(0.5),
),
),
controller: txtusername,
),
TextField(
obscureText: true,
decoration: InputDecoration(
errorText: validation ? 'Must Fill Input Field' : null,
border: InputBorder.none,
hintText: "Enter Password",
hintStyle: TextStyle(
fontSize: 12,
color: Colors.pink[800].withOpacity(0.5),
),
),
controller: txtpassword,
),
],
),
),
SizedBox(height: 20),
Center(
child: AnimatedContainer(
duration: const Duration(
milliseconds: 50,
),
child: getWidgetByStatus(),
),
),
],
),
),
),
);
}
Future<void> saveData(dynamic jsonOfUser) async {
setState(() {
isUploading = true;
});
await Future.delayed(Duration(seconds: MyConstant.VAL_SECONDS), () {});
return post(
MyConstant.URL_USER_INSERT_LOGIN,
body: jsonOfUser,
).then((onRecievedResponse) {
print(onRecievedResponse.body);
setState(() {
isUploading = false;
});
return null;
});
}
Widget getWidgetByStatus() {
if (isUploading) {
return CircularProgressIndicator(
backgroundColor: Colors.pink,
);
} else {
return RaisedButton(
onPressed: () {
User user = User(
username: txtusername.text,
password: txtpassword.text,
type: 11,
);
saveData(User.toJson(user));
checkValidationOfData();
userInputValidation();
},
child: Text('Submit'),
);
}
}
void checkValidationOfData() {
setState(() {
if (txtusername.text.isEmpty) {
return validation = true;
}
if (txtpassword.text.isEmpty) {
return validation = true;
} else {
return validation = false;
}
});
}
void userInputValidation() {
if (validation == false) {
if (User.Key_status == '0') {
setState(() {
return 'Invalid Values';
});
} else if (User.Key_status == '1') {
setState(() {
return Navigator.push(context, MaterialPageRoute(builder: (context) {
return UserProfile();
}));
});
}
}
}
}
类用户：
class User {
int id;
static const String Key_id = 'id';
String name;
static const String Key_name = 'name';
String username;
static const String Key_username = 'username';
String password;
static const String Key_password = 'password';
int type;
static const String Key_type = 'type';
int status;
static const String Key_status = 'status';
User({
this.id,
this.name,
this.username,
this.password,
this.type,
});
static Map<String, String> toJson(User user) {
return {
Key_id: user.id.toString(),
Key_name: user.name,
Key_username: user.username,
Key_password: user.password,
Key_type: user.type.toString(),
};
}
static List<User> fromJsonArray(dynamic jsonArray) {
List<User> listOfUserDetail = List<User>();
for (var jsonObject in jsonArray) {
User newUser = User();
newUser.status = jsonObject[Key_status];
listOfUserDetail.add(newUser);
}
return listOfUserDetail;
}
}
服务器端连接文件
class MyConstant {
static const String BASE_URL_ = 'http://23e30b0d5027.ngrok.io/';
static const String URL_API = BASE_URL_ + 'fyp/';
static const String URL_USER_INSERT = URL_API + 'user_signup.php';
static const String URL_USER_INSERT_LOGIN = URL_API + 'user_login.php';
static const int VAL_SECONDS = 0;
}
php代码
<?php
require_once('conn.php');
$username=$_POST['username'];
$password=$_POST['password'];
$query="SELECT * FROM user WHERE username='$username' and password='$password'";
$query_result= mysqli_query($con,$query);
$count= mysqli_num_rows($query_result);
if($query_result){
$array_of_records = array();
if($count>0) {
while ($row = mysqli_fetch_assoc($query_result)) {
$obj= array(
"id"=> $row['id'],
"name"=> $row['name'],
"username"=> $row['username'],
"password"=> $row['password'],
"type"=> $row['type'],
);
array_push($array_of_records, $obj);
}
echo json_encode(array('status'=>1, 'data'=> $array_of_records));
}
else
{
echo json_encode(array('status'=>0, 'data'=> $array_of_records));
}
}
?>
【问题讨论】：
-
什么时候出现这个错误？如果您分享错误行会有所帮助吗？保存时会出现这种情况吗？
-
因为我在onpressed中应用了userinputValidation函数，所以我无法理解原因。
-
我认为在你的代码中你试图获取 List 的空引用的长度，分享错误的行