不可为 null 的局部变量 'title' 必须先赋值,然后才能使用。颤动错误
The non-nullable local variable 'title' must be assigned before it can be used. Flutter error
我想从 textfield widget
获取用户输入并在 title
变量中赋值,如此代码
import 'package:flutter/material.dart';
class AddTaskScreen extends StatelessWidget {
final Function callBack; // function
AddTaskScreen({required this.callBack});
@override
Widget build(BuildContext context) {
String title; // declaring the variable
return Container(
height: 350,
color: Color(0xff757575),
child: Container(
padding: EdgeInsets.all(20),
decoration: BoxDecoration(
color: Colors.white,
borderRadius: BorderRadius.only(
topRight: Radius.circular(20),
topLeft: Radius.circular(20),
),
),
child: Column(
crossAxisAlignment: CrossAxisAlignment.stretch,
children: [
Text(
'Add Task',
textAlign: TextAlign.center,
style: TextStyle(
color: Colors.lightBlueAccent,
fontSize: 24,
),
),
SizedBox(
height: 20,
),
TextField(
textAlign: TextAlign.center,
autofocus: false,
style: TextStyle(
color: Colors.black,
fontSize: 15,
),
onChanged: (value) {
title = value; // taking the user input
},
decoration: InputDecoration(
focusColor: Colors.lightBlueAccent,
hintText: 'Type a Task',
hintStyle: TextStyle(
color: Colors.grey,
fontSize: 12,
),
),
),
SizedBox(
height: 20,
),
TextButton(
onPressed: () {
callBack(title); // error
},
style: TextButton.styleFrom(
backgroundColor: Colors.lightBlueAccent,
),
child: Text(
'Add',
style: TextStyle(
color: Colors.white,
fontSize: 18,
),
),
),
],
),
),
);
}
}
如您所见,有一个函数 callBack
变量。当我在此函数中传递 title
时,它具有 String
参数,它会给我一个警告。
这是发送此信息时的回调代码。
import 'package:flutter/material.dart';
import 'package:todoey/models/Task.dart';
import 'package:todoey/screens/add_task_screen.dart';
import 'package:todoey/screens/bottom_section.dart';
import 'package:todoey/screens/top_section.dart';
class TasksScreen extends StatefulWidget {
@override
State<TasksScreen> createState() => _TasksScreenState();
}
class _TasksScreenState extends State<TasksScreen> {
List<Task> tasks = [];
@override
Widget build(BuildContext context) {
return Scaffold(
floatingActionButton: FloatingActionButton(
onPressed: () {
showModalBottomSheet(
context: context,
builder: (context) => SingleChildScrollView(
child: Container(
padding: EdgeInsets.only(
bottom: MediaQuery.of(context).viewInsets.bottom),
child: AddTaskScreen(
callBack: (String taskTitle) {
setState(() {
print(taskTitle);
tasks.add(
Task(
name: taskTitle,
),
);
});
Navigator.pop(context);
},
).build(context),
),
),
);
},
child: Icon(
Icons.add,
color: Colors.white,
size: 35,
),
elevation: 5,
backgroundColor: Colors.lightBlueAccent,
),
backgroundColor: Colors.lightBlueAccent,
body: SafeArea(
child: Column(
mainAxisAlignment: MainAxisAlignment.start,
crossAxisAlignment: CrossAxisAlignment.start,
children: [
TopSection(),
SizedBox(
height: 30,
),
BottomSection(tasks: tasks),
],
),
),
);
}
}
对于主要 class,这是代码。
import 'package:flutter/material.dart';
import 'package:todoey/screens/task_screen.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
const MyApp({Key? key}) : super(key: key);
@override
Widget build(BuildContext context) {
return MaterialApp(
debugShowCheckedModeBanner: false,
home: TasksScreen(),
);
}
}
在 flutter 中,您想将可变对象(例如您的 title
)存储在 State
中。这是因为构建函数理论上可以多次调用,你无法控制它。因此,将变量存储在对象的 State
中可以让您在多次调用 build
时保持它“活着”。
这是您的示例,已按预期工作:
import 'package:flutter/material.dart';
class AddTaskScreen extends StatefulWidget {
final Function callBack; // function
AddTaskScreen({required this.callBack});
@override
State<AddTaskScreen> createState() => _AddTaskScreenState();
}
class _AddTaskScreenState extends State<AddTaskScreen> {
/// Declare title in the state, so that its a long lived object
var title = ""; // Initialize it (likely as empty)
@override
Widget build(BuildContext context) {
return Container(
height: 350,
color: Color(0xff757575),
child: Container(
padding: EdgeInsets.all(20),
decoration: BoxDecoration(
color: Colors.white,
borderRadius: BorderRadius.only(
topRight: Radius.circular(20),
topLeft: Radius.circular(20),
),
),
child: Column(
crossAxisAlignment: CrossAxisAlignment.stretch,
children: [
Text(
'Add Task',
textAlign: TextAlign.center,
style: TextStyle(
color: Colors.lightBlueAccent,
fontSize: 24,
),
),
SizedBox(
height: 20,
),
TextField(
textAlign: TextAlign.center,
autofocus: false,
style: TextStyle(
color: Colors.black,
fontSize: 15,
),
onChanged: (value) {
title = value; // taking the user input
},
decoration: InputDecoration(
focusColor: Colors.lightBlueAccent,
hintText: 'Type a Task',
hintStyle: TextStyle(
color: Colors.grey,
fontSize: 12,
),
),
),
SizedBox(
height: 20,
),
TextButton(
onPressed: () {
widget.callBack(title); // error
},
style: TextButton.styleFrom(
backgroundColor: Colors.lightBlueAccent,
),
child: Text(
'Add',
style: TextStyle(
color: Colors.white,
fontSize: 18,
),
),
),
],
),
),
);
}
}
因此您使用的是无状态小部件,因此您的 onChanged
函数将不起作用,因为无状态小部件的状态不会更改以呈现 title[=14 的新值=],所以你可以把它改成一个有状态的小部件来查看效果。
我想从 textfield widget
获取用户输入并在 title
变量中赋值,如此代码
import 'package:flutter/material.dart';
class AddTaskScreen extends StatelessWidget {
final Function callBack; // function
AddTaskScreen({required this.callBack});
@override
Widget build(BuildContext context) {
String title; // declaring the variable
return Container(
height: 350,
color: Color(0xff757575),
child: Container(
padding: EdgeInsets.all(20),
decoration: BoxDecoration(
color: Colors.white,
borderRadius: BorderRadius.only(
topRight: Radius.circular(20),
topLeft: Radius.circular(20),
),
),
child: Column(
crossAxisAlignment: CrossAxisAlignment.stretch,
children: [
Text(
'Add Task',
textAlign: TextAlign.center,
style: TextStyle(
color: Colors.lightBlueAccent,
fontSize: 24,
),
),
SizedBox(
height: 20,
),
TextField(
textAlign: TextAlign.center,
autofocus: false,
style: TextStyle(
color: Colors.black,
fontSize: 15,
),
onChanged: (value) {
title = value; // taking the user input
},
decoration: InputDecoration(
focusColor: Colors.lightBlueAccent,
hintText: 'Type a Task',
hintStyle: TextStyle(
color: Colors.grey,
fontSize: 12,
),
),
),
SizedBox(
height: 20,
),
TextButton(
onPressed: () {
callBack(title); // error
},
style: TextButton.styleFrom(
backgroundColor: Colors.lightBlueAccent,
),
child: Text(
'Add',
style: TextStyle(
color: Colors.white,
fontSize: 18,
),
),
),
],
),
),
);
}
}
如您所见,有一个函数 callBack
变量。当我在此函数中传递 title
时,它具有 String
参数,它会给我一个警告。
这是发送此信息时的回调代码。
import 'package:flutter/material.dart';
import 'package:todoey/models/Task.dart';
import 'package:todoey/screens/add_task_screen.dart';
import 'package:todoey/screens/bottom_section.dart';
import 'package:todoey/screens/top_section.dart';
class TasksScreen extends StatefulWidget {
@override
State<TasksScreen> createState() => _TasksScreenState();
}
class _TasksScreenState extends State<TasksScreen> {
List<Task> tasks = [];
@override
Widget build(BuildContext context) {
return Scaffold(
floatingActionButton: FloatingActionButton(
onPressed: () {
showModalBottomSheet(
context: context,
builder: (context) => SingleChildScrollView(
child: Container(
padding: EdgeInsets.only(
bottom: MediaQuery.of(context).viewInsets.bottom),
child: AddTaskScreen(
callBack: (String taskTitle) {
setState(() {
print(taskTitle);
tasks.add(
Task(
name: taskTitle,
),
);
});
Navigator.pop(context);
},
).build(context),
),
),
);
},
child: Icon(
Icons.add,
color: Colors.white,
size: 35,
),
elevation: 5,
backgroundColor: Colors.lightBlueAccent,
),
backgroundColor: Colors.lightBlueAccent,
body: SafeArea(
child: Column(
mainAxisAlignment: MainAxisAlignment.start,
crossAxisAlignment: CrossAxisAlignment.start,
children: [
TopSection(),
SizedBox(
height: 30,
),
BottomSection(tasks: tasks),
],
),
),
);
}
}
对于主要 class,这是代码。
import 'package:flutter/material.dart';
import 'package:todoey/screens/task_screen.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
const MyApp({Key? key}) : super(key: key);
@override
Widget build(BuildContext context) {
return MaterialApp(
debugShowCheckedModeBanner: false,
home: TasksScreen(),
);
}
}
在 flutter 中,您想将可变对象(例如您的 title
)存储在 State
中。这是因为构建函数理论上可以多次调用,你无法控制它。因此,将变量存储在对象的 State
中可以让您在多次调用 build
时保持它“活着”。
这是您的示例,已按预期工作:
import 'package:flutter/material.dart';
class AddTaskScreen extends StatefulWidget {
final Function callBack; // function
AddTaskScreen({required this.callBack});
@override
State<AddTaskScreen> createState() => _AddTaskScreenState();
}
class _AddTaskScreenState extends State<AddTaskScreen> {
/// Declare title in the state, so that its a long lived object
var title = ""; // Initialize it (likely as empty)
@override
Widget build(BuildContext context) {
return Container(
height: 350,
color: Color(0xff757575),
child: Container(
padding: EdgeInsets.all(20),
decoration: BoxDecoration(
color: Colors.white,
borderRadius: BorderRadius.only(
topRight: Radius.circular(20),
topLeft: Radius.circular(20),
),
),
child: Column(
crossAxisAlignment: CrossAxisAlignment.stretch,
children: [
Text(
'Add Task',
textAlign: TextAlign.center,
style: TextStyle(
color: Colors.lightBlueAccent,
fontSize: 24,
),
),
SizedBox(
height: 20,
),
TextField(
textAlign: TextAlign.center,
autofocus: false,
style: TextStyle(
color: Colors.black,
fontSize: 15,
),
onChanged: (value) {
title = value; // taking the user input
},
decoration: InputDecoration(
focusColor: Colors.lightBlueAccent,
hintText: 'Type a Task',
hintStyle: TextStyle(
color: Colors.grey,
fontSize: 12,
),
),
),
SizedBox(
height: 20,
),
TextButton(
onPressed: () {
widget.callBack(title); // error
},
style: TextButton.styleFrom(
backgroundColor: Colors.lightBlueAccent,
),
child: Text(
'Add',
style: TextStyle(
color: Colors.white,
fontSize: 18,
),
),
),
],
),
),
);
}
}
因此您使用的是无状态小部件,因此您的 onChanged
函数将不起作用,因为无状态小部件的状态不会更改以呈现 title[=14 的新值=],所以你可以把它改成一个有状态的小部件来查看效果。