dart - 颤振组合框
问题描述
我正在尝试创建一个下拉/组合框小部件并且很难弄清楚。下面是我尝试过的代码。基本上创建一个我从这里和那里收集的列表,但无法将它相应地放入我自己的代码中。
import 'package:flutter/material.dart';
class Signup extends StatefulWidget{
Signup({Key key}) : super (key:key);
final String title;
@override
SignupPage createState() => new SignupPage();
}
class SignupPage extends State<Signup> {
@override
Widget build(BuildContext context) {
List _countrycodes = ["+65","+91"];
List<DropdownMenuItem<String>> _dropDownMenuItems;
String _selectedCountryCode;
List<String> _colors = <String>['', 'red', 'green', 'blue', 'orange'];
String _color = '';
List<DropdownMenuItem<String>> getDropDownMenuItems() {
List<DropdownMenuItem<String>> items = new List();
for (String code in _countrycodes) {
items.add(new DropdownMenuItem(
value: code,
child: new Text(code)
));
}
return items;
}
/**return new Material(
color: Colors.blueAccent,
child: new Scaffold(
body: new Stack(
children: <Widget>[
new Container(
decoration: new BoxDecoration(
image: new DecorationImage(image: new AssetImage("assets/images/download.jpg"), fit: BoxFit.cover,),
),
),
new Center(
child: new Text("Hello LandingPage"),
)
],
)
));**/
final name = TextFormField(
keyboardType: TextInputType.text,
autofocus: false,
initialValue: 'Techie Quickie',
decoration: InputDecoration(
hintText: 'Name',
contentPadding: EdgeInsets.fromLTRB(20.0, 10.0, 20.0, 10.0),
border: OutlineInputBorder(
borderRadius: BorderRadius.circular(32.0)
),
),
);
final email = TextFormField(
keyboardType: TextInputType.emailAddress,
autofocus: false,
initialValue: 'tq@gmail.com',
decoration: InputDecoration(
hintText: 'Email',
contentPadding: EdgeInsets.fromLTRB(20.0, 10.0, 20.0, 10.0),
border: OutlineInputBorder(
borderRadius: BorderRadius.circular(32.0)
),
),
);
final password = TextFormField(
keyboardType: TextInputType.text,
obscureText: true,
autofocus: false,
initialValue: 'password',
decoration: InputDecoration(
hintText: 'Password',
contentPadding: EdgeInsets.fromLTRB(20.0, 10.0, 20.0, 10.0),
border: OutlineInputBorder(
borderRadius: BorderRadius.circular(32.0)
),
),
);
final passwordConfirmation = TextFormField(
keyboardType: TextInputType.text,
obscureText: true,
autofocus: false,
initialValue: 'password',
decoration: InputDecoration(
hintText: 'Password',
contentPadding: EdgeInsets.fromLTRB(20.0, 10.0, 20.0, 10.0),
border: OutlineInputBorder(
borderRadius: BorderRadius.circular(32.0)
),
),
);
final loginButton = Padding(
padding: EdgeInsets.symmetric(vertical: 16.0),
child: Material(
borderRadius: BorderRadius.circular(30.0),
shadowColor: Colors.lightBlueAccent.shade100,
elevation: 5.0,
child: MaterialButton(
minWidth: 200.0,
height: 42.0,
onPressed: (){
print("Signup button clicked");
},
color: Colors.lightBlueAccent,
child:
Text('Sign Up',
style: TextStyle(color: Colors.white, fontSize: 20.0),
),
),
)
);
final countryCode = DropdownButton(
value: _selectedCountryCode,
items: _dropDownMenuItems,
onChanged: null
);
final inputDecoratoring =InputDecorator(
decoration: const InputDecoration(
icon: const Icon(Icons.color_lens),
labelText: 'Color',
),
isEmpty: _color == '',
child: new DropdownButtonHideUnderline(
child: new DropdownButton<String>(
value: _color,
isDense: true,
onChanged: (String newValue) {
setState(() {
_color = newValue;
});
},
items: _colors.map((String value) {
return new DropdownMenuItem<String>(
value: value,
child: new Text(value),
);
}).toList(),
),
),
);
return Scaffold(
backgroundColor: Colors.white,
body: Center (
child: ListView(
padding: EdgeInsets.only(left: 24.0, right: 24.0),
shrinkWrap: true,
children: <Widget>[
name,
SizedBox(height: 18.0),
email,
SizedBox(height: 18.0),
password,
SizedBox(height: 18.0),
passwordConfirmation,
SizedBox(height: 18.0),
// countryCode,
// SizedBox(height: 18.0),
loginButton,
SizedBox(height: 38.0),
],
)
)
);
}
}
这是我的 main.dart 课程
import 'package:flutter/material.dart';
import 'package:elegal_dart/screens/landing_page.dart';
import 'package:elegal_dart/screens/home.dart';
import 'package:elegal_dart/screens/devices.dart';
import 'package:elegal_dart/screens/signup.dart';
import 'package:elegal_dart/screens/signin.dart';
/*
void main()
{
Devices( new MaterialApp(
home: new Devices(),
));
}*/
void main() => runApp(new MyApp());
class MyApp extends StatelessWidget
{
@override
Widget build(BuildContext context)
{
return new MaterialApp(
title: 'Navigation',
routes: <String, WidgetBuilder>
{
'/Home' : (BuildContext context) => new Home(),
'/LandingPage' : (BuildContext context) => new LandingPage(),
'/Devices' : (BuildContext context) => new Devices(),
'/Signup' : (BuildContext context) => new Signup(),
'/Signin' : (BuildContext context) => new Signin(),
},
//home: new Signin(onSubmit: null),
home: new Signup(),
);
}
}
更新代码:
import 'package:flutter/material.dart';
class Signup extends StatefulWidget{
Signup({Key key}) : super (key:key);
final String title;
@override
SignupPage createState() => new SignupPage();
}
class SignupPage extends State<Signup> {
@override
Widget build(BuildContext context) {
List _countrycodes = ["+65","+91"];
List<DropdownMenuItem<String>> _dropDownMenuItems;
String _selectedCountryCode;
List<String> _ccodes = <String>['', '+65', '+91', '+60', 'orange'];
String _ccode = '';
List<DropdownMenuItem<String>> getDropDownMenuItems() {
List<DropdownMenuItem<String>> items = new List();
for (String code in _countrycodes) {
items.add(new DropdownMenuItem(
value: code,
child: new Text(code)
));
}
return items;
}
final name = TextFormField(
keyboardType: TextInputType.text,
autofocus: false,
initialValue: 'Techie Quickie',
decoration: InputDecoration(
hintText: 'Name',
contentPadding: EdgeInsets.fromLTRB(20.0, 10.0, 20.0, 10.0),
border: OutlineInputBorder(
borderRadius: BorderRadius.circular(32.0)
),
),
);
final email = TextFormField(
keyboardType: TextInputType.emailAddress,
autofocus: false,
initialValue: 'tq@gmail.com',
decoration: InputDecoration(
hintText: 'Email',
contentPadding: EdgeInsets.fromLTRB(20.0, 10.0, 20.0, 10.0),
border: OutlineInputBorder(
borderRadius: BorderRadius.circular(32.0)
),
),
);
final password = TextFormField(
keyboardType: TextInputType.text,
obscureText: true,
autofocus: false,
initialValue: 'password',
decoration: InputDecoration(
hintText: 'Password',
contentPadding: EdgeInsets.fromLTRB(20.0, 10.0, 20.0, 10.0),
border: OutlineInputBorder(
borderRadius: BorderRadius.circular(32.0)
),
),
);
final passwordConfirmation = TextFormField(
keyboardType: TextInputType.text,
obscureText: true,
autofocus: false,
initialValue: 'password',
decoration: InputDecoration(
hintText: 'Password',
contentPadding: EdgeInsets.fromLTRB(20.0, 10.0, 20.0, 10.0),
border: OutlineInputBorder(
borderRadius: BorderRadius.circular(32.0)
),
),
);
final loginButton = Padding(
padding: EdgeInsets.symmetric(vertical: 16.0),
child: Material(
borderRadius: BorderRadius.circular(30.0),
shadowColor: Colors.lightBlueAccent.shade100,
elevation: 5.0,
child: MaterialButton(
minWidth: 200.0,
height: 42.0,
onPressed: (){
print("Signup button clicked");
},
color: Colors.lightBlueAccent,
child:
Text('Sign Up',
style: TextStyle(color: Colors.white, fontSize: 20.0),
),
),
)
);
final countryCode = DropdownButtonHideUnderline (
child: DropdownButton<String>(
value: _ccode,
isDense: true,
items: _ccodes.map((String value) {
return new DropdownMenuItem<String>(
value: value,
child: new Text(value),
);
}).toList(),
onChanged: (String newValue) {
setState(() {
_ccode = newValue;
});
}
)
);
return Scaffold(
backgroundColor: Colors.white,
body: Center (
child: ListView(
padding: EdgeInsets.only(left: 24.0, right: 24.0),
shrinkWrap: true,
children: <Widget>[
name,
SizedBox(height: 18.0),
email,
SizedBox(height: 18.0),
password,
SizedBox(height: 18.0),
passwordConfirmation,
SizedBox(height: 18.0),
countryCode,
SizedBox(height: 18.0),
loginButton,
SizedBox(height: 38.0),
],
)
)
);
}
}
我在 AVD 中遇到的错误
解决方案
断言告诉你items
不能为空。它指的是这个:items: _dropDownMenuItems,
。你永远不会分配_dropDownMenuItems
.
这是 Dart-y 模式的一个很好的用途。将其替换为:
final countryCode = DropdownButton(
value: _selectedCountryCode,
items: _countrycodes
.map((code) =>
new DropdownMenuItem(value: code, child: new Text(code)))
.toList(),
onChanged: null,
);
并且您不再需要该字段或该getDropDownMenuItems
功能。如果您不熟悉此语法,请将其阅读为“将 countryCodes 中的每个代码映射到 DropdownMenuItem 并将它们重新组合成列表”
此外,您的变量范围是错误的。将它们移出构建函数并进入状态,如下所示:
class SignupPage extends State<Signup> {
List<String> _countrycodes = ["+65", "+91"];
List<String> _colors = ['', 'red', 'green', 'blue', 'orange'];
String _selectedCountryCode;
String _color = '';
推荐阅读
- c# - 如何捕捉解码问题。System.FormatException:“Base-64 字符数组或字符串的长度无效。”
- javascript - Angular 9 - 错误类型错误:无法读取未定义的属性“名称”
- python - 遍历字典的 values()
- python - 如何在 Redis 中从 hmset() 切换到 hset()?
- powershell - 如何在同一文件夹中使用 powershell 生成 Zip
- jquery - 画布中精灵左右无限循环
- c++ - 在 BAZEL 中,有没有办法防止依赖 C/C++ 标头传播到依赖库?
- vuetify.js - 如何在多个启用的 v-select 组件中预先选择选项?
- python - Python中的真假测验功能
- php - 调用未定义的方法 App\Mail\OrderPlaced::views()