颤振组合框

Question

我正在尝试创建一个下拉列表/组合框窗口小部件，并且很难计算。下面是我尝试过的代码。基本上创建了一个列表，该列表是我从这里到那里收集的，但是无法将其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

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(),
      );
  }
}

更新的代码：

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

Answer 1

断言告诉您，items不能为null。指的是这个：items: _dropDownMenuItems,。您永远不会分配_dropDownMenuItems。

这是Dart-y模式的好用法。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getDropDownMenuItems功能。如果您不熟悉此语法，请将其读为“将countryCodes中的每个代码映射到DropdownMenuItem并将它们重新组成一个List”

另外，变量的范围是错误的。将它们移出构建函数并移入状态，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 = '';