具有相同值的Flutter下拉项有问题

Question

是不熟悉的人，在为某些下拉菜单项赋予相同值时遇到问题。这是下面的下拉菜单的代码

Expanded(
  child: Padding(
    padding: const EdgeInsets.only(
        left: 20.0, right: 5.0, top: 5.0),
    child: new Container(
      alignment: Alignment.center,
      height: 40.0,
      decoration: new BoxDecoration(
          color: Colors.cyan,
          borderRadius: new BorderRadius.circular(30.0)),
      child: DropdownButton<int>(
        items: [
          DropdownMenuItem<int>(
            child: Text('S&P500'),
            value: 10,
          ),
          DropdownMenuItem<int>(
            child: Text('WS30'),
            value: 1,
          ),
          DropdownMenuItem<int>(
            child: Text('TV-NDX'),
            value: 1,
          ),
          DropdownMenuItem<int>(
            child: Text('AUS200'),
            value: 1,
          ),
          DropdownMenuItem<int>(
            child: Text('JS225'),
            value: 100,
          ),
          DropdownMenuItem<int>(
            child: Text('UK100'),
            value: 1,
          ),DropdownMenuItem<int>(
            child: Text('FCH1'),
            value: 1,
          ),
          DropdownMenuItem<int>(
            child: Text('STOXX50E'),
            value: 1,
          ),
          DropdownMenuItem<int>(
            child: Text('GDAX'),
            value: 1,
          ),DropdownMenuItem<int>(
            child: Text('BITCOIN'),
            value: 1,
          ),
          DropdownMenuItem<int>(
            child: Text('ETHEREUM'),
            value: 1,
          ),
          DropdownMenuItem<int>(
            child: Text('NGS'),
            value: 1000,
          ),
          DropdownMenuItem<int>(
            child: Text('CRUDEOIL'),
            value: 100,
          ),
          DropdownMenuItem<int>(
            child: Text('UKOIL'),
            value: 100,
          ),
          DropdownMenuItem<int>(
            child: Text('SPX-NDX'),
            value: 10,
          ),
          DropdownMenuItem<int>(
            child: Text('SMI'),
            value: 1,
          ),

        ],
        onChanged: (int value) {
          setState(() {
            asset = value;
          });
        },
        hint: Text("TRADEVIEW",
          style: new TextStyle(
              fontSize: 15.0, color: Colors.black),),
        value: asset,
      ),
    ),
  ),
),

Answer 1

class Data {
  String name;
  int value;

  Data({
    this.name,
    this.value,
  });

  factory Data.fromJson(Map<String, dynamic> json) => Data(
    name: json["name"] == null ? null : json["name"],
    value: json["value"] == null ? null : json["value"],
  );

  Map<String, dynamic> toJson() => {
    "name": name == null ? null : name,
    "value": value == null ? null : value,
  };
}

List<Data> dataList = [
    Data(name: 'CRUDEOIL', value: 100),
    Data(name: "UKOIL", value: 100)
  ];

    DropdownButton<Data>(
                    items: dataList.map<DropdownMenuItem<Data>>((Data value) {
                      return DropdownMenuItem<Data>(
                        value: value,
                        child: Text(value.name),
                      );
                    }).toList(),
                    onChanged: (Data value) {
                      setState(() {
                        asset = value;
                        print('${asset.name}  ${asset.value}');
                      });
                    }

完整代码

import 'package:flutter/material.dart';
// To parse this JSON data, do
//
//     final data = dataFromJson(jsonString);

import 'dart:convert';

Data dataFromJson(String str) => Data.fromJson(json.decode(str));

String dataToJson(Data data) => json.encode(data.toJson());

class Data {
  String name;
  int value;

  Data({
    this.name,
    this.value,
  });

  factory Data.fromJson(Map<String, dynamic> json) => Data(
        name: json["name"] == null ? null : json["name"],
        value: json["value"] == null ? null : json["value"],
      );

  Map<String, dynamic> toJson() => {
        "name": name == null ? null : name,
        "value": value == null ? null : value,
      };
}

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        // This is the theme of your application.
        //
        // Try running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;
  Data asset;
  List<Data> dataList = [
    Data(name: 'CRUDEOIL', value: 100),
    Data(name: "UKOIL", value: 100)
  ];
  void _incrementCounter() {
    setState(() {
      // This call to setState tells the Flutter framework that something has
      // changed in this State, which causes it to rerun the build method below
      // so that the display can reflect the updated values. If we changed
      // _counter without calling setState(), then the build method would not be
      // called again, and so nothing would appear to happen.
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    return Scaffold(
      appBar: AppBar(
        // Here we take the value from the MyHomePage object that was created by
        // the App.build method, and use it to set our appbar title.
        title: Text(widget.title),
      ),
      body: Center(
        // Center is a layout widget. It takes a single child and positions it
        // in the middle of the parent.
        child: Column(
          // Column is also a layout widget. It takes a list of children and
          // arranges them vertically. By default, it sizes itself to fit its
          // children horizontally, and tries to be as tall as its parent.
          //
          // Invoke "debug painting" (press "p" in the console, choose the
          // "Toggle Debug Paint" action from the Flutter Inspector in Android
          // Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
          // to see the wireframe for each widget.
          //
          // Column has various properties to control how it sizes itself and
          // how it positions its children. Here we use mainAxisAlignment to
          // center the children vertically; the main axis here is the vertical
          // axis because Columns are vertical (the cross axis would be
          // horizontal).
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Expanded(
              child: Padding(
                padding:
                    const EdgeInsets.only(left: 20.0, right: 5.0, top: 5.0),
                child: new Container(
                  alignment: Alignment.center,
                  height: 40.0,
                  decoration: new BoxDecoration(
                      color: Colors.cyan,
                      borderRadius: new BorderRadius.circular(30.0)),
                  child: DropdownButton<Data>(
                    items: dataList.map<DropdownMenuItem<Data>>((Data value) {
                      return DropdownMenuItem<Data>(
                        value: value,
                        child: Text(value.name),
                      );
                    }).toList(),
                    onChanged: (Data value) {
                      setState(() {
                        asset = value;
                        print('${asset.name}  ${asset.value}');
                      });
                    },
                    hint: Text(
                      "TRADEVIEW",
                      style: new TextStyle(fontSize: 15.0, color: Colors.black),
                    ),
                    value: asset,
                  ),
                ),
              ),
            )
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}