flutter - 当类用作值而不是字符串时，flutter下拉列表失败

我有以下代码。

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        title: 'My App',
        theme: ThemeData(
          primarySwatch: Colors.blue,
        ),
        home: SimpleScreen());
  }
}

class SimpleScreen extends StatefulWidget {
  @override
  _SimpleScreenState createState() => _SimpleScreenState();
}

class _SimpleScreenState extends State<SimpleScreen> {
  ItemCls currentValue = ItemCls(name: 'one', price: 1);

  List<DropdownMenuItem> _menuItems = <DropdownMenuItem>[
    DropdownMenuItem(
        child: new Container(
          child: new Text("Item#1"),
          width: 200.0,
        ),
        value: ItemCls(name: 'one', price: 1)),
    DropdownMenuItem(
        child: new Container(
          child: new Text("Item#2"),
          width: 200.0,
        ),
        value: ItemCls(name: 'two', price: 2))
  ];

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
        body: Center(
      child: DropdownButton(
        value: currentValue,
        items: _menuItems,
        onChanged: onChanged,
        style: Theme.of(context).textTheme.title,
      ),
    ));
  }

  void onChanged(value) {
    setState(() {
      currentValue = value;
    });
   // print(value);
  }
}

class ItemCls {
  final String name;
  final double price;

  const ItemCls({
    @required this.name,
    @required this.price,
  })  : assert(name != null),
        assert(price != null);
}

失败与

最佳答案

原因
实例ItemCls currentValue = ItemCls(name: 'one', price: 1);不等于value: ItemCls(name: 'one', price: 1)),他们有不同的地址，并视为不同的值(value)
所以DropdownButton认为选择列表中不存在默认值ItemCls(name: 'one', price: 1)
解决方案
使用List<ItemCls>包含您的选择，并将currentValue指向List的第一个值

ItemCls currentValue;
  static List<ItemCls> itemList = [
    ItemCls(name: 'one', price: 1),
    ItemCls(name: 'two', price: 2)
  ];
  ...
 DropdownMenuItem<ItemCls>(
    child: new Container(
      child: new Text("Item#1"),
      width: 200.0,
    ),
    value: itemList[0]),
...
void initState() {
// TODO: implement initState
currentValue = itemList[0];
}

工作演示



完整的代码

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        title: 'My App',
        theme: ThemeData(
          primarySwatch: Colors.blue,
        ),
        home: SimpleScreen());
  }
}

class SimpleScreen extends StatefulWidget {
  @override
  _SimpleScreenState createState() => _SimpleScreenState();
}

class _SimpleScreenState extends State<SimpleScreen> {
  ItemCls currentValue;
  static List<ItemCls> itemList = [
    ItemCls(name: 'one', price: 1),
    ItemCls(name: 'two', price: 2)
  ];

  List<DropdownMenuItem<ItemCls>> _menuItems = <DropdownMenuItem<ItemCls>>[
    DropdownMenuItem<ItemCls>(
        child: new Container(
          child: new Text("Item#1"),
          width: 200.0,
        ),
        value: itemList[0]),
    DropdownMenuItem<ItemCls>(
        child: new Container(
          child: new Text("Item#2"),
          width: 200.0,
        ),
        value: itemList[1])
  ];

  @override
  void initState() {
    // TODO: implement initState
    currentValue = itemList[0];
  }

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
        body: Center(
      child: DropdownButton<ItemCls>(
        value: currentValue,
        items: _menuItems,
        onChanged: onChanged,
        style: Theme.of(context).textTheme.title,
      ),
    ));
  }

  void onChanged(value) {
    setState(() {
      currentValue = value;
    });
    // print(value);
  }
}

class ItemCls {
  final String name;
  final double price;

  const ItemCls({
    @required this.name,
    @required this.price,
  })  : assert(name != null),
        assert(price != null);
}

关于flutter - 当类用作值而不是字符串时，flutter下拉列表失败，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/59278640/