当类用作值而不是字符串时，颤动下拉失败

Question

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);
}

失败了

在构建 SimpleScreen(dirty, 依赖项：[_LocalizationsScope-[GlobalKey#b1ff3]， _InheritedTheme]，状态：_SimpleScreenState#d473f)：'package:flutter/src/material/dropdown.dart'：断言失败：行 620 位置 15: '项目 == 空 ||项目.isEmpty ||值 == 空 || items.where((DropdownMenuItem item) => item.value == value).length == 1'：不正确。

Answer 1

每个DropDownMenuItem 中的value 必须是唯一的。 Dart 通过两种方式确保唯一性：定义 == 运算符和在对象上调用 hashCode。您需要将两者都添加到 ItemCls 类中：

class ItemCls {
  final String name;
  final double price;

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

  bool operator ==(dynamic other) {
    return other is ItemCls && 
           this.name == other.name && 
           this.price == other.price;
  }

  @override
  int get hashCode {
    // Hash code algorithm derived from https://www.sitepoint.com/how-to-implement-javas-hashcode-correctly/
    int hashCode = 1;
    hashCode = (23 * hashCode) + this.name.hashCode;
    hashCode = (23 * hashCode) + this.price.hashCode;
    return hashCode;
  }
}

Answer 2

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被视为不同的价值
所以 DropdownButton 认为默认值 ItemCls(name: 'one', price: 1) 在选择列表中不存在

解决方案
使用List<ItemCls> 包含您的选择并将currentValue 指向列表的第一个值

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);
}