当类用作值而不是字符串时， flutter 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

The following assertion was thrown building SimpleScreen(dirty, dependencies: [_LocalizationsScope-[GlobalKey#b1ff3], _InheritedTheme], state: _SimpleScreenState#d473f): 'package:flutter/src/material/dropdown.dart': Failed assertion: line 620 pos 15: 'items == null || items.isEmpty || value == null || items.where((DropdownMenuItem item) => item.value == value).length == 1': is not true.

Answer 1

每个 DropDownMenuItem 中的 value 都必须是唯一的。 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;
  }
}