flutter - 如何在 Flutter 中创建包含对象的下拉列表
问题描述
我尝试创建一个下拉列表并用少数对象填充它,这些对象代表用户可以从中选择的少数服务器,但是当我运行应用程序时,我收到一条错误消息:
The following assertion was thrown building DropdownWidget(dirty, state: _DropdownWidgetState#1f58f): There should be exactly one item with [DropdownButton]'s value: Instance of 'ServerModel'. Either zero or 2 or more [DropdownMenuItem]s were detected with the same value
你能帮我找出我在代码中做错了什么吗?
import 'package:flutter/material.dart';
class ServerSettingsPage extends StatefulWidget {
@override
_ServerSettingsPageState createState() => _ServerSettingsPageState();
}
class _ServerSettingsPageState extends State<ServerSettingsPage> {
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: Text("Server Settings")),
body: _buildUI(),
);
}
Widget _buildUI() {
return Padding(
padding: const EdgeInsets.fromLTRB(0, 20, 0, 0),
child: Center(
child: Column(
children: <Widget>[
Text(
'Select a server:',
style: TextStyle(fontSize: 18, fontWeight: FontWeight.bold),
),
DropdownWidget(),
],
),
),
);
}
}
class DropdownWidget extends StatefulWidget {
DropdownWidget({Key key}) : super(key: key);
@override
_DropdownWidgetState createState() => _DropdownWidgetState();
}
class _DropdownWidgetState extends State<DropdownWidget> {
ServerModel dropdownValue =
ServerModel(name: 'Default', url: 'https://defaultServer.com/');
@override
Widget build(BuildContext context) {
return DropdownButton<ServerModel>(
value: dropdownValue,
icon: Icon(Icons.arrow_downward),
iconSize: 24,
elevation: 16,
style: TextStyle(color: Colors.purple[700]),
underline: Container(
height: 2,
color: Colors.purple[700],
),
onChanged: (ServerModel newServer) {
setState(() {
dropdownValue = newServer;
});
},
items: <ServerModel>[
ServerModel(name: 'Default', url: 'https:defaultServer.com/'),
ServerModel(name: 'Alpha', url: 'https://alphaServer.com/'),
ServerModel(name: 'Beta', url: 'https://betaServer.com/'),
].map<DropdownMenuItem<ServerModel>>((ServerModel server) {
return DropdownMenuItem<ServerModel>(
value: server,
child: Text(server.name, style: TextStyle(fontSize: 20)),
);
}).toList(),
);
}
}
这是 ServerModel 类:
class ServerModel {
ServerModel({this.name, this.url});
ServerModel.empty() {
this.name = null;
this.url = null;
}
String name;
String url;
}
非常感谢您阅读这篇文章。
解决方案
应该只有一项具有 [DropdownButton] 的值:“ServerModel”的实例。检测到 0 个或 2 个或多个 [DropdownMenuItem] 具有相同的值
发生这种情况是因为下拉列表中的选定值必须指向现有列表项(显然该列表中不应有任何重复项)。您现在设置它的方式是在您的小部件构建期间生成 ServerModel 列表,并且一旦构建它,就不会在小部件的状态内引用该列表。我希望我的答案足够清楚,还请看下面的正确代码:
class _DropdownWidgetState extends State<DropdownWidget> {
List<ServerModel> serverModels = <ServerModel>[
ServerModel(name: 'Default', url: 'https:defaultServer.com/'),
ServerModel(name: 'Alpha', url: 'https://alphaServer.com/'),
ServerModel(name: 'Beta', url: 'https://betaServer.com/'),
];
ServerModel selectedServer;
@override
initState() {
super.initState();
selectedServer = serverModels[0];
}
@override
Widget build(BuildContext context) {
return DropdownButton<ServerModel>(
value: selectedServer,
icon: Icon(Icons.arrow_downward),
iconSize: 24,
elevation: 16,
style: TextStyle(color: Colors.purple[700]),
underline: Container(
height: 2,
color: Colors.purple[700],
),
onChanged: (ServerModel newServer) {
setState(() {
selectedServer = newServer;
});
},
items: serverModels.map((ServerModel map) {
return new DropdownMenuItem<ServerModel>(
value: map, child: Text(map.name));
}).toList(),
);
}
}
在 dartpad 上经过测试的有效交互式答案: https ://dartpad.dev/153bad9baac64382e27bc41cdc8131c9
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