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嘿，伙计们，我面临一个问题，我正在使用此代码从 api 检索数据

class carLists{

carLists();
getCarsFromNetwork(String jsonUrl) async {
List<Cars> list;
String link = jsonUrl;
var res = await http.get(Uri.encodeFull(link),headers: {"Accept":"Application/json"});
if (res.statusCode == 200) {
var data = json.decode(res.body);
var list = data["cars"] as List;
List<Cars> imagesList = list.map((i) => Cars.fromJson(i)).toList();

return imagesList;
}
}
List<DropdownMenuItem> getCars(List<Cars> carPlates){
List<DropdownMenuItem<String>> ordersType = new List<DropdownMenuItem<String>>();

for(var i = 0 ; i<carPlates.length;i++ ) {


    ordersType.add(DropdownMenuItem(
      value: carPlates[i].carPlate, child: Text(carPlates[i].carPlate),));

}
return ordersType;
}

}

并使用这两个函数实现一个下拉按钮来获取数据并返回下拉菜单列表：`

Future<List<Cars>> foo(String link) async{

carPlates =  await carLists().getCarsFromNetwork(link);
print(carPlates.length);
return carPlates;
}

getItems(List values) {
List<DropdownMenuItem<String>> ordersType = new List<
    DropdownMenuItem<String>>();
for (var i = 0; i > values.length; i++) {
  ordersType.add(DropdownMenuItem(value: values[i], child: values[i],));

  return ordersType;
}
}

`并使用此代码填充 DropdownButton

Container(

    child: FutureBuilder(
        future: foo(url) ,
        builder: (context, snapshot){
     if(snapshot.hasData)
       {
         return new DropdownButton(
           iconDisabledColor: Colors.black,
           isExpanded: true,
           icon: Icon(FontAwesomeIcons.arrowCircleDown),
           iconSize: 14,
           style: TextStyle(fontSize: 16, color: Colors.black),
           iconEnabledColor: Colors.deepOrange,
           items: carLists().getCars(carPlates),
           value: dropTyreBrand,
           onChanged: (val) {
             setState(() {
               dropTyreBrand = val;

             });
           },
         );
       }else{

          return CircularProgressIndicator();

     }
        }
    ),
  ) ,

值已填充，一切都很好，但是当我从下拉列表中选择一个值时，它会因此错误而崩溃

I/flutter (4578): 在构建 FutureBuilder>(dirty, state: I/flutter (4578): _FutureBuilderState>#f394d): I/flutter (4578): 'package:flutter/src/material/dropdown .dart'：断言失败：第 608 行 pos 15：'items == null || 我/颤振（4578）：items.isEmpty || 值 == 空 || items.where((DropdownMenuItem item) => item.value == I/flutter (4578): value).length == 1': 不正确。I/flutter (4578): I/flutter (4578): 要么断言表明框架本身有错误，要么我们应该提供实质性的 I/flutter (4578): 此错误消息中的更多信息可帮助您确定和修复根本原因。I/flutter (4578)：在任何一种情况下，请通过在 GitHub 上提交错误报告此断言：I/flutter (4578)： https://github.com/flutter/flutter/issues/new?template=BUG.md I/flutter（4578）：I/flutter（4578）：抛出异常时，这是堆栈：I/flutter（ 4578): #2 新下拉按钮

4

1 回答 1

2

如果您使用 FutureBuilder 并且在 items 中您需要使用 snapshot.data
这就是错误说 items == null 或 items 为空的原因

演示代码片段

return DropdownButton<Designations>(
                    items: snapshot.data
                        .map((designation) => DropdownMenuItem<Designations>(
                              child: Text(designation.designation),
                              value: designation,
                            ))
                        .toList(),

演示完整代码

import 'package:flutter/material.dart';
import 'dart:convert';
import 'package:http/http.dart' as http;

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

// To parse this JSON data, do
//
//     final designations = designationsFromJson(jsonString);

List<Designations> designationsFromJson(String str) => List<Designations>.from(
    json.decode(str).map((x) => Designations.fromJson(x)));

String designationsToJson(List<Designations> data) =>
    json.encode(List<dynamic>.from(data.map((x) => x.toJson())));

class Designations {
  String designationId;
  String designation;

  Designations({
    this.designationId,
    this.designation,
  });

  factory Designations.fromJson(Map<String, dynamic> json) => Designations(
        designationId: json["DesignationId"],
        designation: json["Designation"],
      );

  Map<String, dynamic> toJson() => {
        "DesignationId": designationId,
        "Designation": designation,
      };
}

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: JsonApiDropdown(),
    );
  }
}

class JsonApiDropdown extends StatefulWidget {
  @override
  JsonApiDropdownState createState() {
    return new JsonApiDropdownState();
  }
}

class JsonApiDropdownState extends State<JsonApiDropdown> {
  Designations _currentDesignation;

  final String uri = 'https://jsonplaceholder.typicode.com/users';

  Future<List<Designations>> _fetchDesignation() async {
    String jsonString =
        '[ { "DesignationId": "CDG0008", "Designation": "Avp - Associate Vice President" }, { "DesignationId": "CDG0004", "Designation": "Ceo - Chief Executive Officer" } ]';
    final designations = designationsFromJson(jsonString);
    return designations;
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Fetching data from JSON - DropdownButton'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          mainAxisSize: MainAxisSize.max,
          children: <Widget>[
            FutureBuilder<List<Designations>>(
                future: _fetchDesignation(),
                builder: (BuildContext context,
                    AsyncSnapshot<List<Designations>> snapshot) {
                  if (!snapshot.hasData) return CircularProgressIndicator();
                  return DropdownButton<Designations>(
                    items: snapshot.data
                        .map((designation) => DropdownMenuItem<Designations>(
                              child: Text(designation.designation),
                              value: designation,
                            ))
                        .toList(),
                    onChanged: (Designations value) {
                      setState(() {
                        _currentDesignation = value;
                      });
                    },
                    isExpanded: false,
                    //value: _currentUser,
                    hint: Text('Select User'),
                  );
                }),
            SizedBox(height: 20.0),
            _currentDesignation != null
                ? Text("designation: " +
                    _currentDesignation.designation +
                    "\n id: " +
                    _currentDesignation.designationId)
                : Text("No selected"),
          ],
        ),
      ),
    );
  }
}

在此处输入图像描述

于 2019-09-18T00:44:52.533 回答