Flutter 选择值[使用api和FutureBuilder中的数据]时出现颤振下拉按钮错误

嘿，伙计们，我正面临一个问题，我正在用这段代码从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;
}

}

值已填充，一切正常，但当我从下拉列表中选择一个值时，它会因此错误而崩溃

I/Flatter 4578:以下断言被抛出building FutureBuilder>dirty，状态： I/颤振4578:_FutureBuilderState>f394d: I/flatter 4578:“包：flatter/src/material/dropdown.dart”：失败的断言：第608行位置15:“items==null”|| I/flatter 4578:items.isEmpty | | value==null | | | items.whereDropdownMenuItem=>item.value== I/颤振4578:value.length==1'：不正确。 I/颤振4578： I/flatter4578：要么断言表明框架本身存在错误，要么我们应该提供 I/颤振4578：此错误消息中的更多信息可帮助您确定并修复根本原因。 I/Flatter 4578：无论哪种情况，请在GitHub上提交一个bug来报告这一断言： I/颤振4578： I/颤振4578： I/flatter 4578:抛出异常时，这是堆栈： I/颤振4578:2个新的下拉按钮

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();

     }
        }
    ),
  ) ,

---

如果使用FutureBuilder和in items，则需要使用snapshot.data 这就是为什么说items==null或者items是空的

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"),
          ],
        ),
      ),
    );
  }
}

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"),
          ],
        ),
      ),
    );
  }
}