Not able to prepopulate dropdown with object in flutter

Question

I've a dropdown which is populated with objects received from API. The problem is I'm not able to pre-populate the dropdown with the object when the user takes edit.

Here is the sample JSON:

[{"_id":"435463","userId":"3423423","username":"ma","categoryId":"5656756","insurerId":"567544","packageId":"5675","categoryName":"vehicle insurance","insurerName":"lic","packageName":"family"},      {"_id":"4564644","userId":"2342344","username":"ma","categoryId":"6575744","insurerId":"567567","packageId":"3455","categoryName":"life insurance","insurerName":"lic","packageName":"family"}]
``


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

    UserPackages selectedPckg;
    class AddOrEditClaim extends StatefulWidget {
      @override
      AddOrEditClaimState createState() => AddOrEditClaimState();
    }

    class AddOrEditClaimState extends State<AddOrEditClaim> {
      List<UserPackages> packageList = List();
      final formKey = GlobalKey<FormState>();

      @override
      void initState() {
            fetchPackages();
            selectedPckg= UserPackages();
            UserPackages packgToEdit = new UserPackages();
            packgToEdit.packageId="123";
            packgToEdit.packageName="family";
            selectedPckg=packgToEdit;

             super.initState();
      }

      @override
      Widget build(BuildContext context) {
        // TODO: implement build
        return new Scaffold(
            appBar: AppBar(
              title: Text('Add Claim'),
            ),
            body:
                Form(
                  key:formKey,
                  child:ListView(
                      children: <Widget>[
                        new DropdownButton(
                          items: packageList.map((item) {
                            return new DropdownMenuItem<UserPackages>(
                              child: new Text(item.packageName),
                              value: item,
                            );
                          }).toList(),
                          onChanged: (newVal) {
                            setState(() {
                              selectedPckg = newVal;
                            });
                          },
                          value: selectedPckg,
                        ),
                      ])
                )

        );;
      }
       fetchPackages() async {
        SharedPreferences prefs = await SharedPreferences.getInstance();
        var resp = await http.get('http://www.mocky.io/v2/5da94ca131000058004e0769');
        setState(() {
          packageList= parsePackages(resp.body);
        });
      }

      List<UserPackages> parsePackages(String responseBody) {
        final parsedj = json.decode(responseBody);
        final parsed = parsedj.cast<Map<String, dynamic>>();
        return parsed.map<UserPackages>((json) => UserPackages.fromJson(json)).toList();
      }
    }


Here is the Userpackages class:


    class UserPackages{
       String packageName;
       String packageId;
      UserPackages({this.packageName,this.packageId});
      factory UserPackages.fromJson(Map<String, dynamic> json){
        return UserPackages(
          packageId:json['packageId'] as String,
          packageName: json['packageName'] as String,
        );
      }
    }


The following is being thrown


    'package:flutter/src/material/dropdown.dart': Failed assertion: line 560 pos 15: 'items == null ||items.isEmpty || value == null || items.where((DropdownMenuItem<T> item) => item.value ==value).length == 1': is not true.

Answer 1

Screenshot:

---

Code:

There were too many mistakes in your code, I just created a minimal reproducible code showing you what you want to achieve. You should use FutureBuilder if you don't want to create a local dummy DropdownMenuItem, for simplicity I created it.

class AddOrEditClaim extends StatefulWidget {
  @override
  AddOrEditClaimState createState() => AddOrEditClaimState();
}

class AddOrEditClaimState extends State<AddOrEditClaim> {
  UserPackages _selectedPackage = UserPackages(categoryName: "vehicle insurance", packageId: "5675");
  List<UserPackages> packageList = [];

  @override
  void initState() {
    super.initState();
    fetchPackages();
  }

  @override
  Widget build(BuildContext context) {
    var items = packageList.map((item) {
      return DropdownMenuItem<UserPackages>(
        child: Text(item.categoryName),
        value: item,
      );
    }).toList();

    // if list is empty, create a dummy item
    if (items.isEmpty) {
      items = [
        DropdownMenuItem(
          child: Text(_selectedPackage.categoryName),
          value: _selectedPackage,
        )
      ];
    }

    return Scaffold(
      appBar: AppBar(title: Text('Add Claim')),
      body: Center(
        child: DropdownButton<UserPackages>(
          items: items,
          onChanged: (newVal) => setState(() => _selectedPackage = newVal!),
          value: _selectedPackage,
        ),
      ),
    );
  }

  void fetchPackages() async {
    var resp = await http.get(Uri.parse('http://www.mocky.io/v2/5da94ca131000058004e0769'));
    if (resp.statusCode == 200) {
      setState(() {
        packageList = parsePackages(resp.body);
        _selectedPackage = packageList[0];
      });
    } else {
      print("Error occurred");
    }
  }

  List<UserPackages> parsePackages(String responseBody) {
    final parsedJson = json.decode(responseBody);
    final parsed = parsedJson.cast<Map<String, dynamic>>();
    return parsed.map<UserPackages>((json) => UserPackages.fromJson(json)).toList();
  }
}

class UserPackages {
  final String categoryName;
  final String packageId;

  UserPackages({required this.categoryName, required this.packageId});

  factory UserPackages.fromJson(Map<String, dynamic> json) {
    return UserPackages(
      packageId: json['packageId'] as String,
      categoryName: json['categoryName'] as String,
    );
  }
}

Answer 2

add null / empty check for packageList. because packageList is updaed after fetchPackages network call is resolved so wrap that DropdownButton in a if block as below

Form(
    key:formKey,
    child:ListView(
        children: <Widget>[
          if(packageList.isNotEmpty){
          new DropdownButton(
            items: packageList.map((item) {
              return new DropdownMenuItem<UserPackages>(
                child: new Text(item.packageName),
                value: item,
              );
            }).toList(),
            onChanged: (newVal) {
              setState(() {
                selectedPckg = newVal;
              });
            },
            value: selectedPckg ?? packageList[0],
          ),
          }
        ])
  )