I make Seprate DropDownButton custom StatelessWidget class but its give me error.

Error=>

There should be exactly one item with [DropdownButton]'s value: Instance of 'Company'. Either zero or 2 or more [DropdownMenuItem]s were detected with the same value 'package:flutter/src/material/dropdown.dart': Failed assertion: line 882 pos 15: 'items == null || items.isEmpty || value == null || items.where((DropdownMenuItem item) { return item.value == value; }).length == 1'

**

Code **

This is my custom class

import "package:flutter/material.dart";

class MyDropDown<T> extends StatelessWidget {
  List<DropdownMenuItem<T>> items;
  final T value;
  String hintText;
  ValueChanged<T?> onChanged;

  MyDropDown(
      {Key? key,
      required this.items,
      required this.value,
      required this.hintText,
      required this.onChanged})
      : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Container(
      padding: EdgeInsets.symmetric(horizontal: 10.0),
      decoration: BoxDecoration(
        color: Colors.white,
        border: Border.all(
          color: Colors.grey,
          width: 0.3,
        ),
        borderRadius: BorderRadius.all(
          Radius.circular(
            30.0,
          ),
        ),
      ),
      child: Row(
        children: [
          Expanded(
            // T
            child: DropdownButton<T>(
              hint: Text(
                hintText,
                style: TextStyle(),
              ),
              isExpanded: true,
              value: value,
              items: items,
              onChanged: onChanged,
              underline: Container(),
            ),
          )
        ],
      ),
    );
  }
}

=> This is my model class

class Company {
  int? cId;
  String? cName;

  Company({ this.cId,  this.cName});
}

=> Where i am use this class

import 'package:flutter/material.dart';
import 'package:sqllite_chart/widget/custom_dropdown_widget.dart';
import 'package:sqllite_chart/widget/custom_textformfield_widget.dart';

class MachineDataAddDialog extends StatefulWidget {

  MachineDataAddDialog({Key? key}) : super(key: key);

  @override
  State<MachineDataAddDialog> createState() => _MachineDataAddDialogState();
}

class _MachineDataAddDialogState extends State<MachineDataAddDialog> {

  TextEditingController textEditingControllerMId = TextEditingController();
  TextEditingController textEditingControllerMName = TextEditingController();
  TextEditingController textEditingControllerMTypeId = TextEditingController();
  TextEditingController textEditingControllerMTypeName =
      TextEditingController();
  TextEditingController textEditingControllerCWGId = TextEditingController();
  TextEditingController textEditingControllerCWGName = TextEditingController();
  TextEditingController textEditingControllerMDesc = TextEditingController();

  List<Company> companies = [
    Company(cId: 1,  cName: 'ABC'),
    Company(cId: 2,  cName: 'PQR'),
    Company(cId: 3,  cName: 'RST'),
    Company(cId: 4,  cName: 'GFI'),
    Company(cId: 5,  cName: 'XYZ')
  ];

  List<DropdownMenuItem<Company>> companyListDropDownItems = [];
  Company? selectedCompany;
  int selectedCompanyId = 1;
  String selectedCompanyTitle = 'ABC';

  List<DropdownMenuItem<Company>> buildCompanyList(List company) {
    List<DropdownMenuItem<Company>> items = [];
    for (Company companyList in company) {
      items.add(
        DropdownMenuItem(
          value: companyList,
          child: Row(
            children: [
              Text(
                '${companyList.cName}',
                style: TextStyle(
                  color: Color.fromRGBO(49, 87, 110, 1.0),
                ),
              ),
            ],
          ),
        ),
      );
    }
    return items;
  }

  void onChangeActivityListDropDownItem(Company? selected) {
    setState(() {
      selectedCompany = selected!;
      selectedCompanyId = selected.cId!;
      selectedCompanyTitle = selected.cName!;
    });
  }

  @override
  void initState() {
    selectedCompany=Company();
    companyListDropDownItems = buildCompanyList(companies);
    super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return AlertDialog(
      title: const Align(
        alignment: Alignment.bottomCenter,
        child: Text("Enter Machine Details",
            style: TextStyle(
              shadows: [
                Shadow(
                    color: Color.fromRGBO(49, 87, 110, 1.0),
                    offset: Offset(0, -5))
              ],
              color: Colors.transparent,
              decorationColor: Color.fromRGBO(49, 87, 110, 1.0),
              decoration: TextDecoration.underline,
              decorationThickness: 1,
              decorationStyle: TextDecorationStyle.double,
            )),
      ),
      content: SingleChildScrollView(
        child: Column(
          children: [
            CustomTextFormFieldWidget(
              controller: textEditingControllerMId,
              keyboardType: TextInputType.number,
              hintText: "Enter Machine Id",
            ),
            SizedBox(
              height: 5
            ),
            CustomTextFormFieldWidget(
              controller: textEditingControllerMName,
              keyboardType: TextInputType.text,
              hintText: "Enter Machine Name",
            ),
            SizedBox(
              height: 5
            ),
            CustomTextFormFieldWidget(
              controller: textEditingControllerMTypeId,
              keyboardType: TextInputType.number,
              hintText: "Enter Machine Type ID",
            ),
            SizedBox(
              height: 5
            ),
            CustomTextFormFieldWidget(
              controller: textEditingControllerMTypeName,
              keyboardType: TextInputType.text,
              hintText: "Enter Machine Type Name",
            ),
            SizedBox(
              height: 5
            ),
            MyDropDown<Company>(
              hintText: 'Company',
              value: selectedCompany!,
              items: companyListDropDownItems,
              onChanged:onChangeActivityListDropDownItem,
            ),
            SizedBox(
                height: 5
            ),
            CustomTextFormFieldWidget(
              controller: textEditingControllerCWGId,
              keyboardType: TextInputType.number,
              hintText: "Enter CWG Id",
            ),
            SizedBox(
              height: 5
            ),
            CustomTextFormFieldWidget(
              controller: textEditingControllerCWGName,
              keyboardType: TextInputType.text,
              hintText: "Enter CWG Name",
            ),
            SizedBox(
              height: 5
            ),
            CustomTextFormFieldWidget(
              controller: textEditingControllerMDesc,
              keyboardType: TextInputType.text,
              hintText: "Enter Machine Description",
              maxLines: 2,
            ),
          ],
        ),
      ),
      actions: [
        Row(
          children: [
            Expanded(
              flex: 4,
              child: MaterialButton(
                onPressed: () {
                  Navigator.of(context).pop();
                },
                color: Color.fromRGBO(49, 87, 110, 1.0),
                child: Text("CANCEL", style: TextStyle(color: Colors.white)),
              ),
            ),
            SizedBox(
              width: 10,
            ),
            Expanded(
              flex: 4,
              child: MaterialButton(
                onPressed:() {},
                child: Text("ADD", style: TextStyle(color: Colors.white)),
                color: Color.fromRGBO(49, 87, 110, 1.0),
              ),
            )
          ],
        ),
      ],
    );
  }
}

CodePudding user response：

Problem is while creating your companylist to pass to your custom widget, you pass companyList as a value, which you can not do because value expects String?. You need to find a unique string that belongs to that company

  DropdownMenuItem(
          value: companyList,
          child: Row(
            children: [
              Text(
                '${companyList.cName}',
                style: TextStyle(
                  color: Color.fromRGBO(49, 87, 110, 1.0),
                ),
              ),
            ],
          ),