Set default value for dropdown button in flutter

Question

I have a dropdown button which works fine, but when I try to set a default value it will fail with the following error:

'package:flutter/src/material/dropdown.dart': Failed assertion: line 620 pos 15: 'items == null || items.isEmpty || value == null || items.where((DropdownMenuItem item) => item.value == value).length == 1': is not true.

This is my dropdown button:

 Widget changeWorkspace() {
return StatefulBuilder(
    builder: (BuildContext context, StateSetter setState) {
  return Column(mainAxisSize: MainAxisSize.min, children: <Widget>[
    Padding(
      padding: EdgeInsets.all(8.0),
      child: DropdownButton<AssignedWorkspace>(
          isExpanded: true,
          hint: Text("SELECT WORKSPACE"),
          value: selectedWorkspace,
          onChanged: (dropdownValueSelected) {
            setState(() {
              selectedWorkspace = dropdownValueSelected;
            });
          },
          items: workspaces != null && workspaces.length > 0
              ? workspaces.map((AssignedWorkspace workspace) {
                  return new DropdownMenuItem<AssignedWorkspace>(
                    value: workspace,
                    child: new Text(workspace.name,
                        style: new TextStyle(color: Colors.black)),
                  );
                }).toList()
              : null),
    ),
  ]);
});

}

I've tried to set the value of selectedWorkspace onInit as follows but it fails.

selectedWorkspace = new AssignedWorkspace(
id: userSettings.currentWorkspaceId,
name: userSettings.currentWorkspaceName);

Is there a way of setting a default value in a dropdown button?

Can Karabag · Accepted Answer · 2019-09-10 11:28:37Z

22

import 'package:flutter/material.dart';

import '../config/app_theme.dart';

class DropdownWidget extends StatefulWidget {
  final String title;
  final List<String> items;
  final ValueChanged<String> itemCallBack;
  final String currentItem;
  final String hintText;

  DropdownWidget({
    this.title,
    this.items,
    this.itemCallBack,
    this.currentItem,
    this.hintText,
  });

  @override
  State<StatefulWidget> createState() => _DropdownState(currentItem);
}

class _DropdownState extends State<DropdownWidget> {
  List<DropdownMenuItem<String>> dropDownItems = [];
  String currentItem;
  AppTheme appTheme;

  _DropdownState(this.currentItem);

  @override
  void initState() {
    super.initState();
    for (String item in widget.items) {
      dropDownItems.add(DropdownMenuItem(
        value: item,
        child: Text(
          item,
          style: TextStyle(
            fontSize: 16,
          ),
        ),
      ));
    }
  }

  @override
  void didUpdateWidget(DropdownWidget oldWidget) {
    if (this.currentItem != widget.currentItem) {
      setState(() {
        this.currentItem = widget.currentItem;
      });
    }
    super.didUpdateWidget(oldWidget);
  }

  @override
  Widget build(BuildContext context) {
    appTheme = AppTheme(Theme.of(context).brightness);

    return Container(
      margin: EdgeInsets.symmetric(vertical: 10),
      child: Column(
        crossAxisAlignment: CrossAxisAlignment.stretch,
        children: <Widget>[
          Container(
            margin: EdgeInsets.only(left: 6),
            child: Text(
              widget.title,
              style: appTheme.activityAddPageTextStyle,
            ),
          ),
          Container(
            padding: EdgeInsets.symmetric(vertical: 3, horizontal: 15),
            margin: EdgeInsets.only(top: 10),
            decoration: BoxDecoration(
              borderRadius: BorderRadius.circular(6),
              color: Colors.white,
              boxShadow: [
                BoxShadow(
                  offset: Offset(0, 2),
                  blurRadius: 10,
                  color: Color(0x19000000),
                ),
              ],
            ),
            child: DropdownButtonHideUnderline(
              child: DropdownButton(
                icon: appTheme.activityAddPageDownArrowSVG,
                value: currentItem,
                isExpanded: true,
                items: dropDownItems,
                onChanged: (selectedItem) => setState(() {
                  currentItem = selectedItem;
                  widget.itemCallBack(currentItem);
                }),
                hint: Container(
                  child: Text(widget.hintText, style: appTheme.hintStyle),
                ),
              ),
            ),
          ),
        ],
      ),
    );
  }
}

This is my dropDownWidget without optimization. It has currentItem. You could use it like:

           DropdownWidget(
                title: kStatus,
                items: state.customerStepInfo.statusList,
                currentItem: status,
                hintText: kCommonPick,
                itemCallBack: (String status) {
                  this.status = status;
                },
              )

edited Sep 10, 2019 at 11:28

answered Sep 10, 2019 at 11:20

Can Karabag

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4 Comments

FoxDonut Over a year ago

An explanation would be nice.

mauricev Over a year ago

This code will generate the warning, “Don’t put any logic in createState”.

Falchio Over a year ago

All magic in @override void didUpdateWidget. Its update Widget after build state. (see: flutterbyexample.com/lesson/stateful-widget-lifecycle). Doesn't look like good practice.

Farrukh Najmi Over a year ago

Where is AppTheme defined? An explanation of key parts of code would be nice.

alabra · Accepted Answer · 2021-02-16 19:17:09Z

4

You need implement "equals" in class AssignedWorkspace. I used equatable package.

Example class AssignedWorkspace

class AssignedWorkspace extends Equatable {
  final String id;
  final String name;

  AssignedWorkspace(this.id, this.name);
   
  @override
  List<Object> get props => [id];
}

answered Feb 16, 2021 at 19:17

alabra

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3 Comments

Mehrdad Over a year ago

i didn't unsderstamd why do youd did that?please explain more

alabra Over a year ago

@Mehrdad because when assigning instance class as a value, then you must overload the equals to correctly evaluate if two instances are equal. with this you don't need to have another state (like currentItem in the top voted example). Only instances of the AssignedWorkspace class.

Manoranjan Over a year ago

That's a nice answer, actually solving the issue from the root level. Thanks :)

User09061968 · Accepted Answer · 2020-04-17 13:00:14Z

1

I changed the value of the dropdown var to 1 initially var _value = '1'; So when the dropdown button has to display its value it displays the one whose value I have set 1 as in the items list in DropDownButton

DropdownButton(
                underline: Container(),
                onChanged: (value) {
                  setState(() {
                    _value = value;
                  });
                },
                value: _value,
                items: [
                  DropdownMenuItem(
                    value: "1",
                    child: Row(
                      mainAxisAlignment: MainAxisAlignment.spaceAround,
                      children: <Widget>[
                        Icon(MaterialCommunityIcons.devices),
                        SizedBox(width: 10),
                        Text(
                          "Consumption",
                          style: TextStyle(
                              fontSize: 18.0, fontWeight: FontWeight.w600),
                        ),
                      ],
                    ),
                  ),
                  DropdownMenuItem(
                    value: "2",
                    child: Row(
                      mainAxisAlignment: MainAxisAlignment.spaceAround,
                      children: <Widget>[
                        Icon(MaterialCommunityIcons.solar_panel),
                        SizedBox(width: 10),
                        Text(
                          "Generation",
                          style: TextStyle(
                              fontSize: 18.0, fontWeight: FontWeight.w600),
                        ),
                      ],
                    ),
                  ),
                ],
              ),

answered Apr 17, 2020 at 13:00

User09061968

436 bronze badges

2 Comments

Kamal Over a year ago

Please can you add your questions the name of the SDK ?

User09061968 Over a year ago

Its flutter SDK

sandeep gurram · Accepted Answer · 2021-05-08 15:54:50Z

0

For me id of one of the element is null, once added id is made non-null issue got fixed.

answered May 8, 2021 at 15:54

sandeep gurram

1111 silver badge2 bronze badges

1 Comment

Pointer Over a year ago

Can you explain, or put some example when is one of element null

ack ismayil · Accepted Answer · 2021-06-18 17:33:09Z

0

if you want to see only an initial value you can use hint text named parameter of drop down button and set a text widget. i dont know whether it is a good practice or not.

answered Jun 18, 2021 at 17:33

ack ismayil

313 bronze badges

Comments

reza purwowidodo · Accepted Answer · 2023-11-21 02:21:19Z

-2

 DropdownButtonFormField(
          value: '12',
          isExpanded: true,
          icon: const HeroIcon(
            HeroIcons.chevronDown,
            // solid: true,
            color: Color(0xff656565),
            size: 30,
          ),
          decoration: InputDecoration(
            fillColor: Colors.white,
            contentPadding: const EdgeInsets.fromLTRB(15, 0, 15, 0),
            border: OutlineInputBorder(
              borderRadius: BorderRadius.circular(8),
            ),
            focusedBorder: OutlineInputBorder(
              borderRadius: BorderRadius.circular(8),
              borderSide: const BorderSide(
                color: Colors.grey,
              ),
            ),
            enabledBorder: OutlineInputBorder(
              borderRadius: BorderRadius.circular(8),
              borderSide: const BorderSide(
                color: Colors.grey,
              ),
            ),
          ),
          style: tBodyTextNew,
          onChanged: (value) {
            print(value);
            controller.provinsi.value = int.parse(value.toString());
            kabupaten.kabupaten(req: value);
          },
          items: controller.listProvice.map((value) {
            return DropdownMenuItem<String>(
              value: value.idwilayahProvinsi.toString(),
              child: Text(
                value.namaProvinsi,
                style: tBodyTextNew,
              ),
            );
          }).toList(),
        ),

convert value to string

answered Nov 21, 2023 at 2:21

reza purwowidodo

1

1 Comment

Community Over a year ago

As it’s currently written, your answer is unclear. Please edit to add additional details that will help others understand how this addresses the question asked. You can find more information on how to write good answers in the help center.

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