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 · 2019-09-10T11:28:37.037

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;
                },
              )

This code will generate the warning, “Don’t put any logic in createState”. — mauricev, Dec 10 '22 at 23:48
All magic in `@override void didUpdateWidget`. Its update Widget after build state. (see: https://flutterbyexample.com/lesson/stateful-widget-lifecycle). Doesn't look like good practice. — Falchio, May 31 '23 at 11:11
Where is AppTheme defined? An explanation of key parts of code would be nice. — Farrukh Najmi, Aug 18 '23 at 19:24

score 2 · Answer 2 · answered Feb 16 '21 at 19:17

2

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 '21 at 19:17

alabra

131
1
4

i didn't unsderstamd why do youd did that?please explain more – Mehrdad Apr 04 '21 at 08:02
@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. – alabra Apr 14 '21 at 20:57
That's a nice answer, actually solving the issue from the root level. Thanks :) – Manoranjan May 06 '21 at 19:28

score 0 · Answer 3 · answered Apr 17 '20 at 13:00

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),
                        ),
                      ],
                    ),
                  ),
                ],
              ),

Please can you add your questions the name of the SDK ? – Kamal Apr 17 '20 at 16:54 — Kamal, Apr 17 '20 at 16:54

score 0 · Answer 4 · answered May 08 '21 at 15:54

0

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

answered May 08 '21 at 15:54

sandeep gurram

111
1
2

Can you explain, or put some example when is one of element null – Pointer Aug 04 '21 at 14:59

score 0 · Answer 5 · answered Jun 18 '21 at 17:33

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 '21 at 17:33

ack ismayil

31
3

Set default value for dropdown button in flutter

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