Dart / flutter: DropdownButton causes exception when value is changed

Question

I have written a pretty extensive form using DropdownButton and TextField widgets. The concept is that I have a StatefulWidget, where the class of State<StatefulWidget> contains 2 methods that return the widget I want to build. This way I can easily access and use the entered data and pass it along a function to compose an e-mail out of them.

However, when I select an item from the options, the framework throws an exception during the rebuild. I put in some log functions, and it shows that the setState() method successfully saves the value to selectedValue variable.

Widget buildMultiChoiceInputRow(var label, List<String> values) {
    final List<String> options = values.toList();
    selection = options.first;

    final dropDownMenuOptions = options.map((String value) {
      return new DropdownMenuItem<String>(
        value: value,
        child: new Text(value),
      );
    }).toList();

    return new Column(
      children: <Widget>[
        new Row(
          children: <Widget>[
            new Expanded(
              child: new Container(
                  padding:
                      const EdgeInsets.only(left: 5.0, top: 2.0, right: 5.0),
                  child: new Text(label, style: commonInfoCardInfoTextBlack16Bold)),
            ),
          ],
        ),
        new Row(
          children: <Widget>[
            new Expanded(
              child: new Container(
                padding: const EdgeInsets.only(left: 5.0, right: 5.0),
                child: new DropdownButton(
                value: selectedValue,
                items: dropDownMenuOptions,
                onChanged: (selection) {
                  setState(() {
                    selectedValue = selection;
                    switch (label) {
                      case labelVirtualAdoption:
                        tempAdoptionType =
                            composeMultiChoiceAnswer(label, selection);
                            print(selection);
                            print(selectedValue);
                        break;
                          case labelAskedAboutSpecies:
                            tempAskedAboutSpecies =
                                composeMultiChoiceAnswer(label, selection);
                            break;
                          case labelHouseOrFlat:
                            tempHouseOrFlat =
                                composeMultiChoiceAnswer(label, selection);
                            break;
                            ....
                          default:
                            break;
                        }
                      });
                    }),
              ),
            )
          ],
        ),
        new Divider(color: Colors.transparent)
      ],
    );
  }

Here is the exception:

I/flutter (20998): The following assertion was thrown building AdoptionInput(dirty, state: AdoptionInputState#3cc80):
I/flutter (20998): 'package:flutter/src/material/dropdown.dart': Failed assertion: line 481 pos 15: 'value == null ||
I/flutter (20998): items.where((DropdownMenuItem<T> item) => item.value == value).length == 1': is not true.

And here is the stack, showing that the exception is thrown during the rebuild:

I/flutter (20998): #2      new DropdownButton (package:flutter/src/material/dropdown.dart)
I/flutter (20998): #3      AdoptionInputState.buildMultiChoiceInputRow (package:osszefogasaszanhuzokert/adoptionPageUtilities.dart:443:28)
I/flutter (20998): #4      AdoptionInputState.build (package:osszefogasaszanhuzokert/adoptionPageUtilities.dart:639:11)
I/flutter (20998): #5      StatefulElement.build (package:flutter/src/widgets/framework.dart:3730:27)
I/flutter (20998): #6      ComponentElement.performRebuild (package:flutter/src/widgets/framework.dart:3642:15)
I/flutter (20998): #7      Element.rebuild (package:flutter/src/widgets/framework.dart:3495:5)
I/flutter (20998): #8      BuildOwner.buildScope (package:flutter/src/widgets/framework.dart:2242:33)

The problem seems really similar to a former bug in flutter, but if I try to initialize the selection and selectedValue in initState(), the same exception will be thrown right as the form is built for the first time.

What am I missing here?

In the meantime, I found that the exception is thrown when a widget is built that has different values that "igen" "nem" (or I think different from the first one I used) — Zoltán Györkei
– Zoltán Györkei, Commented May 18, 2018 at 5:57

Evgeniy Volk · Accepted Answer · 2018-08-10 08:23:26Z

54

Your "value" for DropdownButton should be set to 'null' or or be one from the values list.

DropdownButton(
      value: null,
      isDense: true,
      onChanged: (String newValue) {
        // somehow set here selected 'value' above whith 
       // newValue
       // via setState or reactive.
      },
      items: ['yellow', 'brown', 'silver'].map((String value) {
        return DropdownMenuItem(
          value: value,
          child: Text(value),
        );
      }).toList(),
    ),

So for my example DropdownButton value should be set to null or be 'yellow' or 'brown' or 'silver'.

answered Aug 10, 2018 at 8:23

Evgeniy Volk

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2 Comments

Jorden Over a year ago

But after make select: null, the selected value is not showing into the dropdown

s.j Over a year ago

yes selected value not shown? is there any solution to fix this issue

vivek yadav · Accepted Answer · 2019-02-03 08:34:23Z

14

Extending above answer with the second case where I was stuck.

Your "value" for DropdownButton should be set to 'null' or be one from the values list.

Your 'values' should also be different in every item value.

for example: Avoid this

 items.add(DropdownMenuItem(
  value: 1.toString(),
  child: Text(1.toString()),
));
items.add(DropdownMenuItem(
  value: 1.toString(),
  child: Text(1.toString()),
));

Avoid repeating the values.

answered Feb 3, 2019 at 8:34

vivek yadav

1,5772 gold badges13 silver badges17 bronze badges

Comments

Rodrigo Ortega · Accepted Answer · 2019-03-22 20:16:36Z

0

A bit off time, but, the code worked when I passed null as the "value" but I was having this problem when I placed a value that was included on the "Items". The problem was that the "Items" had a duplicate value, so it seems that you should provide all different items in the list passed to the "Items".

answered Mar 22, 2019 at 20:16

Rodrigo Ortega

7367 silver badges10 bronze badges

Comments

Rashid Iqbal · Accepted Answer · 2019-08-16 17:25:37Z

here is the right way:

 import 'package:flutter/material.dart';

class RegisterFragments extends StatefulWidget {
  RegisterFragments({Key key, this.step}) : super(key: key);
  final int step;

  _RegisterFragmentsState createState() => _RegisterFragmentsState();
}

class _RegisterFragmentsState extends State<RegisterFragments> {
  Map<String, bool> values = {"abc": false, "def": true, "ghi": false};
  List<String> _do = ['One', 'Two', 'Free', 'Four'];
  String _dropdownValue = 'One';

  @override
  Widget build(BuildContext context) {
    switch (widget.step) {
      case 0:
        return buildDo();
        break;
      case 1:
        return Container(
          child: ListView.builder(
            shrinkWrap: true,
            itemCount: values.length,
            itemBuilder: (BuildContext context, int index) {
              switch (widget.step) {
                case 0:
                  return buildDo();
                  break;
                case 1:
                  return buildService(context, index);
                  break;
                default:
                  return Container();
                  break;
              }
            },
          ),
        );
        break;
      default:
        return Container();
        break;
    }
  }

  Widget buildService(BuildContext context, int index) {
    String _key = values.keys.elementAt(index);

    return Container(
      child: Card(
        child: CheckboxListTile(
          title: Text(_key),
          onChanged: (bool value) {
            setState(() {
              values[_key] = value;
            });
          },
          value: values[_key],
        ),
      ),
    );
  }

  Widget buildDo() {
    return DropdownButton<String>(
      isExpanded: true,
      hint: Text("Service"),
      items: _do.map<DropdownMenuItem<String>>((String value) {
        return DropdownMenuItem<String>(
          value: value,
          child: Text(value),
        );
      }).toList(),
      onChanged: (String newValue) {
        setState(() {
          this._dropdownValue = newValue;
        });
      },
      value: _dropdownValue,
    );
  }
}

list and dorpdown value

   List<String> _do = ['One', 'Two', 'Free', 'Four'];
   String _dropdownValue = 'One';

dropdown items

tems: _do.map<DropdownMenuItem<String>>((String value) {   
  return DropdownMenuItem<String>( ** 
     value: value,
          child: Text(value),
        );
      }).toList(),
      onChanged: (String newValue) {
        setState(() {
          this._dropdownValue = newValue;
        });
      },
      value: _dropdownValue,

Please add comments and highlight the parts that you did differently than poster.

Amandeep · Accepted Answer · 2020-01-29 17:08:55Z

Use var to declare variable instead of String. Now you don't need to set the default value to null.

var dropdownvalue;

DropdownButton<String>(
                    value: dropdownvalue,
                    icon: Icon(Icons.keyboard_arrow_down),
                    iconSize: 28,
                    elevation: 20,
                    onChanged: (String newval){
                      setState((){
                        dropdownvalue = newval;
                      });

                    },
                    items: <String>["Registration","Verification", "ArenaRun"]
                        .map<DropdownMenuItem<String>>((String value){
                      return DropdownMenuItem<String>(
                        value: value,
                        child: Text(value),
                        );
                    }).toList(),
                  ),

atreeon · Accepted Answer · 2021-07-05 16:02:55Z

0

I was doing a value.toString() and the null was being converted to "null"!

answered Jul 5, 2021 at 16:02

atreeon

24.4k14 gold badges95 silver badges117 bronze badges

Comments

Casual Engineer Zombie · Accepted Answer · 2023-03-05 04:34:53Z

0

Although this question was asked four years ago and the Flutter SDK may have changed since then, if someone comes across this issue, one solution is to remove duplicate values in the DropdownMenuItem. Alternatively, you can add key: ValueKey(item) in your DropdownMenuItem widget. For instance, your DropdownButton code should look like this:

DropdownButton(
     value: selectedValue,
     items: dropDownMenuOptions,
     key: ValueKey(selectedValue),
)

Also make sure that the initial value assigned to "value:" for selectedValue is present in the list passed to "items:".

answered Mar 5, 2023 at 4:34

Casual Engineer Zombie

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