Create Item list for DropdownButton in Flutter

Question

I'm back with another issue with the DropdownButton.

The DropdownButton is not enabled. I found this in api.flutter.dev

If the onChanged callback is null or the list of items is null then the dropdown button will be disabled, i.e. its arrow will be displayed in grey and it will not respond to input.

Here is my code now:

return new DropdownButton<String>(
                            hint: new Text("Select Agency"),
                            value: _currentAgency,
                            onChanged: changedDropDownAgency,
                            items: snapshot.data.docs.forEach((document) {
                              return new DropdownMenuItem<String>(
                                value: document.data()['name'],
                                child: new Text(document.data()['name']),
                              );
                            }),
                          );

void changedDropDownAgency(String selected_agency) {
    setState(() {
      _currentAgency = selected_agency;
    });
    globals.selectedAgency = selected_agency;
  }

The forEach loop runs fine and in debug mode I can see that there is data in the document object. I don't know how to debug the DropdownButton code to see why the button is not active. Any suggestions would be helpful. Thanks.

puelo · Accepted Answer · 2021-05-13 13:47:01Z

1

forEach() on Iterables does not return any value (see: https://api.dart.dev/stable/2.10.5/dart-core/Iterable/forEach.html), and thus items is null and the DropdownButton is disabled. Use map instead (https://api.dart.dev/stable/2.10.5/dart-core/Iterable/map.html). Example:

snapshot.data.docs.map<DropdownMenuItem<String>>((document) {
    return new DropdownMenuItem<String>(
        value: document.data()['name'],
        child: new Text(document.data()['name']),
    );
}).toList(),

edited May 13, 2021 at 13:47

answered May 12, 2021 at 22:07

puelo

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10 Comments

LostTexan Over a year ago

thank you for your response. However, when I use your code I get the following: The following _TypeError was thrown building StreamBuilder<QuerySnapshot>(dirty, state: _StreamBuilderBaseState<QuerySnapshot, AsyncSnapshot<QuerySnapshot>>#5bb78): type 'List<dynamic>' is not a subtype of type 'List<DropdownMenuItem<String>>?'

puelo Over a year ago

I've edited my answer, please see if this works. I am explicitly telling dart what the return type of Map should be.

LostTexan Over a year ago

The following assertion was thrown building StreamBuilder<QuerySnapshot>(dirty, state: _StreamBuilderBaseState<QuerySnapshot, AsyncSnapshot<QuerySnapshot>>#b9a5e): There should be exactly one item with [DropdownButton]'s value: Select Agency. Either zero or 2 or more [DropdownMenuItem]s were detected with the same value 'package:flutter/src/material/dropdown.dart': Failed assertion: line 855 pos 15: 'items == null || items.isEmpty || value == null || items.where((DropdownMenuItem<T> item) { return item.value == value; }).length == 1'

LostTexan Over a year ago

return new DropdownButton<String>( //hint: new Text("Select Agency"), value: _currentAgency, onChanged: changedDropDownAgency, items: snapshot.data.docs .map<DropdownMenuItem<String>>((document) { return new DropdownMenuItem<String>( value: document.data()['name'], child: new Text(document.data()['name']), ); }).toList(), );

puelo Over a year ago

_currentAgency seems to have the value of 'Select Agency' and none or 2 or more of your DropdownMenuItems seem to have this value. You should use hint instead and set the initial value (so _currentAgency) to null.

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