CODEWITHSUNDEEP
flutterdartgenerics
Dusan Milosevic
Dusan Milosevic

Reputation: 89

type '(String) => void' is not a subtype of type '(dynamic) => void'

This is where I use the dropdown:

 class _JobFunctionState extends State<JobFunction> {
  static const jobList = <String>["Item 1", "Item 2", "Item 3"];
  String dropdownValue = jobList[0];

  @override
  Widget build(BuildContext context) {
    return Dropdown<String>(
      hint: const Text("Choose job function"),
      labelText: "Job Function",
      value: dropdownValue,
      dropdownMenuItemList: jobList
          .map<DropdownMenuItem<String>>(
              (String job) => DropdownMenuItem<String>(
                    value: job,
                    child: Text(job),
                  ))
          .toList(),
      onChanged: (newDropdownValue) {
        setState(() {
          dropdownValue = newDropdownValue;
        });
      },
    );
  }

Here's the full dropdown class:

    class Dropdown<T> extends StatefulWidget {
  final List<DropdownMenuItem<T>> dropdownMenuItemList;
  final ValueChanged<T> onChanged;
  final T value;
  final bool isBorder;
  final double radius;
  final TextStyle? textStyle;
  final Color? color;
  final Widget hint;
  final String labelText;

  const Dropdown(
      {Key? key,
      required this.dropdownMenuItemList,
      required this.onChanged,
      required this.value,
      this.isBorder = true,
      this.radius = 10.0,
      this.textStyle,
      this.color,
      required this.hint,
      required this.labelText})
      : super(key: key);

  _DropdownState<T> createState() => _DropdownState();
}



 class _DropdownState<T> extends State<Dropdown> {
  @override
  Widget build(BuildContext context) {
    return FormField<T>(
      builder: (FormFieldState<T> state) {
        return SingleChildScrollView(
          child: DropdownButtonFormField<T>(
            isExpanded: true,
            itemHeight: 50.0,
            items: widget.dropdownMenuItemList as List<DropdownMenuItem<T>>,
            onChanged: widget.onChanged,
            value: widget.value,
            dropdownColor: Colors.white,
            iconEnabledColor: Colors.grey,
            icon: const Icon(Icons.arrow_drop_down),
            hint: widget.hint,
          ),
        );
      },
    );
  }
}

and ValueChanged is:

typedef ValueChanged<T> = void Function(T value);

Dropdown is generic, I've made everything string in the jobFunction widget, and I get this error. if I add dynamic to onChanged parameter(enter image description here), but the type of the parameter should not be dynamic, but String. Any ideas?

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Upvotes: 4

Views: 3694

Answers (3)

Stefano Saitta
Stefano Saitta

Reputation: 2024

Remember also to change the overrided createState method:

@override
_BasicDropdownState<T> createState() => _BasicDropdownState<T>();

Upvotes: 9

Aina27
Aina27

Reputation: 206

I got the same issue and what did work for me was replacing :

class _DropdownState<T> extends State<Dropdown>

by this:

class _DropdownState<T> extends State<Dropdown<T>>

Upvotes: 19

pmatatias
pmatatias

Reputation: 4404

Dropdown is generic, I've made everything string in the jobFunction widget, and I get this error.

ValueChanged should be nullable value. If you specific to a String , then the onchange will specific to String? : means String or null. Because if there is no changes, it must be null.

how to assign the String? to your variable that non-null String is by add a condition like below.

 onChanged: (newDropdownValue) {
       if( newDropdownValue != null){
         setState(() {
          dropdownValue = newDropdownValue!;
        });
      }   
    },

if no changes happend, then you dont need to set new value to you dropdownValue

hope this solve your issue.

Upvotes: 1

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