Flutter - The argument type 'void Function({required String? value})' can't be assigned to the parameter type 'void Function(String?)?'

TheMortiestMorty
September 19, 2023
234 views
0 votes
2 Answers

I have a function called onServerChanged. This was working fine when I did the below:

void onServerChanged(String? value) {
    //...
}

DropdownButton<String>(
  //...
  onChanged: onServerChanged,
),

However, when I changed the function to this, it stopped working:

void onServerChanged({required String? value}) {
    //...
}

This gives an error pointing the function call in the DropdownButton:

The argument type ‘void Function({required String? value})’ can’t be assigned to the parameter type ‘void Function(String?)?’

I really like forcing myself to write the parameter name whenever I call a function so that refactoring doesn’t create silent errors. I tried adding the parameter to the function call in the DropdownButton but it didn’t work. How would I fix this so I can still use named parameters here?

Tags: dart flutter

Answers

- HachemMohamedMahmoud
- September 12, 2023 at 9:05 pm
- 0 votes
0
you need to pass the value through the onChanged arguments like this
```
  DropdownButton<String>(
          //...
          onChanged:(String? val){
        //to convert the value to string do this
          String stringValue  = val.toString();

         debugPrint(stringValue);

  } ,
  )
```
let me know in the comments if this works for you
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- ImaginaryProgramming
- September 12, 2023 at 11:02 pm
- 0 votes
0
Changing your function signature to use a named parameter instead of a positional one causes the DropdownButton to call it incorrectly, since internally it’s doing onChanged.call(value), whereas your signature would require onChanged.call(value: value).

You can easily fix this by slightly adjusting how you handle the callback. Try something like this:
```
void onServerChanged({required String? value}) {
    //...
}

DropdownButton<String>(
  //...
  onChanged: (value) => onServerChanged(value: value),
),
```
Now, you’re passing a callback that takes a positional value. That callback then calls your onServerChanged function with a named parameter, and everyone’s happy.
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