CODEWITHSUNDEEP
flutterflutter-layoutdart-null-safety
Arijeet
Arijeet

Reputation: 1223

The argument type 'Function?' can't be assigned to the parameter type 'void Function()?'

I have a refactored widget which has an onPressed. But whenever I try to access that function, I get this error :

The argument type Function? can't be assigned to the parameter type void Function()?

Here's the code for the refactored widget:

class DialogBox extends StatelessWidget {
  const DialogBox(
      {@required this.contentTitle, @required this.labelText, this.onpressed});
  final String? contentTitle;
  final String? labelText;
  final Function? onpressed;

  @override
  Widget build(BuildContext context) {
    return new AlertDialog(
      title: Text(
        contentTitle!,
        style: TextStyle(
          color: Theme.of(context).colorScheme.secondary,
        ),
      ),
      content: new SingleChildScrollView(
        child: ListBody(
          children: <Widget>[
            TextFormField(
              decoration: InputDecoration(
                border: OutlineInputBorder(),
                labelText: labelText,
              ),
              keyboardType: TextInputType.text,
            ),
          ],
        ),
      ),
      actions: [
        new TextButton(
          child: new Text(
            'Confirm',
            style: TextStyle(
              color: Theme.of(context).colorScheme.secondary,
            ),
          ),
          onPressed: 
            onpressed, //getting the error here.
         
        ),
        new TextButton(
          child: new Text(
            'Cancel',
            style: TextStyle(
              color: Theme.of(context).colorScheme.secondary,
            ),
          ),
          onPressed: () {
            Navigator.of(context).pop();
          },
        ),
      ],
    );
  }
}

Please someone explain the reason for this problem. Any help will be appreciated.

Upvotes: 1

Views: 776

Answers (2)

John Oyekanmi
John Oyekanmi

Reputation: 386

It's stating that an argument of type Function? is different from type void Function()?. This is because a parameter of type Function? is quite different from that of type void Function()? since the former can have any return value but the later explicitly returns non (note: non is different from null).

To solve this issue: try changing the declaration Function? at the above code from Function? to void Function()?.

like so:

class DialogBox extends StatelessWidget {
   const DialogBox(
    {@required this.contentTitle, @required this.labelText, 
this.onpressed});
final String? contentTitle;
final String? labelText;
final void Function()? onpressed;

 ...
}

Upvotes: 2

Tirth Patel
Tirth Patel

Reputation: 5736

You need to use Function()? data-type instead of Function? for onpressed variable.

final Function()? onpressed;

Also, remove ; and put , in following line:

onPressed: onpressed,

Upvotes: 1

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