I'm trying to call this widget which contains FormField with it's validator as a variable.

    import 'package:flutter/material.dart';

class InputField extends StatelessWidget {
  final String label, label2;
  final String content, content2;
  var fieldController = TextEditingController();
  void fieldValidator =() {
    
  };

  InputField(
      {this.label,
      this.content,
      this.label2,
      this.content2,
      this.fieldValidator
      });

  @override
  Widget build(BuildContext context) {
    return LayoutBuilder(
      builder: (BuildContext context, BoxConstraints constraints) {
        return Row(
          children: <Widget>[
            Expanded(
              flex: 1,
              child: Container(
                width: 50.0,
                child: Text(
                  "$label",
                  textAlign: TextAlign.left,
                  style: TextStyle(
                    fontWeight: FontWeight.w900,
                    color: Color.fromARGB(255, 255, 255, 255),
                  ),
                ),
              ),
            ),
            SizedBox(
              width: 5.0,
            ),
            Expanded(
              flex: 3,
              child: Container(
                width: MediaQuery.of(context).size.width / 3.7,
                color: Color.fromARGB(255, 255, 255, 255),
                child: TextFormField(
                  controller: fieldController,
                  validator: fieldValidator,
                  style: TextStyle(
                    fontSize: 15.0,
                    color: Colors.black,
                  ),
                  decoration: InputDecoration(
                    contentPadding: EdgeInsets.all(10.0),
                    hintText: "$content",
                    hintStyle: TextStyle(
                        color: Color.fromARGB(255, 190, 190, 190),
                        fontSize: 14),
                    fillColor: Color.fromARGB(255, 0, 0, 0),
                  ),
                ),
              ),
            ),
          ],
        );
      },
    );
  }
}

I'm setting the validator in the widget constractor , and I want to give my own validator function every time I call the widget like this :

InputField( label: "City",
           content: "Please provide us your city name",
         fieldValidator:(value) {
                            if (value == null || value.isEmpty) {
                              return 'This one is required';
                            }
                            return null;
                          },),

I'm getting this error on the first page where I created my widget:

    This expression has a type of 'void' so its value can't be used.
Try checking to see if you're using the correct API; there might be a function or call that returns void you didn't expect. Also check type parameters and variables which might also be void.

CodePudding user response：

the validator should be from the type of FormFieldValidator<String>? instead of void so your validator should look like:

FormFieldValidator<String>? fieldValidator =(fieldValue) { 
  return "result";
  };

CodePudding user response：

If you look at the TextFormField, you will see that validator is defined like this, a function that may receive a String and returns another.

String? Function(String?)? validator

I would suggest that you make the same for your fieldValidator, so that your InputField definition would look like this:

class InputField extends StatelessWidget {
  final String label, label2;
  final String content, content2;
  final String Function(String?)? fieldValidator;

  var fieldController = TextEditingController();
...