Reusable Widget - Error with parameter - can not add optional parameter

Question

I am trying to create a reusable widget. But if I am OK to set up all the parameters, I do not want to make all of them mandatory. I just want to use 4 or 5 parameters. The other ones will be optional. I have done some research and try to add [] on the optional parameter, but I am getting an error.

ERROR : The parameter 'parametersValidate' can't have a value of 'null' because of its type, but the implicit default value is 'null'.

If you could help me fixing this it would be appreciated.

class TextFormFieldWidget extends StatefulWidget {

   String hintText;
   Widget prefixIcon;
   String defaultText;
   TextInputType textInputType;
   FocusNode focusNode;
   bool obscureText;
   TextEditingController controller;
   Function functionValidate;
   String parametersValidate;
   TextInputAction actionKeyboard;
   Function onSubmitField;
   Function onFieldTap;

    TextFormFieldWidget(

        this.hintText,
        this.textInputType,
        this.controller,
        [this.parametersValidate],
        this.onSubmitField,
        this.onFieldTap,
        this.defaultText,

      {Key? key,
        required this.actionKeyboard,
        required this.prefixIcon,
        required this.focusNode,
        required this.obscureText,
        required this.functionValidate,
      }
      ) : super(key: key);

  @override
  _TextFormFieldWidgetState createState() => _TextFormFieldWidgetState();
}

class _TextFormFieldWidgetState extends State<TextFormFieldWidget> {
  double bottomPaddingToError = 12;

  @override
  Widget build(BuildContext context) {
    return Theme(
      data: Theme.of(context).copyWith(
        primaryColor: Colors.black,
      ),
      child: TextFormField(
        cursorColor: Colors.black,
       // obscureText: widget.obscureText,
        keyboardType: widget.textInputType,
        textInputAction: widget.actionKeyboard,
      //  focusNode: widget.focusNode,
        style: const TextStyle(
          color: Colors.black,
          fontSize: 16.0,
          fontWeight: FontWeight.w200,
          fontStyle: FontStyle.normal,
          letterSpacing: 1.2,
        ),
        initialValue: widget.defaultText,
        decoration: InputDecoration(
          prefixIcon: widget.prefixIcon,
          hintText: widget.hintText,
          enabledBorder: const OutlineInputBorder(
            borderSide: BorderSide(color: Colors.black),
          ),
          focusedBorder: const OutlineInputBorder(
            borderSide: BorderSide(color: Colors.black),
          ),
          hintStyle: const TextStyle(
            color: Colors.grey,
            fontSize: 14.0,
            fontWeight: FontWeight.w300,
            fontStyle: FontStyle.normal,
            letterSpacing: 1.2,
          ),
          contentPadding: EdgeInsets.only(
              top: 12, bottom: bottomPaddingToError, left: 8.0, right: 8.0),
          isDense: true,
          errorStyle: const TextStyle(
            color: Colors.red,
            fontSize: 12.0,
            fontWeight: FontWeight.w300,
            fontStyle: FontStyle.normal,
            letterSpacing: 1.2,
          ),
          errorBorder: const OutlineInputBorder(
            borderSide: BorderSide(color: Colors.black),
          ),
          focusedErrorBorder: const OutlineInputBorder(
            borderSide: BorderSide(color: Colors.black),
          ),
        ),
        controller: widget.controller,
        validator: (value) {
          if (widget.functionValidate != null) {
            String resultValidate =
            widget.functionValidate(value, widget.parametersValidate);
            if (resultValidate != null) {
              return resultValidate;
            }
          }
          return null;
        },
        onFieldSubmitted: (value) {
          if (widget.onSubmitField != null) widget.onSubmitField();
        },
        onTap: () {
          if (widget.onFieldTap != null) widget.onFieldTap();
        },
      ),
    );
  }
}

String? commonValidation(String value, String messageError) {
  var required = requiredValidator(value, messageError);
  if (required != null) {
    return required;
  }
  return null;
}

String? requiredValidator(value, messageError) {
  if (value.isEmpty) {
    return messageError;
  }
  return null;
}

void changeFocus(
    BuildContext context, FocusNode currentFocus, FocusNode nextFocus) {
  currentFocus.unfocus();
  FocusScope.of(context).requestFocus(nextFocus);
}

Answer 1

Try the following code:

class TextFormFieldWidget extends StatefulWidget {

   String hintText;
   Widget? prefixIcon;
   String? defaultText;
   TextInputType? textInputType;
   FocusNode? focusNode;
   bool? obscureText;
   TextEditingController controller;
   Function? functionValidate;
   String? parametersValidate;
   TextInputAction? actionKeyboard;
   Function onSubmitField;
   Function? onFieldTap;

    TextFormFieldWidget({
        Key? key,
        required this.hintText,
        this.textInputType,
        required this.controller,
        this.parametersValidate,
        required this.onSubmitField,
        this.onFieldTap,
        this.defaultText,
        this.actionKeyboard,
        this.prefixIcon,
        this.focusNode,
        this.obscureText,
        this.functionValidate,
      }) : super(key: key);

  @override
  _TextFormFieldWidgetState createState() => _TextFormFieldWidgetState();
}

class _TextFormFieldWidgetState extends State<TextFormFieldWidget> {
  double bottomPaddingToError = 12;

  @override
  Widget build(BuildContext context) {
    return Theme(
      data: Theme.of(context).copyWith(
        primaryColor: Colors.black,
      ),
      child: TextFormField(
        cursorColor: Colors.black,
       // obscureText: widget.obscureText,
        keyboardType: widget.textInputType,
        textInputAction: widget.actionKeyboard,
      //  focusNode: widget.focusNode,
        style: const TextStyle(
          color: Colors.black,
          fontSize: 16.0,
          fontWeight: FontWeight.w200,
          fontStyle: FontStyle.normal,
          letterSpacing: 1.2,
        ),
        initialValue: widget.defaultText,
        decoration: InputDecoration(
          prefixIcon: widget.prefixIcon,
          hintText: widget.hintText,
          enabledBorder: const OutlineInputBorder(
            borderSide: BorderSide(color: Colors.black),
          ),
          focusedBorder: const OutlineInputBorder(
            borderSide: BorderSide(color: Colors.black),
          ),
          hintStyle: const TextStyle(
            color: Colors.grey,
            fontSize: 14.0,
            fontWeight: FontWeight.w300,
            fontStyle: FontStyle.normal,
            letterSpacing: 1.2,
          ),
          contentPadding: EdgeInsets.only(
              top: 12, bottom: bottomPaddingToError, left: 8.0, right: 8.0),
          isDense: true,
          errorStyle: const TextStyle(
            color: Colors.red,
            fontSize: 12.0,
            fontWeight: FontWeight.w300,
            fontStyle: FontStyle.normal,
            letterSpacing: 1.2,
          ),
          errorBorder: const OutlineInputBorder(
            borderSide: BorderSide(color: Colors.black),
          ),
          focusedErrorBorder: const OutlineInputBorder(
            borderSide: BorderSide(color: Colors.black),
          ),
        ),
        controller: widget.controller,
        validator: (value) {
          if (widget.functionValidate != null) {
            String resultValidate =
            widget.functionValidate(value, widget.parametersValidate);
            if (resultValidate != null) {
              return resultValidate;
            }
          }
          return null;
        },
        onFieldSubmitted: (value) {
          if (widget.onSubmitField != null) widget.onSubmitField();
        },
        onTap: () {
          if (widget.onFieldTap != null) widget.onFieldTap();
        },
      ),
    );
  }
}

String? commonValidation(String value, String messageError) {
  var required = requiredValidator(value, messageError);
  if (required != null) {
    return required;
  }
  return null;
}

String? requiredValidator(value, messageError) {
  if (value.isEmpty) {
    return messageError;
  }
  return null;
}

void changeFocus(
    BuildContext context, FocusNode currentFocus, FocusNode nextFocus) {
  currentFocus.unfocus();
  FocusScope.of(context).requestFocus(nextFocus);
}