Flutter error "type 'String' is not a subtype of type '((String?) => String?)?'" when using a reusable textformfield

Question

I am creating a reusable TextFormFields with a reusable validations for its validator. However I am facing an error which is type 'String' is not a subtype of type '((String?) => String?)?

Here is the code for my reusable TextFormFields

class FormFields {
  static textFormFieldWidget({
    TextEditingController? controller,
    String? hintText,
    String? helpText,
    IconData? prefixIcon,
    IconData? suffixIcon,
    bool? isPassword,
    bool? enabled,
    bool? readOnly,
    Color? borderColor,
    Color? borderSide,
    Color? colorFilled,
    FocusNode? textFieldFocus,
    TextStyle? hintStyle,
    String? labelText,
    TextStyle? labelStyle,
    TextInputType? textInputType,
    double rightValue = 0,
    double leftValue = 0,
    dynamic validateFunction,
  }) {
    return Padding(
      padding: EdgeInsets.only(right: rightValue, left: leftValue),
      child: TextFormField(
        keyboardType: textInputType,
        autovalidateMode: AutovalidateMode.onUserInteraction,
        controller: controller,
        readOnly: readOnly == null ? false : true,
        obscureText: isPassword == null ? false : true,
        decoration: InputDecoration(
          fillColor: colorFilled,
          filled: true,
          focusedBorder: OutlineInputBorder(
            borderRadius: const BorderRadius.all(Radius.circular(10.0)),
            borderSide: BorderSide(
                color: borderSide ?? const Color(0xFF6949FF), width: 1.15),
          ),
          enabledBorder: OutlineInputBorder(
            borderRadius: const BorderRadius.all(Radius.circular(10.0)),
            borderSide: BorderSide(
              color: borderSide ?? const Color(0xFFCDD1E0),
              width: 1.15,
            ),
          ),
          labelText: labelText ?? '',
          labelStyle: labelStyle ?? const TextStyle(),
          // dont forget this line
          hintText: hintText ?? '',
          hintStyle: hintStyle ?? const TextStyle(),
          helperText: helpText ?? '',
          prefixIcon: null == prefixIcon ? null : Icon(prefixIcon),
          suffix: null == suffixIcon ? null : Icon(suffixIcon),
          enabled: null == enabled ? true : false,
        ),
        focusNode: textFieldFocus,
        textAlignVertical: TextAlignVertical.center,
        validator: validateFunction,
      ),
    );
  }
}

and I am trying to implement a EmailValidator for it using this code.

class EmailValidator {
  static String? validate(String? value) {
    bool emailValid = 
    RegExp(r"^[a-zA-Z0-9.a-zA-Z0-9.!#$%&'*+-/=?^_`{|}~]+@[a-zA-Z0-9]+\.[a-zA-Z]+")
      .hasMatch(value!);
    // return value==null ||  value.isEmpty ? "Email can't be empty" : null;
    if(value.isEmpty) {
      return "E-mail field is empty, please type your email.";
    }
    if(!emailValid){
      return "The email is invalid, please check carefully!";
    }
    return null;
  }
}

and I am calling it the FormFields.textFormFieldWidget and inside of my login_page.dart

FormFields.textFormFieldWidget(
                          textInputType: TextInputType.emailAddress,
                          leftValue: 2.w,
                          rightValue: 2.w,
                          borderColor: ColourPalette.primaryColor,
                          borderSide: ColourPalette.secondaryColor,
                          colorFilled: colorEmailField,
                          controller: userEmail,
                          textFieldFocus: userEmailFocusNode,
                          helpText:
                              "Type your email that you've registered with us!",
                          hintStyle: CustomStyleText.reusableTextStyle(
                              customColor: Colors.black.withOpacity(0.5),
                              customFontSize: 14,
                              customFontWeight: FontWeight.normal),
                          hintText: "Enter your email.",
                          labelStyle: CustomStyleText.reusableTextStyle(
                            customColor: Colors.black.withOpacity(0.5),
                            customFontSize: 14,
                            customFontWeight: FontWeight.normal,
                          ),
                          labelText: "Email",
                          // here is my validate function and it does not work and it gives me
                          // the error
                          validateFunction:
                              EmailValidator.validate(userEmail.text)),

Answer 1

The validateFunction needs a function and not a String.

Change this:

validateFunction: EmailValidator.validate(userEmail.text)

to this:

validateFunction: EmailValidator.validate