Form Validator

This package is intended to help create informative validation for flutter form fields.


The FormValidator class is inteneded to be used to chain validation functions together to produce one validator function as required by Widgets such as TextFormField. The final closure using the build method matches the type required by the validator attribute for form fields 'String? Function(String?)'.

Getting started

To use this pacakge add text_form_validator as a package in your pub file.


import 'package:text_form_validator/form_validator.dart';
// Basic, the field can't be null or empty
TextFormField(validator: FormValidator.required().build());

// All the messages which will be returned from validation can't be customised
// as required. Each validator is run one after the other, so the message
// to the end user can be as specific as possible
TextFormField(validator: FormValidator.builder()
    .notNull(nullError: "Woah enter something!")

// More complex validation can also be added on the fly
String customErorr = "Number must be a multiple of 100!";

ValidationFn validator = FormValidator.builder()
    .notNull(nullError: "Woah enter something!")
    // If it hits the custom we can assume the types will convert properly
    .custom((p0) => (p0 == null || (num.parse(p0) % 100) != 0) ? customErorr : null)

TextFormField(validator: validator);

Current Default Functions

Below is a list of all current default functions provided, as specified these are here to assist the user, and may not be exhaustive, see FormValidator.custom for adding custom functions. The descriptions provided here are breif, check the functions themselves in utils/string_validation_functions.dart for full details.

Default Functions List

  • notNull: Not equal to null
  • notEmpty: Not equal to ""
  • required: Combines notNull and notEmpty
  • isNumeric: num.tryParse != null
  • isDateTime: The value can be formatted into a date (can specify custom format)
  • equals: == provided value
  • notEquals: != provided value
  • lengthGt: value.length > providedValue
  • lengthGtEq: value.length >= providedValue
  • lengthLt: value.length < providedValue
  • lengthLtEq: value.length <= providedValue
  • lengthEq: value.length == providedValue
  • inList: The value is in the provided list
  • notInList: The value is not in the provided list
  • matches: The value matches the supplied regex pattern
  • isAnEmail: The value matches the StraingValidationFunctions.emailRegExp pattern

Additional information

The benefit of the chaining of functions allow user messages to be as customized as possible.

For StraingValidationFunctions errors are provided in line. This allows for errors like in StraingValidationFunctions.equals where it specifies the equal value in the default error.


form_validator provides a way of composing different validation functions into one result for use in TextFormFields