validate_kit #

A zero-dependency, fluent form validation toolkit for Dart and Flutter.

Every Flutter developer writes the same validation code over and over: emails, passwords, matching fields, numeric ranges, custom regex. validate_kit turns that repetitive, nested if soup into clean, readable chains that return the first error — or every error when you want them all.

final error = FieldValidator()
    .required('Field cannot be blank')
    .email('Enter a valid email address')
    .custom((val) =>
        val!.contains('admin') ? 'Admin emails not allowed' : null)
    .validate(controller.text);

Demo #

The example app in action:

The example/ directory contains a full registration form and a live validation playground.

Highlights #

• Fluent chaining. Compose rules in the order they should be evaluated.
• Zero dependencies. Pure Dart. Works in Flutter, Dart CLI, and on the server. No transitive packages to audit.
• Short-circuits by default. The chain stops at the first failure, with an opt-in aggregate() mode to collect every error instead.
• Sensible optional fields. Every format rule skips empty values; only required forces a value, so optional fields just work.
• Localization-ready. Override any default message per call or globally via ValidationMessages.
• Drop-in for TextFormField. A FieldValidator is callable and exposes an asFunction tear-off, so it plugs straight into validator:.
• Rich results. validateAll() and validateResult() give you every error and a typed result object.
• Handy extensions. 'a@b.com'.isValidEmail, input.isBlank, and more.
• Fully documented and tested. Public API docs on every member and a broad unit-test suite covering null safety, boundaries, and edge cases.

Installation #

Add the dependency to your pubspec.yaml:

dependencies:
  validate_kit: ^1.0.0

Then run:

flutter pub get   # or: dart pub get

Quick start #

import 'package:validate_kit/validate_kit.dart';

final emailValidator = FieldValidator()
    .required('Email is required')
    .email('Enter a valid email address');

// In a form:
TextFormField(
  controller: emailController,
  validator: emailValidator.asFunction,
);

// Anywhere else:
final error = emailValidator.validate(emailController.text);
if (error != null) {
  // Show `error` to the user.
}

A FieldValidator is callable, so you can also write validator: FieldValidator().required().email().asFunction.

Built-in rules #

Rules that validate a format or length skip empty values, so they are safe to chain on optional fields. Add required() when the field is mandatory.

Common recipes #

Confirm password #

final confirmValidator = FieldValidator()
    .required('Please confirm your password')
    .matches(() => passwordController.text, 'Passwords do not match');

The matches callback is evaluated at validation time, so it always reads the current password.

Strong password #

final passwordValidator = FieldValidator().strongPassword(
  minLength: 10,
  requireSpecialCharacter: false,
);

By default it requires at least 8 characters plus an uppercase letter, a lowercase letter, a number, and a special character. Each requirement can be toggled.

Age / numeric range #

final ageValidator = FieldValidator()
    .required('Age is required')
    .integer('Whole numbers only')
    .numericRange(18, 120, message: 'You must be between 18 and 120');

Custom rule #

final noAdminValidator = FieldValidator().custom(
  (value) =>
      value!.contains('admin') ? 'Admin emails are not allowed' : null,
);

Collecting every error #

By default validate() returns only the first failing message. Enable aggregation to return them all joined by a separator:

final validator = FieldValidator(
  aggregate: true,
  separator: '\n',
)
    .required('Field cannot be blank')
    .email('Enter a valid email address');

// Or configure it fluently:
final same = FieldValidator()
    .required('Field cannot be blank')
    .email('Enter a valid email address')
    .aggregate(separator: ' • ');

For structured access, validateAll() always returns the full list and validateResult() returns a ValidationResult:

final result = validator.validateResult(input);
if (result.isInvalid) {
  for (final message in result.errors) {
    debugPrint(message);
  }
}

Localization #

Every default message lives on ValidationMessages. Override the ones you need once at start-up, then continue to pass per-call overrides where it makes sense:

void main() {
  ValidationMessages.required = 'Este campo es obligatorio.';
  ValidationMessages.email = 'Introduce un correo válido.';
  ValidationMessages.mismatch = 'Los valores no coinciden.';
  runApp(const MyApp());
}

String extensions #

For quick checks outside a form, import the library and use the extensions:

'a@b.com'.isValidEmail;      // true
'https://a.com'.isValidUrl;  // true
'3.14'.isNumeric;            // true
'42'.isInteger;              // true
'Abcd123!'.isStrongPassword; // true
'   '.isBlank;               // true (null-aware)
'ab1'.isAlphanumeric;        // true

// Normalize optional input:
final value = input.trimmedOrNull; // null when blank

You can also delegate to a validator with validateWith:

final error = input.validateWith(
  FieldValidator().required().email(),
);

Single-rule helpers #

When one rule is all you need, Validators returns a plain String? Function(String?):

TextFormField(validator: Validators.email());
TextFormField(validator: Validators.minLength(3, 'Too short'));

Example #

The example/ directory contains a runnable app with a full registration form (every built-in rule in context) and a live playground that validates as you type:

cd example
flutter run

API overview #

• FieldValidator (alias MultiValidator) — the fluent builder.
• ValidationRule — a single immutable rule; extend the package with addRule.
• ValidationResult — isValid, errors, firstError.
• ValidationMessages — overridable default messages.
• Validators — single-rule factory helpers.
• String extensions on String and String?.

Author and Contact #

Developed and maintained by Ebrahim Joy.

• Email: eebrahimjoy@gmail.com
• Website: eebrahimjoy.com
• GitHub: @eebrahimjoy

Contributing #

Please report bugs and request features through the issue tracker. Pull requests are welcome.

License #

Released under the MIT License.