A ValueNotifier that validates a value against a collection of customizable rules.

Create a rule

You can create your own set of rules by inheriting ValidationRule<T>

Let's say we create a rule that validates inputs that are required callled RequiredRule

import 'package:validation_notifier/validation_notifier.dart';

class RequiredRule<T extends Object> extends ValidationRule<T> {
  RequiredRule({String errorMessage = 'This field is required'}) : super(errorMessage);

  void validate(T? value) {
    isValid = value != null;

    if (value is String?) {
      final strValue = value as String?;
      isValid = strValue?.isNotEmpty ?? false;

Another rule that validates string inputs requiring them to have a email format called EmailFormatRule

class EmailFormatRule extends ValidationRule<String> {
  EmailFormatRule({String errorMessage = 'Invalid email format'}) : super(errorMessage);

 static final emailRegex = RegExp(

  void validate(String? value) {
    if (value == null) {
      isValid = false;
    } else {
      isValid = emailRegex.hasMatch(value);

Add the rules

You can add your previously created rules to a ValidationNotifier<T>

final email = ValidationNotifier<String>(rules: [

Use email in a ValueListenableBuilder

  valueListenable: email,
  builder: (context, value, child) {
    return TextFormField(
      onChanged: (v) => email.valueToValidate = v,
      keyboardType: TextInputType.emailAddress,
      autocorrect: false,
      decoration: InputDecoration(
        labelText: 'Email',
        errorText: value.errorMessage,

Call ValidationNotifier<T>.validate() to validate valueToValidate using the rules you defined.

// A callback of a button for logging-in

void onLogin() {
  final validationResult = email.validate();

  if (evalidationResult == ValidationState.valid) {
    // Valid
  } else {
    // Invalid

Important Notes

  • When overriding ValidationRule.validate(), you must set the value of isValid here.

  • When a rule is invalid, ValidationNotifier<T>.value will be set and other calls to remaining rules will be canceled.

  • The sequence of the rules to be called during validation is determined by the sequence of the rules in ValidationNotifier<T> constructor.