Standalone (Dart-only)

You don't need Flutter to use code generation. This page shows how to set up and use validasi_gen in a pure Dart project — CLI tools, servers, or backend applications.

Project setup

dart pub add validasi validasi_annotation
dart pub add dev:build_runner dev:validasi_gen

No flutter dependency — just Dart.

Model definition

// lib/models/user.dart
import 'package:validasi/validasi.dart';
import 'package:validasi_annotation/validasi_annotation.dart';

part 'user.g.dart';

@ValidateClass()
class User {
  @Validate<String>([Required(), MinLength(2), MaxLength(50)])
  final String username;

  @Validate<String>([MinLength(6)])
  final String password;

  @Validate<int>([MinLength(18)])
  final int age;

  const User({required this.username, required this.password, required this.age});
}

Run the generator:

dart run build_runner build --delete-conflicting-outputs

Validating objects

import 'models/user.dart';

void handleRequest() {
  final user = User(username: 'alice', password: 'secret', age: 25);

  final result = user.validate();

  if (!result.isValid) {
    for (final error in result.errors) {
      // error.rule   = 'MinLength'
      // error.message = 'Password must be at least 6 characters'
      // error.path    = ['password']
    }
    throw ValidationException(result.errors);
  }

  // Use result.data (the validated User instance)
  final valid = result.requireValue();
  saveUser(valid);
}

Validating single fields

When you only need to check one field at a time:

// Check just the username
final usernameResult = user.validateField(UserFields.username);
if (!usernameResult.isValid) {
  print(usernameResult.errors.first.message);
}

// Check just the age
final ageResult = user.validateField(UserFields.age);

Async validation (Dart-only)

import 'package:validasi_annotation/validasi_annotation.dart';

Future<bool> _usernameAvailable(String? username) async {
  if (username == null) return false;
  final taken = await database.lookup(username);
  return !taken;
}

@ValidateClass()
class User {
  @Validate<String>([Required(), MinLength(3), AsyncInline(_usernameAvailable)])
  final String username;

  const User({required this.username});
}

final result = await user.validateAsync();

Continuous generation

During development, use build_runner watch to regenerate on every file change:

dart run build_runner watch --delete-conflicting-outputs

Integration with Dart backends

Framework	Pattern
Shelf	Validate in middleware or handler body; return errors as JSON
Serverpod	Annotate model classes; call validate() before insertRow()
Dart Frog	Validate in route handler; return response.json(errors) on failure
Custom	Validate at API boundary; map ValidationError → your error schema

No special framework setup needed — just call validate() and handle the result.