Implementing OpenAPI 3.0 in Dart

Given an OpenAPI 3.0 spec, implement a type-safe Dart API layer with:

• Freezed (or Equatable) for models
• Dio for HTTP with interceptors and error handling
• json_serializable for JSON serialisation

Process

1. Read the OpenAPI spec — parse components/schemas for models, paths for endpoints.
2. Map schemas → Dart models (Freezed or Equatable — see Model Strategy).
3. Map paths → Dio service methods.
4. Add a repository layer that wraps the service.
5. Wire error handling via DioException → domain ApiError.

Dependencies

dependencies:
  dio: ^5.9.2
  freezed_annotation: ^3.1.0   # if using Freezed
  json_annotation: ^4.11.0
  equatable: ^2.0.5             # if using Equatable

dev_dependencies:
  build_runner: ^2.14.1
  freezed: ^3.2.5               # if using Freezed
  json_serializable: ^6.13.1

Directory Structure

lib/
└── data/
    ├── network/
    │   ├── dio_client.dart          # Dio instance + interceptors
    │   ├── api_error.dart           # global error model
    │   └── services/
    │       └── product_service.dart # one file per resource tag
    └── models/
        ├── product.dart
        └── ...

Dio Client Setup

import 'package:dio/dio.dart';

Dio createDioClient({required String baseUrl, String? accessToken}) {
  final dio = Dio(BaseOptions(
    baseUrl: baseUrl,
    connectTimeout: const Duration(seconds: 10),
    receiveTimeout: const Duration(seconds: 10),
    headers: {'Content-Type': 'application/json'},
  ));

  dio.interceptors.add(InterceptorsWrapper(
    onRequest: (options, handler) {
      if (accessToken != null) {
        options.headers['Authorization'] = 'Bearer $accessToken';
      }
      handler.next(options);
    },
    onError: (error, handler) {
      // Convert DioException to domain ApiError before propagating
      handler.next(error);
    },
  ));

  return dio;
}

Model Strategy

Freezed Model (complex / union types)

// OpenAPI: components/schemas/Product
import 'package:freezed_annotation/freezed_annotation.dart';

part 'product.freezed.dart';
part 'product.g.dart';

@freezed
sealed class Product with _$Product {
  const factory Product({
    required String id,
    required String title,
    required double price,
    @JsonKey(name: 'created_at') DateTime? createdAt, // snake_case from spec
  }) = _Product;

  factory Product.fromJson(Map<String, dynamic> json) =>
      _$ProductFromJson(json);
}

Equatable Model (simple DTO)

// OpenAPI: components/schemas/CreateProductRequest
import 'package:equatable/equatable.dart';
import 'package:json_annotation/json_annotation.dart';

part 'create_product_request.g.dart';

@JsonSerializable()
class CreateProductRequest extends Equatable {
  const CreateProductRequest({required this.title, required this.price});

  final String title;
  final double price;

  Map<String, dynamic> toJson() => _$CreateProductRequestToJson(this);

  @override
  List<Object?> get props => [title, price];
}

Type Mapping

Primitives

Nullability

• Property in required array → non-nullable (String id)
• Property NOT in required → nullable (String? name)

Enum

// OpenAPI: type: string, enum: [active, draft, archived]
@JsonEnum(fieldRename: FieldRename.snake)
enum ProductStatus { active, draft, archived }

oneOf / Union Types → Freezed sealed

// OpenAPI: oneOf: [{$ref: Cat}, {$ref: Dog}]
@freezed
sealed class Pet with _$Pet {
  const factory Pet.cat(Cat data) = _PetCat;
  const factory Pet.dog(Dog data) = _PetDog;

  factory Pet.fromJson(Map<String, dynamic> json) => _$PetFromJson(json);
}

// Exhaustive pattern matching at the call site:
switch (pet) {
  case Pet(:final cat): ...
  case Pet(:final dog): ...
}

$ref Resolution

{"$ref": "#/components/schemas/Product"} → use Product class directly. Never inline duplicate definitions.

Service Layer (Dio)

One service class per OpenAPI tag. Method signatures follow: {httpMethod}{Resource}.

// Covers: GET /products, POST /products, GET /products/{id}
class ProductService {
  const ProductService(this._dio);
  final Dio _dio;

  Future<List<Product>> getProducts({int page = 1, int limit = 20}) async {
    final response = await _dio.get<List<dynamic>>(
      '/products',
      queryParameters: {'page': page, 'limit': limit},
    );
    return response.data!.map((e) => Product.fromJson(e as Map<String, dynamic>)).toList();
  }

  Future<Product> getProduct(String id) async {
    final response = await _dio.get<Map<String, dynamic>>('/products/$id');
    return Product.fromJson(response.data!);
  }

  Future<Product> createProduct(CreateProductRequest request) async {
    final response = await _dio.post<Map<String, dynamic>>(
      '/products',
      data: request.toJson(),
    );
    return Product.fromJson(response.data!);
  }
}

Error Handling

Define a global error model matching the spec's error schema:

@freezed
sealed class ApiError with _$ApiError {
  const factory ApiError({
    required int status,
    required String message,
    String? detail,
  }) = _ApiError;

  factory ApiError.fromJson(Map<String, dynamic> json) =>
      _$ApiErrorFromJson(json);
}

Wrap Dio calls in the repository to catch DioException:

class ProductRepository {
  const ProductRepository(this._service);
  final ProductService _service;

  Future<List<Product>> fetchProducts({int page = 1}) async {
    try {
      return await _service.getProducts(page: page);
    } on DioException catch (e) {
      if (e.response?.data != null) {
        throw ApiError.fromJson(e.response!.data as Map<String, dynamic>);
      }
      rethrow;
    }
  }
}

Naming Conventions

Constraints

• No `!` operator: rely on nullability — if spec says required, make it non-nullable.
• Never duplicate `$ref` schemas: reference the class; do not inline.
• Use `@JsonKey(name: ...)` for naming mismatches — do not rename the JSON key, rename the Dart field.
• Equatable vs Freezed: default to Freezed for domain models; use Equatable for pure request DTOs that don't need copyWith.
• DioException always caught at repository level — services throw raw, repositories map to ApiError.
• Run dart run build_runner build --delete-conflicting-outputs after generating models.

Old Patterns (pre-Freezed 3)

Freezed 2.x used abstract class instead of sealed class. Both patterns still work, but prefer sealed for exhaustive pattern matching with Dart 3.

# Freezed 2.x (no longer recommended)
dependencies:
  freezed_annotation: ^2.4.1

dev_dependencies:
  freezed: ^2.5.2