Swagger/OpenAPI Generation for Swift (iOS/macOS)

Developing mobile applications for iOS and macOS often requires integration with REST APIs. Manually writing the networking layer is a time-consuming and error-prone process. Code generation tools based on OpenAPI (Swagger) specifications allow you to automatically create typed Swift models, network requests, and even full-fledged API clients. This speeds up development, reduces the number of bugs, and simplifies code maintenance when the API changes.

In this article, we will review the main tools for generating Swift code from OpenAPI specifications, show examples of their use, and provide recommendations for integration into iOS/macOS projects.

Main Generation Tools

Today, there are several popular solutions that support Swift as a target language:

OpenAPI Generator — the most popular open-source tool. Supports many languages, including Swift 5 and Combine.
Swagger Codegen — the predecessor of OpenAPI Generator, still supported but less flexible.
Create API — a generator from Apple, built into Xcode 15+, for creating clients from OpenAPI specifications.

Let's look at each of them in more detail.

OpenAPI Generator for Swift

OpenAPI Generator is a universal tool that can be used as a CLI, Docker image, or plugin for Gradle/Maven. Several generators are available for Swift:

swift5 — generation in Swift 5 with support for Codable, URLSession, and Combine.
swift5-deprecated — an older version for backward compatibility.

Installation and Usage

The easiest way is to use Docker:

docker run --rm -v "${PWD}:/local" \  openapitools/openapi-generator-cli generate \  -i /local/spec.yaml \  -g swift5 \  -o /local/SwiftClient

After execution, a generated Xcode project with models, API requests, and tests will appear in the SwiftClient folder.

Example of Generated Code

Suppose we have a specification with an endpoint for getting a user:

openapi: 3.0.0info:  title: User API  version: 1.0.0paths:  /users/{id}:    get:      operationId: getUserById      parameters:        - name: id          in: path          required: true          schema:            type: integer      responses:        '200':          description: OK          content:            application/json:              schema:                $ref: '#/components/schemas/User'components:  schemas:    User:      type: object      properties:        id:          type: integer        name:          type: string        email:          type: string      required:        - id        - name        - email

OpenAPI Generator will create the following Swift model:

import Foundation

public struct User: Codable {    public let id: Int    public let name: String    public let email: String

    enum CodingKeys: String, CodingKey {        case id        case name        case email    }}

And the corresponding API method:

import Foundationimport Combine

public class UserAPI {    public static func getUserById(id: Int) -> AnyPublisher<User, Error> {        var request = URLRequest(url: URL(string: "https://api.example.com/users/\\(id)")!)        request.httpMethod = "GET"        return URLSession.shared.dataTaskPublisher(for: request)            .map(\\.data)            .decode(type: User.self, decoder: JSONDecoder())            .eraseToAnyPublisher()    }}