⌘ kean.blog

Introducing CreateAPI


If you’ve tried OpenAPI spec generators, you know how it goes. They get you about 60-80% there, but you end up having to modify the code by hand. For one of the specs that I tested – GitHub REST API spec – a popular code generator produces more than 300 compile-time errors, which is not ideal. With CreateAPI, I pushed well beyond just making sure the generated code compiles.

  • It’s Fast: processes specs with 100K lines of YAML in less than a second
  • Reliable: tested on 1KK lines of publicly available OpenAPI specs producing correct code every time
  • Smart: generates Swift code that looks like it’s carefully written by hand
  • Customizable: offers a ton of customization options

I started with a powerful foundation: OpenAPIKit by Mathew Polzin. It takes care of the parsing and validation, so CreateAPI can focus purely on code generation.

Snapshot Testing #

I went to the extreme when testing CreateAPI. I wrote a simple snapshot testing utility that takes a couple of hundred publicly available OpenAPI specs from APIs Guru and other sources and feeds them to CreateAPI. It then makes sure that the generated code doesn’t miss any schemas, paths, or properties from the input specs, and that all of the generated code compiles successfully.

One of the main examples I used was GitHub REST API spec. It’s a massive spec with more than 70K lines of YAML. For this spec, I went further than just making sure it compiles. I went through all (or almost all) paths and entities making sure they match the documentation. I also wrote unit tests that take mock JSONs provided by GitHub and make sure the Codable models handle them correctly at runtime. I’m planning to release the generated code as a separate framework – OctoKit.

Optimizations #

I just have to talk about performance. CreateAPI is optimized to run on multicore processors like M1 Pro/Max. There is parallelization at every stage; even parsing is partially parallelized. And code generation saturates all available code. As a result, it processes 100K lines of OpenAPI specs in less than a second.

CreateAPI was built entirely on a 10-Core M1 Pro, which made it feasible to work with these massive test inputs: 1KK of OpenAPI specs and 500K of generated Swift code. This beast compiles 500K lines of Swift code (heavily modularized) in less than 2 minutes – absolutely insane.

Code Generation #

And finally, let’s talk about some of the optimizations CreateAPI does to the produced code. Here are just a few examples:

Swifty Booleans #

Uses of Boolean methods and properties should read as assertions about the receiver when the use is nonmutating, e.g. x.isEmpty

From https://www.swift.org/documentation/api-design-guidelines/

Unfortunately, it’s uncommon for web APIs to follow this convention. This is why CreateAPI automatically generates Swifty property names for you.

  type: boolean


public var isVisible: Bool

It’s also smart enough not to add is when it’s not needed:

  type: boolean
public var hasIssues: Bool

For every “smart” feature, CreateAPI has an option to either fully disable it, or add exceptions. For example, you can rename anything in the generated coded: properties, types, enums, etc:

  # Rename properties, example:
  #   - name: firstName
  #   - SimpleUser.name: firstName
  properties: {}

Abbreviations #

CreateAPI automatically capitalizes common abbreviations, making them look more “Swifty”.


public var repoId: String
public var repoUrl: URL


public var repoID: String
public var repoURL: URL

Ordering Properties #

Most generators ignore the original order of properties from the specs. Yet they are often designed with a particular order in mind. CreateAPI preserves the order of the properties (thanks to the ordered dictionaries in OpenAPIKit).

CreateAPI also has an option to order properties alphabetically.

Removing Noise #

Readability is important. CreateAPI makes sure that the generated code is clean and concise. For example, most generators add all enum case names by default – it’s just simpler to implement it this way:

public enum Status: String, Codable, Equatable, CaseIterable {
    case placed = "placed"
    case approved = "approved"
    case delivered = "delivered"

CreateAPI doesn’t add redundant names. It’s a small thing, but all these small things add up:

public enum Status: String, Codable, CaseIterable {
    case placed
    case approved
    case delivered

Another example is requests and queries. For simple requests, CreateAPI doesn’t generate a dedicated class or struct to represent a request and generates a compact version instead. It significantly reduces the amount of generated code.

public func post(accessToken: String) -> Request<OctoKit.Reaction>
    .post(path, body: ["accessToken": accessToken])

The same applies to simple URL queries:

private func makeGetQuery(_ perPage: Int, _ cursor: String) -> [(String, String?)] {
    [("per_page", String(perPage)), ("cursor", cursor))]

Inlining Typealiases #

Authors of OpenAPI specs often define arrays with items defined inline:

  title: Search Result Text Matches
  type: array
    type: object
        type: string
        nullable: true
        type: string

Many tools fails to produce anything useful for these specs. Here’s one example:

public typealias SearchResultTextMatches = [SearchResultTextMatches]

It doesn’t compile - the element definition is completely missing and the typealias refers to itself.

In that situation, CreateAPI uses multiple techniques that together produce the following output:

public struct SearchResultTextMatch: Codable {
    public var objectURL: String?
    public var objectType: String?
    public var property: String?
    // ...

    public init(objectURL: String? = nil, objectType: String? = nil, ...) {
        // ...

    private enum CodingKeys: String, CodingKey {
        case objectURL = "object_url"
        case objectType = "object_type"
        case property
        // ...

Here is what happens under the hood:

  1. CreateAPI encounts a schema named "search-result-text-matches" of an "array" type.
  2. It checks the type of the element – it’s an anonymous object defined inline.
  3. It generates a struct declaration for the element schema and names it SearchResultTextMatch - a singularized form of "search-result-text-matches".
  4. It skips adding a typealias because it can be inlined later (can be disabled with isInliningTypealiases)

When the "search-result-text-matches" schema is used as a reference anywhere else in the spec, CreateAPI inlines the typealias value:

// Usage
public struct TopicSearchResultItem: Codable {
    public var textMatches: [SearchResultTextMatch]
    // ...

Comments #

CreateAPI adds all comments and examples from the spec, but it’s also smart enough not to add redundant comments.

For example, in the following schema, the title matches the description, so it can keep only the title. But the title also matches the name of the entity, so the comments are completely redundant.

  title: Validation Error
  description: Validation Error
  type: object
    - message
    - documentation_url
public struct ValidationError: Codable {
    public var message: String
    public var documentationURL: String
    // ...

Edge Cases #

After going through 1KK lines of OpenAPI specs, I hit a ton of edge cases. I just want to add one example. Here’s what I found in Telegram Bot spec:

  default: 🎲
  type: string
    - 🎲
    - 🎯
    - 🏀
    - 🎰

None of the generators I tried were able to handle it correctly. The problem is that a naive approach doesn’t work because you can’t use emojis as enum cases in Swift. CreateAPI uses String Unicode APIs to generate the following case names:

public enum Emoji: String, Codable, CaseIterable {
    case gameDie = "🎲"
    case directHit = "🎯"
    case basketballAndHoop = "🏀"
    case soccerBall = "⚽"
    case slotMachine = "🎰"

Pre-Release #

I pushed the first pre-release version of CreateAPI, and it joined the rest of the frameworks for working with web APIs:

  • Nuke - image loading and caching
  • NukeUI - UI components for image loading
  • Get - web API client built using async/await
  • CreateAPI - code generator for OpenAPI specs
  • Pulse - network logger and inspector
  • URLQueryEncoder - URL query encoder based on Codable
  • NaiveDate - working with dates without timezones
  • HTTPHeaders - simple handling of HTTP response headers

There are still a ton of improvements in the backlog, and I would also appreciate any community contributions!