OpenAPIKit

A library containing Swift types that encode to- and decode from OpenAPI 3.0.x and OpenAPI 3.1.x Documents and their components.

The single most confusing thing you will grapple with out of the gate is explained by the following grid of what OpenAPIKit versions support which OpenAPI specification versions.

OpenAPIKit	OpenAPI v3.0	OpenAPI v3.1
v2.x	✅	❌
v3.x	✅	✅

Usage
Curated Integrations
Notes
Contributing
Security
Specification Coverage & Type Reference

Usage

Migration

1.x to 2.x

If you are migrating from OpenAPIKit 1.x to OpenAPIKit 2.x, check out the v2 migration guide.

2.x to 3.x

If you are migrating from OpenAPIKit 2.x to OpenAPIKit 3.x, check out the v3 migration guide.

You will need to start being explicit about which of the two new modules you want to use in your project: OpenAPIKit (now supports OpenAPI spec v3.1) and/or OpenAPIKit30 (continues to support OpenAPI spec v3.0 like the previous versions of OpenAPIKit did).

In package manifests, dependencies will be one of:

// v3.0 of spec:
dependencies: [.product(name: "OpenAPIKit30", package: "OpenAPIKit")]

// v3.1 of spec:
dependencies: [.product(name: "OpenAPIKit", package: "OpenAPIKit")]

Your imports need to be specific as well:

// v3.0 of spec:
import OpenAPIKit30

// v3.1 of spec:
import OpenAPIKit

It is recommended that you build your project against the OpenAPIKit module and only use OpenAPIKit30 to support reading OpenAPI 3.0.x documents in and then converting them to OpenAPI 3.1.x documents. The situation not supported yet by this strategy is where you need to write out an OpenAPI 3.0.x document (as opposed to 3.1.x). That is a planned feature but it has not yet been implemented. If your use-case benefits from reading in an OpenAPI 3.0.x document and also writing out an OpenAPI 3.0.x document then you can operate entirely against the OpenAPIKit30 module.

Decoding OpenAPI Documents

Most documentation will focus on what it looks like to work with the OpenAPIKit module and OpenAPI 3.1.x documents. If you need to support OpenAPI 3.0.x documents, take a look at the section on supporting OpenAPI 3.0.x documents before you get too deep into this library's docs.

You can decode a JSON OpenAPI document (i.e. using the JSONDecoder from Foundation library) or a YAML OpenAPI document (i.e. using the YAMLDecoder from the Yams library) with the following code:

import OpenAPIKit

let decoder = ... // JSONDecoder() or YAMLDecoder()
let openAPIDoc = try decoder.decode(OpenAPI.Document.self, from: ...)

Decoding Errors

You can wrap any error you get back from a decoder in OpenAPI.Error to get a friendlier human-readable description from localizedDescription.

do {
  try decoder.decode(OpenAPI.Document.self, from: ...)
} catch let error {
  print(OpenAPI.Error(from: error).localizedDescription)  
}

Encoding OpenAPI Documents

You can encode a JSON OpenAPI document (i.e. using the JSONEncoder from the Foundation library) or a YAML OpenAPI document (i.e. using the YAMLEncoder from the Yams library) with the following code:

let openAPIDoc: OpenAPI.Document = ...
let encoder = ... // JSONEncoder() or YAMLEncoder()
let encodedOpenAPIDoc = try encoder.encode(openAPIDoc)

Validating OpenAPI Documents

Thanks to Swift's type system, the vast majority of the OpenAPI Specification is represented by the types of OpenAPIKit -- you cannot create bad OpenAPI docuements in the first place and decoding a document will fail with generally useful errors.

That being said, there are a small number of additional checks that you can perform to really put any concerns to rest.

let openAPIDoc: OpenAPI.Document = ...
// perform additional validations on the document:
try openAPIDoc.validate()

You can use this same validation system to dig arbitrarily deep into an OpenAPI Document and assert things that the OpenAPI Specification does not actually mandate. For more on validation, see the OpenAPIKit Validation Documentation.

Supporting OpenAPI 3.0.x Documents

If you need to operate on OpenAPI 3.0.x documents and only 3.0.x documents, you can use the OpenAPIKit30 module throughout your code.

However, if you need to operate on both OpenAPI 3.0.x and 3.1.x documents, the recommendation is to use the OpenAPIKit compatibility layer to read in a 3.0.x document and convert it to a 3.1.x document so that you can use just the one set of Swift types throughout most of your program. An example of that follows.

In this example, only one file in the whole project needs to import OpenAPIKit30 or OpenAPIKitCompat. Every other file would just import OpenAPIKit and work with the document in the 3.1.x format.

Converting from 3.0.x to 3.1.x

// import OpenAPIKit30 for OpenAPI 3.0 document support
import OpenAPIKit30
// import OpenAPIKit for OpenAPI 3.1 document support
import OpenAPIKit
// import OpenAPIKitCompat to convert between the versions
import OpenAPIKitCompat

// if most of your project just works with OpenAPI v3.1, most files only need to import OpenAPIKit.
// Only in the file where you are supporting converting from OpenAPI v3.0 to v3.1 do you need the
// other two imports.

// we can support either version by attempting to parse an old version and then a new version if the old version fails
let oldDoc: OpenAPIKit30.OpenAPI.Document?
let newDoc: OpenAPIKit.OpenAPI.Document

oldDoc = try? JSONDecoder().decode(OpenAPI.Document.self, from: someFileData)

newDoc = oldDoc?.convert(to: .v3_1_0) ??
  (try! JSONDecoder().decode(OpenAPI.Document.self, from: someFileData))
// ^ Here we simply fall-back to 3.1.x if loading as 3.0.x failed. You could do a more
//   graceful job of this by determining up front which version to attempt to load or by 
//   holding onto errors for each decode attempt so you can tell the user why the document 
//   failed to decode as neither 3.0.x nor 3.1.x if it fails in both cases.

A note on dictionary ordering

The Foundation library's JSONEncoder and JSONDecoder do not make any guarantees about the ordering of keyed containers. This means decoding a JSON OpenAPI Document and then encoding again might result in the document's various hashed structures being in a different order.

If retaining order is important for your use-case, I recommend the Yams and FineJSON libraries for YAML and JSON respectively. Also keep in mind that JSON is entirely valid YAML and therefore you will likely get good results from Yams decoding of JSON as well (it just won't encode as valid JSON).

The Foundation JSON encoding and decoding will be the most stable and battle-tested option with Yams as a pretty well established and stable option as well. FineJSON is lesser used (to my knowledge) but I have had success with it in the past.

OpenAPI Document structure

The types used by this library largely mirror the object definitions found in the OpenAPI specification version 3.1.0 (OpenAPIKit module) and version 3.0.3 (OpenAPIKit30 module). The Project Status lists each object defined by the spec and the name of the respective type in this library. The project status page currently focuses on OpenAPI 3.1.x but for the purposes of determining what things are named and what is supported you can mostly infer the status of the OpenAPI 3.0.x support as well.

Document Root

At the root there is an OpenAPI.Document. In addition to some information that applies to the entire API, the document contains OpenAPI.Components (essentially a dictionary of reusable components that can be referenced with JSONReferences and OpenAPI.References) and an OpenAPI.PathItem.Map (a dictionary of routes your API defines).

Routes

Each route is an entry in the document's OpenAPI.PathItem.Map. The keys of this dictionary are the paths for each route (i.e. /widgets). The values of this dictionary are OpenAPI.PathItems which define any combination of endpoints (i.e. GET, POST, PATCH, etc.) that the given route supports. In addition to accessing endpoints on a path item under the name of the method (.get, .post, etc.), you can get an array of pairs matching endpoint methods to operations with the .endpoints method on PathItem.

Endpoints

Each endpoint on a route is defined by an OpenAPI.Operation. Among other things, this operation can specify the parameters (path, query, header, etc.), request body, and response bodies/codes supported by the given endpoint.

Request/Response Bodies

Request and response bodies can be defined in great detail using OpenAPI's derivative of the JSON Schema specification. This library uses the JSONSchema type for such schema definitions.

Schemas

Fundamental types are specified as JSONSchema.integer, JSONSchema.string, JSONSchema.boolean, etc.

Schema attributes are given as arguments to static constructors. By default, schemas are non-nullable, required, and generic. The below examples are not comprehensive and you can pass any number of the optional attributes to the static constructors as arguments.

A schema can be made optional (i.e. it can be omitted) with JSONSchema.integer(required: false) or an existing schema can be asked for an optionalSchemaObject().

A schema can be made nullable with JSONSchema.number(nullable: true) or an existing schema can be asked for a nullableSchemaObject().

Nullability highlights an important decision OpenAPIKit makes. The JSON Schema specification that dictates how OpenAPI v3.1 documents encode nullability states that a nullable property is encoded as having the null type in addition to whatever other type(s) it has. So in OpenAPIKit you set nullability as a property of a schema, but when encoded/decoded it will represent the inclusion of absence of null in the list of types of the schema. If you are using the OpenAPIKit30 module then nullability is encoded as a nullable property per the OpenAPI 3.0.x specification.

Some types of schemas can be further specialized with a format. For example, JSONSchema.number(format: .double) or JSONSchema.string(format: .dateTime).

You can specify a schema's allowed values (e.g. for an enumerated type) with JSONSchema.string(allowedValues: "hello", "world").

Each type of schema has its own additional set of properties that can be specified. For example, integers can have a minimum value: JSONSchema.integer(minimum: (0, exclusive: true)). exclusive: true in this context means the number must be strictly greater than 0 whereas exclusive: false means the number must be greater-than or equal-to 0.

Compound objects can be built with JSONSchema.array, JSONSchema.object, JSONSchema.all(of:), etc.

For example, perhaps a person is represented by the schema:

JSONSchema.object(
  title: "Person",
  properties: [
    "first_name": .string(minLength: 2),
    "last_name": .string(nullable: true),
    "age": .integer,
    "favorite_color": .string(allowedValues: "red", "green", "blue")
  ]
)

Take a look at the OpenAPIKit Schema Object documentation for more information.

OpenAPI References

The OpenAPI.Reference type represents the OpenAPI specification's reference support that is essentially just JSON Reference specification compliant but with the ability to override summaries and descriptions at the reference site where appropriate.

For details on the underlying reference support, see the next section on the JSONReference type.

JSON References

The JSONReference type allows you to work with OpenAPIDocuments that store some of their information in the shared Components Object dictionary or even external files. Only documents where all references point to the Components Object can be dereferenced currently, but you can encode and decode all references.

You can create an external reference with JSONReference.external(URL). Internal references usually refer to an object in the Components Object dictionary and are constructed with JSONReference.component(named:). If you need to refer to something in the current file but not in the Components Object, you can use JSONReference.internal(path:).

You can check whether a given JSONReference exists in the Components Object with document.components.contains(). You can access a referenced object in the Components Object with document.components[reference].

You can create references from the Components Object with document.components.reference(named:ofType:). This method will throw an error if the given component does not exist in the ComponentsObject.

You can use document.components.lookup() or the Components type's subscript to turn an Either containing either a reference or a component into an optional value of that component's type (having either pulled it out of the Either or looked it up in the Components Object). The lookup() method throws when it can't find an item whereas subscript returns nil.

For example,

let apiDoc: OpenAPI.Document = ...
let addBooksPath = apiDoc.paths["/cloudloading/addBook"]

let addBooksParameters: [OpenAPI.Parameter]? = addBooksPath?.parameters.compactMap { apiDoc.components[$0] }

Note that this looks a component up in the Components Object but it does not transform it into an entirely derefernced object in the same way as is described below in the Dereferencing & Resolving section.

Security Requirements

In the OpenAPI specifcation, a security requirement (like can be found on the root Document or on Operations) is a dictionary where each key is the name of a security scheme found in the Components Object and each value is an array of applicable scopes (which is of course only a non-empty array when the security scheme type is one for which "scopes" are relevant).

OpenAPIKit defines the SecurityRequirement typealias as a dictionary with JSONReference keys; These references point to the Components Object and provide a slightly stronger contract than the String values required by the OpenAPI specification. Naturally, these are encoded to JSON/YAML as String values rather than JSON References to maintain compliance with the OpenAPI Specification.

To give an example, let's say you want to describe OAuth 2.0 authentication via the implicit flow. First, define a Security Scheme:

let oauthScheme = OpenAPI.SecurityScheme.oauth2(
    flows: .init(
        implicit: .init(
            authorizationUrl: URL(string: "https://my-api.com/oauth2/auth")!,
            scopes: ["read:widget": "read widget"]
        )
    )
)

Next, store it in your OpenAPI document's Components Object (which likely has other entries but we'll only specify the security schemes for this example):

let components = OpenAPI.Components(
    securitySchemes: ["implicit-oauth": oauthScheme]
)

Finally, your OpenAPI Document should use the Components Object we just created and also reference the OAuth implicit scheme via an internal JSON reference:

let document = OpenAPI.Document(
    info: .init(title: "API", version: "1.0"),
    servers: [],
    paths: [:],
    components: components,
    security: [[.component( named: "implicit-oauth"): ["read:widget"]]]
)

If your API supports multiple alternative authentication strategies (only one of which needs to be used), you might have additional entries in your Document's Security array:

let document = OpenAPI.Document(
    info: .init(title: "API", version: "1.0"),
    servers: [],
    paths: [:],
    components: components,
    security: [
        [.component( named: "implicit-oauth"): ["read:widget"]],
        [.component( named: "auth-code-oauth"): ["read:widget"]]
    ]
)

Specification Extensions

Many OpenAPIKit types support Specification Extensions. As described in the OpenAPI Specification, these extensions must be objects that are keyed with the prefix "x-". For example, a property named "specialProperty" on the root OpenAPI Object (OpenAPI.Document) is invalid but the property "x-specialProperty" is a valid specification extension.

You can get or set specification extensions via the vendorExtensions property on any object that supports this feature. The keys are Strings beginning with the aforementioned "x-" prefix and the values are AnyCodable. If you set an extension without using the "x-" prefix, the prefix will be added upon encoding.

AnyCodable can be constructed from literals or explicitly. The following are all valid.

var document = OpenAPI.Document(...)

document.vendorExtensions["x-specialProperty1"] = true
document.vendorExtensions["x-specialProperty2"] = "hello world"
document.vendorExtensions["x-specialProperty3"] = ["hello", "world"]
document.vendorExtensions["x-specialProperty4"] = ["hello": "world"]
document.vendorExtensions["x-specialProperty5"] = AnyCodable("hello world")

Dereferencing & Resolving

In addition to looking something up in the Components object, you can entirely derefererence many OpenAPIKit types. A dereferenced type has had all of its references looked up (and all of its properties' references, all the way down).

You use a value's dereferenced(in:) method to fully dereference it.

You can even dereference the whole document with the OpenAPI.Document locallyDereferenced() method. As the name implies, you can only derefence whole documents that are contained within one file (which is another way of saying that all references are "local"). Specifically, all references must be located within the document's Components Object.

Unlike what happens when you lookup an individual component using the lookup() method on Components, dereferencing a whole OpenAPI.Document will result in type-level changes that guarantee all references are removed. OpenAPI.Document's locallyDereferenced() method returns a DereferencedDocument which exposes DereferencedPathItems which have DereferencedParameters and DereferencedOperations and so on.

Anywhere that a type would have had either a reference or a component, the dereferenced variety will simply have the component. For example, PathItem has an array of parameters, each of which is Either<OpenAPI.Reference<Parameter>, Parameter> whereas a DereferencedPathItem has an array of DereferencedParameters. The dereferenced variant of each type exposes all the same properties and you can get at the underlying OpenAPI type via an underlying{TypeName} property. This can make for a much more convenient way to traverse a document because you don't need to check for or look up references anywhere the OpenAPI Specification allows them.

For all dereferenced types, dereferencing will store a new vendor extension on the dereferenced value to keep track of the Component Object name the value used to be referenced at. This vendor extension is a string value with the x-component-name key.

You can take things a step further and resolve the document. Calling resolved() on a DereferencedDocument will produce a canonical form of an OpenAPI.Document. The ResolvedRoutes and ResolvedEndpoints that the ResolvedDocument exposes collect all relevant information from the whole document into themselves. For example, a ResolvedEndpoint knows what servers it can be used on, what path it is located at, and which parameters it supports (even if some of those parameters were defined in an OpenAPI.Operation and others were defined in the containing OpenAPI.PathItem).

If your end goal is to analyze the OpenAPI Document or generate something entirely new (like code) from it, the ResolvedDocument is by far more convenient to traverse and query than the original OpenAPI.Document. The downside is, there is not currently support for mutating the ResolvedDocument and then turning it back into an OpenAPI.Document to encode it.

let document: OpenAPI.Document = ...

let resolvedDocument = try document
    .locallyDereferenced()
    .resolved()

for endpoint in resolvedDocument.endpoints {
    // The description found on the PathItem containing the Operation defining this endpoint:
    let routeDescription = endpoint.routeDescription

    // The description found directly on the Operation defining this endpoint:
    let endpointDescription = endpoint.endpointDescription

    // The path, which in the OpenAPI.Document is the key of the dictionary containing
    // the PathItem under which the Operation for this endpoint lives:
    let path = endpoint.path

    // The method, which in the OpenAPI.Document is the way you access the Operation for
    // this endpoint on the PathItem (GET, PATCH, etc.):
    let httpMethod = endpoint.method

    // All parameters defined for the Operation _or_ the PathItem containing it:
    let parameters = endpoint.parameters

    // Per the specification, this is 
    // 1. the list of servers defined on the Operation if one is given.
    // 2. the list of servers defined on the PathItem if one is given _and_ 
    //	no list was found on the Operation.
    // 3. the list of servers defined on the Document if no list was found on
    //	the Operation _or_ the PathItem.
    let servers = endpoint.servers

    // and many more properties...
}

Curated Integrations

Following is a short list of integrations that might be immediately useful or just serve as examples of ways that OpenAPIKit can be used to harness the power of the OpenAPI specification.

If you have a library you would like to propose for this section, please create a pull request and explain a bit about your project.

Generating Swift

The swift-openapi-generator uses OpenAPIKit under the hood and generates Swift code for interfacing with APIs that have OpenAPI descriptions.

Generating OpenAPI Documents

VaporOpenAPI / VaporOpenAPIExample provide an example of generating OpenAPI from a Vapor application's routes.

JSONAPI+OpenAPI is a library that generates OpenAPI schemas from JSON:API types. The library has some rudimentary and experimental support for going the other direction and generating Swift types that represent JSON:API resources described by OpenAPI documentation.

Declarative OpenAPI Documents

The Swift Package Registry API Docs define the OpenAPI documentation for the Swift Package Registry standard using declarative Swift code and OpenAPIKit. This project also provides a useful example of producing a user-friendly ReDoc web interface to the OpenAPI documentation after encoding it as YAML.

Semantic Diffing of OpenAPI Documents

OpenAPIDiff is a library and a CLI that implements semantic diffing; that is, rather than just comparing two OpenAPI documents line-by-line for textual differences, it parses the documents and describes the differences in the two OpenAPI ASTs.

Notes

This library does not currently support file reading at all muchless following $refs to other files and loading them in. You must read OpenAPI documentation into Data or String (depending on the decoder you want to use) and all references must be internal to the same file to be resolved.

This library is opinionated about a few defaults when you use the Swift types, however encoding and decoding stays true to the spec. Some key things to note:

Within schemas, required is specified on the property rather than being specified on the parent object (encoding/decoding still follows the OpenAPI spec).
- ex JSONSchema.object(properties: [ "val": .string(required: true)]) is an "object" type with a required "string" type property.
Within schemas, required defaults to true on initialization (again, encoding/decoding still follows the OpenAPI spec).
- ex. JSONSchema.string is a required "string" type.
- ex. JSONSchema.string(required: false) is an optional "string" type.

See A note on dictionary ordering before deciding on an encoder/decoder to use with this library.

Contributing

Contributions to OpenAPIKit are welcome and appreciated! The project is mostly maintained by one person which means additional contributors have a huge impact on how much gets done how quickly.

Please see the Contribution Guidelines for a few brief notes on contributing the the project.

Security

The OpenAPIKit project takes code security seriously. As part of the Swift Server Workground incubation program, this project follows a shared set of standards around receiving, reporting, and reacting to security vulnerabilies.

Please see Security for information on how to report vulnerabilities to the OpenAPIKit project and what to expect after you do.

Please do not report security vulnerabilities via GitHub issues.

Specification Coverage & Type Reference

For a full list of OpenAPI Specification types annotated with whether OpenAPIKit supports them and relevant translations to OpenAPIKit types, see the Specification Coverage documentation. For detailed information on the OpenAPIKit types, see the full type documentation.

mattpolzin / OpenAPIKit