Swiftpack.co - Package - mattpolzin/JSONAPI


MIT license Swift 4.2 Build Status

A Swift package for encoding to- and decoding from JSON API compliant requests and responses.

See the JSON API Spec here: https://jsonapi.org/format/

Primary Goals

The primary goals of this framework are:

  1. Allow creation of Swift types that are easy to use in code but also can be encoded to- or decoded from JSON API v1.0 Spec compliant payloads without lots of boilerplate code.
  2. Leverage Codable to avoid additional outside dependencies and get operability with non-JSON encoders/decoders for free.
  3. Do not sacrifice type safety.
  4. Be platform agnostic so that Swift code can be written once and used by both the client and the server.


The big caveat is that, although the aim is to support the JSON API spec, this framework ends up being naturally opinionated about certain things that the API Spec does not specify. These caveats are largely a side effect of attempting to write the library in a "Swifty" way.

If you find something wrong with this library and it isn't already mentioned under Project Status, let me know! I want to keep working towards a library implementation that is useful in any application.

Dev Environment


  1. Swift 4.2+ and Swift Package Manager

Xcode project

To create an Xcode project for JSONAPI, run swift package generate-xcodeproj

Running the Playground

To run the included Playground files, create an Xcode project using Swift Package Manager, then create an Xcode Workspace in the root of the repository and add both the generated Xcode project and the playground to the Workspace.

Note that Playground support for importing non-system Frameworks is still a bit touchy as of Swift 4.2. Sometimes building, cleaning and building, or commenting out and then uncommenting import statements (especially in the Entities.swift Playground Source file) can get things working for me when I am getting an error about JSONAPI not being found.

Project Status



  • [x] data
  • [x] included
  • [x] errors
  • [x] meta
  • [x] jsonapi
  • [x] links

Resource Object

  • [x] id
  • [x] type
  • [x] attributes
  • [x] relationships
  • [x] links
  • [x] meta

Relationship Object

  • [x] data
  • [x] links
  • [x] meta

Links Object

  • [x] href
  • [x] meta


  • [x] Support transforms on Attributes values (e.g. to support different representations of Date)
  • [x] Support validation on Attributes.
  • [ ] Create more descriptive errors that are easier to use for troubleshooting.


Entity Validator

  • [x] Disallow optional array in Attribute (should be empty array, not null).
  • [x] Only allow TransformedAttribute and its derivatives as stored properties within Attributes struct. Computed properties can still be any type because they do not get encoded or decoded.
  • [x] Only allow ToManyRelationship and ToOneRelationship within Relationships struct.

Potential Improvements

  • [ ] (Maybe) Use KeyPath to specify Includes thus creating type safety around the relationship between a primary resource type and the types of included resources.
  • [ ] (Maybe) Replace SingleResourceBody and ManyResourceBody with support at the Document level to just interpret PrimaryResource, PrimaryResource?, or [PrimaryResource] as the same decoding/encoding strategies.
  • [ ] Property-based testing (using SwiftCheck).
  • [ ] Error or warning if an included entity is not related to a primary entity or another included entity (Turned off or at least not throwing by default).


In this documentation, in order to draw attention to the difference between the JSONAPI framework (this Swift library) and the JSON API Spec (the specification this library helps you follow), the specification will consistently be referred to below as simply the SPEC.


An EntityDescription is the JSONAPI framework's representation of what the SPEC calls a Resource Object. You might create the following EntityDescription to represent a person in a network of friends:

enum PersonDescription: IdentifiedEntityDescription {
	static var type: String { return "people" }

	struct Attributes: JSONAPI.Attributes {
		let name: Attribute<[String]>
		let favoriteColor: Attribute<String>

	struct Relationships: JSONAPI.Relationships {
		let friends: ToManyRelationship<Person>

The requirements of an EntityDescription are:

  1. A static var "type" that matches the JSON type; The SPEC requires every Resource Object to have a "type".
  2. A struct of Attributes - OR - typealias Attributes = NoAttributes
  3. A struct of Relationships - OR - typealias Relationships = NoRelationships

Note that an enum type is used here for the EntityDescription; it could have been a struct, but EntityDescriptions do not ever need to be created so an enum with no cases is a nice fit for the job.

This readme doesn't go into detail on the SPEC, but the following Resource Object would be described by the above PersonDescription:

  "type": "people",
  "id": "9",
  "attributes": {
    "name": [
    "favoriteColor": "Green"
  "relationships": {
    "friends": {
      "data": [
          "id": "7",
          "type": "people"
          "id": "8",
          "type": "people"


Once you have an EntityDescription, you create, encode, and decode Entities that "fit the description". If you have a CreatableRawIdType (see the section on RawIdTypes below) then you can create new Entities that will automatically be given unique Ids, but even without a CreatableRawIdType you can encode, decode and work with entities.

The Entity and EntityDescription together with a JSONAPI.Meta type and a JSONAPI.Links type embody the rules and properties of a JSON API Resource Object.

An Entity needs to be specialized on four generic types. The first is the EntityDescription described above. The others are a Meta, Links, and MaybeRawId.


The second generic specialization on Entity is Meta. This is described in its own section below. All Meta at any level of a JSON API Document follow the same rules.


The third generic specialization on Entity is Links. This is described in its own section below. All Links at any level of a JSON API Document follow the same rules, although the SPEC makes different suggestions as to what types of links might live on which parts of the Document.


The second is the raw type of Id to use for the Entity. The actual Id of the Entity will not be a RawIdType, though. The Id will package a value of RawIdType with a specialized reference back to the Entity type it identifies. This just looks like Id<RawIdType, Entity<EntityDescription, RawIdType>>.

Having the Entity type associated with the Id makes it easy to store all of your entities in a hash broken out by Entity type; You can pass Ids around and always know where to look for the Entity to which the Id refers.

A RawIdType is the underlying type that uniquely identifies an Entity. This is often a String or a UUID.


MaybeRawId is either a RawIdType that can be used to uniquely identify Entities or it is Unidentified which is used to indicate an Entity does not have an Id (which is useful when a client is requesting that the server create an Entity and assign it a new Id).

Convenient typealiases

Often you can use one RawIdType for many if not all of your Entities. That means you can save yourself some boilerplate by using typealiases like the following:

public typealias Entity<Description: JSONAPI.EntityDescription, Meta: JSONAPI.Meta, Links: JSONAPI.Links> = JSONAPI.Entity<Description, Meta, Links, String>

public typealias NewEntity<Description: JSONAPI.EntityDescription, Meta: JSONAPI.Meta, Links: JSONAPI.Links> = JSONAPI.Entity<Description, Meta, Links, Unidentified>

It can also be nice to create a typealias for each type of entity you want to work with:

typealias Person = Entity<PersonDescription, NoMetadata, NoLinks>

typealias NewPerson = NewEntity<PersonDescription, NoMetadata, NoLinks>

Note that I am assuming an unidentified person is a "new" person. I suspect that is generally an acceptable conflation because the only time the SPEC allows a Resource Object to be encoded without an Id is when a client is requesting the given Resource Object be created by the server and the client wants the server to create the Id for that object.


There are two types of Relationships: ToOneRelationship and ToManyRelationship. An EntityDescription's Relationships type can contain any number of Relationship properties of either of these types. Do not store anything other than Relationship properties in the Relationships struct of an EntityDescription.

In addition to identifying entities by Id and type, Relationships can contain Meta or Links that follow the same rules as Meta and Links elsewhere in the JSON API Document.

To describe a relationship that may be omitted (i.e. the key is not even present in the JSON object), you make the entire ToOneRelationship or ToManyRelationship optional. However, this is not recommended because you can also represent optional relationships as nullable which means the key is always present. A ToManyRelationship can naturally represent the absence of related values with an empty array, so ToManyRelationship does not support nullability at all. A ToOneRelationship can be marked as nullable (i.e. the value could be either null or a resource identifier) like this:

let nullableRelative: ToOneRelationship<Person?, NoMetadata, NoLinks>

An entity that does not have relationships can be described by adding the following to an EntityDescription:

typealias Relationships = NoRelationships

Relationship values boil down to Ids of other entities. To access the Id of a related Entity, you can use the custom ~> operator with the KeyPath of the Relationship from which you want the Id. The friends of the above Person Entity can be accessed as follows (type annotations for clarity):

let friendIds: [Person.Identifier] = person ~> \.friends


The Attributes of an EntityDescription can contain any JSON encodable/decodable types as long as they are wrapped in an Attribute, ValidatedAttribute, or TransformedAttribute struct.

To describe an attribute that may be omitted (i.e. the key might not even be in the JSON object), you make the entire Attribute optional:

let optionalAttribute: Attribute<String>?

To describe an attribute that is expected to exist but might have a null value, you make the value within the Attribute optional:

let nullableAttribute: Attribute<String?>

An entity that does not have attributes can be described by adding the following to an EntityDescription:

typealias Attributes = NoAttributes

Attributes can be accessed via the subscript operator of the Entity type as follows:

let favoriteColor: String = person[\.favoriteColor]


Sometimes you need to use a type that does not encode or decode itself in the way you need to represent it as a serialized JSON object. For example, the Swift Foundation type Date can encode/decode itself to Double out of the box, but you might want to represent dates as ISO 8601 compliant Strings instead. The Foundation library JSONDecoder has a setting to make this adjustment, but for the sake of an example, you could create a Transformer.

A Transformer just provides one static function that transforms one type to another. You might define one for an ISO 8601 compliant Date like this:

enum ISODateTransformer: Transformer {
	public static func transform(_ value: String) throws -> Date {
		// parse Date out of input and return

Then you define the attribute as a TransformedAttribute instead of an Attribute:

let date: TransformedAttribute<String, ISODateTransformer>

Note that the first generic parameter of TransformAttribute is the type you expect to decode from JSON, not the type you want to end up with after transformation.

If you make your Transformer a ReversibleTransformer then your life will be a bit easier when you construct TransformedAttributes because you have access to initializers for both the pre- and post-transformed value types. Continuing with the above example of a ISODateTransformer:

extension ISODateTransformer: ReversibleTransformer {
	public static func reverse(_ value: Date) throws -> String {
		// serialize Date to a String

let exampleAttribute = try? TransformedAttribute<String, ISODateTransformer>(transformedValue: Date())
let otherAttribute = try? TransformedAttribute<String, ISODateTransformer>(rawValue: "2018-12-01 09:06:41 +0000")


You can also creator Validators and ValidatedAttributes. A Validator is just a Transformer that by convention does not perform a transformation. It simply throws if an attribute value is invalid.

Computed Attribute

You can add computed properties to your EntityDescription.Attributes struct if you would like to expose attributes that are not explicitly represented by the JSON. These computed properties should still have the type Attribute because that way you can take advantage of the quick access provided by Entity's subscript operator. Here's an example of how you might take the Person[\.name] attribute from the example above and create a fullName computed property.

public var fullName: Attribute<String> {
	return name.map { $0.joined(separator: " ") }


The entirety of a JSON API request or response is encoded or decoded from- or to a Document. As an example, a JSON API response containing one Person and no included entities could be decoded as follows:

let decoder = JSONDecoder()

let responseStructure = JSONAPI.Document<SingleResourceBody<Person>, NoMetadata, NoLinks, NoIncludes, UnknownJSONAPIError>.self

let document = try decoder.decode(responseStructure, from: data)

A JSON API Document is guaranteed by the SPEC to be "data", "metadata", or "errors." If it is "data", it may also contain "metadata" and/or other "included" resources. If it is "errors," it may also contain "metadata."


The first generic type of a JSONAPIDocument is a ResourceBody. This can either be a SingleResourceBody<PrimaryResource> or a ManyResourceBody<PrimaryResource>. You will find zero or one PrimaryResource values in a JSON API document that has a SingleResourceBody and you will find zero or more PrimaryResource values in a JSON API document that has a ManyResourceBody. You can use the Poly types (Poly1 through Poly6) to specify that a ResourceBody will be one of a few different types of Entity. These Poly types work in the same way as the Include types described below.

If you expect a response to not have a "data" top-level key at all, then use NoResourceBody instead.

nullable PrimaryResource

If you expect a SingleResourceBody to sometimes come back null, you should make your PrimaryResource optional. If you do not make your PrimaryResource optional then a null primary resource will be considered an error when parsing the JSON.

You cannot, however, use an optional PrimaryResource with a ManyResourceBody because the SPEC requires that an empty document in that case be represented by an empty array rather than null.


The second generic type of a JSONAPIDocument is a Meta. This Meta follows the same rules as Meta at any other part of a JSON API Document. It is described below in its own section, but as an example, the JSON API document could contain the following pagination info in its meta entry:

	"meta": {
		"total": 100,
		"limit": 50,
		"offset": 50

You would then create the following Meta type:

struct PageMetadata: JSONAPI.Meta {
	let total: Int
	let limit: Int
	let offset: Int

You can always use NoMetadata if this JSON API feature is not needed.


The third generic type of a JSONAPIDocument is a Links struct. Links are described in their own section below.


The fourth generic type of a JSONAPIDocument is an Include. This type controls which types of Entity are looked for when decoding the "included" part of the JSON API document. If you do not expect any included entities to be in the document, NoIncludes is the way to go. The JSONAPI framework provides Includes for up to six types of included entities. These are named Include1, Include2, Include3, and so on.

IMPORTANT: The number trailing "Include" in these type names does not indicate a number of included entities, it indicates a number of types of included entities. Include1 can be used to decode any number of included entities as long as all the entities are of the same type.

To specify that we expect friends of a person to be included in the above example JSONAPIDocument, we would use Include1<Person> instead of NoIncludes.


The fifth generic type of a JSONAPIDocument is an APIDescription. The type represents the "JSON:API Object" described by the SPEC. This type describes the highest version of the SPEC supported and can carry additional metadata to describe the API.

You can specify this is not part of the document by using the NoAPIDescription type.

You can describe the API by a version with no metadata by using APIDescription<NoMetadata>.

You can supply any JSONAPI.Meta type as the metadata type of the API description.


The final generic type of a JSONAPIDocument is the Error. You should create an error type that can decode all the errors you expect your JSONAPIDocument to be able to decode. As prescribed by the SPEC, these errors will be found in the root document member errors.


A Meta struct is totally open-ended. It is described by the SPEC as a place to put any information that does not fit into the standard JSON API Document structure anywhere else.

You can specify NoMetadata if the part of the document being described should not contain any Meta.


A Links struct must contain only Link properties. Each Link property can either be a URL or a URL and some Meta. Each part of the document has some suggested common Links to include but generally any link can be included.

You can specify NoLinks if the part of the document being described should not contain any Links.


If you want to create new JSONAPI.Entity values and assign them Ids then you will need to conform at least one type to CreatableRawIdType. Doing so is easy; here are two example conformances for UUID and String (via UUID):

extension UUID: CreatableRawIdType {
	public static func unique() -> UUID {
		return UUID()

extension String: CreatableRawIdType {
	public static func unique() -> String {
		return UUID().uuidString


The JSONAPI framework is packaged with a test library to help you test your JSONAPI integration. The test library is called JSONAPITestLib. It provides literal expressibility for Attribute, ToOneRelationship, and Id in many situations so that you can easily write test Entity values into your unit tests. It also provides a check() function for each Entity type that can be used to catch problems with your JSONAPI structures that are not caught by Swift's type system. You can see the JSONAPITestLib in action in the Playground included with the JSONAPI repository.


Stars: 1
Help us keep the lights on


Used By

Total: 0


0.10.0 - Dec 11, 2018

I Removed the many many convenience initializers on Entity and Document. They appeared to be causing the compiler strife and there was a confusing number of them available at any one time. The recommendation is to provide convenience initializers on a per-need basis instead if they offer a lot of utility to you; you can even copy the existing convenience accessors right out of the library (they are just commented out).

I also added the ability to initialize nullable attributes with literals when their wrapped type could also be initialized from that literal.

0.9.0 - Dec 9, 2018

  • Added Pointer typealias for ToOneRelationship with no meta or links.
  • Added some missing Entity initializers.
  • Added Poly7/Include7.
  • Added relationship operator ~> support for optional relationships.
  • Added subscript access to attributes that are not AttributeType (so that computed properties do not need to be AttributeType.
  • Made it possible to add Relationships to values that are not EntityType.
  • Fixed encoding bug when Relationship is nullable (not optional, but nullable).

0.8.1 - Dec 6, 2018

I switched to a super stripped down Result type that is internally scoped. This reduces friction with code that integrates this library, especially given that the Result type was not ever being used for part of the public API of this library.

0.8.0 - Dec 6, 2018

This library now hits all the major points for encoding/decoding JSON API Spec compliant documents. Definitely still room for improvement, and I think I will wait until I have used the library a bit more before I call it v1.0.

0.7.1 - Dec 4, 2018