JSONAPI

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/

:warning: This library provides well-tested type safety when working with JSON:API 1.0, however the Swift compiler can sometimes have difficulty tracking down small typos when initializing ResourceObjects. Once the code is written correctly, it will compile, but tracking down the source of programmer errors can be an annoyance. This is mostly a concern when creating resource objects in-code (servers and test cases must do this). Writing a client that uses this framework to ingest JSON API Compliant API responses is much less painful. :warning:

Table of Contents

• JSONAPI
  • Table of Contents
  • Primary Goals
    • Caveat
  • Dev Environment
    • Prerequisites
    • CocoaPods
    • Xcode project
    • Running the Playground
  • Project Status
    • JSON:API
      • Document
      • Resource Object
      • Relationship Object
      • Links Object
    • Misc
    • Testing
      • Resource Object Validator
    • Potential Improvements
  • Usage
    • JSONAPI.ResourceObjectDescription
    • JSONAPI.ResourceObject
      • Meta
      • Links
      • MaybeRawId
        • RawIdType
      • Convenient typealiases
    • JSONAPI.Relationships
    • JSONAPI.Attributes
      • Transformer
      • Validator
      • Computed Attribute
    • Copying/Mutating ResourceObjects
    • JSONAPI.Document
      • ResourceBody
        • nullable PrimaryResource
      • MetaType
      • LinksType
      • IncludeType
      • APIDescriptionType
      • Error
    • JSONAPI.Meta
    • JSONAPI.Links
    • JSONAPI.RawIdType
    • Sparse Fieldsets
      • Supporting Sparse Fieldset Encoding
      • Sparse Fieldset typealias comparisons
    • Custom Attribute or Relationship Key Mapping
    • Custom Attribute Encode/Decode
    • Meta-Attributes
    • Meta-Relationships
  • Example
    • Preamble (Setup shared by server and client)
    • Server Pseudo-example
    • Client Pseudo-example
• JSONAPI+Testing
• JSONAPI+Arbitrary
• JSONAPI+OpenAPI

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.

Caveat

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

Prerequisites

1. Swift 4.2+
2. Swift Package Manager OR Cocoapods

CocoaPods

To use this framework in your project via Cocoapods instead of Swift Package Manager, add the following dependencies to your Podfile.

	pod 'Poly', :git => 'https://github.com/mattpolzin/Poly.git'
	pod 'JSONAPI', :git => 'https://github.com/mattpolzin/JSONAPI.git'

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

JSON:API

Document

• [x] data
• [x] included
• [x] errors
• [x] meta
• [x] jsonapi (i.e. API Information)
• [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

Misc

• [x] Support transforms on Attributes values (e.g. to support different representations of Date)
• [x] Support validation on Attributes.
• [x] Support sparse fieldsets (encoding only). A client can likely just define a new model to represent a sparse population of another model in a very specific use case for decoding purposes. On the server side, sparse fieldsets of Resource Objects can be encoded without creating one model for every possible sparse fieldset.

Testing

Resource Object 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

These ideas could be implemented in future versions.

• [ ] (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.
• [ ] Support sideposting. JSONAPI spec might become opinionated in the future (https://github.com/json-api/json-api/pull/1197, https://github.com/json-api/json-api/issues/1215, https://github.com/json-api/json-api/issues/1216) but there is also an existing implementation to consider (https://jsonapi-suite.github.io/jsonapi_suite/ruby/writes/nested-writes). At this time, any sidepost implementation would be an awesome tertiary library to be used alongside the primary JSONAPI library. Maybe JSONAPISideloading.
• [ ] Error or warning if an included resource object is not related to a primary resource object or another included resource object (Turned off or at least not throwing by default).

Usage

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.

JSONAPI.ResourceObjectDescription

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

enum PersonDescription: IdentifiedResourceObjectDescription {
	static var jsonType: 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 a ResourceObjectDescription are:

1. A static var "jsonType" 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 ResourceObjectDescription; it could have been a struct, but ResourceObjectDescriptions 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": [
      "Jane",
      "Doe"
    ],
    "favoriteColor": "Green"
  },
  "relationships": {
    "friends": {
      "data": [
        {
          "id": "7",
          "type": "people"
        },
        {
          "id": "8",
          "type": "people"
        }
      ]
    }
  }
}

JSONAPI.ResourceObject

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

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

A ResourceObject needs to be specialized on four generic types. The first is the ResourceObjectDescription described above. The others are a Meta, Links, and MaybeRawId.

Meta

The second generic specialization on ResourceObject is Meta. This is described in its own section below. All Meta at any level of a JSON API Document follow the same rules. You can use NoMetadata if you do not need to package any metadata with the ResourceObject.

Links

The third generic specialization on ResourceObject 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. You can use NoLinks if you do not need to package any links with the ResourceObject.

MaybeRawId

The last generic specialization on ResourceObject is MaybeRawId. This is either a RawIdType that can be used to uniquely identify ResourceObjects or it is Unidentified which is used to indicate a ResourceObject does not have an Id (which is useful when a client is requesting that the server create a ResourceObject and assign it a new Id).

RawIdType

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

Having the ResourceObject type associated with the Id makes it easy to store all of your resource objects in a hash broken out by ResourceObject type; You can pass Ids around and always know where to look for the ResourceObject to which the Id refers. This encapsulation provides some type safety because the Ids of two ResourceObjects with the "raw ID" of "1" but different types will not compare as equal.

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

Convenient typealiases

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

public typealias ResourceObject<Description: JSONAPI.ResourceObjectDescription, Meta: JSONAPI.Meta, Links: JSONAPI.Links> = JSONAPI.ResourceObject<Description, Meta, Links, String>

public typealias NewResourceObject<Description: JSONAPI.ResourceObjectDescription, Meta: JSONAPI.Meta, Links: JSONAPI.Links> = JSONAPI.ResourceObject<Description, Meta, Links, Unidentified>

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

typealias Person = ResourceObject<PersonDescription, NoMetadata, NoLinks>

typealias NewPerson = NewResourceObject<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.

JSONAPI.Relationships

There are two types of Relationships: ToOneRelationship and ToManyRelationship. A ResourceObjectDescription'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 a ResourceObjectDescription.

In addition to identifying resource objects 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>

A ResourceObject that does not have relationships can be described by adding the following to a ResourceObjectDescription:

typealias Relationships = NoRelationships

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

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

JSONAPI.Attributes

The Attributes of a ResourceObjectDescription 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?>

A resource object that does not have attributes can be described by adding the following to an ResourceObjectDescription:

typealias Attributes = NoAttributes

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

let favoriteColor: String = person[\.favoriteColor]

Transformer

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")

Validator

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 ResourceObjectDescription.Attributes struct if you would like to expose attributes that are not explicitly represented by the JSON. These computed properties do not have to be wrapped in Attribute, ValidatedAttribute, or TransformedAttribute. This allows computed attributes to be of types that are not Codable. 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: " ") }
}

If your computed property is wrapped in a AttributeType then you can still use the default subscript operator to access it (as would be the case with the person[\.fullName] example above). However, if you add a property to the Attributes struct that is not wrapped in an AttributeType, you must either access it from its full path (person.attributes.newThing) or with the "direct" subscript accessor (person[direct: \.newThing]). This keeps the subscript access unambiguous enough for the compiler to be helpful prior to explicitly casting, comparing, or storing the result.

Copying/Mutating ResourceObjects

ResourceObject is a value type, so copying is its default behavior. There are two common mutations you might want to make when copying a ResourceObject:

1. Assigning a new Identifier to the copy of an identified ResourceObject.
2. Assigning a new Identifier to the copy of an unidentified ResourceObject.

The above can be accomplished with code like the following:

// use case 1
let person1 = person.withNewIdentifier()

// use case 2
let newlyIdentifiedPerson1 = unidentifiedPerson.identified(byType: String.self)

let newlyIdentifiedPerson2 = unidentifiedPerson.identified(by: "2232")

JSONAPI.Document

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 resource objects 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."

ResourceBody

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 ResourceObject. 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.

MetaType

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.

LinksType

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

IncludeType

The fourth generic type of a JSONAPIDocument is an Include. This type controls which types of ResourceObject are looked for when decoding the "included" part of the JSON API document. If you do not expect any included resource objects to be in the document, NoIncludes is the way to go. The JSONAPI framework provides Includes for up to six types of included resource objects. 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 resource objects, it indicates a number of types of included resource objects. Include1 can be used to decode any number of included resource objects as long as all the resource objects 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.

APIDescriptionType

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.

Error

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.

JSONAPI.Meta

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.

If you need to support metadata with structure that is not pre-determined, consider an "Any Codable" type such as that found at https://github.com/Flight-School/AnyCodable.

JSONAPI.Links

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.

JSONAPI.RawIdType

If you want to create new JSONAPI.ResourceObject 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
	}
}

Sparse Fieldsets

Sparse Fieldsets are currently supported when encoding only. When decoding, Sparse Fieldsets become tricker to support under the current types this library uses and it is assumed that clients will request one or maybe two sparse fieldset combinations for any given model at most so it can simply define the JSONAPI models needed to decode those subsets of all possible fields. A server, on the other hand, likely needs to support arbitrary combinations of sparse fieldsets and this library provides a mechanism for encoding those sparse fieldsets without too much extra footwork.

You can use sparse fieldsets on the primary resources(s) and includes of a JSONAPI.Document.

There is a sparse fieldsets example included with this repository as a Playground page.

Supporting Sparse Fieldset Encoding

1. The JSONAPI ResourceObjectDescription's Attributes struct must conform to JSONAPI.SparsableAttributes rather than JSONAPI.Attributes.
2. The JSONAPI ResourceObjectDescription's Attributes struct must contain a CodingKeys enum that conforms to JSONAPI.SparsableCodingKey instead of Swift.CodingKey.
3. typealiases you may have created for JSONAPI.Document that allow you to decode Documents will not support the "encode-only" nature of sparse fieldsets. See the next section for typealias comparisons.
4. To create a sparse fieldset from a ResourceObject just call its sparse(with: fields) method and pass an array of Attributes.CodingKeys values you would like included in the encoding.
5. Initialize and encode a Document containing one or more sparse or full primary resource(s) and any number of sparse or full includes.

Sparse Fieldset typealias comparisons

You might have found a typealias like the following for encoding/decoding JSONAPI.Documents (note the primary resource body is a JSONAPI.ResourceBody):

typealias Document<PrimaryResourceBody: JSONAPI.ResourceBody, IncludeType: JSONAPI.Include> = JSONAPI.Document<PrimaryResourceBody, NoMetadata, NoLinks, IncludeType, NoAPIDescription, UnknownJSONAPIError>

In order to support sparse fieldsets (which are encode-only), the following companion typealias would be useful (note the primary resource body is a JSONAPI.EncodableResourceBody):

typealias SparseDocument<PrimaryResourceBody: JSONAPI.EncodableResourceBody, IncludeType: JSONAPI.Include> = JSONAPI.Document<PrimaryResourceBody, NoMetadata, NoLinks, IncludeType, NoAPIDescription, UnknownJSONAPIError>

Custom Attribute or Relationship Key Mapping

There is not anything special going on at the JSONAPI.Attributes and JSONAPI.Relationships levels, so you can easily provide custom key mappings by taking advantage of Codable's CodingKeys pattern. Here are two models that will encode/decode equivalently but offer different naming in your codebase:

public enum ResourceObjectDescription1: JSONAPI.ResourceObjectDescription {
	public static var jsonType: String { return "entity" }

	public struct Attributes: JSONAPI.Attributes {
		public let coolProperty: Attribute<String>
	}

	public typealias Relationships = NoRelationships
}

public enum ResourceObjectDescription2: JSONAPI.ResourceObjectDescription {
	public static var jsonType: String { return "entity" }

	public struct Attributes: JSONAPI.Attributes {
		public let wholeOtherThing: Attribute<String>

		enum CodingKeys: String, CodingKey {
			case wholeOtherThing = "coolProperty"
		}
	}

    public typealias Relationships = NoRelationships
}

Custom Attribute Encode/Decode

You can safely provide your own encoding or decoding functions for your Attributes struct if you need to as long as you are careful that your encode operation correctly reverses your decode operation. Although this is generally not necessary, AttributeType provides a convenience method to make your decoding a bit less boilerplate ridden. This is what it looks like:

public enum ResourceObjectDescription1: JSONAPI.ResourceObjectDescription {
	public static var jsonType: String { return "entity" }

	public struct Attributes: JSONAPI.Attributes {
		public let property1: Attribute<String>
		public let property2: Attribute<Int>
		public let property3: Attribute<String>

		public let weirdThing: Attribute<String>

		enum CodingKeys: String, CodingKey {
			case property1
			case property2
			case property3
		}
	}

	public typealias Relationships = NoRelationships
}

extension ResourceObjectDescription1.Attributes {
	public init(from decoder: Decoder) throws {
		let container = try decoder.container(keyedBy: CodingKeys.self)

		property1 = try .defaultDecoding(from: container, forKey: .property1)
		property2 = try .defaultDecoding(from: container, forKey: .property2)
		property3 = try .defaultDecoding(from: container, forKey: .property3)

		weirdThing = .init(value: "hello world")
	}

	public func encode(to encoder: Encoder) throws {
		var container = encoder.container(keyedBy: CodingKeys.self)

		try container.encode(property1, forKey: .property1)
		try container.encode(property2, forKey: .property2)
		try container.encode(property3, forKey: .property3)
	}
}

Meta-Attributes

This advanced feature may not ever be useful, but if you find yourself in the situation of dealing with an API that does not 100% follow the SPEC then you might find meta-attributes are just the thing to make your resource objects more natural to work with.

Suppose, for example, you are presented with the unfortunate situation where a piece of information you need is only available as part of the Id of a resource object. Perhaps a user's Id is formatted "{integer}-{createdAt}" where "createdAt" is the unix timestamp when the user account was created. The following UserDescription will expose what you need as an attribute. Realistically, the following example code is still terrible for its error handling. Using a Result type and/or invariants would clean things up substantially.

enum UserDescription: ResourceObjectDescription {
	public static var jsonType: String { return "users" }

	struct Attributes: JSONAPI.Attributes {
		var createdAt: (User) -> Date {
			return { user in
				let components = user.id.rawValue.split(separator: "-")

				guard components.count == 2 else {
					assertionFailure()
					return Date()
				}

				let timestamp = TimeInterval(components[1])

				guard let date = timestamp.map(Date.init(timeIntervalSince1970:)) else {
					assertionFailure()
					return Date()
				}

				return date
			}
		}
	}

	typealias Relationships = NoRelationships
}

typealias User = JSONAPI.ResourceObject<UserDescription, NoMetadata, NoLinks, String>

Given a value user of the above resource object type, you can access the createdAt attribute just like you would any other:

let createdAt = user[\.createdAt]

This works because createdAt is defined in the form: var {name}: ({ResourceObject}) -> {Value} where {ResourceObject} is the JSONAPI.ResourceObject described by the ResourceObjectDescription containing the meta-attribute.

Meta-Relationships

This advanced feature may not ever be useful, but if you find yourself in the situation of dealing with an API that does not 100% follow the SPEC then you might find meta-relationships are just the thing to make your resource objects more natural to work with.

Similarly to Meta-Attributes, Meta-Relationships allow you to represent non-compliant relationships as computed relationship properties. In the following example, a relationship is created from some attributes on the JSON model.

enum UserDescription: ResourceObjectDescription {
	public static var jsonType: String { return "users" }

	struct Attributes: JSONAPI.Attributes {
		let friend_id: Attribute<String>
	}

	struct Relationships: JSONAPI.Relationships {
		public var friend: (User) -> User.Identifier {
			return { user in
				return User.Identifier(rawValue: user[\.friend_id])
			}
		}
	}
}

typealias User = JSONAPI.ResourceObject<UserDescription, NoMetadata, NoLinks, String>

Given a value user of the above resource object type, you can access the friend relationship just like you would any other:

let friendId = user ~> \.friend

This works because friend is defined in the form: var {name}: ({ResourceObject}) -> {Identifier} where {ResourceObject} is the JSONAPI.ResourceObject described by the ResourceObjectDescription containing the meta-relationship.

Example

The following serves as a sort of pseudo-example. It skips server/client implementation details not related to JSON:API but still gives a more complete picture of what an implementation using this framework might look like. You can play with this example code in the Playground provided with this repo.

Preamble (Setup shared by server and client)

// Make String a CreatableRawIdType.
var globalStringId: Int = 0
extension String: CreatableRawIdType {
	public static func unique() -> String {
		globalStringId += 1
		return String(globalStringId)
	}
}

// Create a typealias because we do not expect JSON:API Resource
// Objects for this particular API to have Metadata or Links associated
// with them. We also expect them to have String Identifiers.
typealias JSONEntity<Description: ResourceObjectDescription> = JSONAPI.ResourceObject<Description, NoMetadata, NoLinks, String>

// Similarly, create a typealias for unidentified entities. JSON:API
// only allows unidentified entities (i.e. no "id" field) for client
// requests that create new entities. In these situations, the server
// is expected to assign the new entity a unique ID.
typealias UnidentifiedJSONEntity<Description: ResourceObjectDescription> = JSONAPI.ResourceObject<Description, NoMetadata, NoLinks, Unidentified>

// Create relationship typealiases because we do not expect
// JSON:API Relationships for this particular API to have
// Metadata or Links associated with them.
typealias ToOneRelationship<Entity: Identifiable> = JSONAPI.ToOneRelationship<Entity, NoMetadata, NoLinks>
typealias ToManyRelationship<Entity: Relatable> = JSONAPI.ToManyRelationship<Entity, NoMetadata, NoLinks>

// Create a typealias for a Document because we do not expect
// JSON:API Documents for this particular API to have Metadata, Links,
// useful Errors, or an APIDescription (The *SPEC* calls this
// "API Description" the "JSON:API Object").
typealias Document<PrimaryResourceBody: JSONAPI.ResourceBody, IncludeType: JSONAPI.Include> = JSONAPI.Document<PrimaryResourceBody, NoMetadata, NoLinks, IncludeType, NoAPIDescription, UnknownJSONAPIError>

// MARK: Entity Definitions

enum AuthorDescription: ResourceObjectDescription {
	public static var jsonType: String { return "authors" }

	public struct Attributes: JSONAPI.Attributes {
		public let name: Attribute<String>
	}

	public typealias Relationships = NoRelationships
}

typealias Author = JSONEntity<AuthorDescription>

enum ArticleDescription: ResourceObjectDescription {
	public static var jsonType: String { return "articles" }

	public struct Attributes: JSONAPI.Attributes {
		public let title: Attribute<String>
		public let abstract: Attribute<String>
	}

	public struct Relationships: JSONAPI.Relationships {
		public let author: ToOneRelationship<Author>
	}
}

typealias Article = JSONEntity<ArticleDescription>

// MARK: Document Definitions

// We create a typealias to represent a document containing one Article
// and including its Author
typealias SingleArticleDocumentWithIncludes = Document<SingleResourceBody<Article>, Include1<Author>>

// ... and a typealias to represent a document containing one Article and
// not including any related entities.
typealias SingleArticleDocument = Document<SingleResourceBody<Article>, NoIncludes>

Server Pseudo-example

// Skipping over all the API and database stuff, here's a chunk of code
// that creates a document. Note that this document is the entirety
// of a JSON:API response body.
func articleDocument(includeAuthor: Bool) -> Either<SingleArticleDocument, SingleArticleDocumentWithIncludes> {
    // Let's pretend all of this is coming from a database:

    let authorId = Author.Identifier(rawValue: "1234")

    let article = Article(id: .init(rawValue: "5678"),
                          attributes: .init(title: .init(value: "JSON:API in Swift"),
                                            abstract: .init(value: "Not yet written")),
                          relationships: .init(author: .init(id: authorId)),
                          meta: .none,
                          links: .none)

    let document = SingleArticleDocument(apiDescription: .none,
                                         body: .init(resourceObject: article),
                                         includes: .none,
                                         meta: .none,
                                         links: .none)

    switch includeAuthor {
    case false:
        return .init(document)

    case true:
        let author = Author(id: authorId,
                            attributes: .init(name: .init(value: "Janice Bluff")),
                            relationships: .none,
                            meta: .none,
                            links: .none)

        let includes: Includes<SingleArticleDocumentWithIncludes.Include> = .init(values: [.init(author)])

        return .init(document.including(includes))
    }
}

let encoder = JSONEncoder()
encoder.keyEncodingStrategy = .convertToSnakeCase
encoder.outputFormatting = .prettyPrinted

let responseBody = articleDocument(includeAuthor: true)
let responseData = try! encoder.encode(responseBody)

// Next step would be setting the HTTP body of a response.
// We will just print it out instead:
print("-----")
print(String(data: responseData, encoding: .utf8)!)

// ... and if we had received a request for an article without
// including the author:
let otherResponseBody = articleDocument(includeAuthor: false)
let otherResponseData = try! encoder.encode(otherResponseBody)
print("-----")
print(String(data: otherResponseData, encoding: .utf8)!)

Client Pseudo-example

enum NetworkError: Swift.Error {
    case serverError
    case quantityMismatch
}

// Skipping over all the API stuff, here's a chunk of code that will
// decode a document. We will assume we have made a request for a
// single article including the author.
func docode(articleResponseData: Data) throws -> (article: Article, author: Author) {
    let decoder = JSONDecoder()
    decoder.keyDecodingStrategy = .convertFromSnakeCase

    let articleDocument = try decoder.decode(SingleArticleDocumentWithIncludes.self, from: articleResponseData)

    switch articleDocument.body {
    case .data(let data):
        let authors = data.includes[Author.self]

        guard authors.count == 1 else {
            throw NetworkError.quantityMismatch
        }

        return (article: data.primary.value, author: authors[0])
    case .errors(let errors, meta: _, links: _):
        throw NetworkError.serverError
    }
}

let response = try! docode(articleResponseData: responseData)

// Next step would be to do something useful with the article and author but we will print them instead.
print("-----")
print(response.article)
print(response.author)

JSONAPI+Testing

The JSONAPI framework is packaged with a test library to help you test your JSONAPI integration.

The test library is called JSONAPITesting. It provides literal expressibility for Attribute, ToOneRelationship, and Id in many situations so that you can easily write test ResourceObject values into your unit tests. It also provides a check() function for each ResourceObject 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 JSONAPITesting in action in the Playground included with the JSONAPI repository.

JSONAPI+Arbitrary

This library has moved into its own Package. See https://github.com/mattpolzin/JSONAPI-Arbitrary for more information.

JSONAPI+OpenAPI

The JSONAPI+OpenAPI library generates OpenAPI compliant JSON Schema for models built with the JSONAPI library. If your Swift code is your preferred source of truth for API information, this is an easy way to document the response schemas of your API.

JSONAPI+OpenAPI also has experimental support for generating JSONAPI Swift code from Open API documentation (this currently lives on the feature/gen-swift branch).

See https://github.com/mattpolzin/JSONAPI-OpenAPI for more information.