Added Meta-Attribute support and documentation

README.md

@@ -52,6 +52,7 @@ See the JSON API Spec here: https://jsonapi.org/format/
 	- [`JSONAPI.RawIdType`](#jsonapirawidtype)
 	- [Custom Attribute or Relationship Key Mapping](#custom-attribute-or-relationship-key-mapping)
 	- [Custom Attribute Encode/Decode](#custom-attribute-encodedecode)
+	- [Meta-attributes](#meta-attributes)
 - [Example](#example)
 	- [Preamble (Setup shared by server and client)](#preamble-setup-shared-by-server-and-client)
 	- [Server Pseudo-example](#server-pseudo-example)
@@ -526,6 +527,51 @@ extension EntityDescription1.Attributes {
 }
 ```
 
+### 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 entities 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 an entity. 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, this code is still terrible for its error handling. Using a `Result` type and/or invariants would clean things up substantially.
+
+```
+enum UserDescription: EntityDescription {
+	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.Entity<UserDescription, NoMetadata, NoLinks, String>
+```
+
+Given a value `user` of the above entity 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}: ({Entity}) -> {Value}` where `{Entity}` is the `JSONAPI.Entity` described by the `EntityDescription` containing the meta-attribute.
+
 ## 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.
 

Sources/JSONAPI/Resource/Entity.swift

@@ -471,6 +471,15 @@ public extension EntityProxy {
 	}
 }
 
+// MARK: Meta-Attribute Access
+public extension EntityProxy {
+	/// Access an attribute requiring a transformation on the RawValue _and_
+	/// a secondary transformation on this entity (self).
+	subscript<T>(_ path: KeyPath<Description.Attributes, (Self) -> T>) -> T {
+		return attributes[keyPath: path](self)
+	}
+}
+
 // MARK: Relationship Access
 public extension EntityProxy {
 	/// Access to an Id of a `ToOneRelationship`.

Tests/JSONAPITests/Entity/EntityTests.swift

@@ -609,6 +609,26 @@ extension EntityTests {
 	}
 }
 
+// MARK: With a Meta Attribute
+
+extension EntityTests {
+	func test_MetaEntityAccessWorks() {
+		let entity1 = TestEntityWithMetaAttribute(id: "even",
+												  attributes: .init(),
+												  relationships: .none,
+												  meta: .none,
+												  links: .none)
+		let entity2 = TestEntityWithMetaAttribute(id: "odd",
+												  attributes: .init(),
+												  relationships: .none,
+												  meta: .none,
+												  links: .none)
+
+		XCTAssertEqual(entity1[\.metaAttribute], true)
+		XCTAssertEqual(entity2[\.metaAttribute], false)
+	}
+}
+
 // MARK: - Test Types
 extension EntityTests {
 
@@ -790,6 +810,22 @@ extension EntityTests {
 
 	typealias UnidentifiedTestEntityWithMetaAndLinks = NewEntity<UnidentifiedTestEntityType, TestEntityMeta, TestEntityLinks>
 
+	enum TestEntityWithMetaAttributeDescription: EntityDescription {
+		public static var jsonType: String { return "meta_attribute_entity" }
+
+		struct Attributes: JSONAPI.Attributes {
+			var metaAttribute: (TestEntityWithMetaAttribute) -> Bool {
+				return { entity in
+					(entity.id.rawValue.count % 2) == 0
+				}
+			}
+		}
+
+		typealias Relationships = NoRelationships
+	}
+
+	typealias TestEntityWithMetaAttribute = BasicEntity<TestEntityWithMetaAttributeDescription>
+
 	enum IntToString: Transformer {
 		public static func transform(_ from: Int) -> String {
 			return String(from)

Tests/JSONAPITests/XCTestManifests.swift

@@ -228,6 +228,7 @@ extension EntityTests {
         ("test_IntOver10_success", test_IntOver10_success),
         ("test_IntToString", test_IntToString),
         ("test_IntToString_encode", test_IntToString_encode),
+        ("test_MetaEntityAccessWorks", test_MetaEntityAccessWorks),
         ("test_NonNullOptionalNullableAttribute", test_NonNullOptionalNullableAttribute),
         ("test_NonNullOptionalNullableAttribute_encode", test_NonNullOptionalNullableAttribute_encode),
         ("test_nullableRelationshipIsNull", test_nullableRelationshipIsNull),