Add ability to specify an Attribute needs to be transformed once it has been decoded.

Sources/JSONAPI/Resource/Attribute.swift

@@ -5,17 +5,28 @@
 //  Created by Mathew Polzin on 11/13/18.
 //
 
-public struct Attribute<Value: Codable>: Codable {
-	public let value: Value
+public struct TransformAttribute<RawValue: Codable, Transformer: JSONAPI.Transformer>: Codable where Transformer.From == RawValue {
+	private let rawValue: RawValue
+	
+	public let value: Transformer.To
+	
+	public init(rawValue: RawValue) throws {
+		self.rawValue = rawValue
+		value = try Transformer.transform(rawValue)
+	}
 }
 
-extension Attribute: Equatable where Value: Equatable {}
+public typealias Attribute<T: Codable> = TransformAttribute<T, IdentityTransformer<T>>
+
+extension TransformAttribute: Equatable where Transformer.From: Equatable, Transformer.To: Equatable {}
 
 // MARK: - Codable
-extension Attribute {
+extension TransformAttribute {
 	public init(from decoder: Decoder) throws {
 		let container = try decoder.singleValueContainer()
 		
+		let rawVal: RawValue
+		
 		// A little trickery follows. If the value is nil, the
 		// container.decode(Value.self) will fail even if Value
 		// is Optional. However, we can check if decoding nil
@@ -23,12 +34,14 @@ extension Attribute {
 		// type at which point we can store nil in `value`.
 		let anyNil: Any? = nil
 		if container.decodeNil(),
-			let val = anyNil as? Value {
-				value = val
-				return
+			let val = anyNil as? Transformer.From {
+				rawVal = val
+		} else {
+			rawVal = try container.decode(Transformer.From.self)
 		}
 		
-		value = try container.decode(Value.self)
+		rawValue = rawVal
+		value = try Transformer.transform(rawVal)
 	}
 	
 	public func encode(to encoder: Encoder) throws {
@@ -37,11 +50,24 @@ extension Attribute {
 		// See note in decode above about the weirdness
 		// going on here.
 		let anyNil: Any? = nil
-		if let _ = anyNil as? Value,
-			(value as Any?) == nil {
+		if let _ = anyNil as? Transformer.From,
+			(rawValue as Any?) == nil {
 			try container.encodeNil()
 		}
 		
-		try container.encode(value)
+		try container.encode(rawValue)
 	}
 }
+
+// MARK: - Transformers
+
+public protocol Transformer {
+	associatedtype From
+	associatedtype To
+	
+	static func transform(_ from: From) throws -> To
+}
+
+public enum IdentityTransformer<T>: Transformer {
+	public static func transform(_ from: T) throws -> T { return from }
+}

Sources/JSONAPI/Resource/Entity.swift

@@ -139,16 +139,23 @@ public extension Entity {
 	/// Access the attribute at the given keypath. This just
 	/// allows you to write `entity[\.propertyName]` instead
 	/// of `entity.relationships.propertyName`.
-	subscript<T>(_ path: KeyPath<EntityType.Attributes, Attribute<T>>) -> T {
+	subscript<T, TFRM: Transformer>(_ path: KeyPath<EntityType.Attributes, TransformAttribute<T, TFRM>>) -> TFRM.To {
 		return attributes[keyPath: path].value
 	}
 	
 	/// Access the attribute at the given keypath. This just
 	/// allows you to write `entity[\.propertyName]` instead
 	/// of `entity.relationships.propertyName`.
-	subscript<T>(_ path: KeyPath<EntityType.Attributes, Attribute<T>?>) -> T? {
+	subscript<T, TFRM: Transformer>(_ path: KeyPath<EntityType.Attributes, TransformAttribute<T, TFRM>?>) -> TFRM.To? {
 		return attributes[keyPath: path]?.value
 	}
+	
+	/// Access the attribute at the given keypath. This just
+	/// allows you to write `entity[\.propertyName]` instead
+	/// of `entity.relationships.propertyName`.
+	subscript<T, TFRM: Transformer, U>(_ path: KeyPath<EntityType.Attributes, TransformAttribute<T, TFRM>?>) -> U? where TFRM.To == U? {
+		return attributes[keyPath: path].flatMap { $0.value }
+	}
 }
 
 // MARK: Relationship Access