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Indeterminate Types with Codable in Swift
import Foundation
struct ImageAttachment: Codable {
let url: URL
let width: Int
let height: Int
}
struct AudioAttachment: Codable {
let title: String
let url: URL
let shouldAutoplay: Bool
}
struct Attachment: Codable {
let type: String
let payload: Any?
// MARK: Codable
private enum CodingKeys: String, CodingKey {
case type
case payload
}
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
type = try container.decode(String.self, forKey: .type)
if let decode = Attachment.decoders[type] {
payload = try decode(container)
} else {
payload = nil
}
}
func encode(to encoder: Encoder) throws {
var container = encoder.container(keyedBy: CodingKeys.self)
try container.encode(type, forKey: .type)
if let payload = self.payload {
guard let encode = Attachment.encoders[type] else {
let context = EncodingError.Context(codingPath: [], debugDescription: "Invalid attachment: \(type).")
throw EncodingError.invalidValue(self, context)
}
try encode(payload, &container)
} else {
try container.encodeNil(forKey: .payload)
}
}
// MARK: Registration
private typealias AttachmentDecoder = (KeyedDecodingContainer<CodingKeys>) throws -> Any
private typealias AttachmentEncoder = (Any, inout KeyedEncodingContainer<CodingKeys>) throws -> Void
private static var decoders: [String: AttachmentDecoder] = [:]
private static var encoders: [String: AttachmentEncoder] = [:]
static func register<A: Codable>(_ type: A.Type, for typeName: String) {
decoders[typeName] = { container in
try container.decode(A.self, forKey: .payload)
}
encoders[typeName] = { payload, container in
try container.encode(payload as! A, forKey: .payload)
}
}
}
struct Message: Codable {
let from: String
let text: String
let attachments: [Attachment]
}
// MARK: Usage
Attachment.register(ImageAttachment.self, for: "image")
Attachment.register(AudioAttachment.self, for: "audio")
let json = """
{
"from": "Guille",
"text": "Look what I just found!",
"attachments": [
{
"type": "image",
"payload": {
"url": "http://via.placeholder.com/640x480",
"width": 640,
"height": 480
}
},
{
"type": "audio",
"payload": {
"title": "Never Gonna Give You Up",
"url": "https://contoso.com/media/NeverGonnaGiveYouUp.mp3",
"shouldAutoplay": true,
}
},
{
"type": "pdf",
"payload": {
"title": "The Swift Programming Language",
"url": "https://contoso.com/media/SwiftBook.pdf"
}
}
]
}
""".data(using: .utf8)!
let decoder = JSONDecoder()
let message = try decoder.decode(Message.self, from: json)
print("\(message.from) says: '\(message.text)'")
for attachment in message.attachments {
switch attachment.payload {
case let image as ImageAttachment:
print("found 'image' at \(image.url) with size \(image.width)x\(image.height)")
case let audio as AudioAttachment:
print("found 'audio' titled \"\(audio.title)\"")
default:
print("unsupported attachment: \(attachment.type)")
}
}
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