如何根据"类型"手动实现多个结构的可编码？

Question

考虑以下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://audio.com/NeverGonnaGiveYouUp.mp3",
"shouldAutoplay": true,
}
}
]
}

以及以下 Swift 结构：

struct ImageAttachment: Codable {
let url: URL
let width: Int
let height: Int
}
struct AudioAttachment: Codable {
let title: String
let url: URL
let shouldAutoplay: Bool
}
enum Attachment {
case image(ImageAttachment)
case audio(AudioAttachment)
case unsupported
}
extension Attachment: Codable {
private enum CodingKeys: String, CodingKey {
case type
case payload
}
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
let type = try container.decode(String.self, forKey: .type)
switch type {
case "image":
let payload = try container.decode(ImageAttachment.self, forKey: .payload)
self = .image(payload)
case "audio":
let payload = try container.decode(AudioAttachment.self, forKey: .payload)
self = .audio(payload)
default:
self = .unsupported
}
}
...
}

如果"有效载荷"键参数是平坦的(又名没有"有效载荷"(，我将如何处理类似的用例，例如：

{
"type": "image",
"url": "http://via.placeholder.com/640x480",
"width": 640,
"height": 480
}
{
"type": "audio",
"title": "Never Gonna Give You Up",
"url": "https://audio.com/NeverGonnaGiveYouUp.mp3",
"shouldAutoplay": true,
}

我不知道如何为平面情况正确实现 init 解码器，因为同时保留附件结构并允许未来的灵活性(添加更多类型的附件(。

Answer 1

你只需要做一个小的改变

extension Attachment: Codable {
private enum CodingKeys: String, CodingKey {
case type
case payload
}
init(from decoder: Decoder) throws {
let container = try decoder.container(keyedBy: CodingKeys.self)
let type = try container.decode(String.self, forKey: .type)
// The attachment data is nested if it has the `payload` key
let isNested = container.allKeys.contains(.payload)
switch type {
case "image":
// If the attachment data is nested inside the `payload` property, decode
// it from that property. Otherwise, decode it from the current decoder
let payload = try isNested ? container.decode(ImageAttachment.self, forKey: .payload) : ImageAttachment(from: decoder)
self = .image(payload)
case "audio":
// Same as image attachment above
let payload = try isNested ? container.decode(AudioAttachment.self, forKey: .payload) : AudioAttachment(from: decoder)
self = .audio(payload)
default:
self = .unsupported
}
}
}