如何使用 Argo 和 Swift 创建子类？

Question

我在 Swift 应用程序中使用 Argo 将 JSON 解码为对象。我有 JSON 这样的：

"activities": [
  {
    "id": "intro-to-the-program",
    "type": "session",
    "audio": "intro-to-the-program.mp3"
  },
  {
    "id": "goal-setting",
    "type": "session",
    "audio": "goal-setting.mp3"
  },
  {
    "id": "onboarding-quiz",
    "type": "quiz"
  }
]

基于'type'，其实我想实例化一个子class of the Activity class (ActivitySession, Activity测验等）并让 subclass 进行自己的解码。

我该怎么做？顶级 decode() 函数需要 return 类型的 Decoded<Activity>，而我目前的 none 方法似乎能够击败它。

Answer 1

这是一种方法，您可以通过打开类型来有条件地对其进行解码，并在给出无效类型时收到一条很好的错误消息。

struct ThingWithActivities: Decodable {
  let activities: [Activity]

  static func decode(json: JSON) -> Decoded<ThingWithActivities> {
    return curry(ThingWithActivities.init)
      <^> json <|| "activities"
  }
}

class Activity: Decodable {
  let id: String

  init(id: String) {
    self.id = id
  }

  class func decode(json: JSON) -> Decoded<Activity> {
    let decodedType: Decoded<String> = json <| "type"
    return decodedType.flatMap { type in
      switch type {
        case "session": return ActivitySession.decode(json)
        case "quiz": return ActivityQuiz.decode(json)
      default:
        return .Failure(.Custom("Expected valid type, found: \(type)"))
      }
    }
  }
}

class ActivitySession: Activity {
  let audio: String

  init(id: String, audio: String) {
    self.audio = audio
    super.init(id: id)
  }

  override static func decode(json: JSON) -> Decoded<Activity> {
    return curry(ActivitySession.init)
      <^> json <| "id"
      <*> json <| "audio"
  }

}

class ActivityQuiz: Activity {
  override static func decode(json: JSON) -> Decoded<Activity> {
    return curry(ActivityQuiz.init)
      <^> json <| "id"
  }
}

let activities: Decoded<ThingWithActivities>? = JSONFromFile("activities").flatMap(decode)
print(activities) 
// => Optional(Success(ThingWithActivities(activities: [ActivitySession, ActivitySession, ActivityQuiz])))

关键部分是提取类型并.flatMap处理它以有条件地解码成它应该是的类型。

如何使用 Argo 和 Swift 创建子类？

How can I create subclasses using Argo and Swift?

swift

argo