当键仅在运行时已知时展平 JSON

Question

假设我们有如下 JSON 结构（通常用于 Firebase 的实时数据库）：

{
  "18348b9b-9a49-4e04-ac35-37e38a8db1e2": {
    "isActive": false,
    "age": 29,
    "company": "BALOOBA"
  },
  "20aca96e-663a-493c-8e9b-cb7b8272f817": {
    "isActive": false,
    "age": 39,
    "company": "QUONATA"
  },
  "bd0c389b-2736-481a-9cf0-170600d36b6d": {
    "isActive": false,
    "age": 35,
    "company": "EARTHMARK"
  }
}

预期的解决方案：

使用Decodable 我想将其转换为一个包含 3 个元素的数组：

struct BoringEntity: Decodable {
    let id: String
    let isActive: Bool
    let age: Int
    let company: String

    init(from decoder: Decoder) throws {
        // ...
    }
}

let entities: [BoringEntity] = try! JSONDecoder()...

id属性对应json对象的根字符串，例如：18348b9b-9a49-4e04-ac35-37e38a8db1e2。

解决方法：

我已经尝试了几种方法，但如果不使用辅助实体（或使用可选项）就无法获得 id 属性：

/// Incomplete BoringEntity version to make Decodable conformance possible.
struct BoringEntityIncomplete: Decodable {
    let isActive: Bool
    let age: Int
    let company: String
}

// Decode to aux struct
let decoded = try! JSONDecoder().decode([String : BoringEntityIncomplete].self, for: jsonData)
// Map aux entities to BoringEntity
let entities = decoded.map { BoringEntity(...) }

使用init(from: Decoder) 并不像其他情况那样简单，因为由于密钥未知，无法使用keyedContainer(,)。

Decodable 不适合这类情况吗？

Answer 1

在我回答你的问题之前有几件事：

1：注释 (// id) 使 JSON 无效。 JSON 不允许 cmets。

2：BoringEntity中的id属性从何而来？

struct BoringEntity: Decodable {
    let id: String          // where is it stored in the JSON???
    let isActive: Bool
    let age: Int
    let company: String
}

---

如果我忽略了这些事情，您可以将BoringEntity 的数组包装在一个结构（BoringEntities）中。不建议直接使用[BoringEntity]，因为您必须覆盖默认的init(from decoder:) 或Array。

这里的技巧是让JSONDecoder 通过container.allKeys 属性返回键列表：

struct BoringEntity: Decodable {
    let isActive: Bool
    let age: Int
    let company: String
}

struct BoringEntities: Decodable {
    var entities = [BoringEntity]()

    // This really is just a stand-in to make the compiler happy.
    // It doesn't actually do anything.
    private struct PhantomKeys: CodingKey {
        var intValue: Int?
        var stringValue: String 
        init?(intValue: Int) { self.intValue = intValue; self.stringValue = "\(intValue)" }
        init?(stringValue: String) { self.stringValue = stringValue }
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: PhantomKeys.self)

        for key in container.allKeys {
            let entity = try container.decode(BoringEntity.self, forKey: key)
            entities.append(entity)
        }
    }
}

用法：

let jsonData = """
{
  "18348b9b-9a49-4e04-ac35-37e38a8db1e2": {
    "isActive": false,
    "age": 29,
    "company": "BALOOBA"
  },
  "20aca96e-663a-493c-8e9b-cb7b8272f817": {
    "isActive": false,
    "age": 39,
    "company": "QUONATA"
  },
  "bd0c389b-2736-481a-9cf0-170600d36b6d": {
    "isActive": false,
    "age": 35,
    "company": "EARTHMARK"
  }
}
""".data(using: .utf8)!

let entities = try JSONDecoder().decode(BoringEntities.self, from: jsonData).entities

Answer 2

基础实体：

struct BoringEntity: Decodable {
    let id: String
    let isActive: Bool
    let age: Int
    let company: String
}

---

解决方案 1：使用没有密钥的额外结构

/// Incomplete BoringEntity version to make Decodable conformance possible.
private struct BoringEntityBare: Decodable {
    let isActive: Bool
    let age: Int
    let company: String
}

// Decode to aux struct
private let decoded = try! JSONDecoder().decode([String : BoringEntityBare].self, from: jsonData)
// Map aux entities to BoringEntity
let entities = decoded.map { BoringEntity(id: $0.key, isActive: $0.value.isActive, age: $0.value.age, company: $0.value.company) }
print(entities)

---

解决方案 2：使用包装器

感谢 Code Different，我能够将我的方法与他的 PhantomKeys 想法结合起来，但没有办法绕过它：必须始终使用额外的实体。

struct BoringEntities: Decodable {
    var entities = [BoringEntity]()

    // This really is just a stand-in to make the compiler happy.
    // It doesn't actually do anything.
    private struct PhantomKeys: CodingKey {
        var intValue: Int?
        var stringValue: String
        init?(intValue: Int) { self.intValue = intValue; self.stringValue = "\(intValue)" }
        init?(stringValue: String) { self.stringValue = stringValue }
    }

    private enum BareKeys: String, CodingKey {
        case isActive, age, company
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: PhantomKeys.self)

        // There's only one key
        for key in container.allKeys {
            let aux = try container.nestedContainer(keyedBy: BareKeys.self, forKey: key)

            let age = try aux.decode(Int.self, forKey: .age)
            let company = try aux.decode(String.self, forKey: .company)
            let isActive = try aux.decode(Bool.self, forKey: .isActive)

            let entity = BoringEntity(id: key.stringValue, isActive: isActive, age: age, company: company)
            entities.append(entity)
        }
    }
}

let entities = try JSONDecoder().decode(BoringEntities.self, from: jsonData).entities
print(entities)