How do I handle decoding two possible types for one key in Swift?

Question

I have a Codable struct that is used to decode incoming JSON. Unfortunately, sometimes one of its key's value is a string, and sometimes it is a float. I was able to cobble a couple of do/try/catch blocks below to get it to work, but is there a better way to handle this?

struct Project: Codable {
    public let version: Float

    init(from decoder: Decoder) throws {
        var decodedVersion: Float = 1.0
        do {
            decodedVersion = try values.decode(Float.self, forKey: .version)
        } catch {
            do {
                if let inVersion = try Float(values.decode(String.self, forKey: .version)) {
                    decodedVersion = inVersion
                }
            } catch {
                throw error
            }
        }
        version = decodedVersion
    }
}

Answer 1

如果在您的JSON中，与密钥相关联的值有时可能为Float，有时为String（除了在后端修复此错误），您也可以采用此方法。

让我们说这是你的“搞笑”JSON

let data = """
[
{
    "magicField": "one"
},
{
    "magicField":1
}
]
""".data(using: .utf8)!

好，我们如何在Swift中优雅地表示这种数据？

struct Element:Decodable {
    let magicField: ???
}

我们希望magicField始终拥有一个值，有时为Float，有时为String。

我们可以使用Quantum Mechanics ...或Enum

来解决这个问题

让我们定义这种类型

enum QuantumValue: Decodable {

    case float(Float), string(String)

    init(from decoder: Decoder) throws {
        if let float = try? decoder.singleValueContainer().decode(Float.self) {
            self = .float(float)
            return
        }

        if let string = try? decoder.singleValueContainer().decode(String.self) {
            self = .string(string)
            return
        }

        throw QuantumError.missingValue
    }

    enum QuantumError:Error {
        case missingValue
    }
}

正如您所看到的，QuantumValue类型的值可以包含Float或String。始终为1且正好为1。

元素

我们现在可以定义JSON的常规元素

struct Element:Decodable {
    let magicField: QuantumValue
}

解码

就是这样。让我们最终解码JSON。

if let elms = try? JSONDecoder().decode([Element].self, from: data) {
    print(elms)
}

结果

[
Element(magicField: QuantumValue.string("one")),
Element(magicField: QuantumValue.float(1.0))
]

更新（回答Rob的评论）

switch magicField {
    case .string(let text):
        println(text)
    case .float(let num):
        println(num)
}