从呈现为字符串的 JSON 解析十进制

Question

使用 Xcode 10.2 和 iOS 12.x，我们能够从 json 字符串中提取 Decimal。对于 Xcode 11.1 和 iOS 13.1，它会抛出异常

应解码 Double，但找到了一个字符串/数据。

class MyClass : Codable {

     var decimal: Decimal?
 }

然后尝试解析它

let json = "{\"decimal\":\"0.007\"}"
let data = json.data(using: .utf8)
let decoder = JSONDecoder()
decoder.nonConformingFloatDecodingStrategy = .convertFromString(positiveInfinity: "s1", negativeInfinity: "s2", nan: "s3")
 do {
   let t = try decoder.decode(MyClass.self, from: data!)
 } catch {
   print(error)
 }

如果我将 json 字符串更改为

let json = "{\"decimal\":0.007}"

它有效，但我们又一次失去了精确度。有什么想法吗？

Answer 1

您需要扩展 KeyedDecodingContainer 并为 Decimal.Type 添加一个实现。

extension KeyedDecodingContainer {
    func decode(_ type: Decimal.Type, forKey key: K) throws -> Decimal {
        let stringValue = try decode(String.self, forKey: key)
        guard let decimalValue = Decimal(string: stringValue) else {
            let context = DecodingError.Context(codingPath: [key], debugDescription: "The key \(key) couldn't be converted to a Decimal value")
            throw DecodingError.typeMismatch(type, context)
        }
        return decimalValue
    }
}

这是一个例子：

let json = """
{
  "capAmount": "123.45"
}
"""

struct Status: Decodable {
    let capAmount: Decimal

    enum CodingKeys: String, CodingKey {
        case capAmount
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        capAmount = try container.decode(Decimal.self, forKey: .capAmount)
    }
}

// Execute it
if let data = json.data(using: .utf8){
    let status = try JSONDecoder().decode(Status.self, from: data)
    print(status.capAmount)
}

Answer 2

struct Root: Codable {
    let decimal: Decimal
}

---

extension Root {
    public init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        decimal = try Decimal(string: container.decode(String.self, forKey: .decimal)) ?? .zero
    }
}

---

let json = #"{"decimal":"0.007"}"# 
do {
    let root = try JSONDecoder().decode(Root.self, from: .init(json.utf8))
    print(root)
} catch {
    print(error)
}

这将打印出来

根（十进制：0.007）

Answer 3

该解码策略与将数字表示为字符串无关。您需要做的是实现init(from:) 并从那里转换字符串

class MyClass : Codable {
    var decimal: Double?

    enum CodingKeys: String, CodingKey {
        case decimal = "test"
    }

    required init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        decimal = Double(try container.decode(String.self, forKey: .decimal)
        //or if Decimal is used:
        //decimal = Decimal(string: try container.decode(String.self, forKey: .decimal)
    }
}

请注意，我在这里使用 Double 而不是 Decimal 以使其更简单

Answer 4

我相信一个更干净的解决方案是声明值不是像一个字符串而是像一个值：

"test": 0.007

有这样的结构：

struct Stuff {
     var test: Decimal
}

然后：

let decoder = JSONDecoder()
let stuff = try decoder.decode(Stuff.self, from: json)

否则你可以使用这个例子：

https://forums.swift.org/t/parsing-decimal-values-from-json/6906/3

Answer 5

类型应该是 Double 并且在解析中也定义为 Double。 Swift 会解决剩下的问题

struct MyClass: Decodable {
        let decimal: Double

        //can be renamed to follow the API name.
        enum CodingKeys: String, CodingKey {
            case decimal
        }
    }
    extension MyClass {
        init(from decoder: Decoder) throws {

            let values = try decoder.container(keyedBy: CodingKeys.self)
            decimal = try values.decode(Double.self, forKey: .decimal)

        }
    }