JSONDecoder data decoding strategy for hexadecimal strings

JSONDecoder+HexData.swift

struct MyStruct: Decodable {
  let data: Data
  
  static func decodeHexData(_ decoder: Decoder) throws -> Data {
    let container = try decoder.singleValueContainer()
    let stringValue = try container.decode(String.self)

    guard let data = Data(hexString: stringValue) else {
      throw MyStructError.invalidHexString(stringValue)
    }

    return data
  }
  
  enum MyStructError: Swift.Error {
    case invalidHexString(String)
  }
}

UsingIt.swift

let jsonDecoder = JSONDecoder()
jsonDecoder.dataDecodingStrategy = .custom(MyStruct.decodeHexData(_:))

let json = """
{
  "data": "3605ac536759ad548700f5f4"
}
"""

let jsonData = json.data(using: .utf8)!
let decodedStruct = try jsonDecoder.decode(MyStruct.self, from: jsonData)

print(decodedStruct.data) // "12 bytes"