Odtnhj

I have the following JSON

"DynamicKey":6410,"Meta":"name":"","page":""

DynamicKey is unknown at compile time.I'm trying to find a reference how to
parse this struct using decodable.

public struct MyStruct: Decodable 
 public let unknown: Double
 public let meta: [String: String]

 private enum CodingKeys: String, CodingKey 
 case meta = "Meta"
 

Any ideas?

To decode an arbitrary string, you need a key like this:

// Arbitrary key
private struct Key: CodingKey, Hashable, CustomStringConvertible 
 static let meta = Key(stringValue: "Meta")!

 var description: String 
 return stringValue
 

 var hashValue: Int return stringValue.hash 

 static func ==(lhs: Key, rhs: Key) -> Bool 
 return lhs.stringValue == rhs.stringValue
 

 let stringValue: String
 init(_ string: String) self.stringValue = string 
 init?(stringValue: String) self.init(stringValue) 
 var intValue: Int? return nil 
 init?(intValue: Int) return nil 

This is a very general-purpose tool (expect for the static let meta) that can be used for all kinds of generic-key problems.

With that, you can find the first key that isn't .meta and use that as your dynamic key.

public init(from decoder: Decoder) throws 
 let container = try decoder.container(keyedBy: Key.self)

 meta = try container.decode([String: String].self, forKey: .meta)

 guard let dynamicKey = container.allKeys.first(where: $0 != .meta ) else 
 throw DecodingError.dataCorrupted(DecodingError.Context(codingPath: ,
 debugDescription: "Could not find dynamic key"))
 

 unknown = try container.decode(Double.self, forKey: dynamicKey)

All together as a playground:

import Foundation

let json = Data("""
"DynamicKey":6410,"Meta":"name":"","page":""
""".utf8)

public struct MyStruct: Decodable 
 public let unknown: Double
 public let meta: [String: String]

 // Arbitrary key
 private struct Key: CodingKey, Hashable, CustomStringConvertible 
 static let meta = Key(stringValue: "Meta")!
 var description: String 
 return stringValue
 

 var hashValue: Int return stringValue.hash 

 static func ==(lhs: Key, rhs: Key) -> Bool 
 return lhs.stringValue == rhs.stringValue
 

 let stringValue: String
 init(_ string: String) self.stringValue = string 
 init?(stringValue: String) self.init(stringValue) 
 var intValue: Int? return nil 
 init?(intValue: Int) return nil 
 

 public init(from decoder: Decoder) throws 
 let container = try decoder.container(keyedBy: Key.self)

 meta = try container.decode([String: String].self, forKey: .meta)

 guard let dynamicKey = container.allKeys.first(where: $0 != .meta ) else 
 throw DecodingError.dataCorrupted(.init(codingPath: ,
 debugDescription: "Could not find dynamic key"))
 

 unknown = try container.decode(Double.self, forKey: dynamicKey)
 



let myStruct = try! JSONDecoder().decode(MyStruct.self, from: json)
myStruct.unknown
myStruct.meta

This technique can be expanded to decode arbitrary JSON. Sometimes it's easier to do that, and then pull out the pieces you want, then to decode each piece. For example, with the JSON gist above, you could implement MyStruct this way:

public struct MyStruct: Decodable 
 public let unknown: Double
 public let meta: [String: String]

 public init(from decoder: Decoder) throws 
 let container = try decoder.singleValueContainer()
 let json = try container.decode(JSON.self)

 guard let meta = json["Meta"]?.dictionaryValue as? [String: String] else 
 throw DecodingError.dataCorrupted(.init(codingPath: ,
 debugDescription: "Could not find meta key"))
 
 self.meta = meta

 guard let (_, unknownJSON) = json.objectValue?.first(where: (key, _) in key != "Meta" ),
 let unknown = unknownJSON.doubleValue
 else 
 throw DecodingError.dataCorrupted(.init(codingPath: ,
 debugDescription: "Could not find dynamic key"))
 
 self.unknown = unknown
 

import UIKit

var str = """
"DynamicKey":6410,"Meta":"name":"","page":""
"""
public struct MyStruct: Decodable 
 public var unknown: Double?
 public var meta: [String: String]?

 public init(from decoder: Decoder) 

 guard let container = try? decoder.container(keyedBy: CodingKeys.self) else 
 fatalError()
 
 for key in container.allKeys 
 unknown = try? container.decode(Double.self, forKey: key)//) ?? 0.0
 if key.stringValue == "Meta" 
 meta = try? container.decode([String: String].self, forKey: key)
 

 
 print(container.allKeys)
 

 struct CodingKeys: CodingKey 
 var stringValue: String
 init?(stringValue: String) 
 self.stringValue = stringValue
 
 var intValue: Int?
 init?(intValue: Int) 
 return nil
 
 

 let jsonData = str.data(using: .utf8)!
 let jsonDecoder = JSONDecoder()
 let myStruct = try! jsonDecoder.decode(MyStruct.self, from: jsonData)
 print("Meta : (myStruct.meta)")
 print("Double : (myStruct.unknown)")

I've already answered a similar question

https://stackoverflow.com/a/48412139/1979882