main.swift

import Foundation

struct AnyCodingKey : CodingKey {
    
    let stringValue: String
    let intValue: Int?
    
    init(_ base: CodingKey) {
        self.stringValue = base.stringValue
        self.intValue = base.intValue
    }
    
    init?(intValue: Int) {
        self.intValue = intValue
        self.stringValue = String(intValue)
    }
    
    init?(stringValue: String) {
        self.stringValue = stringValue
        self.intValue = Int(stringValue)
    }
}

extension JSONDecoder {
    
    private static let rootKeyPathKey = CodingUserInfoKey(rawValue: "rootKeyPath")!
    
    private struct KeyPathDecodable<Value : Decodable> : Decodable {
        
        let value: Value
        
        init(from decoder: Decoder) throws {
            
            let keyPath = decoder.userInfo[JSONDecoder.rootKeyPathKey] as! [AnyCodingKey]
            var container = try decoder.container(keyedBy: AnyCodingKey.self)
            
            for key in keyPath.dropLast() {
                container = try container.nestedContainer(keyedBy: AnyCodingKey.self, forKey: key)
            }
            
            self.value = try container.decode(Value.self, forKey: keyPath.last!)
        }
    }
    
    func decode<T : Decodable>(_ type: T.Type, rootKeyPath: [AnyCodingKey], from data: Data) throws -> T {
        userInfo[JSONDecoder.rootKeyPathKey] = rootKeyPath
        return try decode(KeyPathDecodable<T>.self, from: data).value
    }
}

struct Address : Codable {
    var street: String
    var zip: String
    var city: String
    var state: String
}

let jsonString = """
{
  "foo": "hello world",
  "house": {
    "bar": 67.8,
    "address": {
      "state": "California",
      "street": "Apple Bay Street",
      "zip": "94608",
      "city": "Emeryville"
    }
  }
}
"""

enum PersonToAddressCodingKeys : CodingKey {
    case house, address
}

let jsonData = jsonString.data(using: .utf8)!

do {
    print(try JSONDecoder().decode(Address.self, rootKeyPath: [PersonToAddressCodingKeys.house, .address].map(AnyCodingKey.init), from: jsonData))
} catch {
    print(error)
}

// Address(street: "Apple Bay Street", zip: "94608", city: "Emeryville", state: "California")