main.swift

struct ServerResponse {
    var id: Int
    var username: String
    var fullName: String
    var reviewCounts = [Int]()
}

extension ServerResponse : Decodable {
    
    private enum CodingKeys : String, CodingKey {
        case id, user, reviewsCount = "reviews_count"
        
        enum User : String, CodingKey {
            case username = "user_name", realInfo = "real_info"
            
            enum RealInfo : String, CodingKey {
                case fullName = "full_name"
            }
        }
        
        enum ReviewsCount : String, CodingKey {
            case count
        }
    }
    
    init(from decoder: Decoder) throws {
        
        // top-level container
        let container = try decoder.container(keyedBy: CodingKeys.self)
        self.id = try container.decode(Int.self, forKey: .id)
        
        // container for { "user_name": "Tester", "real_info": { "full_name": "Jon Doe" } }
        let userContainer = try container.nestedContainer(keyedBy: CodingKeys.User.self, forKey: .user)
        self.username = try userContainer.decode(String.self, forKey: .username)
        
        // container for { "full_name": "Jon Doe" }
        let realInfoContainer = try userContainer.nestedContainer(keyedBy: CodingKeys.User.RealInfo.self, forKey: .realInfo)
        self.fullName = try realInfoContainer.decode(String.self, forKey: .fullName)
        
        // container for [{ "count": 4 }, { "count": 5 }]
        var reviewCountContainer = try container.nestedUnkeyedContainer(forKey: .reviewsCount)
        
        if let count = reviewCountContainer.count {
            self.reviewCounts.reserveCapacity(count)
        }
        
        while !reviewCountContainer.isAtEnd {
            let nestedReviewCountContainer = try reviewCountContainer.nestedContainer(keyedBy: CodingKeys.ReviewsCount.self)
            self.reviewCounts.append(try nestedReviewCountContainer.decode(Int.self, forKey: .count))
        }
    }
}
let jsonData = """
{
  "id": 1,
  "user": {
    "user_name": "Tester",
    "real_info": {
    "full_name":"Jon Doe"
  }
  },
  "reviews_count": [
    {
      "count": 4
    },
    {
      "count": 5
    }
  ]
}
""".data(using: .utf8)!

do {
    let response = try JSONDecoder().decode(ServerResponse.self, from: jsonData)
    print(response)
} catch {
    print(error)
}

// ServerResponse(id: 1, username: "Tester", fullName: "Jon Doe", reviewCounts: [4, 5])