JSONDecoder+keypath

Contents.swift

//: Playground - noun: a place where people can play

import UIKit


var json: String = """
{
    "result": {
        "persons": [
            {
                "name": "taro",
                "age": 25
            },
            {
                "name": "hanako",
                "age": 23
            }
        ],
        "code": 0,
        "flag": true
    }
}
"""

struct PersonsResponse: Decodable {
    struct Result: Decodable {
        struct Person: Decodable {
            let name: String
            let age: Int
        }
        
        let persons: [Person]
        let code: Int
        let flag: Bool
    }
    
    let result: Result
}



let data = json.data(using: .utf8)!
let decoder = JSONDecoder()

var date = Date()

for _ in 0..<1000 {
    do {
        let _ = try decoder.decode(PersonsResponse.self, from: data)
    } catch let error {
        print(error)
    }
}
print("1:", Date().timeIntervalSince(date))

extension JSONDecoder {
    func decode<T: Decodable>(_ type: T.Type, from data: Data, keyPath: String) throws -> T {
        let toplevel = try JSONSerialization.jsonObject(with: data)
        if let nestedJson = (toplevel as AnyObject).value(forKeyPath: keyPath) {
            let nestedJsonData = try JSONSerialization.data(withJSONObject: nestedJson)
            return try decode(type, from: nestedJsonData)
        } else {
            throw DecodingError.dataCorrupted(.init(codingPath: [], debugDescription: "Nested json not found for key path \"\(keyPath)\""))
        }
    }
}

struct Person: Decodable {
    let name: String
    let age: Int
}

date = Date()
for _ in 0..<1000 {
    do {
        let _ = try decoder.decode([Person].self, from: data, keyPath: "result.persons")
    } catch let error {
        print(error)
    }
}
print("2:", Date().timeIntervalSince(date))