Codable protocol for JSON parsing in iOS Swift

Introduction

The Codable protocol helps your app easily convert data between Swift objects and JSON format. This makes reading and saving data simple and safe.

When you want to load data from a web service that sends JSON.

When you need to save app data as JSON files on the device.

When you want to send data from your app to a server in JSON format.

When you want to convert JSON data into Swift objects to use in your app.

When you want to convert Swift objects into JSON to share or store.

Syntax

iOS Swift

struct MyData: Codable {
  var name: String
  var age: Int
}

Use Codable on structs or classes to make them able to convert to/from JSON.

Properties must be standard types like String, Int, Double, Bool, or other Codable types.

Examples

A simple user struct that can be encoded to or decoded from JSON.

iOS Swift

struct User: Codable {
  var username: String
  var score: Int
}

This struct can represent a product and be converted to JSON easily.

iOS Swift

struct Product: Codable {
  var id: Int
  var name: String
  var price: Double
}

Optional properties like message can be nil if JSON does not include them.

iOS Swift

struct Response: Codable {
  var success: Bool
  var message: String?
}

Sample App

This example shows how to decode a JSON string into a Person object using Codable. It prints the person's name and age.

iOS Swift

import Foundation

struct Person: Codable {
  var name: String
  var age: Int
}

let jsonString = "{\"name\": \"Alice\", \"age\": 30}"

if let jsonData = jsonString.data(using: .utf8) {
  let decoder = JSONDecoder()
  if let person = try? decoder.decode(Person.self, from: jsonData) {
    print("Name: \(person.name), Age: \(person.age)")
  } else {
    print("Failed to decode JSON")
  }
}

Important Notes

Always handle decoding errors safely using try? or do-catch.

Use JSONEncoder to convert Swift objects back to JSON data.

Codable works best with simple data types and nested Codable structs.

Summary

Codable makes converting between Swift objects and JSON easy.

Use JSONDecoder to read JSON into Swift objects.

Use JSONEncoder to write Swift objects as JSON.