swift-codable-json

Swift Codable for JSON Parsing

Overview

Codable provides type-safe JSON parsing but strict typing means any mismatch crashes decoding. One date strategy per decoder, CodingKeys for every naming mismatch, custom decoders for nested structures.

Core principle: Design for API reality (not ideal JSON), fail gracefully with error handling, use optionals for unreliable data.

Basic Patterns

Pattern 1: Simple Mapping

// JSON: {"id": 123, "name": "Alice", "email": "[email protected]"}

struct User: Codable {
    let id: Int
    let name: String
    let email: String
}

// Usage:
let data = jsonString.data(using: .utf8)!
let user = try JSONDecoder().decode(User.self, from: data)

Auto-synthesis works when:

• Property names match JSON keys exactly
• All types match (String → String, Int → Int)
• All required properties present in JSON

Pattern 2: CodingKeys for Name Mapping

Problem: API uses snake_case, Swift uses camelCase.

// JSON: {"user_id": 123, "first_name": "Alice", "created_at": "2026-01-15"}

struct User: Codable {
    let userID: Int
    let firstName: String
    let createdAt: String

    enum CodingKeys: String, CodingKey {
        case userID = "user_id"
        case firstName = "first_name"
        case createdAt = "created_at"
    }
}

Rule: Every property must appear in CodingKeys, even if name matches.

// ❌ WRONG: Compiler error (missing properties in CodingKeys)
enum CodingKeys: String, CodingKey {
    case userID = "user_id"  // Missing firstName and createdAt
}

// ✅ CORRECT: All properties listed
enum CodingKeys: String, CodingKey {
    case userID = "user_id"
    case firstName = "first_name"
    case createdAt = "created_at"
}

Date Handling

Problem: Multiple Date Formats in Same Response

CRITICAL: JSONDecoder supports ONE date strategy at a time.

// JSON with mixed formats:
{
    "created": "2026-01-15T10:30:00Z",      // ISO8601
    "published": "15/01/2026",              // Custom format
    "timestamp": 1705316400                  // Unix timestamp
}

// ❌ WRONG: Can't set multiple strategies
let decoder = JSONDecoder()
decoder.dateDecodingStrategy = .iso8601  // Only applies to ONE field