axiom-codable

Swift Codable Patterns

Comprehensive guide to Codable protocol conformance for JSON and PropertyList encoding/decoding in Swift 6.x.

Quick Reference

Decision Tree: When to Use Each Approach

Has your type...
├─ All properties Codable? → Automatic synthesis (just add `: Codable`)
├─ Property names differ from JSON keys? → CodingKeys customization
├─ Needs to exclude properties? → CodingKeys customization
├─ Enum with associated values? → Check enum synthesis patterns
├─ Needs structural transformation? → Manual implementation + bridge types
├─ Needs data not in JSON? → DecodableWithConfiguration (iOS 15+)
└─ Complex nested JSON? → Manual implementation + nested containers

Common Triggers

Error	Solution
"Type 'X' does not conform to protocol 'Decodable'"	Ensure all stored properties are Codable
"No value associated with key X"	Check CodingKeys match JSON keys
"Expected to decode X but found Y instead"	Type mismatch; check JSON structure or use bridge type
"keyNotFound"	JSON missing expected key; make property optional or provide default
"Date parsing failed"	Configure dateDecodingStrategy on decoder

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Part 1: Automatic Synthesis

Swift automatically synthesizes Codable conformance when all stored properties are Codable.

Struct Synthesis

// ✅ Automatic synthesis
struct User: Codable {
    let id: UUID              // Codable
    var name: String          // Codable
    var membershipPoints: Int // Codable
}

// JSON: {"id":"...", "name":"Alice", "membershipPoints":100}

Requirements:

• All stored properties must conform to Codable
• Properties use standard Swift types or other Codable types
• No custom initialization logic needed

Enum Synthesis Patterns

Pattern 1: Raw Value Enums

enum Direction: String, Codable {
    case north, south, east, west
}

// Encodes as: "north"

The raw value itself becomes the JSON representation.

Pattern 2: Enums Without Associated Values