Swift Academy · Lesson

Manual encode(to:) and init(from:)

Take full control of serialization logic.

Manual encode(to:) and init(from:) is a free Swift Academy lesson on CoddyKit — lesson 3 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Swift Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

When Synthesis Is Not Enough

Sometimes the JSON shape does not line up with your properties — you need computed transforms, default fallbacks, or flattening. Then you implement encode(to:) and init(from:) by hand.

import Foundation

struct Temperature: Codable {
    var celsius: Double
}

print("Manual coding gives full control")

Declaring CodingKeys First

Manual coding starts with a CodingKeys enum. The keyed containers use these keys to read and write values.

import Foundation

struct User {
    var name: String
    var age: Int
    enum CodingKeys: String, CodingKey {
        case name
        case age
    }
}

print(User.CodingKeys.name.stringValue)

Implementing init(from:)

init(from decoder:) obtains a keyed container with container(keyedBy:), then decodes each property by its key.

import Foundation

struct User: Decodable {
    var name: String
    var age: Int
    enum CodingKeys: String, CodingKey { case name, age }
    init(from decoder: Decoder) throws {
        let c = try decoder.container(keyedBy: CodingKeys.self)
        name = try c.decode(String.self, forKey: .name)
        age = try c.decode(Int.self, forKey: .age)
    }
}

let u = try JSONDecoder().decode(User.self, from: "{\"name\":\"Ada\",\"age\":36}".data(using: .utf8)!)
print(u.name, u.age)

Implementing encode(to:)

encode(to encoder:) gets a mutable keyed container with container(keyedBy:) and writes each property with encode(_:forKey:).

import Foundation

struct User: Encodable {
    var name: String
    var age: Int
    enum CodingKeys: String, CodingKey { case name, age }
    func encode(to encoder: Encoder) throws {
        var c = encoder.container(keyedBy: CodingKeys.self)
        try c.encode(name, forKey: .name)
        try c.encode(age, forKey: .age)
    }
}

let data = try JSONEncoder().encode(User(name: "Ada", age: 36))
print(String(data: data, encoding: .utf8)!)

Full Codable by Hand

Implementing both methods makes the type fully Codable with custom logic on both ends.

import Foundation

struct Point: Codable {
    var x: Int
    var y: Int
    enum CodingKeys: String, CodingKey { case x, y }
    init(x: Int, y: Int) { self.x = x; self.y = y }
    init(from d: Decoder) throws {
        let c = try d.container(keyedBy: CodingKeys.self)
        x = try c.decode(Int.self, forKey: .x)
        y = try c.decode(Int.self, forKey: .y)
    }
    func encode(to e: Encoder) throws {
        var c = e.container(keyedBy: CodingKeys.self)
        try c.encode(x, forKey: .x)
        try c.encode(y, forKey: .y)
    }
}

let back = try JSONDecoder().decode(Point.self, from: try JSONEncoder().encode(Point(x: 1, y: 2)))
print(back.x, back.y)

Providing Default Values

Use decodeIfPresent with the nil-coalescing operator to supply a default when a key is missing, instead of throwing.

import Foundation

struct Settings: Decodable {
    var theme: String
    enum CodingKeys: String, CodingKey { case theme }
    init(from decoder: Decoder) throws {
        let c = try decoder.container(keyedBy: CodingKeys.self)
        theme = try c.decodeIfPresent(String.self, forKey: .theme) ?? "light"
    }
}

let s = try JSONDecoder().decode(Settings.self, from: "{}".data(using: .utf8)!)
print(s.theme)

Transforming Values

Manual decoding lets you transform raw JSON into a richer representation — here a Fahrenheit input is stored as Celsius.

import Foundation

struct Reading: Decodable {
    var celsius: Double
    enum CodingKeys: String, CodingKey { case fahrenheit }
    init(from decoder: Decoder) throws {
        let c = try decoder.container(keyedBy: CodingKeys.self)
        let f = try c.decode(Double.self, forKey: .fahrenheit)
        celsius = (f - 32) * 5 / 9
    }
}

let r = try JSONDecoder().decode(Reading.self, from: "{\"fahrenheit\":212}".data(using: .utf8)!)
print(r.celsius)

Flattening Nested JSON

You can read a nested container with nestedContainer(keyedBy:forKey:) and lift its values up onto a flat Swift struct.

import Foundation

struct Profile: Decodable {
    var city: String
    enum Keys: String, CodingKey { case address }
    enum AddressKeys: String, CodingKey { case city }
    init(from decoder: Decoder) throws {
        let c = try decoder.container(keyedBy: Keys.self)
        let a = try c.nestedContainer(keyedBy: AddressKeys.self, forKey: .address)
        city = try a.decode(String.self, forKey: .city)
    }
}

let json = "{\"address\":{\"city\":\"Rome\"}}"
let p = try JSONDecoder().decode(Profile.self, from: json.data(using: .utf8)!)
print(p.city)

Encoding into a Nested Container

Encoding can do the reverse: write a flat struct out into a nested JSON object using nestedContainer(keyedBy:forKey:).

import Foundation

struct Profile: Encodable {
    var city: String
    enum Keys: String, CodingKey { case address }
    enum AddressKeys: String, CodingKey { case city }
    func encode(to encoder: Encoder) throws {
        var c = encoder.container(keyedBy: Keys.self)
        var a = c.nestedContainer(keyedBy: AddressKeys.self, forKey: .address)
        try a.encode(city, forKey: .city)
    }
}

let data = try JSONEncoder().encode(Profile(city: "Rome"))
print(String(data: data, encoding: .utf8)!)

Validating During Decode

Because init(from:) is just an initializer, you can validate values and throw a custom error when they are out of range.

import Foundation

struct Score: Decodable {
    var value: Int
    enum CodingKeys: String, CodingKey { case value }
    init(from decoder: Decoder) throws {
        let c = try decoder.container(keyedBy: CodingKeys.self)
        let v = try c.decode(Int.self, forKey: .value)
        guard (0...100).contains(v) else {
            throw DecodingError.dataCorruptedError(forKey: .value, in: c, debugDescription: "out of range")
        }
        value = v
    }
}

let s = try JSONDecoder().decode(Score.self, from: "{\"value\":80}".data(using: .utf8)!)
print(s.value)

Mixing Manual and Synthesized

A type can implement just one method manually and let the compiler synthesize the other — for example a custom init(from:) with synthesized encoding.

import Foundation

struct Flag: Codable {
    var on: Bool
    enum CodingKeys: String, CodingKey { case on }
    init(from decoder: Decoder) throws {
        let c = try decoder.container(keyedBy: CodingKeys.self)
        on = (try? c.decode(Bool.self, forKey: .on)) ?? false
    }
    init(on: Bool) { self.on = on }
}

let data = try JSONEncoder().encode(Flag(on: true))
print(String(data: data, encoding: .utf8)!)

Quick Check: Manual Coding

Test your understanding of hand-written Codable methods.

Recap: Manual encode(to:) and init(from:)

You learned full control over coding:

Declare CodingKeys, then implement init(from:) and encode(to:).
Use container(keyedBy:) and encode/decode(_:forKey:).
decodeIfPresent plus ?? supplies defaults; nestedContainer flattens or nests JSON.
You can validate, transform, and mix manual with synthesized methods.

Frequently asked questions

Is the “Manual encode(to:) and init(from:)” lesson free?

Yes — the full text of “Manual encode(to:) and init(from:)” is free to read here on the web, and the Swift Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Swift Academy course, upgrade to CoddyKit PRO.

What will I learn in “Manual encode(to:) and init(from:)”?

Take full control of serialization logic. You practise Swift Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Swift Academy?

No prior experience is required. Swift Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.

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