With JSONDecoder in Swift 4, can missing keys use a default value instead of having to be optional properties?

Question

Swift 4 added the new Codable protocol. When I use JSONDecoder it seems to require all the non-optional properties of my Codable class to have keys in the JSON or it throws an error.

Making every property of my class optional seems like an unnecessary hassle since what I really want is to use the value in the json or a default value. (I don't want the property to be nil.)

Is there a way to do this?

class MyCodable: Codable {
    var name: String = "Default Appleseed"
}

func load(input: String) {
    do {
        if let data = input.data(using: .utf8) {
            let result = try JSONDecoder().decode(MyCodable.self, from: data)
            print("name: \(result.name)")
        }
    } catch  {
        print("error: \(error)")
        // `Error message: "Key not found when expecting non-optional type
        // String for coding key \"name\""`
    }
}

let goodInput = "{\"name\": \"Jonny Appleseed\" }"
let badInput = "{}"
load(input: goodInput) // works, `name` is Jonny Applessed
load(input: badInput) // breaks, `name` required since property is non-optional

Answer 1

You can implement the init(from decoder: Decoder) method in your type instead of using the default implementation:

class MyCodable: Codable {
    var name: String = "Default Appleseed"

    required init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        if let name = try container.decodeIfPresent(String.self, forKey: .name) {
            self.name = name
        }
    }
}

You can also make name a constant property (if you want to):

class MyCodable: Codable {
    let name: String

    required init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        if let name = try container.decodeIfPresent(String.self, forKey: .name) {
            self.name = name
        } else {
            self.name = "Default Appleseed"
        }
    }
}

or

required init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    self.name = try container.decodeIfPresent(String.self, forKey: .name) ?? "Default Appleseed"
}

Re your comment: With a custom extension

extension KeyedDecodingContainer {
    func decodeWrapper<T>(key: K, defaultValue: T) throws -> T
        where T : Decodable {
        return try decodeIfPresent(T.self, forKey: key) ?? defaultValue
    }
}

you could implement the init method as

required init(from decoder: Decoder) throws {
    let container = try decoder.container(keyedBy: CodingKeys.self)
    self.name = try container.decodeWrapper(key: .name, defaultValue: "Default Appleseed")
}

but that is not much shorter than

    self.name = try container.decodeIfPresent(String.self, forKey: .name) ?? "Default Appleseed"