Swift Decodable Optional Key

Question

(This is a follow-up from this question: Using Decodable protocol with multiples keys.)

I have the following Swift code:

let additionalInfo = try values.nestedContainer(keyedBy: UserInfoKeys.self, forKey: .age)
age = try additionalInfo.decodeIfPresent(Int.self, forKey: .realage)

I know that if I use decodeIfPresent and don't have the property it will still handle it correctly if it's an optional variable.

For example the following JSON works to parse it using the code above.

{
    "firstname": "Test",
    "lastname": "User",
    "age": {"realage": 29}
}

And the following JSON works as well.

{
    "firstname": "Test",
    "lastname": "User",
    "age": {"notrealage": 30}
}

But the following doesn't work.

{
    "firstname": "Test",
    "lastname": "User"
}

How can I make all 3 examples work? Is there something similar to decodeIfPresent for nestedContainer?

Answer 1

You can use the following KeyedDecodingContainer function:

func contains(_ key: KeyedDecodingContainer.Key) -> Bool

Returns a Bool value indicating whether the decoder contains a value associated with the given key. The value associated with the given key may be a null value as appropriate for the data format.

For instance, to check if the "age" key exists before requesting the corresponding nested container:

struct Person: Decodable {
    let firstName, lastName: String
    let age: Int?

    enum CodingKeys: String, CodingKey {
        case firstName = "firstname"
        case lastName = "lastname"
        case age
    }

    enum AgeKeys: String, CodingKey {
        case realAge = "realage"
        case fakeAge = "fakeage"
    }

    init(from decoder: Decoder) throws {
        let values = try decoder.container(keyedBy: CodingKeys.self)
        self.firstName = try values.decode(String.self, forKey: .firstName)
        self.lastName = try values.decode(String.self, forKey: .lastName)

        if values.contains(.age) {
            let age = try values.nestedContainer(keyedBy: AgeKeys.self, forKey: .age)
            self.age = try age.decodeIfPresent(Int.self, forKey: .realAge)
        } else {
            self.age = nil
        }
    }
}