Decodable, doesn't decode optional enum with invalid value

The line

self.clubLevel = try container.decode(ClubLevel?.self, forKey: .clubLevel)

doesn't try to decode ClubLevel, assigning nil if unsuccessful. What it does is:

1. Try to decode nil (represented in JSON as null) for the clubLevel key. If unsuccessful,
2. Try to decode a ClubLevel for the clubLevel key. If unsuccessful,
3. Throw an error

So if the value for the clubLevel key is neither nil nor a valid ClubLevel representation, you'll get an error thrown. You'll note that this also means you'll get an error thrown if the clubLevel key is missing entirely (rather than being present with a value of nil).

Ignoring missing keys is done with decodeIfPresent:

self.clubLevel = try container.decodeIfPresent(ClubLevel.self, forKey: .clubLevel)

This will now:

1. Return nil if the clubLevel key is missing from the container. If they key is present,
2. Try to decode nil (represented in JSON as null) for the clubLevel key. If unsuccessful,
3. Try to decode a ClubLevel for the clubLevel key. If unsuccessful,
4. Throw an error

This is the default behaviour for decoding optionals in a compiler-generated implementation of init(from:). It will still throw an error in your case as the value for the clubLevel key is not a valid ClubLevel.

If you want to just try and decode a ClubLevel, assigning nil on the decoding failing for any reason (key missing, invalid value, etc.), then you want to use try?:

self.clubLevel = try? container.decode(ClubLevel.self, forKey: .clubLevel)

I came across the same problem and thought I'd add my solution for anyone interested.

The idea is to wrap the enum inside the following struct:

struct OptionalDecodableEnum<T>: Decodable where T: RawRepresentable, T.RawValue: Decodable {
    let value: T?

    init(from decoder: Decoder) throws {
        value = T(rawValue: try decoder.singleValueContainer().decode(T.RawValue.self))
    }
}

The main benefit is that you don't have to implement Decodable every time you want an optional enum. You also don't need the optional chaining with extra parentheses.

However, you will have to return the inner value property when you want to use it. E.g.

struct Foo: Decodable {
    enum ClubLevel: Int, Codable {
        case golden = 1, silver, bronze
    }

    let clubLevel: OptionalDecodableEnum<ClubLevel>
}

let foo = try jsonDecoder.decode(Foo.self, from: data)
print(String(describing: foo.clubLevel.value))