example.swift

public protocol JSONRepresentable: Encodable {

    func asJsonData() throws -> Data

    func asJson() throws -> [String : Any]?

}

extension JSONRepresentable {

    public func asJsonData() throws -> Data {
        return try JSONEncoder().encode(self)
    }
    
    public func asJson() throws -> [String : Any]? {
        return (try JSONSerialization.jsonObject(with: asJsonData(), options: .allowFragments)) as? [String : Any]
    }

}

//
// Somewhere inside a serialization function
//
default:
    let innerSkipFields = filterSkipFields(skipFields, property: label)
    if  let jsonRepresentable = value as? JSONRepresentable, // This forces static dispatch
        let maybeJson = try? jsonRepresentable.asJson(),
        let json = maybeJson,
        let mapValue = serializeFields(object: json, skipFields: innerSkipFields) {
        fields[label] = .mapValue(MapValue(fields: mapValue))
    } else {
        print("WARN - Unable to serialize property '\(label)' with value '\(value)' into FirestoreDocument.Value")
    }