Swift - Codable struct with generic Dictionary var?

Question

I have a Swift struct that looks like this:

struct MyStruct: Codable {

  var id: String
  var name: String
  var createdDate: Date

}

To that, I would like to add another property: a [String:Any] dictionary. The result would look like this:

struct MyStruct: Codable {

  var id: String
  var name: String
  var createdDate: Date

  var attributes: [String:Any] = [:]
}

In the end, I would like to be able to serialize my MyStruct instance to a JSON string, and vice versa. However, when I go to build I get an error saying,

Type 'MyStruct' does not conform to protocol 'Codable'
Type 'MyStruct' does not conform to protocol 'Decodable'

It's clearly the attributes var that is tripping up my build, but I'm not sure how I can get the desired results. Any idea how I can code my struct to support this?

Answer 1

Since the comments already point out that Any type has nothing to do with generics, let me jump straight into the solution.

First thing you need is some kind of wrapper type for your Any attribute values. Enums with associated values are great for that job. Since you know best what types are to be expected as an attribute, feel free to add/remove any case from my sample implementation.

enum MyAttrubuteValue {
    case string(String)
    case date(Date)
    case data(Data)
    case bool(Bool)
    case double(Double)
    case int(Int)
    case float(Float)
}

We will be later wrapping attribute values from the [String: Any] dictionary into the wrapper enum cases, but first we need to make the type conform to the Codable protocols. I am using singleValueContainer() for the decoding/encoding so the final json will produce a regular json dicts.

extension MyAttrubuteValue: Codable {

    init(from decoder: Decoder) throws {
        let container = try decoder.singleValueContainer()
        if let string = try? container.decode(String.self) {
            self = .string(string)
        } else if let date = try? container.decode(Date.self) {
            self = .date(date)
        } else if let data = try? container.decode(Data.self) {
            self = .data(data)
        } else if let bool = try? container.decode(Bool.self) {
            self = .bool(bool)
        } else if let double = try? container.decode(Double.self) {
            self = .double(double)
        } else if let int = try? container.decode(Int.self) {
            self = .int(int)
        } else if let float = try? container.decode(Float.self) {
            self = .float(float)
        } else {
            fatalError()
        }
    }

    func encode(to encoder: Encoder) throws {
        var container = encoder.singleValueContainer()
        switch self {
        case .string(let string):
            try? container.encode(string)
        case .date(let date):
            try? container.encode(date)
        case .data(let data):
            try? container.encode(data)
        case .bool(let bool):
            try? container.encode(bool)
        case .double(let double):
            try? container.encode(double)
        case .int(let int):
            try? container.encode(int)
        case .float(let float):
            try? container.encode(float)
        }
    }

}

At this point we are good to go, but before we will decode/encode the attributes, we can use some extra interoperability between [String: Any] and [String: MyAttrubuteValue] types. To map easily between Any and MyAttrubuteValue lets add the following:

extension MyAttrubuteValue {

    var value: Any {
        switch self {
        case .string(let value):
            return value
        case .date(let value):
            return value
        case .data(let value):
            return value
        case .bool(let value):
            return value
        case .double(let value):
            return value
        case .int(let value):
            return value
        case .float(let value):
            return value
        }
    }

    init?(_ value: Any) {
        if let string = value as? String {
            self = .string(string)
        } else if let date = value as? Date {
            self = .date(date)
        } else if let data = value as? Data {
            self = .data(data)
        } else if let bool = value as? Bool {
            self = .bool(bool)
        } else if let double = value as? Double {
            self = .double(double)
        } else if let int = value as? Int {
            self = .int(int)
        } else if let float = value as? Float {
            self = .float(float)
        } else {
            return nil
        }
    }

}

Now, with the quick value access and new init, we can map values easily. We are also making sure that the helper properties are only available for the dictionaries of concrete types, the ones we are working with.

extension Dictionary where Key == String, Value == Any {
    var encodable: [Key: MyAttrubuteValue] {
        compactMapValues(MyAttrubuteValue.init)
    }
}

extension Dictionary where Key == String, Value == MyAttrubuteValue {
    var any: [Key: Any] {
        mapValues(\.value)
    }
}

Now the final part, a custom Codable implementation for MyStruct

extension MyStruct: Codable {

    enum CodingKeys: String, CodingKey {
        case id = "id"
        case name = "name"
        case createdDate = "createdDate"
        case attributes = "attributes"
    }

    public func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(id, forKey: .id)
        try container.encode(name, forKey: .name)
        try container.encode(createdDate, forKey: .createdDate)
        try container.encode(attributes.encodable, forKey: .attributes)
    }

    public init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        id = try container.decode(String.self, forKey: .id)
        name = try container.decode(String.self, forKey: .name)
        createdDate = try container.decode(Date.self, forKey: .createdDate)
        attributes = try container.decode(
            [String: MyAttrubuteValue].self, forKey: .attributes
        ).any
    }

}

This solution is fairly long, but pretty straight-forward at the same time. We lose automatic Codable implementation, but we got exactly what we wanted. Now you are able to encode ~Any~ type that already conforms to Codable easily, by adding an extra case to your new MyAttrubuteValue enum. One final thing to say is that we use similar approach to this one in production, and we have been happy so far.

That's a lot of code, here is a gist.