Swift - Decode/encode an array of generics with different types

Question

How can I decode/encode an array of different generic types?

I have a data structure, which has properties, that conform to a protocol Connection, thus I use generics:

// Data structure which saves two objects, which conform to the Connection protocol
struct Configuration<F: Connection, T: Connection>: Codable {
    var from: F
    var to: T
    private var id: String = UUID.init().uuidString

    enum CodingKeys: String, CodingKey {
        case from, to, id
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)
        self.from = try container.decode(F.self, forKey: .from)
        self.to = try container.decode(T.self, forKey: .to)
        self.id = try container.decode(String.self, forKey: .id)
    }

    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(from, forKey: .from)
        try container.encode(to, forKey: .to)
        try container.encode(id, forKey: .id)
    }
}

protocol Connection: Codable {
    var path: String { get set }
}


// Two implementations of the Connection protocol
struct SFTPConnection: Connection, Codable {
    var path: String
    var user: String
    var sshKey: String
}

struct FTPConnection: Connection, Codable {
    var path: String
    var user: String
    var password: String
}

This works fine when I know of what type the connections F and T are. But I have cases, where I want to load a configuration, not knowing which type F and T are.

public static func load<F: Connection, T: Connection>(for key: String) throws -> Configuration<F, T>? {
    // Load from UserDefaults
    guard let configurationData = defaults.object(forKey: key) as? Data else {
        return nil
    }

    // Decode
    guard let configuration = try? PropertyListDecoder().decode(Configuration<F, T>.self, from: configurationData) else {
        return nil
    }

    return configuration
}

// OR

func loadAll<F:Connection, T: Connection>() -> [String: Configuration<F, T>]? {
    return UserDefaults.standard.dictionaryRepresentation() as? [String: Configuration<F, T>]
}

In the above cases F and T could be of any unknown type, that conforms to the Connection protocol. So the above functions wouldn't work, since I would need to specify a specific type for F and T when calling the function, which I don't know.

In the second function, F alone could actually be of different types. That's where it gets difficult. I figured I need to somehow store the types of F and T in the User Defaults as well and then use them in the decode and encode function (thus discarding the generics). But I have no idea how I would elegantly do that.

I would appreciate any ideas on how to solve this problem!

Short story: You have to know the type. How many different types do you have? You could save a type string along with the data (for example as a dictionary) and in a switch statement map the string to the concrete type when retrieving the data. — vadian
– vadian, Commented Dec 16, 2019 at 22:39
@vadian I've added an answer, which is the best I could come up with. But it's not very pretty, since there is a lot of duplicate code. Do you have an idea how to shorten it? — Codey
– Codey, Commented Dec 17, 2019 at 11:51
How do you make use of Connection? The way you've written this, the caller has to know the precise type of the configuration in order to create the generic. That doesn't make a lot of sense. What calling code can access .password for example? The protocol doesn't include that, so Configuration can't use it. I don't understand why Configuration is generic. It feels like Connection needs more methods. — Rob Napier
– Rob Napier, Commented Dec 17, 2019 at 13:51
More critically to this particular problem, is the list of Connection implementations likely to be small and static (like these 2), or are there likely to be an arbitrary number implementations that you don't control? Both are solvable, but the latter is more complex. — Rob Napier
– Rob Napier, Commented Dec 17, 2019 at 13:53
@RobNapier yes that's correct, the caller needs to know the type of Connection and exactly that is the problem. The approach with using generics comes from another question (see stackoverflow.com/q/59353454/3272409) where my original problem was, that I couldn't use protocol with Codable. Now I see that using generics won't work. In the end, a caller should receive a Configuration object and has to know of which type both Connection's are. So now I know, that I have to store the type in some var in Configuration. With this information, the caller could access .password. — Codey
– Codey, Commented Dec 17, 2019 at 15:34

Codey · Accepted Answer · 2019-12-27 16:30:55Z

The following solutions resolves all the issues, that I had with generics and not knowing the specific type of Connection. The key to the solution was

saving the type of a Connection implementation in the implementation itself and
Using superEncoder and superDecoder to encode/decode the from and to properties.

This is the solution:

import Foundation

protocol Connection: Codable {
    var type: ConnectionType { get }
    var path: String { get set }
}


struct LocalConnection: Connection {
    let type: ConnectionType = ConnectionType.local

    var path: String
}


struct SFTPConnection : Connection {
    let type: ConnectionType = ConnectionType.sftp

    var path: String
    var user: String
    var sshKey: String

    init(path: String, user: String, sshKey: String) {
        self.path = path
        self.user = user
        self.sshKey = sshKey
    }
}


struct FTPConnection: Connection {
    let type: ConnectionType = ConnectionType.ftp

    var path: String
    var user: String
    var password: String
}


struct TFTPConnection: Connection {
    let type: ConnectionType = ConnectionType.tftp

    var path: String
}




enum ConnectionType : Int, Codable {
    case local
    case sftp
    case ftp
    case tftp

    func getType() -> Connection.Type {
        switch self {
        case .local: return LocalConnection.self
        case .sftp: return SFTPConnection.self
        case .ftp: return FTPConnection.self
        case .tftp: return TFTPConnection.self
        }
    }
}




struct Configuration {
    var from : Connection
    var to : Connection
    private var id = UUID.init().uuidString

    var fromType : ConnectionType { return from.type }
    var toType : ConnectionType { return to.type }

    init(from: Connection, to: Connection) {
        self.from = from
        self.to = to
    }
}


extension Configuration : Codable {

    enum CodingKeys: String, CodingKey {
        case id, from, to, fromType, toType
    }

    init(from decoder: Decoder) throws {
        let container = try decoder.container(keyedBy: CodingKeys.self)

        self.id = try container.decode(String.self, forKey: .id)

        var type : ConnectionType

        type = try container.decode(ConnectionType.self, forKey: .fromType)
        let fromDecoder = try container.superDecoder(forKey: .from)
        self.from = try type.getType().init(from: fromDecoder)

        type = try container.decode(ConnectionType.self, forKey: .toType)
        let toDecoder = try container.superDecoder(forKey: .to)
        self.to = try type.getType().init(from: toDecoder)
    }

    func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)

        try container.encode(self.id, forKey: .id)

        try container.encode(self.fromType, forKey: .fromType)
        let fromContainer = container.superEncoder(forKey: .from)
        try from.encode(to: fromContainer)

        try container.encode(self.toType, forKey: .toType)
        let toContainer = container.superEncoder(forKey: .to)
        try to.encode(to: toContainer)
    }
}

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