Swift JSON decoding with dependent types

My suggestion is to add two lazy instantiated properties to get the Player instances from the array.

The benefit of a lazy property over a computed property is that the value is calculated once and not until it’s being accessed the first time. And a custom init(from:) method is not needed.

struct Game: Decodable {
    let name: String
    let playerId1: Int
    let playerId2: Int

    enum CodingKeys: String, CodingKey { case name, playerId1, playerId2 }

    lazy var player1 : Player? = players.first{ $0.id == playerId1 }
    lazy var player2 : Player? = players.first{ $0.id == playerId2 }
   
}

Alternatively create a CodingUserInfoKey

extension CodingUserInfoKey {
    static let players = CodingUserInfoKey(rawValue: "players")!
}

and an extension of JSONDecoder

extension JSONDecoder {
    convenience init(players: [Player]) {
        self.init()
        self.userInfo[.players] = players
    }
}

and pass the players array in the userInfo object of the JSON decoder

let decoder = JSONDecoder(players: players)
let games = try! decoder.decode([Game].self, from: Data(gamesResponse.utf8))
dump(games[0].player1)

Now you can get the actual players in the init(from: method.

struct Game: Decodable {
    let name: String
    let player1: Player
    let player2: Player
    
    enum CodingKeys: String, CodingKey {
        case name, playerId1, playerId2
    }
}

extension Game {
    
    init(from decoder: Decoder) throws {
        let values = try decoder.container(keyedBy: CodingKeys.self)
        guard let players = decoder.userInfo[.players] as? [Player] else { fatalError("No players array available") }
        name = try values.decode(String.self, forKey: .name)
        let playerId1 = try values.decode(Int.self, forKey: .playerId1)
        let playerId2 = try values.decode(Int.self, forKey: .playerId2)
        player1 = players.first{ $0.id == playerId1 }!
        player2 = players.first{ $0.id == playerId2 }!
    }
}

The code assumes that the players array contains all Player instances corresponding to the playerId values. If not then you have to declare player1 and player2 as optional and remove the exclamation marks.