Convert a custom object to Data to be saved in NSUserDefauts

Question

We have a custom object setup like so:

struct BallPark: Codable,Equatable  {
    static func == (lhs: LeanVenue, rhs: LeanVenue) -> Bool {
        return lhs.id == rhs.id
    }
    let id: String
    let name: String?
    let location: VenueCoordinates?
    let mapEnabled: Bool?
    let facilityStatus: String?
    }

struct VenueCoordinates: Codable {
    let latitude: String?
    let longitude: String?
    
    var lat: Double? {
        guard let latitud = latitude else { return nil}
        return Double(latitud)
    }
    var lon: Double? {
        guard let longitude = longitude else { return nil}
        return Double(longitude)
    }
}

We are trying to convert it to type Data so it can be saved to user defaults like so:

guard let bestBallParkToSave = theBestBallPark // this is of type BallPark, after fetching data

else {return}

 let encodedBestBallPark = NSKeyedArchiver.archivedData(withRootObject: bestBallParkToSave)
            
 UserDefaults.standard.set(encodedBestBallPark, forKey: "favoriteBallPark")

The problem is it causes an error when trying to convert:

'NSInvalidArgumentException', reason: '-[__SwiftValue encodeWithCoder:]: unrecognized selector sent to instance 0x6000027dca80'

I've tried using this stack overflow answer: How to convert custom object to Data Swift but there wasn't really a clear answer except making sure it conforms to Codable which I believe my struct and everything in it does. If you have any suggestions please let me know.

Answer 1

The issue there. is that you are mixing up Codable with NSCoding. To use NSKeyedArchiver's archivedData method you need to have a class that conforms to NSCoding. In you case you have a structure that conforms to Codable so you need to use JSONEncoder encode method. Note: Your equatable method declaration was wrong:

struct BallPark: Codable, Equatable  {
    static func == (lhs: Self, rhs: Self) -> Bool {
        return lhs.id == rhs.id
    }
    let id: String
    let name: String?
    let location: VenueCoordinates?
    let mapEnabled: Bool?
    let facilityStatus: String?
}

---

struct VenueCoordinates: Codable {
    let latitude: String?
    let longitude: String?
    
    var lat: Double? {
        guard let latitud = latitude else { return nil}
        return Double(latitud)
    }
    var lon: Double? {
        guard let longitude = longitude else { return nil}
        return Double(longitude)
    }
}

---

let bestBallParkToSave = BallPark.init(id: "1", name: "Steve", location: .init(latitude: "20.0", longitude: "40.0"), mapEnabled: true, facilityStatus: "a status")

do {
    let encodedBestBallPark = try JSONEncoder().encode(bestBallParkToSave)
    UserDefaults.standard.set(encodedBestBallPark, forKey: "favoriteBallPark")
    if let ballParkData = UserDefaults.standard.data(forKey: "favoriteBallPark") {
        let loadedBallPark = try JSONDecoder().decode(BallPark.self, from: ballParkData)
        print(loadedBallPark)  // BallPark(id: "1", name: Optional("Steve"), location: Optional(VenueCoordinates(latitude: Optional("20.0"), longitude: Optional("40.0"))), mapEnabled: Optional(true), facilityStatus: Optional("a status"))
    }
} catch {
     print(error)
}

---

You can also make your life easier extending UserDefaults and create customs encoding and decoding methods:

extension UserDefaults {
    func decodeObject<T: Decodable>(forKey defaultName: String, using decoder: JSONDecoder = .init()) throws -> T {
        try decoder.decode(T.self, from: data(forKey: defaultName) ?? .init())
    }
    func encode<T: Encodable>(_ value: T, forKey defaultName: String, using encoder: JSONEncoder = .init()) throws {
        try set(encoder.encode(value), forKey: defaultName)
    }
}

---

Usage:

let bestBallParkToSave = BallPark(id: "1", name: "Steve", location: .init(latitude: "20.0", longitude: "40.0"), mapEnabled: true, facilityStatus: "a status")
do {

    try UserDefaults.standard.encode(bestBallParkToSave, forKey: "favoriteBallPark")
    let loadedBallPark: BallPark = try UserDefaults.standard.decodeObject(forKey: "favoriteBallPark")
    print(loadedBallPark)  // BallPark(id: "1", name: Optional("Steve"), location: Optional(VenueCoordinates(latitude: Optional("20.0"), longitude: Optional("40.0"))), mapEnabled: Optional(true), facilityStatus: Optional("a status"))
    
} catch {
     print(error)
}