Attempt to insert non-property list object when trying to save a custom object in Swift 3

I have a simple object which conforms to the NSCoding protocol.

import Foundation

class JobCategory: NSObject, NSCoding {
    var id: Int
    var name: String
    var URLString: String

    init(id: Int, name: String, URLString: String) {
        self.id = id
        self.name = name
        self.URLString = URLString
    }

    // MARK: - NSCoding
    required init(coder aDecoder: NSCoder) {
        id = aDecoder.decodeObject(forKey: "id") as? Int ?? aDecoder.decodeInteger(forKey: "id")
        name = aDecoder.decodeObject(forKey: "name") as! String
        URLString = aDecoder.decodeObject(forKey: "URLString") as! String
    }

    func encode(with aCoder: NSCoder) {
        aCoder.encode(id, forKey: "id")
        aCoder.encode(name, forKey: "name")
        aCoder.encode(URLString, forKey: "URLString")
    }
}

I'm trying to save an instance of it in UserDefaults but it keeps failing with the following error.

Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Attempt to insert non-property list object for key jobCategory'

This is the code where I'm saving in UserDefaults.

enum UserDefaultsKeys: String {
    case jobCategory
}

class ViewController: UIViewController {

    @IBAction func didTapSaveButton(_ sender: UIButton) {
        let category = JobCategory(id: 1, name: "Test Category", URLString: "http://www.example-job.com")

        let userDefaults = UserDefaults.standard
        userDefaults.set(category, forKey: UserDefaultsKeys.jobCategory.rawValue)
        userDefaults.synchronize()
    }
}

I replaced the enum value to key with a normal string but the same error still occurs. Any idea what's causing this?

You need to create Data instance from your JobCategory model using JSONEncoder and store that Data instance in UserDefaults and later decode using JSONDecoder.

struct JobCategory: Codable {
    let id: Int
    let name: String
}

// To store in UserDefaults
if let encoded = try? JSONEncoder().encode(category) {
    UserDefaults.standard.set(encoded, forKey: UserDefaultsKeys.jobCategory.rawValue)
}

// Retrieve from UserDefaults
if let data = UserDefaults.standard.object(forKey: UserDefaultsKeys.jobCategory.rawValue) as? Data, 
   let category = try? JSONDecoder().decode(JobCategory.self, from: data) {
     print(category.name)
}

Old Answer

You need to create Data instance from your JobCategory instance using archivedData(withRootObject:) and store that Data instance in UserDefaults and later unarchive using unarchiveTopLevelObjectWithData(_:), So try like this.

For Storing data in UserDefaults

let category = JobCategory(id: 1, name: "Test Category", URLString: "http://www.example-job.com")
let encodedData = NSKeyedArchiver.archivedData(withRootObject: category, requiringSecureCoding: false)
let userDefaults = UserDefaults.standard
userDefaults.set(encodedData, forKey: UserDefaultsKeys.jobCategory.rawValue)

For retrieving data from UserDefaults

let decoded  = UserDefaults.standard.object(forKey: UserDefaultsKeys.jobCategory.rawValue) as! Data
let decodedTeams = NSKeyedUnarchiver.unarchiveTopLevelObjectWithData(decoded) as! JobCategory
print(decodedTeams.name)