UserDefaults+Extras.swift

import Foundation

extension UserDefaults {
    enum Key: String {
        case customKey
    }
    
    //can be used for KVO
    @objc dynamic var customKey: Int {
       return get(.customKey) ?? 0
    }

    func set<T>(_ value: T?, for key: Key) {
        if let value = value {
            set(value, forKey: key.rawValue)
        } else {
            removeObject(forKey: key.rawValue)
        }
    }

    func get<T>(_ key: Key) -> T? {
        return object(forKey: key.rawValue) as? T
    }
    
    func removeAllObjects() {
        dictionaryRepresentation().keys.forEach(removeObject(forKey:))
    }
}

UserDefaults.set(true, for: .customKey)
let bool: Bool? = UserDefaults.get(.customKey) //Optional<True>
let int: Int? = UserDefaults.get(.customKey) //nil
UserDefaults.set(123, for: .customKey)
let int: Int? = UserDefaults.get(.customKey) //Optional<123>