@DiskBacked - Easy computed properties backed by UserDefaults.

DiskBacked.swift

/// This propety wrapper generates a variable that is backed by UserDefaults.
/// This can be used with any Codable type, and saves the item to disk as Data.
/// At initialization, you supply the key at which the the variable will be
/// saved, and the instance of UserDefaults in which to save it.
///
/// Like any other variable, it can be made get-only using `private(set)`, to
/// allow for better encapsulation, if so desired.
///
/// Example usage:
/// ```
/// @DiskBacked(key: "myObject", userDefaults: .standard)
/// private(set) var myObject: MyObject?
/// ```
@propertyWrapper
struct DiskBacked<T: Codable> {{
    private let key: String
    private let userDefaults: UserDefaults
    
    var wrappedValue: T? {
        get {
            userDefaults.codable(forKey: key)
        }
        set {
            try? userDefaults.saveCodable(newValue, forKey: key)
        }
    }
    
    init(key: String, userDefaults: UserDefaults) {
        self.key = key
        self.userDefaults = userDefaults
    }
}

private extension UserDefaults {
    
    func saveCodable<T: Codable>(_ object: T, forKey key: String) throws {
        let encoded = try JSONEncoder().encode(object)
        setValue(encoded, forKey: key)
    }
    
    func codable<T: Codable>(forKey key: String) -> T? {
        guard let encoded = data(forKey: key) else {
            return nil
        }
        return try? JSONDecoder().decode(T.self, from: encoded)
    }
    
}