Creating a expiry date for UserDefaults values - swift

Question

So I had this idea of creating an expiry for UserDefaults. This is the approach I was starting to take but I'm stuck.
struct TimedObject<T: Codable>: Codable {
let object: T
let expireDate: Date
}
and then:
extension UserDefaults {
func set<T: Codable>(_ value: T, forKey key: String, expireDate: Date) {
let timedObject = TimedObject<T>(object: value, expireDate: expireDate)
let encoder = JSONEncoder()
if let encoded = try? encoder.encode(timedObject) {
UserDefaults.standard.set(encoded, forKey: key)
}
override open class func value(forKey key: String) -> Any? {
guard let value = self.value(forKey: key) else {
return nil
}
if TimedObject<???>.self == type(of: value) { // This is where I'm stuck
...
}
}
So if I would name the type and not use generics I would easily solve this. But naturally I prefer to use generics. Can this be done?

Answer 1

I know OP is using a struct to wrap the stored value but I would still like to offer a different protocol based solution where any type that should be stored with an expiration date needs to conform to this protocol.
Here is the protocol for I am using
protocol TimedObject: Codable {
associatedtype Value: Codable
var value: Value { get }
var expirationDate: Date { get }
}
and the functions to store and retrieve from UserDefaults
extension UserDefaults {
func set<Timed: TimedObject>(_ value: Timed, forKey key: String) {
if let encoded = try? JSONEncoder().encode(value) {
self.set(encoded, forKey: key)
}
}
func value<Timed: TimedObject>(_ type: Timed.Type, forKey key: String) -> Timed.Value? {
guard let data = self.value(forKey: key) as? Data, let object = try? JSONDecoder().decode(Timed.self, from: data) else {
return nil
}
return object.expirationDate > .now ? object.value : nil
}
}
Finally an example
struct MyStruct: Codable {
let id: Int
let name: String
}
extension MyStruct: TimedObject {
typealias Value = Self
var value: MyStruct { self }
var expirationDate: Date {
.now.addingTimeInterval(24 * 60 * 60)
}
}
let my = MyStruct(id: 12, name: "abc")
UserDefaults.standard.set(my, forKey: "my")
let my2 = UserDefaults.standard.value(MyStruct.self, forKey: "my")

Answer 2

Since you're returning Any?, it is best to create another struct to point to TimedObject as you don't need the object property when returning Any?:
struct Expiry: Codable {
var expireDate: Date
}
struct TimedObject<T: Codable>: Timable {
let object: T
var expireDate: Date
}
override open class func value(forKey key: String) -> Any? {
guard let value = self.value(forKey: key) as? Data else {
return nil
}
if let timed = try? JSONDecoder().decode(Expiry.self, from: value) {
//do anything with timed
}
//make sure to return value
}
And here's a method to access TimedObject:
func timedObject<T: Codable>(forKey key: String) -> TimedObject<T>? {
guard let value = self.data(forKey: key) as? Data, let timed = try? JSONDecoder().decode(TimedObject<T>.self, from: value) else {
return nil
}
return value
}

Answer 3

If you make the value() method generic then you can reverse the process done in the set() method: retrieve the data and decode it as a TimedObject<T>.
However, I would choose a different name to avoid possible ambiguities with the exisiting value(forKey:) method. Also I see no reason why this should be a class method.
Note also that your generic set() method should call the non-generic version on the same instance.
extension UserDefaults {
func set<T: Codable>(_ value: T, forKey key: String, expireDate: Date) {
let timedObject = TimedObject(object: value, expireDate: expireDate)
let encoder = JSONEncoder()
if let encoded = try? encoder.encode(timedObject) {
set(encoded, forKey: key)
}
}
func expiringValue<T: Codable>(forKey key: String) -> T? {
guard let data = self.data(forKey: key) else {
return nil
}
let decoder = JSONDecoder()
guard let decoded = try? decoder.decode(TimedObject<T>.self, from: data) else {
return nil
}
// check expire date ...
return decoded.object
}
}
Example usage:
let val1 = UserDefaults.standard.expiringValue(forKey: "foo") as String?
let val2: String? = UserDefaults.standard.expiringValue(forKey: "bar")
In both cases, expiringValue(forKey:) is called with the inferred type.
Or in combination with optional binding:
if let val: String = UserDefaults.standard.expiringValue(forKey: "test") {
print(val)
}
Another option is to pass the desired type as an additional argument:
func value<T: Codable>(forKey key: String, as: T.Type) -> T? {
guard let data = self.data(forKey: key) else {
return nil
}
let decoder = JSONDecoder()
guard let decoded = try? decoder.decode(TimedObject<T>.self, from: data) else {
return nil
}
// check expire date ...
return decoded.object
}
which is then used as
let val = UserDefaults.standard.value(forKey: "foo", as: String.self)