Property Wrappers With Generic (Optional) User Defaults - ios

Question

With due reference:
https://github.com/apple/swift-evolution/blob/master/proposals/0258-property-wrappers.md#user-defaults
We've started to use property wrappers for the UserDefaults, it works seamlessly with non-optional properties.
However, setting nil of an optional property crashes with:
[User Defaults] Attempt to set a non-property-list object as an
NSUserDefaults/CFPreferences value for key "someKeyThatWeSet"
Terminating app due to uncaught exception
'NSInvalidArgumentException', reason: 'Attempt to insert non-property
list object null for key "someKeyThatWeSet"'
The code below can be tested on Playground directly:
#propertyWrapper
struct C2AppProperty<T> {
let key: String
let defaultValue: T
init(_ key: String, defaultValue: T) {
self.key = key
self.defaultValue = defaultValue
}
var wrappedValue: T {
get {
return UserDefaults.standard.object(forKey: key) as? T ?? defaultValue
}
set {
UserDefaults.standard.set(newValue, forKey: key)
}
}
}
struct C2User {
#C2AppProperty("userID", defaultValue: nil)
public static var publicUserID: String?
}
print(C2User.publicUserID)
C2User.publicUserID = "edusta"
print(C2User.publicUserID)
C2User.publicUserID = nil
print(C2User.publicUserID)
Expected:
nil
Optional<"edusta">
nil
Found:
nil
Optional<"edusta">
libc++abi.dylib: terminating with uncaught exception of type
NSException
What I've tried so far:
set {
// Comparing non-optional value of type 'T' to nil always returns false.
if newValue == nil {
UserDefaults.standard.removeObject(forKey: combinedKey)
} else {
UserDefaults.standard.set(newValue, forKey: combinedKey)
}
}
What kind of a check is needed here to catch that newValue is nil? Or an Optional<nil>?

Answer 1

This code works for me:
#propertyWrapper
struct UserDefault<T> {
private let key: String
private let defaultValue: T
private let userDefaults: UserDefaults
init(_ key: String, defaultValue: T, userDefaults: UserDefaults = .standard) {
self.key = key
self.defaultValue = defaultValue
self.userDefaults = userDefaults
}
var wrappedValue: T {
get {
guard let value = userDefaults.object(forKey: key) else {
return defaultValue
}
return value as? T ?? defaultValue
}
set {
if let value = newValue as? OptionalProtocol, value.isNil() {
userDefaults.removeObject(forKey: key)
} else {
userDefaults.set(newValue, forKey: key)
}
}
}
}
fileprivate protocol OptionalProtocol {
func isNil() -> Bool
}
extension Optional : OptionalProtocol {
func isNil() -> Bool {
return self == nil
}
}

Answer 2

Thanks to Michcio for providing the solution，
If we read property very frequently, the following code may have better performance. Use storage property can save value in memory, it won't be read value from disk every time.
#propertyWrapper
struct ReadOptimizeStorage<T> {
private let key: String
private let defaultValue: T
private let userDefaults: UserDefaults
init(key: String, defaultValue: T, userDefaults: UserDefaults = .standard) {
self.key = key
self.defaultValue = defaultValue
self.userDefaults = userDefaults
wrappedValue = userDefaults.object(forKey: key) as? T ?? defaultValue
}
var wrappedValue: T {
didSet {
if let value = wrappedValue as? OptionalProtocol, value.isNil() {
userDefaults.removeObject(forKey: key)
} else {
userDefaults.set(wrappedValue, forKey: key)
}
}
}
}
fileprivate protocol OptionalProtocol {
func isNil() -> Bool
}
extension Optional : OptionalProtocol {
func isNil() -> Bool {
return self == nil
}
}

Answer 3

I made a tiny spm package inspired by the ripples answers above:
https://github.com/mezhevikin/userdefault.git
import UserDefault
extension UserDefaults {
// Optional property
#UserDefault("login", .standard)
static var login: String?
// Property with default value
#UserDefault("isDarkMode", false)
static var isDarkMode: Bool
}
UserDefaults.login = "mezhevikin"
UserDefaults.isDarkMode = true