The user’s defaults database is a key-value store that let’s you persist data across app launches. It is meant to be used to store user preferences. Using the Codeable protocol and a JSONEncoder you can very easily convert Swift types to JSON data in order to store them.
Save/Load Codeable structs
To save data into UserDefaults you must first encode it as JSON, and then save it using a specified key like this:
struct Settings: Codeable {
...
}
let userSettings = Settings()
let encoder = JSONEncoder()
if let encoded = try? encoder.encode(userSettings) {
UserDefaults.standard.set(encoded, forKey: "archiveKey")
}
Then you can load it like this:
if let savedSettingsData = UserDefaults.standard.object(forKey: "archiveKey") as? Data else {
let decoder = JSONDecoder()
let loadedSettings = try? decoder.decode(Settings.self, from: savedSettingsData) // Settings?
}
Reusability
Using Swift protocols and protocol inheritance, we can define aUserDefaultsPeristable protocol:
protocol DefaultInitializable {
init()
}
protocol UserDefaultsPeristable: Codable, DefaultInitializable {
static var key: String {get}
}
We also defined another protocol called DefaultInitializable for types than can be initialized with a default value using a default initializer. The default value will be used in cases where we’re looking for the specific type in UserDefaults and it does not exist.
In order to provide a default implementation of save and load functions we will also create a UserDefaultsPersister:
struct UserDefaultsPersister<A: UserDefaultsPeristable> {
static func save(_ value: A) {
let encoder = JSONEncoder()
if let encoded = try? encoder.encode(value) {
UserDefaults.standard.set(encoded, forKey: A.key)
}
}
static func load() -> A {
guard let savedSettingsData = UserDefaults.standard.object(forKey: A.key) as? Data else {
return A()
}
let decoder = JSONDecoder()
let loadedSettings = try? decoder.decode(A.self, from: savedSettingsData)
return loadedSettings ?? A()
}
}
And provide the default implementations for UserDefaultsPersistable protocol:
extension UserDefaultsPeristable {
func save() {
UserDefaultsPersister.save(self)
}
static func load() -> Self {
return UserDefaultsPersister.load()
}
}
Usage
For example, if you want to persist your app’s Audio settings:
struct AudioPreferences: UserDefaultsPeristable {
static var key = "userAudioPreferences"
var volume = 1.0 // default value
var bass = 0.5
var treble = 0.5
var quality = 1.0
}
var audioPreferences = AudioPreferences.load()
// user modifies volume
audioPreferences.volume = 1.0
audioPreferences.save() // Audio preferences are saved in user defaults
Declaring a type to be UserDefaultsPersistable allows you to easily store it in UserDefaults by only specifying a key. Obviously with this approach you can’t store more than one instance of the same type but when it comes to storing user preferences this is a safety mechanism rather than a limitation.
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Thank you for reading!