What is singleton object?

Source: Hacking with Swift

Singletons are objects that should only ever be created once, then shared everywhere they need to be used. They are common on Apple’s platforms: FileManager, UserDefaults, UIApplication, and UIAccelerometer are all mostly used through their singleton implementations.

• The basic implementation of a Swift singleton looks like this:

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// create singleton object
class Settings {
    static let shared = Settings()

    var username: String?
    var email: String?

    private init() { }
}
// create inctences from a singleton object
let userSettings = Settings.shared
userSettings.username = "ohaok"

let userSettings_Two = Settings.shared
print(userSettings_Two.username)
// prints: ohaok

Adding a private initializer is important, because it stops other parts of our code from trying to create a Settings class instance. However, the class creates its own instance of itself as a static variable, which means the only instance of the Settings class is the one it created: Settings.shared.