What is a singleton?

Swift version: 5.4

Paul Hudson @twostraws May 28th 2019

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:

struct Settings {
    static let shared = Settings()
    var username: String?

    private init() { }
}

Adding an initializer to the Settings struct automatically disables the default memberwise initializer, and it’s marked private so that it also can’t be called from outside the class. However, the struct creates an instance of itself as a static variable, which means the only instance of the Settings struct is the one it created: Settings.shared.