Singleton

Junior PRO

Essence

Singleton is a creational pattern: a class has exactly one instance, created lazily, with global access. The defining ingredients are static let shared and private init(): without a private initializer it is not a singleton but a "shared instance", a convenient default next to which other instances can be created. UserDefaults.standard, FileManager.default, URLSession.shared and NotificationCenter.default are shared instances (you can create UserDefaults(suiteName:) or FileManager()); true singletons are UIApplication.shared and UIDevice.current. This is a classic trick question.