Weak Singleton in Swift

Countless articles have been written arguing whether the Singleton pattern is an anti-pattern or not. I'm not going to get into that argument, though I do think there are valid use-cases for it from a pragmatic point of view. I recently wanted to implement such a singleton in Swift, but with the twist that if it isn't referenced from anywhere it will be released from memory; a so-called weak singleton. Fire it up in a playground, and see how it works :)

// The type being stored in the singleton
class Resource {  
  let name: String

  init () {
    print("init")     // Called once
    name = "I'm a resource"

  deinit {
    print("deinit")   // Called once

// The class serving up the singleton
class SharedResource {  
  static weak var weakInstance: Resource?

  static var sharedInstance: Resource {
    get {
      if let instance = weakInstance {
        return instance
      } else {
        let newInstance = Resource()
        weakInstance = newInstance
        return newInstance

// Reference the singleton from 3 different variables
// Note that init() will be called just one time
var str: Resource? = SharedResource.sharedInstance  
var str2: Resource? = SharedResource.sharedInstance  
var str3: Resource? = SharedResource.sharedInstance

// Dereference the singleton
// After the last variable dereferences it, deinit will be called
str = nil  
str2 = nil  
str3 = nil  

Frederik Nakstad

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Tokyo, Japan