Thread safe shared instance "singletons" in Swift.

gistfile1.swift

class Singleton {
    class var sharedInstance: Singleton {
        struct Static {
            static var token: dispatch_once_t = 0
            static var instance: Singleton!
        }
        dispatch_once(&Static.token) {
            Static.instance = Singleton()
        }
        return Static.instance
    }
    
    init() {
        println("Init the Singleton!")
    }
}

//
// UPDATE
//

// Static variables in a struct are lazily initialized, so there's no need for the dispatch once.

class Singleton {
    class var sharedInstance: Singleton {
        struct Static {
            static let instance: Singleton! = Singleton()
        }
        return Static.instance
    }
    
    init() {
        println("Init the Singleton!")
    }
}

// Alternatively, a global variable, which is also lazily initialized, could be used to store the shared instance.
// A class method could optionally be added to provide a more object-oriented access to the shared instance.

let SingletoneSharedInstance: Singleton = Singleton()

class Singleton {
    class var sharedInstance: Singleton {
        return SingletoneSharedInstance
    }

    init() {
        println("Init the Singleton!")
    }
}

I think its worth mentioning one-liner singleton pattern like from here since this gist is topmost in google search

Hi, what part of this makes it thread safe?

@ilandbt Swift initialize global variables (line 40) in an atomic way when they are first accessed :)

In Swift 3.0, you can simplify things even further:

class Singleton {
    static let sharedInstance = Singleton()
    private init() { }
}

Hi, what part of this makes it thread safe?

True! :)