Using a dispatch_once singleton model in Swift

tl;dr: Use the class constant approach if you are using Swift 1.2 or above and the nested struct approach if you need to support earlier versions.

From my experience with Swift there are three approaches to implement the Singleton pattern that support lazy initialization and thread safety.

Class constant

class Singleton  {
   static let sharedInstance = Singleton()
}

This approach supports lazy initialization because Swift lazily initializes class constants (and variables), and is thread safe by the definition of let. This is now officially recommended way to instantiate a singleton.

Class constants were introduced in Swift 1.2. If you need to support an earlier version of Swift, use the nested struct approach below or a global constant.

Nested struct

class Singleton {
    class var sharedInstance: Singleton {
        struct Static {
            static let instance: Singleton = Singleton()
        }
        return Static.instance
    }
}

Here we are using the static constant of a nested struct as a class constant. This is a workaround for the lack of static class constants in Swift 1.1 and earlier, and still works as a workaround for the lack of static constants and variables in functions.

dispatch_once

The traditional Objective-C approach ported to Swift. I'm fairly certain there's no advantage over the nested struct approach but I'm putting it here anyway as I find the differences in syntax interesting.

class Singleton {
    class var sharedInstance: Singleton {
        struct Static {
            static var onceToken: dispatch_once_t = 0
            static var instance: Singleton? = nil
        }
        dispatch_once(&Static.onceToken) {
            Static.instance = Singleton()
        }
        return Static.instance!
    }
}

See this GitHub project for unit tests.

Since Apple has now clarified that static struct variables are initialized both lazy and wrapped in dispatch_once (see the note at the end of the post), I think my final solution is going to be:

class WithSingleton {
    class var sharedInstance: WithSingleton {
        struct Singleton {
            static let instance = WithSingleton()
        }

        return Singleton.instance
    }
}

This takes advantage of the automatic lazy, thread-safe initialization of static struct elements, safely hides the actual implementation from the consumer, keeps everything compactly compartmentalized for legibility, and eliminates a visible global variable.

Apple has clarified that lazy initializer are thread-safe, so there's no need for dispatch_once or similar protections

The lazy initializer for a global variable (also for static members of structs and enums) is run the first time that global is accessed, and is launched as dispatch_once to make sure that the initialization is atomic. This enables a cool way to use dispatch_once in your code: just declare a global variable with an initializer and mark it private.

From here