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.

An exploration of the Singleton pattern in Swift. All approaches below support lazy initialization and thread safety.

Issues and pull requests welcome.

class SingletonA {
    
    static let sharedInstance = SingletonA()
    
    init() {
        println("AAA");
    }
    
}

This approach supports lazy initialization because Swift lazily initializes class constants (and variables), and is thread safe by the definition of let.

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.

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

The traditional Objective-C approach ported to Swift.

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

I'm fairly certain there's no advantage over the nested struct approach but I'm including it anyway as I find the differences in syntax interesting.