tl;dr: Use the nested struct approach outlined below.

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

Issues and pull requests welcome.

let _SingletonASharedInstance = SingletonA()

class SingletonA  {

    class var sharedInstance : SingletonA {
        return _SingletonASharedInstance
    }
    
}

We use a global constant because class constants are not yet supported.

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

In a future version of Swift, we should be able to make this global constant private, which would make this the best approach.

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

Unlike classes, structs do support static constants. By using a nested struct we can leverage its static constant as a class constant.

This is the approach I recommend.

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.