Get singleton instance by Swift

swift_singleton_constant

/*
 * Way 1: Class constant
 * 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 SWManagement: NSObject {
    static let sharedInstance = SWManagement()
}

swift_singleton_gcd

/*
 * Way 2: 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 SWManagement: NSObject {
    class var sharedInstance: SWManagement {
        struct Static {
            static var onceToken: dispatch_once_t = 0
            static var instance: SWManagement? = nil
        }
        dispatch_once(&Static.onceToken) {
            Static.instance = SWManagement()
        }
        return Static.instance!
    }
}

swift_singleton_nestedStruct

/*
 * Way 3: Nested struct
 * 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.
 */
class SWManagement: NSObject {
    class var sharedInstance: SWManagement {
        struct Singleton {
            static let instance = SWManagement()
        }
        return Singleton.instance
    }
}