GCD with static functions of a struct

Question

how\'d you apply thread safe functionality to static functions of a struct

class SingleSome {

    struct Static {
        private static var instance: Singl         


        

Answer 1

Use a semaphore, dispatch_sync isn't appropriate because you need a synchronous return value from getInstance:

class SingleSome {

    struct Static {
        private static var instance: SingleSome?
        private static let lock = dispatch_semaphore_create(1)

        //need barrier sync
        static func getInstance(block: () -> SingleSome) -> SingleSome {
            dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER)
            var value = instance
            if value == nil {
                instance = block()
                value = instance
            }
            dispatch_semaphore_signal(lock)
            return value!
        }

        static func remove() { //need barrier sync
            dispatch_semaphore_wait(lock, DISPATCH_TIME_FOREVER)
            instance = nil
            dispatch_semaphore_signal(lock)
        }
    }

}

Also note that as written this is subject to deadlocks if block results in either remove or getInstance being called as dispatch_semaphore_t is not thread recursive.

Answer 2

You can use a private serial queue to ensure that only one thread can be in any of the critical sections at any instant.

class SingleSome {

    struct Static {
        private static let queue = dispatch_queue_create("SingleSome.Static.queue", nil)
        private static var instance: SingleSome?

        static func getInstance(block: () -> SingleSome) -> SingleSome {
            var myInstance: SingleSome?
            dispatch_sync(queue) {
                if self.instance == nil {
                    self.instance = block()
                }
                myInstance = self.instance
            }
            // This return has to be outside the dispatch_sync block,
            // so there's a race condition if I return instance directly.
            return myInstance!
        }

        static func remove() {
            dispatch_sync(queue) {
                self.instance = nil
            }
        }
    }

}