replacement of @synchronized objective-c in swift

synchronized.swift

/**
    Runs a closure in a synchronized way.

    :param: lock    The object to be used to synchronize
    :param: closure The closure that will be run in a synchronized way
*/
private func synchronized(lock: AnyObject, closure: @autoclosure () -> ()) {
    objc_sync_enter(lock)
    closure()
    objc_sync_exit(lock)
}