iOS Tutorial => Dispatch Semaphore

Example

DispatchSemaphore provides an efficient implementation of a traditional counting semaphore, which can be used to control access to a resource across multiple execution contexts.

A scenario for when to use a semaphore could be if you are doing some file reading/writing, if multiple tasks are trying to read and write from file at the same time, it could increase your performance to make each task wait its turn so as to not overburden the I/O controller.

Swift 3

func do2TasksAtATime () {
    print("starting long running tasks (2 at a time)")
    let sem = DispatchSemaphore(value: 2)            //this semaphore only allows 2 tasks to run at the same time (the resource count)
    for i in 0...7 {                                 //launch a bunch of tasks
        DispatchQueue.global().async {               //run tasks on a background thread
            sem.wait()                               //wait here if no resources available
            sleep(2)                                 //do some long task eg file access (here we are just sleeping for a 2 seconds for demonstration purposes)
            print("long task \(i) done! \(Date())")
            sem.signal()                             //let the semaphore know this resource is now available
        }
    }
}

Example output: (notice the time stamps)

starting long running tasks (2 at a time)
long task 0 done! 2017-02-16 07:11:53 +0000
long task 1 done! 2017-02-16 07:11:53 +0000
long task 2 done! 2017-02-16 07:11:55 +0000
long task 3 done! 2017-02-16 07:11:55 +0000
long task 5 done! 2017-02-16 07:11:57 +0000
long task 4 done! 2017-02-16 07:11:57 +0000
long task 6 done! 2017-02-16 07:11:59 +0000
long task 7 done! 2017-02-16 07:11:59 +0000

For more info, refer to the Apple Docs

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