派遣訊號量

DispatchSemaphore 提供傳統計數訊號量的有效實現，可用於控制跨多個執行上下文的資源訪問。

何時使用訊號量的情況可能是，如果你正在進行一些檔案讀/寫，如果多個任務同時嘗試從檔案讀取和寫入，則可以提高效能以使每個任務等待輪到所以不要讓 I / O 控制器負擔過重。

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
        }
    }
}

示例輸出:(注意時間戳）

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

有關詳細資訊，請參閱 Apple Docs